EARLY DETECTION METHOD OF CORROSION ON

BURIED STEEL GAS PIPELINE USING WIRELESS

SENSOR NETWORK

A PROJECT REPORT

Submited by

MANOJ KUMAR 348074016

RAGHAV KUMAR JHA 348074020

ALOK KUMAR 348074005

In partial fulfilment for the award of the degree

Of

BACHELOR OF ENGINEERING

IN

ELECTRONICS AND INSTRUMENTATION ENGINEERING

AARUPADAI VEEDU INSTITUTE OF TECHNOLOGY(VINAYAKA MISSIONS UNIVERSITY)

OLD MAHABALIPURAM ROADPAIYANOORDIST-KANCHIPURAM

APRIL2011

1

VINAYAKA MISSIONS UNIVERSITYAARUPADAI VEEDU INSTITUTE OF TECHNOLOGY

CHENNAI-603104

BONAFIDE CERTIFICATE

This is to certified that this project report ldquoIMPROVEMENT OF

HEAT FURNACE TEMPERATURE CONTROL VIA PLC METHODrdquo is

the bonafide work of ldquoMANOJ KUMAR (348074016) RAGHAV KUMAR

JHA (348074020) and ALOK KUMAR(348074005) rdquo in partial fulfillement

of the requirements for the award of Degree of Bachelor of Engineering in

Electronics and Instrumentation Engineering from Aarupadai Veedu Institute of

Technology under our guidance and supervision

SIGNATURE SIGNATURE

DrNVEERAPPAN ME PhD MIE MISTE AssoProf LChitra

HEAD OF THE DEPARTMENT ASSOCIATE PROF

Department of Electrical and Department of Electronics and Electronics Engineering Instrumentation Engineering AVITpaiyanoor AVITpaiyanoor Chennai-603104 Chennai-603104 Certified that the candidates was examined in the viva-voce Examination

held on helliphelliphelliphelliphelliphelliphellip

INTERNAL EXAMINER EXTERNAL EXAMINER

2

ACKNOWLEDGEMENT

At the outset we thank ldquo THE ALMIGHTYrdquo for his divine guidance and blessings

throughout our project work

We are grateful to DrASGanesan Pro-Chancellor Vinayaka Missions University

and Dr(Mrs)NRAlameluMEPhDPrincipal AVITPaiyanoor for providing us

adequate academic facilities

We are very mush indebted to DrNVeerappanMEPhDMIE MISTE HOD

Department of EEE for granting permission to realize this project and for his valuable

suggestions during the review of the project work We wish to express our deep sense of

gratitude to our project guide MrsLChithra ME Associate Professor Department of

EEE for her excellent guidance and continous encouragement in completion of the project

work

We are also grateful to Mrs MChithra BEMBA Lecture Mrs TManjula

ME Assistant Professor Department of EEE for their valuable suggestions during this

project and co-opeartion in finalizing the project report

We are also thankful to all other Faculty members and non-teaching staff of EEE

Department for their co-operartion for the successful completion of this project Finally we

extend our professed thanks to our parents family members and friends for their help and

perennial encouragement towards realizing this project successfully

MANOJ KUMAR -348074016

RAGHAV KUMAR JHA -348074020

ALOK KUMAR -348074005

3

ABSTRACT

This Project deal with the early detection on any abnormality

on the pipeline such as leakage is essential for efficient management As to date

wireless sensors have been widely used to gather information in monitoring

reliability of the pipeline The commonly used sensors are corrosion and

pressure sensors One of the issues that relate to pipeline monitoring is the

reactive rather than proactive maintenance approach to leakage and

abnormality The objective of this project is to develop a method for early

detection of corrosion which the buried pipeline is exposed to changing

temperatures and gas flow pressures This will ensure that maintenance works

can be done quickly to prevent pipe burst as well as to minimize operational

cost Also to shut down the process imediately by using a control action ie

solenoid valve

4

TABLE OF CONTENTS

CHAPTER No TITLE PAGE No

ABSTRACT

LIST OF SYMBOLS

1 INTRODUCTION

11 CORROSION DETECTION

12 BLOCK DIAGRAM

2 PROGRAMABLE LOGIC CONTROLLER

21 INTRODUCTION

22 PLC HISTORY

23 PLC HARDWARE

24 WORKING OF PLC

25 PROGRAMMING THE PLC

3 CORROSION

31 INTRODUCTION

32 TYPES OF CORROSION

33 EFFECT OF CORROSION

34 CORROSION PREVENTION

4 SENSORS

41 CORROSION SENSOR

5

42 RELAY

43 PRESSURE SENSOR

5 WIRELESS NETWORK

51 ENCODER

52 TRANSMITTER

53 RECEIVER

54 DECODER

6 POWER SUPPLY

61 RECTFIER

62 FILTER

63 VOLTAGE REGULATOR

7 CONTROL ACTION

71 BUZZER

72 BUFFER AMPLIFIER

73 SOLONIOD VALVE

REFERENCES

6

CHAPTER ndash 1

INTRODUCTION

7

1 INTRODUCTION

11 Oil gas and water are the natural resources that have been

the key of life and source of economy for most countries in the world

These natural resources are transported from its original plants or storage

through extended pipelines which spreads throughout the countries In

some countries pipelines also being used to supply gas directly to users

These pipelines serve as the backbone between producers and

consumers Maintaining pipelines is essential to sustain economic growth

political stability and also safety Delays in detecting leakage on pipeline

may lead to more serious matters such as fire and fatality Wireless

technologies have evolved so rapidly now-a-days and widely used in many

applications and services This mainly because it can remotely placed and

uses its own power source such as battery to power itself There are

number of technologies to monitor and protect pipelines Most of these

are designed specifically for detecting and locating pipeline leakages

These technologies allow a remote facility to detect and to report the

positions of any leakage Most of these solutions rely on the availability of

a network to transfer the information and report leakages Pipeline

monitoring systems have been using wireless devices as part of the

system communication and information transfer

One of the industries that have taken advantage of wireless

sensors is oil and gas industry Gas main distribution method is using

pipeline Monitoring of pipeline is very crucial because of its valuable

resources as well as for safety precautions Wireless sensors have been

widely used to monitor the health and the condition of the pipeline

Continuous monitoring of pipelines is necessary to ensure the safety

operation of pipelines transmission However several issues and

problems have been discussed related to the usage of wireless sensors in

pipeline monitoring system One of the issues is to detect leakage or

abnormality of the pipeline as early as possible particularly in gas

pipeline It is very important that we detect the signs of pipeline defects

as early as possible because if that allow it to go on it could cause some

8

serious problems later on such as endanger the environment and if the

pipeline near human population area it can be a disaster In the section

of pipeline

ruptures at 146 millions of cubic feet per day (MMCFD) of gas for pressure

of 1198 psi and 544deg Celsius was flowing through the 18-inch diameter

pipe and caused one life Structural defects on gas pipeline may caused

by several factors such as corrosion Several factors have been identified

that lead to the event of buried pipeline corrosion such as pipe coatings

soil conditions changing of temperature stresses pipe pressure and

cyclic loading effects

The objective of this project is to develop a method for early

detection of corrosion which the buried pipeline is exposed to changing

temperatures and gas flow pressures so that prompt actions can be done

to prevent any incidents or fatality It is a necessity to have system that

detects signs or abnormalities that will resulted to leakage event In a

pipeline monitoring and inspection system has several tasks to be

performed for natural gas pipelines environment to ensure the integrity of

the pipeline These include measuring pipe wall thickness measuring

velocity and flow of gas detecting gas contamination in pipeline and also

determining structural defects on pipes In gas pipelines commonly

placed in hazardous environmental such as deserts underwater and

buried deep These conditions can cause deterioration or even damage

due to corrosion erosion and fatigue Major problem is to detect corrosion

cracks

9

Fig of full circuit

10

RF Receiver Decoder PLC Load

Power Supply

12 BLOCK DIAGRAM

TRANSMITTER SECTION

RECEIVER SECTION

11

CHAPTER ndash 2

PROGRAMMABLE LOGIC CONTROLLER

12

2 PROGRAMMABLE LOGIC CONTROLLER

21 INTRODUCTION

Programmable logic controllers (PLCs) are members of the

computer family capable of storing instructions to control functions such as sequencing

timing and counting which control a machine or a process The PLC is composed of two

basic sections the Central Processing Unit (CPU) and the InputOutput (IO) interface

system The PLC measures input signals coming from a machine and through the internal

program provides output or control back to the machine Ladder logic is the

programming language used to represent electrical sequences of operation In hardwired

circuits the electrical wiring is connected from one device to another according to logic of

operation In a PLC the devices are connected to the input interface the outputs are

connected to the output interface and the actual wiring of the components is done

electronically inside the PLC using ladder logic This is known as soft wired PLC is a

device that is capable of being programmed to perform a controlling function Before the

advent of PLC the problem of industrial control was usually solved by relays or

hardwired solid-state logic blocks These are very flexible in design and easy for

maintenance personal to understand However they involved a vast amount of

interconnection For the wiring cost to be minimized relays and logic blocks had to be

kept together This led to development of control panel concept for larger and more

complex logic control system The PLC was first conceived by group of engineers from

hydramatic division of GM in 1968This was designed to provide flexibility in control

based on programming and executing logic instruction Adopting the ladder diagram

programming language simplifying maintenance and reducing the cost of spare parts

inventories realized major advantages

22PLC HISTORY

In the late 1960s PLCs were first introduced The primary reason for designing such a

device was eliminating the large cost involved in replacing the complicated relay based

machine control systems Bedford Associates (Bedford MA) proposed something called a

Modular Digital Controller (MODICON) to a major US car manufacturer Other companies

at the time proposed computer based schemes one of which was based upon the PDP-8 The

MODICON 084 brought the worlds first PLC into commercial production

13

When production requirements changed so did the control system This becomes very

expensive when the change is frequent Since relays are mechanical devices they also have a

limited lifetime which required strict adhesion to maintenance schedules Troubleshooting

was also quite tedious when so many relays are involved Now picture a machine control

panel that included many possibly hundreds or thousands of individual relays The size

could be mind boggling How about the complicated initial wiring of so many individual

devices These relays would be individually wired together in a manner that would yield the

desired outcome Were there problems You bet These new controllers also had to be

easily programmed by maintenance and plant engineers The lifetime had to be long and

programming changes easily performed They also had to survive the harsh industrial

environment Thats a lot to ask The answers were to use a programming technique most

people were already familiar with and replace mechanical parts with solid-state ones

In the mid70acircbdquocents the dominant PLC technologies were sequencer state-machines and the bit-

slice based CPU The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs

Conventional microprocessors lacked the power to quickly solve PLC logic in all but the

smallest PLCs As conventional microprocessors evolved larger and larger PLCs were being

based upon them However even today some are still based upon the 2903(ref A-Bs PLC-3)

Modicon has yet to build a faster PLC than their 984ABX which was based upon the 2901

Communications abilities began to appear in approximately 1973 The first such system was

Modicons Modbus The PLC could now talk to other PLCs and they could be far away from

the actual machine they were controlling They could also now be used to send and receive

varying voltages to allow them to enter the analog world Unfortunately the lack of

standardization coupled with continually changing technology has made PLC

communications a nightmare of incompatible protocols and physical networks Still it was a

great decade for the PLC The 80acircbdquocents saw an attempt to standardize communications with

General Motors manufacturing automation protocol(MAP) It was also a time for reducing

the size of the PLC and making them software programmable through symbolic programming

on personal computers instead of dedicated programming terminals or handheld

programmers Today the worlds smallest PLC is about the size of a single control relay

The 90acircbdquocents have seen a gradual reduction in the introduction of new protocols and the

modernization of the physical layers of some of the more popular protocols that survived the

1980s The latest standard (IEC 1131-3) has tried to merge plc programming languages

under one international standard We now have PLCs that are programmable in function

block diagrams instruction lists C and structured text all at the same time PCs are also

14

being used to replace PLCs in some applications The original company who commissioned

the MODICON 084 has actually switched to a PC based control system

23PLC HARDWARE

A programmable logic controller consists of the following components

Central Processing Unit (CPU) Memory Input modules Output modules and Power

supply A PLC hardware block diagram is shown in Figure The programming terminal in the

diagram is not a part of the PLC but it is essential to have a terminal for programming or

monitoring a PLC In the diagram the arrows between blocks indicate the information and

power-flowing-directions

Fig PLC-Hardware-Block-Diagram

CPU

Like other computerized devices there is a Central Processing Unit (CPU) in a PLC The

CPU which is the brain of a PLC does the following operations

Updating inputs and outputs This function allows a PLC to read the status of its input

terminals and energize or deenergize its output terminals

Performing logic and arithmetic operations A CPU conducts all the mathematic and logic

operations involved in a PLC

Communicating with memory The PLCacircbdquocents programs and data are stored in memory

When a PLC is operating its CPU may read or change the contents of memory locations

Scanning application programs An application program which is called a ladder logic

program is a set of instructions written by a PLC programmer The scanning function allows

the PLC to execute the application program as specified by the programmer

15

PROCESS

Programming Terminal

CPU MemoryPowerSupply

InputModule

OutputModule

InputDevices

OutputDevices

PLC

Communicating with a programming terminal The CPU transfers program and data

between itself and the programming terminal A PLC CPU is controlled by operating system

software The operating system software is a group of supervisory programs that are loaded

and stored permanently in the PLC memory by the PLC manufacturer

Memory

Memory is the component that stores information programs and data in a PLC The process

of putting new information into a memory location is called writing The process of retrieving

information from a memory location is called reading The common types of memory used in

PLCs are Read Only Memory (ROM) and Random Access Memory (RAM) A ROM

location can be read but not written ROM is used to store programs and data that should not

be altered For example the PLCs operating programs are stored in ROM

A RAM location can be read or written This means the information stored in a RAM

location can be retrieved andor altered Ladder logic programs are stored in RAM When a

new ladder logic program is loaded into a PLCs memory the old program that was stored in

the same locations is over-written and essentially erased The memory capacities of PLCs

vary Memory capacities are often expressed in terms of kilo-bytes (K) One byte is a group

of 8 bits One bit is a memory location that may store one binary number that has the value of

either 1 or 0 (Binary numbers are addressed in Module 2) 1K memory means that there are

1024 bytes of RAM 16K memory means there are 16 x 1024 =16384 bytes of RAM

Input modules and output modules

A PLC is a control device It takes information from inputs and makes decisions to

energize or de-energize outputs The decisions are made based on the statuses of inputs and

outputs and the ladder logic program that is being executed The input devices used with a

PLC include pushbuttons limit switches relay contacts photo sensors proximity switches

temperature sensors and the like These input devices can be AC (alternating current) or DC

(direct current) The input voltages can be high or low The input signals can be digital or

analog Differing inputs require different input modules An input module provides an

interface between input devices and a PLCs CPU which uses only a low DC voltage The

input moduleacircbdquocents function is to convert the input signals to DC voltages that are acceptable

to the CPU Standard discrete input modules include 24 V AC 48 V AC 120 V AC 220 V

AC 24 V DC 48 V DC 120 V DC 220 V DC and transistor-transistor logic (TTL) level

The devices controlled by a PLC include relays alarms solenoids fans lights and motor

starters These devices may require different levels of AC or DC voltages Since the signals

16

processed in a PLC are low DC voltages it is the function of the output module to convert

PLC control signals to the voltages required by the controlled circuits or devices Standard

discrete output modules include 24 V AC 48 V AC 120 V AC 220 V AC 24 V DC 48 V

DC 120 V DC 220 V DC and TTL level Power Supply -PLCs are powered by standard

commercial AC power lines However many PLC components such as the CPU and

memory utilize 5 volts or another level of DC power The PLC power supply converts AC

power into DC power to support those components of the PLC

Programming Terminal -A PLC requires a programming terminal and programming

software for operation The programming terminal can be a dedicated terminal or a generic

computer purchased anywhere The programming terminal is used for programming the PLC

and monitoring the PLCs operation It may also download a ladder logic program (the

sending of a program from the programming terminal to the PLC) or upload a ladder logic

program (the sending of a program from the PLC to the programming terminal) The terminal

uses programming software for programming and talking to a PLC

24 WORKING OF PLC

Bringing input signal status to the internal memory of CPU

The field signals are connected to the IP module At the output of IP module the field

status converted into the voltage level required by the CPU is always available

At the beginning of each cycle the CPU brings in all the field IP signals from IP module amp

stores into its internal memory called as PII meaning process image input

The programmable controller operates cyclically meaning when complete program has been

scanned it starts again at the beginning of the program

IOBUS

A PLC works by continually scanning a program We can think of this scan cycle as

consisting of 3 important steps There are typically more than 3 but we can focus on the

important parts and not worry about the others Typically the others are checking the system

and updating the current internal counter and timer values

Step 1-Check Input Status-First the PLC takes a look at each input to determine if it is on or

off In other words is the sensor connected to the first input on How about the second input

How about the third It records this data into its memory to be used during the next step

17

Step 2-Execute Program-Next the PLC executes your program one instruction at a time

Maybe your program said that if the first input was on then it should turn on the first output

Since it already knows which inputs are onoff from the previous step it will be able to decide

whether the first output should be turned on based on the state of the first input It will store

the execution results for use later during the next step

Step 3-Update Output Status-Finally the PLC updates the status of the outputs It updates the

outputs based on which inputs were on during the first step and the results of executing your

program during the second step Based on the example in step 2 it would now turn on the first

output because the first input was on and your program said to turn on the first output when

this condition is trueProcess Control and Automation Process Control

The process of recognizing the state of the process at all times analyze the information

according to the set rules and guidelines and accordingly actuate the control elements is

referred to as process control

RECOGNISING THE STATUS

In control of process all these actions can be taken manually with human involvement or in a

semiautomatic or fully automatic manner Automation -Automation is basically the

delegation of human control functions to technical equipment aimed towards achieving

- Higher-productivity

-Superior quality of end product

-Efficient usage of energy and raw materials

-Improved safety in working conditions etc

Methods adopted for Process Control and Automation

- Manual control

- Hard wired logic control

- Electronics control

-PLC control

- Manual Control

Hardwired Control

-This was considered to be the first step towards automation

- Here the contractor amp relays together with timers amp counters were used

Electronics Control

18

-With the advent of electronics the logic gates started replacing the relays amp auxiliary

contractors in the control circuits amp timers

- With changes the benefits are

1) Reduced space requirements

2) Energy saving

3) Less maintenance and hence greater reliability etc

-With electronics the implementation of changes in the control logic as well as reducing the

project lead-time was not possible

Programmable Logic Controller

- With microprocessor and associated peripherals chips the process of control and

automation went a radical change

- Instead of achieving the desired control or automation through physical wiring of control

devices in PLC it is through a program or software Thus these controllers are referred to as

programmable logic controllers

- The programmable controllers have experienced an unprecedented growth as universal

element It can be effectively used in applications ranging from simple control like replacing

small number relays to complex automation problem

25 PROGRAMMING THE PLC

Ladder Logic

Ladder logic is the main programming method used for PLCs The ladder logic has been

developed to mimic relay logic The decision to use the relay logic diagrams was a strategic

one By selecting ladder logic as the main programming method the amount of retraining

needed for engineers and trades people was greatly reduced

Modern control systems still include relays but these are rarely used for logic A relay is a

simple device that uses a magnetic field to control a switch as pictured in Fig When a

voltage is applied to the input coil the resulting current creates a magnetic field The

magnetic field pulls a metal switch (or reed) towards it and the contacts touch closing the

switch The contact that closes when the coil is energized is called normally open The

normally closed contacts touch when the input coil is not energized Relays are normally

drawn in schematic form using a circle to represent the input coil The output contacts are

shown with two parallel lines Normally open contacts are shown as two lines and will be

open (non-conducting) when the input is not energized Normally closed contacts are shown

with two lines with a diagonal line through them When the input coil is not energized the

19

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

VINAYAKA MISSIONS UNIVERSITYAARUPADAI VEEDU INSTITUTE OF TECHNOLOGY

CHENNAI-603104

BONAFIDE CERTIFICATE

This is to certified that this project report ldquoIMPROVEMENT OF

HEAT FURNACE TEMPERATURE CONTROL VIA PLC METHODrdquo is

the bonafide work of ldquoMANOJ KUMAR (348074016) RAGHAV KUMAR

JHA (348074020) and ALOK KUMAR(348074005) rdquo in partial fulfillement

of the requirements for the award of Degree of Bachelor of Engineering in

Electronics and Instrumentation Engineering from Aarupadai Veedu Institute of

Technology under our guidance and supervision

SIGNATURE SIGNATURE

DrNVEERAPPAN ME PhD MIE MISTE AssoProf LChitra

HEAD OF THE DEPARTMENT ASSOCIATE PROF

Department of Electrical and Department of Electronics and Electronics Engineering Instrumentation Engineering AVITpaiyanoor AVITpaiyanoor Chennai-603104 Chennai-603104 Certified that the candidates was examined in the viva-voce Examination

held on helliphelliphelliphelliphelliphelliphellip

INTERNAL EXAMINER EXTERNAL EXAMINER

2

ACKNOWLEDGEMENT

At the outset we thank ldquo THE ALMIGHTYrdquo for his divine guidance and blessings

throughout our project work

We are grateful to DrASGanesan Pro-Chancellor Vinayaka Missions University

and Dr(Mrs)NRAlameluMEPhDPrincipal AVITPaiyanoor for providing us

adequate academic facilities

We are very mush indebted to DrNVeerappanMEPhDMIE MISTE HOD

Department of EEE for granting permission to realize this project and for his valuable

suggestions during the review of the project work We wish to express our deep sense of

gratitude to our project guide MrsLChithra ME Associate Professor Department of

EEE for her excellent guidance and continous encouragement in completion of the project

work

We are also grateful to Mrs MChithra BEMBA Lecture Mrs TManjula

ME Assistant Professor Department of EEE for their valuable suggestions during this

project and co-opeartion in finalizing the project report

We are also thankful to all other Faculty members and non-teaching staff of EEE

Department for their co-operartion for the successful completion of this project Finally we

extend our professed thanks to our parents family members and friends for their help and

perennial encouragement towards realizing this project successfully

MANOJ KUMAR -348074016

RAGHAV KUMAR JHA -348074020

ALOK KUMAR -348074005

3

ABSTRACT

This Project deal with the early detection on any abnormality

on the pipeline such as leakage is essential for efficient management As to date

wireless sensors have been widely used to gather information in monitoring

reliability of the pipeline The commonly used sensors are corrosion and

pressure sensors One of the issues that relate to pipeline monitoring is the

reactive rather than proactive maintenance approach to leakage and

abnormality The objective of this project is to develop a method for early

detection of corrosion which the buried pipeline is exposed to changing

temperatures and gas flow pressures This will ensure that maintenance works

can be done quickly to prevent pipe burst as well as to minimize operational

cost Also to shut down the process imediately by using a control action ie

solenoid valve

4

TABLE OF CONTENTS

CHAPTER No TITLE PAGE No

ABSTRACT

LIST OF SYMBOLS

1 INTRODUCTION

11 CORROSION DETECTION

12 BLOCK DIAGRAM

2 PROGRAMABLE LOGIC CONTROLLER

21 INTRODUCTION

22 PLC HISTORY

23 PLC HARDWARE

24 WORKING OF PLC

25 PROGRAMMING THE PLC

3 CORROSION

31 INTRODUCTION

32 TYPES OF CORROSION

33 EFFECT OF CORROSION

34 CORROSION PREVENTION

4 SENSORS

41 CORROSION SENSOR

5

42 RELAY

43 PRESSURE SENSOR

5 WIRELESS NETWORK

51 ENCODER

52 TRANSMITTER

53 RECEIVER

54 DECODER

6 POWER SUPPLY

61 RECTFIER

62 FILTER

63 VOLTAGE REGULATOR

7 CONTROL ACTION

71 BUZZER

72 BUFFER AMPLIFIER

73 SOLONIOD VALVE

REFERENCES

6

CHAPTER ndash 1

INTRODUCTION

7

1 INTRODUCTION

11 Oil gas and water are the natural resources that have been

the key of life and source of economy for most countries in the world

These natural resources are transported from its original plants or storage

through extended pipelines which spreads throughout the countries In

some countries pipelines also being used to supply gas directly to users

These pipelines serve as the backbone between producers and

consumers Maintaining pipelines is essential to sustain economic growth

political stability and also safety Delays in detecting leakage on pipeline

may lead to more serious matters such as fire and fatality Wireless

technologies have evolved so rapidly now-a-days and widely used in many

applications and services This mainly because it can remotely placed and

uses its own power source such as battery to power itself There are

number of technologies to monitor and protect pipelines Most of these

are designed specifically for detecting and locating pipeline leakages

These technologies allow a remote facility to detect and to report the

positions of any leakage Most of these solutions rely on the availability of

a network to transfer the information and report leakages Pipeline

monitoring systems have been using wireless devices as part of the

system communication and information transfer

One of the industries that have taken advantage of wireless

sensors is oil and gas industry Gas main distribution method is using

pipeline Monitoring of pipeline is very crucial because of its valuable

resources as well as for safety precautions Wireless sensors have been

widely used to monitor the health and the condition of the pipeline

Continuous monitoring of pipelines is necessary to ensure the safety

operation of pipelines transmission However several issues and

problems have been discussed related to the usage of wireless sensors in

pipeline monitoring system One of the issues is to detect leakage or

abnormality of the pipeline as early as possible particularly in gas

pipeline It is very important that we detect the signs of pipeline defects

as early as possible because if that allow it to go on it could cause some

8

serious problems later on such as endanger the environment and if the

pipeline near human population area it can be a disaster In the section

of pipeline

ruptures at 146 millions of cubic feet per day (MMCFD) of gas for pressure

of 1198 psi and 544deg Celsius was flowing through the 18-inch diameter

pipe and caused one life Structural defects on gas pipeline may caused

by several factors such as corrosion Several factors have been identified

that lead to the event of buried pipeline corrosion such as pipe coatings

soil conditions changing of temperature stresses pipe pressure and

cyclic loading effects

The objective of this project is to develop a method for early

detection of corrosion which the buried pipeline is exposed to changing

temperatures and gas flow pressures so that prompt actions can be done

to prevent any incidents or fatality It is a necessity to have system that

detects signs or abnormalities that will resulted to leakage event In a

pipeline monitoring and inspection system has several tasks to be

performed for natural gas pipelines environment to ensure the integrity of

the pipeline These include measuring pipe wall thickness measuring

velocity and flow of gas detecting gas contamination in pipeline and also

determining structural defects on pipes In gas pipelines commonly

placed in hazardous environmental such as deserts underwater and

buried deep These conditions can cause deterioration or even damage

due to corrosion erosion and fatigue Major problem is to detect corrosion

cracks

9

Fig of full circuit

10

RF Receiver Decoder PLC Load

Power Supply

12 BLOCK DIAGRAM

TRANSMITTER SECTION

RECEIVER SECTION

11

CHAPTER ndash 2

PROGRAMMABLE LOGIC CONTROLLER

12

2 PROGRAMMABLE LOGIC CONTROLLER

21 INTRODUCTION

Programmable logic controllers (PLCs) are members of the

computer family capable of storing instructions to control functions such as sequencing

timing and counting which control a machine or a process The PLC is composed of two

basic sections the Central Processing Unit (CPU) and the InputOutput (IO) interface

system The PLC measures input signals coming from a machine and through the internal

program provides output or control back to the machine Ladder logic is the

programming language used to represent electrical sequences of operation In hardwired

circuits the electrical wiring is connected from one device to another according to logic of

operation In a PLC the devices are connected to the input interface the outputs are

connected to the output interface and the actual wiring of the components is done

electronically inside the PLC using ladder logic This is known as soft wired PLC is a

device that is capable of being programmed to perform a controlling function Before the

advent of PLC the problem of industrial control was usually solved by relays or

hardwired solid-state logic blocks These are very flexible in design and easy for

maintenance personal to understand However they involved a vast amount of

interconnection For the wiring cost to be minimized relays and logic blocks had to be

kept together This led to development of control panel concept for larger and more

complex logic control system The PLC was first conceived by group of engineers from

hydramatic division of GM in 1968This was designed to provide flexibility in control

based on programming and executing logic instruction Adopting the ladder diagram

programming language simplifying maintenance and reducing the cost of spare parts

inventories realized major advantages

22PLC HISTORY

In the late 1960s PLCs were first introduced The primary reason for designing such a

device was eliminating the large cost involved in replacing the complicated relay based

machine control systems Bedford Associates (Bedford MA) proposed something called a

Modular Digital Controller (MODICON) to a major US car manufacturer Other companies

at the time proposed computer based schemes one of which was based upon the PDP-8 The

MODICON 084 brought the worlds first PLC into commercial production

13

When production requirements changed so did the control system This becomes very

expensive when the change is frequent Since relays are mechanical devices they also have a

limited lifetime which required strict adhesion to maintenance schedules Troubleshooting

was also quite tedious when so many relays are involved Now picture a machine control

panel that included many possibly hundreds or thousands of individual relays The size

could be mind boggling How about the complicated initial wiring of so many individual

devices These relays would be individually wired together in a manner that would yield the

desired outcome Were there problems You bet These new controllers also had to be

easily programmed by maintenance and plant engineers The lifetime had to be long and

programming changes easily performed They also had to survive the harsh industrial

environment Thats a lot to ask The answers were to use a programming technique most

people were already familiar with and replace mechanical parts with solid-state ones

In the mid70acircbdquocents the dominant PLC technologies were sequencer state-machines and the bit-

slice based CPU The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs

Conventional microprocessors lacked the power to quickly solve PLC logic in all but the

smallest PLCs As conventional microprocessors evolved larger and larger PLCs were being

based upon them However even today some are still based upon the 2903(ref A-Bs PLC-3)

Modicon has yet to build a faster PLC than their 984ABX which was based upon the 2901

Communications abilities began to appear in approximately 1973 The first such system was

Modicons Modbus The PLC could now talk to other PLCs and they could be far away from

the actual machine they were controlling They could also now be used to send and receive

varying voltages to allow them to enter the analog world Unfortunately the lack of

standardization coupled with continually changing technology has made PLC

communications a nightmare of incompatible protocols and physical networks Still it was a

great decade for the PLC The 80acircbdquocents saw an attempt to standardize communications with

General Motors manufacturing automation protocol(MAP) It was also a time for reducing

the size of the PLC and making them software programmable through symbolic programming

on personal computers instead of dedicated programming terminals or handheld

programmers Today the worlds smallest PLC is about the size of a single control relay

The 90acircbdquocents have seen a gradual reduction in the introduction of new protocols and the

modernization of the physical layers of some of the more popular protocols that survived the

1980s The latest standard (IEC 1131-3) has tried to merge plc programming languages

under one international standard We now have PLCs that are programmable in function

block diagrams instruction lists C and structured text all at the same time PCs are also

14

being used to replace PLCs in some applications The original company who commissioned

the MODICON 084 has actually switched to a PC based control system

23PLC HARDWARE

A programmable logic controller consists of the following components

Central Processing Unit (CPU) Memory Input modules Output modules and Power

supply A PLC hardware block diagram is shown in Figure The programming terminal in the

diagram is not a part of the PLC but it is essential to have a terminal for programming or

monitoring a PLC In the diagram the arrows between blocks indicate the information and

power-flowing-directions

Fig PLC-Hardware-Block-Diagram

CPU

Like other computerized devices there is a Central Processing Unit (CPU) in a PLC The

CPU which is the brain of a PLC does the following operations

Updating inputs and outputs This function allows a PLC to read the status of its input

terminals and energize or deenergize its output terminals

Performing logic and arithmetic operations A CPU conducts all the mathematic and logic

operations involved in a PLC

Communicating with memory The PLCacircbdquocents programs and data are stored in memory

When a PLC is operating its CPU may read or change the contents of memory locations

Scanning application programs An application program which is called a ladder logic

program is a set of instructions written by a PLC programmer The scanning function allows

the PLC to execute the application program as specified by the programmer

15

PROCESS

Programming Terminal

CPU MemoryPowerSupply

InputModule

OutputModule

InputDevices

OutputDevices

PLC

Communicating with a programming terminal The CPU transfers program and data

between itself and the programming terminal A PLC CPU is controlled by operating system

software The operating system software is a group of supervisory programs that are loaded

and stored permanently in the PLC memory by the PLC manufacturer

Memory

Memory is the component that stores information programs and data in a PLC The process

of putting new information into a memory location is called writing The process of retrieving

information from a memory location is called reading The common types of memory used in

PLCs are Read Only Memory (ROM) and Random Access Memory (RAM) A ROM

location can be read but not written ROM is used to store programs and data that should not

be altered For example the PLCs operating programs are stored in ROM

A RAM location can be read or written This means the information stored in a RAM

location can be retrieved andor altered Ladder logic programs are stored in RAM When a

new ladder logic program is loaded into a PLCs memory the old program that was stored in

the same locations is over-written and essentially erased The memory capacities of PLCs

vary Memory capacities are often expressed in terms of kilo-bytes (K) One byte is a group

of 8 bits One bit is a memory location that may store one binary number that has the value of

either 1 or 0 (Binary numbers are addressed in Module 2) 1K memory means that there are

1024 bytes of RAM 16K memory means there are 16 x 1024 =16384 bytes of RAM

Input modules and output modules

A PLC is a control device It takes information from inputs and makes decisions to

energize or de-energize outputs The decisions are made based on the statuses of inputs and

outputs and the ladder logic program that is being executed The input devices used with a

PLC include pushbuttons limit switches relay contacts photo sensors proximity switches

temperature sensors and the like These input devices can be AC (alternating current) or DC

(direct current) The input voltages can be high or low The input signals can be digital or

analog Differing inputs require different input modules An input module provides an

interface between input devices and a PLCs CPU which uses only a low DC voltage The

input moduleacircbdquocents function is to convert the input signals to DC voltages that are acceptable

to the CPU Standard discrete input modules include 24 V AC 48 V AC 120 V AC 220 V

AC 24 V DC 48 V DC 120 V DC 220 V DC and transistor-transistor logic (TTL) level

The devices controlled by a PLC include relays alarms solenoids fans lights and motor

starters These devices may require different levels of AC or DC voltages Since the signals

16

processed in a PLC are low DC voltages it is the function of the output module to convert

PLC control signals to the voltages required by the controlled circuits or devices Standard

discrete output modules include 24 V AC 48 V AC 120 V AC 220 V AC 24 V DC 48 V

DC 120 V DC 220 V DC and TTL level Power Supply -PLCs are powered by standard

commercial AC power lines However many PLC components such as the CPU and

memory utilize 5 volts or another level of DC power The PLC power supply converts AC

power into DC power to support those components of the PLC

Programming Terminal -A PLC requires a programming terminal and programming

software for operation The programming terminal can be a dedicated terminal or a generic

computer purchased anywhere The programming terminal is used for programming the PLC

and monitoring the PLCs operation It may also download a ladder logic program (the

sending of a program from the programming terminal to the PLC) or upload a ladder logic

program (the sending of a program from the PLC to the programming terminal) The terminal

uses programming software for programming and talking to a PLC

24 WORKING OF PLC

Bringing input signal status to the internal memory of CPU

The field signals are connected to the IP module At the output of IP module the field

status converted into the voltage level required by the CPU is always available

At the beginning of each cycle the CPU brings in all the field IP signals from IP module amp

stores into its internal memory called as PII meaning process image input

The programmable controller operates cyclically meaning when complete program has been

scanned it starts again at the beginning of the program

IOBUS

A PLC works by continually scanning a program We can think of this scan cycle as

consisting of 3 important steps There are typically more than 3 but we can focus on the

important parts and not worry about the others Typically the others are checking the system

and updating the current internal counter and timer values

Step 1-Check Input Status-First the PLC takes a look at each input to determine if it is on or

off In other words is the sensor connected to the first input on How about the second input

How about the third It records this data into its memory to be used during the next step

17

Step 2-Execute Program-Next the PLC executes your program one instruction at a time

Maybe your program said that if the first input was on then it should turn on the first output

Since it already knows which inputs are onoff from the previous step it will be able to decide

whether the first output should be turned on based on the state of the first input It will store

the execution results for use later during the next step

Step 3-Update Output Status-Finally the PLC updates the status of the outputs It updates the

outputs based on which inputs were on during the first step and the results of executing your

program during the second step Based on the example in step 2 it would now turn on the first

output because the first input was on and your program said to turn on the first output when

this condition is trueProcess Control and Automation Process Control

The process of recognizing the state of the process at all times analyze the information

according to the set rules and guidelines and accordingly actuate the control elements is

referred to as process control

RECOGNISING THE STATUS

In control of process all these actions can be taken manually with human involvement or in a

semiautomatic or fully automatic manner Automation -Automation is basically the

delegation of human control functions to technical equipment aimed towards achieving

- Higher-productivity

-Superior quality of end product

-Efficient usage of energy and raw materials

-Improved safety in working conditions etc

Methods adopted for Process Control and Automation

- Manual control

- Hard wired logic control

- Electronics control

-PLC control

- Manual Control

Hardwired Control

-This was considered to be the first step towards automation

- Here the contractor amp relays together with timers amp counters were used

Electronics Control

18

-With the advent of electronics the logic gates started replacing the relays amp auxiliary

contractors in the control circuits amp timers

- With changes the benefits are

1) Reduced space requirements

2) Energy saving

3) Less maintenance and hence greater reliability etc

-With electronics the implementation of changes in the control logic as well as reducing the

project lead-time was not possible

Programmable Logic Controller

- With microprocessor and associated peripherals chips the process of control and

automation went a radical change

- Instead of achieving the desired control or automation through physical wiring of control

devices in PLC it is through a program or software Thus these controllers are referred to as

programmable logic controllers

- The programmable controllers have experienced an unprecedented growth as universal

element It can be effectively used in applications ranging from simple control like replacing

small number relays to complex automation problem

25 PROGRAMMING THE PLC

Ladder Logic

Ladder logic is the main programming method used for PLCs The ladder logic has been

developed to mimic relay logic The decision to use the relay logic diagrams was a strategic

one By selecting ladder logic as the main programming method the amount of retraining

needed for engineers and trades people was greatly reduced

Modern control systems still include relays but these are rarely used for logic A relay is a

simple device that uses a magnetic field to control a switch as pictured in Fig When a

voltage is applied to the input coil the resulting current creates a magnetic field The

magnetic field pulls a metal switch (or reed) towards it and the contacts touch closing the

switch The contact that closes when the coil is energized is called normally open The

normally closed contacts touch when the input coil is not energized Relays are normally

drawn in schematic form using a circle to represent the input coil The output contacts are

shown with two parallel lines Normally open contacts are shown as two lines and will be

open (non-conducting) when the input is not energized Normally closed contacts are shown

with two lines with a diagonal line through them When the input coil is not energized the

19

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

ACKNOWLEDGEMENT

At the outset we thank ldquo THE ALMIGHTYrdquo for his divine guidance and blessings

throughout our project work

We are grateful to DrASGanesan Pro-Chancellor Vinayaka Missions University

and Dr(Mrs)NRAlameluMEPhDPrincipal AVITPaiyanoor for providing us

adequate academic facilities

We are very mush indebted to DrNVeerappanMEPhDMIE MISTE HOD

Department of EEE for granting permission to realize this project and for his valuable

suggestions during the review of the project work We wish to express our deep sense of

gratitude to our project guide MrsLChithra ME Associate Professor Department of

EEE for her excellent guidance and continous encouragement in completion of the project

work

We are also grateful to Mrs MChithra BEMBA Lecture Mrs TManjula

ME Assistant Professor Department of EEE for their valuable suggestions during this

project and co-opeartion in finalizing the project report

We are also thankful to all other Faculty members and non-teaching staff of EEE

Department for their co-operartion for the successful completion of this project Finally we

extend our professed thanks to our parents family members and friends for their help and

perennial encouragement towards realizing this project successfully

MANOJ KUMAR -348074016

RAGHAV KUMAR JHA -348074020

ALOK KUMAR -348074005

3

ABSTRACT

This Project deal with the early detection on any abnormality

on the pipeline such as leakage is essential for efficient management As to date

wireless sensors have been widely used to gather information in monitoring

reliability of the pipeline The commonly used sensors are corrosion and

pressure sensors One of the issues that relate to pipeline monitoring is the

reactive rather than proactive maintenance approach to leakage and

abnormality The objective of this project is to develop a method for early

detection of corrosion which the buried pipeline is exposed to changing

temperatures and gas flow pressures This will ensure that maintenance works

can be done quickly to prevent pipe burst as well as to minimize operational

cost Also to shut down the process imediately by using a control action ie

solenoid valve

4

TABLE OF CONTENTS

CHAPTER No TITLE PAGE No

ABSTRACT

LIST OF SYMBOLS

1 INTRODUCTION

11 CORROSION DETECTION

12 BLOCK DIAGRAM

2 PROGRAMABLE LOGIC CONTROLLER

21 INTRODUCTION

22 PLC HISTORY

23 PLC HARDWARE

24 WORKING OF PLC

25 PROGRAMMING THE PLC

3 CORROSION

31 INTRODUCTION

32 TYPES OF CORROSION

33 EFFECT OF CORROSION

34 CORROSION PREVENTION

4 SENSORS

41 CORROSION SENSOR

5

42 RELAY

43 PRESSURE SENSOR

5 WIRELESS NETWORK

51 ENCODER

52 TRANSMITTER

53 RECEIVER

54 DECODER

6 POWER SUPPLY

61 RECTFIER

62 FILTER

63 VOLTAGE REGULATOR

7 CONTROL ACTION

71 BUZZER

72 BUFFER AMPLIFIER

73 SOLONIOD VALVE

REFERENCES

6

CHAPTER ndash 1

INTRODUCTION

7

1 INTRODUCTION

11 Oil gas and water are the natural resources that have been

the key of life and source of economy for most countries in the world

These natural resources are transported from its original plants or storage

through extended pipelines which spreads throughout the countries In

some countries pipelines also being used to supply gas directly to users

These pipelines serve as the backbone between producers and

consumers Maintaining pipelines is essential to sustain economic growth

political stability and also safety Delays in detecting leakage on pipeline

may lead to more serious matters such as fire and fatality Wireless

technologies have evolved so rapidly now-a-days and widely used in many

applications and services This mainly because it can remotely placed and

uses its own power source such as battery to power itself There are

number of technologies to monitor and protect pipelines Most of these

are designed specifically for detecting and locating pipeline leakages

These technologies allow a remote facility to detect and to report the

positions of any leakage Most of these solutions rely on the availability of

a network to transfer the information and report leakages Pipeline

monitoring systems have been using wireless devices as part of the

system communication and information transfer

One of the industries that have taken advantage of wireless

sensors is oil and gas industry Gas main distribution method is using

pipeline Monitoring of pipeline is very crucial because of its valuable

resources as well as for safety precautions Wireless sensors have been

widely used to monitor the health and the condition of the pipeline

Continuous monitoring of pipelines is necessary to ensure the safety

operation of pipelines transmission However several issues and

problems have been discussed related to the usage of wireless sensors in

pipeline monitoring system One of the issues is to detect leakage or

abnormality of the pipeline as early as possible particularly in gas

pipeline It is very important that we detect the signs of pipeline defects

as early as possible because if that allow it to go on it could cause some

8

serious problems later on such as endanger the environment and if the

pipeline near human population area it can be a disaster In the section

of pipeline

ruptures at 146 millions of cubic feet per day (MMCFD) of gas for pressure

of 1198 psi and 544deg Celsius was flowing through the 18-inch diameter

pipe and caused one life Structural defects on gas pipeline may caused

by several factors such as corrosion Several factors have been identified

that lead to the event of buried pipeline corrosion such as pipe coatings

soil conditions changing of temperature stresses pipe pressure and

cyclic loading effects

The objective of this project is to develop a method for early

detection of corrosion which the buried pipeline is exposed to changing

temperatures and gas flow pressures so that prompt actions can be done

to prevent any incidents or fatality It is a necessity to have system that

detects signs or abnormalities that will resulted to leakage event In a

pipeline monitoring and inspection system has several tasks to be

performed for natural gas pipelines environment to ensure the integrity of

the pipeline These include measuring pipe wall thickness measuring

velocity and flow of gas detecting gas contamination in pipeline and also

determining structural defects on pipes In gas pipelines commonly

placed in hazardous environmental such as deserts underwater and

buried deep These conditions can cause deterioration or even damage

due to corrosion erosion and fatigue Major problem is to detect corrosion

cracks

9

Fig of full circuit

10

RF Receiver Decoder PLC Load

Power Supply

12 BLOCK DIAGRAM

TRANSMITTER SECTION

RECEIVER SECTION

11

CHAPTER ndash 2

PROGRAMMABLE LOGIC CONTROLLER

12

2 PROGRAMMABLE LOGIC CONTROLLER

21 INTRODUCTION

Programmable logic controllers (PLCs) are members of the

computer family capable of storing instructions to control functions such as sequencing

timing and counting which control a machine or a process The PLC is composed of two

basic sections the Central Processing Unit (CPU) and the InputOutput (IO) interface

system The PLC measures input signals coming from a machine and through the internal

program provides output or control back to the machine Ladder logic is the

programming language used to represent electrical sequences of operation In hardwired

circuits the electrical wiring is connected from one device to another according to logic of

operation In a PLC the devices are connected to the input interface the outputs are

connected to the output interface and the actual wiring of the components is done

electronically inside the PLC using ladder logic This is known as soft wired PLC is a

device that is capable of being programmed to perform a controlling function Before the

advent of PLC the problem of industrial control was usually solved by relays or

hardwired solid-state logic blocks These are very flexible in design and easy for

maintenance personal to understand However they involved a vast amount of

interconnection For the wiring cost to be minimized relays and logic blocks had to be

kept together This led to development of control panel concept for larger and more

complex logic control system The PLC was first conceived by group of engineers from

hydramatic division of GM in 1968This was designed to provide flexibility in control

based on programming and executing logic instruction Adopting the ladder diagram

programming language simplifying maintenance and reducing the cost of spare parts

inventories realized major advantages

22PLC HISTORY

In the late 1960s PLCs were first introduced The primary reason for designing such a

device was eliminating the large cost involved in replacing the complicated relay based

machine control systems Bedford Associates (Bedford MA) proposed something called a

Modular Digital Controller (MODICON) to a major US car manufacturer Other companies

at the time proposed computer based schemes one of which was based upon the PDP-8 The

MODICON 084 brought the worlds first PLC into commercial production

13

When production requirements changed so did the control system This becomes very

expensive when the change is frequent Since relays are mechanical devices they also have a

limited lifetime which required strict adhesion to maintenance schedules Troubleshooting

was also quite tedious when so many relays are involved Now picture a machine control

panel that included many possibly hundreds or thousands of individual relays The size

could be mind boggling How about the complicated initial wiring of so many individual

devices These relays would be individually wired together in a manner that would yield the

desired outcome Were there problems You bet These new controllers also had to be

easily programmed by maintenance and plant engineers The lifetime had to be long and

programming changes easily performed They also had to survive the harsh industrial

environment Thats a lot to ask The answers were to use a programming technique most

people were already familiar with and replace mechanical parts with solid-state ones

In the mid70acircbdquocents the dominant PLC technologies were sequencer state-machines and the bit-

slice based CPU The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs

Conventional microprocessors lacked the power to quickly solve PLC logic in all but the

smallest PLCs As conventional microprocessors evolved larger and larger PLCs were being

based upon them However even today some are still based upon the 2903(ref A-Bs PLC-3)

Modicon has yet to build a faster PLC than their 984ABX which was based upon the 2901

Communications abilities began to appear in approximately 1973 The first such system was

Modicons Modbus The PLC could now talk to other PLCs and they could be far away from

the actual machine they were controlling They could also now be used to send and receive

varying voltages to allow them to enter the analog world Unfortunately the lack of

standardization coupled with continually changing technology has made PLC

communications a nightmare of incompatible protocols and physical networks Still it was a

great decade for the PLC The 80acircbdquocents saw an attempt to standardize communications with

General Motors manufacturing automation protocol(MAP) It was also a time for reducing

the size of the PLC and making them software programmable through symbolic programming

on personal computers instead of dedicated programming terminals or handheld

programmers Today the worlds smallest PLC is about the size of a single control relay

The 90acircbdquocents have seen a gradual reduction in the introduction of new protocols and the

modernization of the physical layers of some of the more popular protocols that survived the

1980s The latest standard (IEC 1131-3) has tried to merge plc programming languages

under one international standard We now have PLCs that are programmable in function

block diagrams instruction lists C and structured text all at the same time PCs are also

14

being used to replace PLCs in some applications The original company who commissioned

the MODICON 084 has actually switched to a PC based control system

23PLC HARDWARE

A programmable logic controller consists of the following components

Central Processing Unit (CPU) Memory Input modules Output modules and Power

supply A PLC hardware block diagram is shown in Figure The programming terminal in the

diagram is not a part of the PLC but it is essential to have a terminal for programming or

monitoring a PLC In the diagram the arrows between blocks indicate the information and

power-flowing-directions

Fig PLC-Hardware-Block-Diagram

CPU

Like other computerized devices there is a Central Processing Unit (CPU) in a PLC The

CPU which is the brain of a PLC does the following operations

Updating inputs and outputs This function allows a PLC to read the status of its input

terminals and energize or deenergize its output terminals

Performing logic and arithmetic operations A CPU conducts all the mathematic and logic

operations involved in a PLC

Communicating with memory The PLCacircbdquocents programs and data are stored in memory

When a PLC is operating its CPU may read or change the contents of memory locations

Scanning application programs An application program which is called a ladder logic

program is a set of instructions written by a PLC programmer The scanning function allows

the PLC to execute the application program as specified by the programmer

15

PROCESS

Programming Terminal

CPU MemoryPowerSupply

InputModule

OutputModule

InputDevices

OutputDevices

PLC

Communicating with a programming terminal The CPU transfers program and data

between itself and the programming terminal A PLC CPU is controlled by operating system

software The operating system software is a group of supervisory programs that are loaded

and stored permanently in the PLC memory by the PLC manufacturer

Memory

Memory is the component that stores information programs and data in a PLC The process

of putting new information into a memory location is called writing The process of retrieving

information from a memory location is called reading The common types of memory used in

PLCs are Read Only Memory (ROM) and Random Access Memory (RAM) A ROM

location can be read but not written ROM is used to store programs and data that should not

be altered For example the PLCs operating programs are stored in ROM

A RAM location can be read or written This means the information stored in a RAM

location can be retrieved andor altered Ladder logic programs are stored in RAM When a

new ladder logic program is loaded into a PLCs memory the old program that was stored in

the same locations is over-written and essentially erased The memory capacities of PLCs

vary Memory capacities are often expressed in terms of kilo-bytes (K) One byte is a group

of 8 bits One bit is a memory location that may store one binary number that has the value of

either 1 or 0 (Binary numbers are addressed in Module 2) 1K memory means that there are

1024 bytes of RAM 16K memory means there are 16 x 1024 =16384 bytes of RAM

Input modules and output modules

A PLC is a control device It takes information from inputs and makes decisions to

energize or de-energize outputs The decisions are made based on the statuses of inputs and

outputs and the ladder logic program that is being executed The input devices used with a

PLC include pushbuttons limit switches relay contacts photo sensors proximity switches

temperature sensors and the like These input devices can be AC (alternating current) or DC

(direct current) The input voltages can be high or low The input signals can be digital or

analog Differing inputs require different input modules An input module provides an

interface between input devices and a PLCs CPU which uses only a low DC voltage The

input moduleacircbdquocents function is to convert the input signals to DC voltages that are acceptable

to the CPU Standard discrete input modules include 24 V AC 48 V AC 120 V AC 220 V

AC 24 V DC 48 V DC 120 V DC 220 V DC and transistor-transistor logic (TTL) level

The devices controlled by a PLC include relays alarms solenoids fans lights and motor

starters These devices may require different levels of AC or DC voltages Since the signals

16

processed in a PLC are low DC voltages it is the function of the output module to convert

PLC control signals to the voltages required by the controlled circuits or devices Standard

discrete output modules include 24 V AC 48 V AC 120 V AC 220 V AC 24 V DC 48 V

DC 120 V DC 220 V DC and TTL level Power Supply -PLCs are powered by standard

commercial AC power lines However many PLC components such as the CPU and

memory utilize 5 volts or another level of DC power The PLC power supply converts AC

power into DC power to support those components of the PLC

Programming Terminal -A PLC requires a programming terminal and programming

software for operation The programming terminal can be a dedicated terminal or a generic

computer purchased anywhere The programming terminal is used for programming the PLC

and monitoring the PLCs operation It may also download a ladder logic program (the

sending of a program from the programming terminal to the PLC) or upload a ladder logic

program (the sending of a program from the PLC to the programming terminal) The terminal

uses programming software for programming and talking to a PLC

24 WORKING OF PLC

Bringing input signal status to the internal memory of CPU

The field signals are connected to the IP module At the output of IP module the field

status converted into the voltage level required by the CPU is always available

At the beginning of each cycle the CPU brings in all the field IP signals from IP module amp

stores into its internal memory called as PII meaning process image input

The programmable controller operates cyclically meaning when complete program has been

scanned it starts again at the beginning of the program

IOBUS

A PLC works by continually scanning a program We can think of this scan cycle as

consisting of 3 important steps There are typically more than 3 but we can focus on the

important parts and not worry about the others Typically the others are checking the system

and updating the current internal counter and timer values

Step 1-Check Input Status-First the PLC takes a look at each input to determine if it is on or

off In other words is the sensor connected to the first input on How about the second input

How about the third It records this data into its memory to be used during the next step

17

Step 2-Execute Program-Next the PLC executes your program one instruction at a time

Maybe your program said that if the first input was on then it should turn on the first output

Since it already knows which inputs are onoff from the previous step it will be able to decide

whether the first output should be turned on based on the state of the first input It will store

the execution results for use later during the next step

Step 3-Update Output Status-Finally the PLC updates the status of the outputs It updates the

outputs based on which inputs were on during the first step and the results of executing your

program during the second step Based on the example in step 2 it would now turn on the first

output because the first input was on and your program said to turn on the first output when

this condition is trueProcess Control and Automation Process Control

The process of recognizing the state of the process at all times analyze the information

according to the set rules and guidelines and accordingly actuate the control elements is

referred to as process control

RECOGNISING THE STATUS

In control of process all these actions can be taken manually with human involvement or in a

semiautomatic or fully automatic manner Automation -Automation is basically the

delegation of human control functions to technical equipment aimed towards achieving

- Higher-productivity

-Superior quality of end product

-Efficient usage of energy and raw materials

-Improved safety in working conditions etc

Methods adopted for Process Control and Automation

- Manual control

- Hard wired logic control

- Electronics control

-PLC control

- Manual Control

Hardwired Control

-This was considered to be the first step towards automation

- Here the contractor amp relays together with timers amp counters were used

Electronics Control

18

-With the advent of electronics the logic gates started replacing the relays amp auxiliary

contractors in the control circuits amp timers

- With changes the benefits are

1) Reduced space requirements

2) Energy saving

3) Less maintenance and hence greater reliability etc

-With electronics the implementation of changes in the control logic as well as reducing the

project lead-time was not possible

Programmable Logic Controller

- With microprocessor and associated peripherals chips the process of control and

automation went a radical change

- Instead of achieving the desired control or automation through physical wiring of control

devices in PLC it is through a program or software Thus these controllers are referred to as

programmable logic controllers

- The programmable controllers have experienced an unprecedented growth as universal

element It can be effectively used in applications ranging from simple control like replacing

small number relays to complex automation problem

25 PROGRAMMING THE PLC

Ladder Logic

Ladder logic is the main programming method used for PLCs The ladder logic has been

developed to mimic relay logic The decision to use the relay logic diagrams was a strategic

one By selecting ladder logic as the main programming method the amount of retraining

needed for engineers and trades people was greatly reduced

Modern control systems still include relays but these are rarely used for logic A relay is a

simple device that uses a magnetic field to control a switch as pictured in Fig When a

voltage is applied to the input coil the resulting current creates a magnetic field The

magnetic field pulls a metal switch (or reed) towards it and the contacts touch closing the

switch The contact that closes when the coil is energized is called normally open The

normally closed contacts touch when the input coil is not energized Relays are normally

drawn in schematic form using a circle to represent the input coil The output contacts are

shown with two parallel lines Normally open contacts are shown as two lines and will be

open (non-conducting) when the input is not energized Normally closed contacts are shown

with two lines with a diagonal line through them When the input coil is not energized the

19

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

ABSTRACT

This Project deal with the early detection on any abnormality

on the pipeline such as leakage is essential for efficient management As to date

wireless sensors have been widely used to gather information in monitoring

reliability of the pipeline The commonly used sensors are corrosion and

pressure sensors One of the issues that relate to pipeline monitoring is the

reactive rather than proactive maintenance approach to leakage and

abnormality The objective of this project is to develop a method for early

detection of corrosion which the buried pipeline is exposed to changing

temperatures and gas flow pressures This will ensure that maintenance works

can be done quickly to prevent pipe burst as well as to minimize operational

cost Also to shut down the process imediately by using a control action ie

solenoid valve

4

TABLE OF CONTENTS

CHAPTER No TITLE PAGE No

ABSTRACT

LIST OF SYMBOLS

1 INTRODUCTION

11 CORROSION DETECTION

12 BLOCK DIAGRAM

2 PROGRAMABLE LOGIC CONTROLLER

21 INTRODUCTION

22 PLC HISTORY

23 PLC HARDWARE

24 WORKING OF PLC

25 PROGRAMMING THE PLC

3 CORROSION

31 INTRODUCTION

32 TYPES OF CORROSION

33 EFFECT OF CORROSION

34 CORROSION PREVENTION

4 SENSORS

41 CORROSION SENSOR

5

42 RELAY

43 PRESSURE SENSOR

5 WIRELESS NETWORK

51 ENCODER

52 TRANSMITTER

53 RECEIVER

54 DECODER

6 POWER SUPPLY

61 RECTFIER

62 FILTER

63 VOLTAGE REGULATOR

7 CONTROL ACTION

71 BUZZER

72 BUFFER AMPLIFIER

73 SOLONIOD VALVE

REFERENCES

6

CHAPTER ndash 1

INTRODUCTION

7

1 INTRODUCTION

11 Oil gas and water are the natural resources that have been

the key of life and source of economy for most countries in the world

These natural resources are transported from its original plants or storage

through extended pipelines which spreads throughout the countries In

some countries pipelines also being used to supply gas directly to users

These pipelines serve as the backbone between producers and

consumers Maintaining pipelines is essential to sustain economic growth

political stability and also safety Delays in detecting leakage on pipeline

may lead to more serious matters such as fire and fatality Wireless

technologies have evolved so rapidly now-a-days and widely used in many

applications and services This mainly because it can remotely placed and

uses its own power source such as battery to power itself There are

number of technologies to monitor and protect pipelines Most of these

are designed specifically for detecting and locating pipeline leakages

These technologies allow a remote facility to detect and to report the

positions of any leakage Most of these solutions rely on the availability of

a network to transfer the information and report leakages Pipeline

monitoring systems have been using wireless devices as part of the

system communication and information transfer

One of the industries that have taken advantage of wireless

sensors is oil and gas industry Gas main distribution method is using

pipeline Monitoring of pipeline is very crucial because of its valuable

resources as well as for safety precautions Wireless sensors have been

widely used to monitor the health and the condition of the pipeline

Continuous monitoring of pipelines is necessary to ensure the safety

operation of pipelines transmission However several issues and

problems have been discussed related to the usage of wireless sensors in

pipeline monitoring system One of the issues is to detect leakage or

abnormality of the pipeline as early as possible particularly in gas

pipeline It is very important that we detect the signs of pipeline defects

as early as possible because if that allow it to go on it could cause some

8

serious problems later on such as endanger the environment and if the

pipeline near human population area it can be a disaster In the section

of pipeline

ruptures at 146 millions of cubic feet per day (MMCFD) of gas for pressure

of 1198 psi and 544deg Celsius was flowing through the 18-inch diameter

pipe and caused one life Structural defects on gas pipeline may caused

by several factors such as corrosion Several factors have been identified

that lead to the event of buried pipeline corrosion such as pipe coatings

soil conditions changing of temperature stresses pipe pressure and

cyclic loading effects

The objective of this project is to develop a method for early

detection of corrosion which the buried pipeline is exposed to changing

temperatures and gas flow pressures so that prompt actions can be done

to prevent any incidents or fatality It is a necessity to have system that

detects signs or abnormalities that will resulted to leakage event In a

pipeline monitoring and inspection system has several tasks to be

performed for natural gas pipelines environment to ensure the integrity of

the pipeline These include measuring pipe wall thickness measuring

velocity and flow of gas detecting gas contamination in pipeline and also

determining structural defects on pipes In gas pipelines commonly

placed in hazardous environmental such as deserts underwater and

buried deep These conditions can cause deterioration or even damage

due to corrosion erosion and fatigue Major problem is to detect corrosion

cracks

9

Fig of full circuit

10

RF Receiver Decoder PLC Load

Power Supply

12 BLOCK DIAGRAM

TRANSMITTER SECTION

RECEIVER SECTION

11

CHAPTER ndash 2

PROGRAMMABLE LOGIC CONTROLLER

12

2 PROGRAMMABLE LOGIC CONTROLLER

21 INTRODUCTION

Programmable logic controllers (PLCs) are members of the

computer family capable of storing instructions to control functions such as sequencing

timing and counting which control a machine or a process The PLC is composed of two

basic sections the Central Processing Unit (CPU) and the InputOutput (IO) interface

system The PLC measures input signals coming from a machine and through the internal

program provides output or control back to the machine Ladder logic is the

programming language used to represent electrical sequences of operation In hardwired

circuits the electrical wiring is connected from one device to another according to logic of

operation In a PLC the devices are connected to the input interface the outputs are

connected to the output interface and the actual wiring of the components is done

electronically inside the PLC using ladder logic This is known as soft wired PLC is a

device that is capable of being programmed to perform a controlling function Before the

advent of PLC the problem of industrial control was usually solved by relays or

hardwired solid-state logic blocks These are very flexible in design and easy for

maintenance personal to understand However they involved a vast amount of

interconnection For the wiring cost to be minimized relays and logic blocks had to be

kept together This led to development of control panel concept for larger and more

complex logic control system The PLC was first conceived by group of engineers from

hydramatic division of GM in 1968This was designed to provide flexibility in control

based on programming and executing logic instruction Adopting the ladder diagram

programming language simplifying maintenance and reducing the cost of spare parts

inventories realized major advantages

22PLC HISTORY

In the late 1960s PLCs were first introduced The primary reason for designing such a

device was eliminating the large cost involved in replacing the complicated relay based

machine control systems Bedford Associates (Bedford MA) proposed something called a

Modular Digital Controller (MODICON) to a major US car manufacturer Other companies

at the time proposed computer based schemes one of which was based upon the PDP-8 The

MODICON 084 brought the worlds first PLC into commercial production

13

When production requirements changed so did the control system This becomes very

expensive when the change is frequent Since relays are mechanical devices they also have a

limited lifetime which required strict adhesion to maintenance schedules Troubleshooting

was also quite tedious when so many relays are involved Now picture a machine control

panel that included many possibly hundreds or thousands of individual relays The size

could be mind boggling How about the complicated initial wiring of so many individual

devices These relays would be individually wired together in a manner that would yield the

desired outcome Were there problems You bet These new controllers also had to be

easily programmed by maintenance and plant engineers The lifetime had to be long and

programming changes easily performed They also had to survive the harsh industrial

environment Thats a lot to ask The answers were to use a programming technique most

people were already familiar with and replace mechanical parts with solid-state ones

In the mid70acircbdquocents the dominant PLC technologies were sequencer state-machines and the bit-

slice based CPU The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs

Conventional microprocessors lacked the power to quickly solve PLC logic in all but the

smallest PLCs As conventional microprocessors evolved larger and larger PLCs were being

based upon them However even today some are still based upon the 2903(ref A-Bs PLC-3)

Modicon has yet to build a faster PLC than their 984ABX which was based upon the 2901

Communications abilities began to appear in approximately 1973 The first such system was

Modicons Modbus The PLC could now talk to other PLCs and they could be far away from

the actual machine they were controlling They could also now be used to send and receive

varying voltages to allow them to enter the analog world Unfortunately the lack of

standardization coupled with continually changing technology has made PLC

communications a nightmare of incompatible protocols and physical networks Still it was a

great decade for the PLC The 80acircbdquocents saw an attempt to standardize communications with

General Motors manufacturing automation protocol(MAP) It was also a time for reducing

the size of the PLC and making them software programmable through symbolic programming

on personal computers instead of dedicated programming terminals or handheld

programmers Today the worlds smallest PLC is about the size of a single control relay

The 90acircbdquocents have seen a gradual reduction in the introduction of new protocols and the

modernization of the physical layers of some of the more popular protocols that survived the

1980s The latest standard (IEC 1131-3) has tried to merge plc programming languages

under one international standard We now have PLCs that are programmable in function

block diagrams instruction lists C and structured text all at the same time PCs are also

14

being used to replace PLCs in some applications The original company who commissioned

the MODICON 084 has actually switched to a PC based control system

23PLC HARDWARE

A programmable logic controller consists of the following components

Central Processing Unit (CPU) Memory Input modules Output modules and Power

supply A PLC hardware block diagram is shown in Figure The programming terminal in the

diagram is not a part of the PLC but it is essential to have a terminal for programming or

monitoring a PLC In the diagram the arrows between blocks indicate the information and

power-flowing-directions

Fig PLC-Hardware-Block-Diagram

CPU

Like other computerized devices there is a Central Processing Unit (CPU) in a PLC The

CPU which is the brain of a PLC does the following operations

Updating inputs and outputs This function allows a PLC to read the status of its input

terminals and energize or deenergize its output terminals

Performing logic and arithmetic operations A CPU conducts all the mathematic and logic

operations involved in a PLC

Communicating with memory The PLCacircbdquocents programs and data are stored in memory

When a PLC is operating its CPU may read or change the contents of memory locations

Scanning application programs An application program which is called a ladder logic

program is a set of instructions written by a PLC programmer The scanning function allows

the PLC to execute the application program as specified by the programmer

15

PROCESS

Programming Terminal

CPU MemoryPowerSupply

InputModule

OutputModule

InputDevices

OutputDevices

PLC

Communicating with a programming terminal The CPU transfers program and data

between itself and the programming terminal A PLC CPU is controlled by operating system

software The operating system software is a group of supervisory programs that are loaded

and stored permanently in the PLC memory by the PLC manufacturer

Memory

Memory is the component that stores information programs and data in a PLC The process

of putting new information into a memory location is called writing The process of retrieving

information from a memory location is called reading The common types of memory used in

PLCs are Read Only Memory (ROM) and Random Access Memory (RAM) A ROM

location can be read but not written ROM is used to store programs and data that should not

be altered For example the PLCs operating programs are stored in ROM

A RAM location can be read or written This means the information stored in a RAM

location can be retrieved andor altered Ladder logic programs are stored in RAM When a

new ladder logic program is loaded into a PLCs memory the old program that was stored in

the same locations is over-written and essentially erased The memory capacities of PLCs

vary Memory capacities are often expressed in terms of kilo-bytes (K) One byte is a group

of 8 bits One bit is a memory location that may store one binary number that has the value of

either 1 or 0 (Binary numbers are addressed in Module 2) 1K memory means that there are

1024 bytes of RAM 16K memory means there are 16 x 1024 =16384 bytes of RAM

Input modules and output modules

A PLC is a control device It takes information from inputs and makes decisions to

energize or de-energize outputs The decisions are made based on the statuses of inputs and

outputs and the ladder logic program that is being executed The input devices used with a

PLC include pushbuttons limit switches relay contacts photo sensors proximity switches

temperature sensors and the like These input devices can be AC (alternating current) or DC

(direct current) The input voltages can be high or low The input signals can be digital or

analog Differing inputs require different input modules An input module provides an

interface between input devices and a PLCs CPU which uses only a low DC voltage The

input moduleacircbdquocents function is to convert the input signals to DC voltages that are acceptable

to the CPU Standard discrete input modules include 24 V AC 48 V AC 120 V AC 220 V

AC 24 V DC 48 V DC 120 V DC 220 V DC and transistor-transistor logic (TTL) level

The devices controlled by a PLC include relays alarms solenoids fans lights and motor

starters These devices may require different levels of AC or DC voltages Since the signals

16

processed in a PLC are low DC voltages it is the function of the output module to convert

PLC control signals to the voltages required by the controlled circuits or devices Standard

discrete output modules include 24 V AC 48 V AC 120 V AC 220 V AC 24 V DC 48 V

DC 120 V DC 220 V DC and TTL level Power Supply -PLCs are powered by standard

commercial AC power lines However many PLC components such as the CPU and

memory utilize 5 volts or another level of DC power The PLC power supply converts AC

power into DC power to support those components of the PLC

Programming Terminal -A PLC requires a programming terminal and programming

software for operation The programming terminal can be a dedicated terminal or a generic

computer purchased anywhere The programming terminal is used for programming the PLC

and monitoring the PLCs operation It may also download a ladder logic program (the

sending of a program from the programming terminal to the PLC) or upload a ladder logic

program (the sending of a program from the PLC to the programming terminal) The terminal

uses programming software for programming and talking to a PLC

24 WORKING OF PLC

Bringing input signal status to the internal memory of CPU

The field signals are connected to the IP module At the output of IP module the field

status converted into the voltage level required by the CPU is always available

At the beginning of each cycle the CPU brings in all the field IP signals from IP module amp

stores into its internal memory called as PII meaning process image input

The programmable controller operates cyclically meaning when complete program has been

scanned it starts again at the beginning of the program

IOBUS

A PLC works by continually scanning a program We can think of this scan cycle as

consisting of 3 important steps There are typically more than 3 but we can focus on the

important parts and not worry about the others Typically the others are checking the system

and updating the current internal counter and timer values

Step 1-Check Input Status-First the PLC takes a look at each input to determine if it is on or

off In other words is the sensor connected to the first input on How about the second input

How about the third It records this data into its memory to be used during the next step

17

Step 2-Execute Program-Next the PLC executes your program one instruction at a time

Maybe your program said that if the first input was on then it should turn on the first output

Since it already knows which inputs are onoff from the previous step it will be able to decide

whether the first output should be turned on based on the state of the first input It will store

the execution results for use later during the next step

Step 3-Update Output Status-Finally the PLC updates the status of the outputs It updates the

outputs based on which inputs were on during the first step and the results of executing your

program during the second step Based on the example in step 2 it would now turn on the first

output because the first input was on and your program said to turn on the first output when

this condition is trueProcess Control and Automation Process Control

The process of recognizing the state of the process at all times analyze the information

according to the set rules and guidelines and accordingly actuate the control elements is

referred to as process control

RECOGNISING THE STATUS

In control of process all these actions can be taken manually with human involvement or in a

semiautomatic or fully automatic manner Automation -Automation is basically the

delegation of human control functions to technical equipment aimed towards achieving

- Higher-productivity

-Superior quality of end product

-Efficient usage of energy and raw materials

-Improved safety in working conditions etc

Methods adopted for Process Control and Automation

- Manual control

- Hard wired logic control

- Electronics control

-PLC control

- Manual Control

Hardwired Control

-This was considered to be the first step towards automation

- Here the contractor amp relays together with timers amp counters were used

Electronics Control

18

-With the advent of electronics the logic gates started replacing the relays amp auxiliary

contractors in the control circuits amp timers

- With changes the benefits are

1) Reduced space requirements

2) Energy saving

3) Less maintenance and hence greater reliability etc

-With electronics the implementation of changes in the control logic as well as reducing the

project lead-time was not possible

Programmable Logic Controller

- With microprocessor and associated peripherals chips the process of control and

automation went a radical change

- Instead of achieving the desired control or automation through physical wiring of control

devices in PLC it is through a program or software Thus these controllers are referred to as

programmable logic controllers

- The programmable controllers have experienced an unprecedented growth as universal

element It can be effectively used in applications ranging from simple control like replacing

small number relays to complex automation problem

25 PROGRAMMING THE PLC

Ladder Logic

Ladder logic is the main programming method used for PLCs The ladder logic has been

developed to mimic relay logic The decision to use the relay logic diagrams was a strategic

one By selecting ladder logic as the main programming method the amount of retraining

needed for engineers and trades people was greatly reduced

Modern control systems still include relays but these are rarely used for logic A relay is a

simple device that uses a magnetic field to control a switch as pictured in Fig When a

voltage is applied to the input coil the resulting current creates a magnetic field The

magnetic field pulls a metal switch (or reed) towards it and the contacts touch closing the

switch The contact that closes when the coil is energized is called normally open The

normally closed contacts touch when the input coil is not energized Relays are normally

drawn in schematic form using a circle to represent the input coil The output contacts are

shown with two parallel lines Normally open contacts are shown as two lines and will be

open (non-conducting) when the input is not energized Normally closed contacts are shown

with two lines with a diagonal line through them When the input coil is not energized the

19

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

TABLE OF CONTENTS

CHAPTER No TITLE PAGE No

ABSTRACT

LIST OF SYMBOLS

1 INTRODUCTION

11 CORROSION DETECTION

12 BLOCK DIAGRAM

2 PROGRAMABLE LOGIC CONTROLLER

21 INTRODUCTION

22 PLC HISTORY

23 PLC HARDWARE

24 WORKING OF PLC

25 PROGRAMMING THE PLC

3 CORROSION

31 INTRODUCTION

32 TYPES OF CORROSION

33 EFFECT OF CORROSION

34 CORROSION PREVENTION

4 SENSORS

41 CORROSION SENSOR

5

42 RELAY

43 PRESSURE SENSOR

5 WIRELESS NETWORK

51 ENCODER

52 TRANSMITTER

53 RECEIVER

54 DECODER

6 POWER SUPPLY

61 RECTFIER

62 FILTER

63 VOLTAGE REGULATOR

7 CONTROL ACTION

71 BUZZER

72 BUFFER AMPLIFIER

73 SOLONIOD VALVE

REFERENCES

6

CHAPTER ndash 1

INTRODUCTION

7

1 INTRODUCTION

11 Oil gas and water are the natural resources that have been

the key of life and source of economy for most countries in the world

These natural resources are transported from its original plants or storage

through extended pipelines which spreads throughout the countries In

some countries pipelines also being used to supply gas directly to users

These pipelines serve as the backbone between producers and

consumers Maintaining pipelines is essential to sustain economic growth

political stability and also safety Delays in detecting leakage on pipeline

may lead to more serious matters such as fire and fatality Wireless

technologies have evolved so rapidly now-a-days and widely used in many

applications and services This mainly because it can remotely placed and

uses its own power source such as battery to power itself There are

number of technologies to monitor and protect pipelines Most of these

are designed specifically for detecting and locating pipeline leakages

These technologies allow a remote facility to detect and to report the

positions of any leakage Most of these solutions rely on the availability of

a network to transfer the information and report leakages Pipeline

monitoring systems have been using wireless devices as part of the

system communication and information transfer

One of the industries that have taken advantage of wireless

sensors is oil and gas industry Gas main distribution method is using

pipeline Monitoring of pipeline is very crucial because of its valuable

resources as well as for safety precautions Wireless sensors have been

widely used to monitor the health and the condition of the pipeline

Continuous monitoring of pipelines is necessary to ensure the safety

operation of pipelines transmission However several issues and

problems have been discussed related to the usage of wireless sensors in

pipeline monitoring system One of the issues is to detect leakage or

abnormality of the pipeline as early as possible particularly in gas

pipeline It is very important that we detect the signs of pipeline defects

as early as possible because if that allow it to go on it could cause some

8

serious problems later on such as endanger the environment and if the

pipeline near human population area it can be a disaster In the section

of pipeline

ruptures at 146 millions of cubic feet per day (MMCFD) of gas for pressure

of 1198 psi and 544deg Celsius was flowing through the 18-inch diameter

pipe and caused one life Structural defects on gas pipeline may caused

by several factors such as corrosion Several factors have been identified

that lead to the event of buried pipeline corrosion such as pipe coatings

soil conditions changing of temperature stresses pipe pressure and

cyclic loading effects

The objective of this project is to develop a method for early

detection of corrosion which the buried pipeline is exposed to changing

temperatures and gas flow pressures so that prompt actions can be done

to prevent any incidents or fatality It is a necessity to have system that

detects signs or abnormalities that will resulted to leakage event In a

pipeline monitoring and inspection system has several tasks to be

performed for natural gas pipelines environment to ensure the integrity of

the pipeline These include measuring pipe wall thickness measuring

velocity and flow of gas detecting gas contamination in pipeline and also

determining structural defects on pipes In gas pipelines commonly

placed in hazardous environmental such as deserts underwater and

buried deep These conditions can cause deterioration or even damage

due to corrosion erosion and fatigue Major problem is to detect corrosion

cracks

9

Fig of full circuit

10

RF Receiver Decoder PLC Load

Power Supply

12 BLOCK DIAGRAM

TRANSMITTER SECTION

RECEIVER SECTION

11

CHAPTER ndash 2

PROGRAMMABLE LOGIC CONTROLLER

12

2 PROGRAMMABLE LOGIC CONTROLLER

21 INTRODUCTION

Programmable logic controllers (PLCs) are members of the

computer family capable of storing instructions to control functions such as sequencing

timing and counting which control a machine or a process The PLC is composed of two

basic sections the Central Processing Unit (CPU) and the InputOutput (IO) interface

system The PLC measures input signals coming from a machine and through the internal

program provides output or control back to the machine Ladder logic is the

programming language used to represent electrical sequences of operation In hardwired

circuits the electrical wiring is connected from one device to another according to logic of

operation In a PLC the devices are connected to the input interface the outputs are

connected to the output interface and the actual wiring of the components is done

electronically inside the PLC using ladder logic This is known as soft wired PLC is a

device that is capable of being programmed to perform a controlling function Before the

advent of PLC the problem of industrial control was usually solved by relays or

hardwired solid-state logic blocks These are very flexible in design and easy for

maintenance personal to understand However they involved a vast amount of

interconnection For the wiring cost to be minimized relays and logic blocks had to be

kept together This led to development of control panel concept for larger and more

complex logic control system The PLC was first conceived by group of engineers from

hydramatic division of GM in 1968This was designed to provide flexibility in control

based on programming and executing logic instruction Adopting the ladder diagram

programming language simplifying maintenance and reducing the cost of spare parts

inventories realized major advantages

22PLC HISTORY

In the late 1960s PLCs were first introduced The primary reason for designing such a

device was eliminating the large cost involved in replacing the complicated relay based

machine control systems Bedford Associates (Bedford MA) proposed something called a

Modular Digital Controller (MODICON) to a major US car manufacturer Other companies

at the time proposed computer based schemes one of which was based upon the PDP-8 The

MODICON 084 brought the worlds first PLC into commercial production

13

When production requirements changed so did the control system This becomes very

expensive when the change is frequent Since relays are mechanical devices they also have a

limited lifetime which required strict adhesion to maintenance schedules Troubleshooting

was also quite tedious when so many relays are involved Now picture a machine control

panel that included many possibly hundreds or thousands of individual relays The size

could be mind boggling How about the complicated initial wiring of so many individual

devices These relays would be individually wired together in a manner that would yield the

desired outcome Were there problems You bet These new controllers also had to be

easily programmed by maintenance and plant engineers The lifetime had to be long and

programming changes easily performed They also had to survive the harsh industrial

environment Thats a lot to ask The answers were to use a programming technique most

people were already familiar with and replace mechanical parts with solid-state ones

In the mid70acircbdquocents the dominant PLC technologies were sequencer state-machines and the bit-

slice based CPU The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs

Conventional microprocessors lacked the power to quickly solve PLC logic in all but the

smallest PLCs As conventional microprocessors evolved larger and larger PLCs were being

based upon them However even today some are still based upon the 2903(ref A-Bs PLC-3)

Modicon has yet to build a faster PLC than their 984ABX which was based upon the 2901

Communications abilities began to appear in approximately 1973 The first such system was

Modicons Modbus The PLC could now talk to other PLCs and they could be far away from

the actual machine they were controlling They could also now be used to send and receive

varying voltages to allow them to enter the analog world Unfortunately the lack of

standardization coupled with continually changing technology has made PLC

communications a nightmare of incompatible protocols and physical networks Still it was a

great decade for the PLC The 80acircbdquocents saw an attempt to standardize communications with

General Motors manufacturing automation protocol(MAP) It was also a time for reducing

the size of the PLC and making them software programmable through symbolic programming

on personal computers instead of dedicated programming terminals or handheld

programmers Today the worlds smallest PLC is about the size of a single control relay

The 90acircbdquocents have seen a gradual reduction in the introduction of new protocols and the

modernization of the physical layers of some of the more popular protocols that survived the

1980s The latest standard (IEC 1131-3) has tried to merge plc programming languages

under one international standard We now have PLCs that are programmable in function

block diagrams instruction lists C and structured text all at the same time PCs are also

14

being used to replace PLCs in some applications The original company who commissioned

the MODICON 084 has actually switched to a PC based control system

23PLC HARDWARE

A programmable logic controller consists of the following components

Central Processing Unit (CPU) Memory Input modules Output modules and Power

supply A PLC hardware block diagram is shown in Figure The programming terminal in the

diagram is not a part of the PLC but it is essential to have a terminal for programming or

monitoring a PLC In the diagram the arrows between blocks indicate the information and

power-flowing-directions

Fig PLC-Hardware-Block-Diagram

CPU

Like other computerized devices there is a Central Processing Unit (CPU) in a PLC The

CPU which is the brain of a PLC does the following operations

Updating inputs and outputs This function allows a PLC to read the status of its input

terminals and energize or deenergize its output terminals

Performing logic and arithmetic operations A CPU conducts all the mathematic and logic

operations involved in a PLC

Communicating with memory The PLCacircbdquocents programs and data are stored in memory

When a PLC is operating its CPU may read or change the contents of memory locations

Scanning application programs An application program which is called a ladder logic

program is a set of instructions written by a PLC programmer The scanning function allows

the PLC to execute the application program as specified by the programmer

15

PROCESS

Programming Terminal

CPU MemoryPowerSupply

InputModule

OutputModule

InputDevices

OutputDevices

PLC

Communicating with a programming terminal The CPU transfers program and data

between itself and the programming terminal A PLC CPU is controlled by operating system

software The operating system software is a group of supervisory programs that are loaded

and stored permanently in the PLC memory by the PLC manufacturer

Memory

Memory is the component that stores information programs and data in a PLC The process

of putting new information into a memory location is called writing The process of retrieving

information from a memory location is called reading The common types of memory used in

PLCs are Read Only Memory (ROM) and Random Access Memory (RAM) A ROM

location can be read but not written ROM is used to store programs and data that should not

be altered For example the PLCs operating programs are stored in ROM

A RAM location can be read or written This means the information stored in a RAM

location can be retrieved andor altered Ladder logic programs are stored in RAM When a

new ladder logic program is loaded into a PLCs memory the old program that was stored in

the same locations is over-written and essentially erased The memory capacities of PLCs

vary Memory capacities are often expressed in terms of kilo-bytes (K) One byte is a group

of 8 bits One bit is a memory location that may store one binary number that has the value of

either 1 or 0 (Binary numbers are addressed in Module 2) 1K memory means that there are

1024 bytes of RAM 16K memory means there are 16 x 1024 =16384 bytes of RAM

Input modules and output modules

A PLC is a control device It takes information from inputs and makes decisions to

energize or de-energize outputs The decisions are made based on the statuses of inputs and

outputs and the ladder logic program that is being executed The input devices used with a

PLC include pushbuttons limit switches relay contacts photo sensors proximity switches

temperature sensors and the like These input devices can be AC (alternating current) or DC

(direct current) The input voltages can be high or low The input signals can be digital or

analog Differing inputs require different input modules An input module provides an

interface between input devices and a PLCs CPU which uses only a low DC voltage The

input moduleacircbdquocents function is to convert the input signals to DC voltages that are acceptable

to the CPU Standard discrete input modules include 24 V AC 48 V AC 120 V AC 220 V

AC 24 V DC 48 V DC 120 V DC 220 V DC and transistor-transistor logic (TTL) level

The devices controlled by a PLC include relays alarms solenoids fans lights and motor

starters These devices may require different levels of AC or DC voltages Since the signals

16

processed in a PLC are low DC voltages it is the function of the output module to convert

PLC control signals to the voltages required by the controlled circuits or devices Standard

discrete output modules include 24 V AC 48 V AC 120 V AC 220 V AC 24 V DC 48 V

DC 120 V DC 220 V DC and TTL level Power Supply -PLCs are powered by standard

commercial AC power lines However many PLC components such as the CPU and

memory utilize 5 volts or another level of DC power The PLC power supply converts AC

power into DC power to support those components of the PLC

Programming Terminal -A PLC requires a programming terminal and programming

software for operation The programming terminal can be a dedicated terminal or a generic

computer purchased anywhere The programming terminal is used for programming the PLC

and monitoring the PLCs operation It may also download a ladder logic program (the

sending of a program from the programming terminal to the PLC) or upload a ladder logic

program (the sending of a program from the PLC to the programming terminal) The terminal

uses programming software for programming and talking to a PLC

24 WORKING OF PLC

Bringing input signal status to the internal memory of CPU

The field signals are connected to the IP module At the output of IP module the field

status converted into the voltage level required by the CPU is always available

At the beginning of each cycle the CPU brings in all the field IP signals from IP module amp

stores into its internal memory called as PII meaning process image input

The programmable controller operates cyclically meaning when complete program has been

scanned it starts again at the beginning of the program

IOBUS

A PLC works by continually scanning a program We can think of this scan cycle as

consisting of 3 important steps There are typically more than 3 but we can focus on the

important parts and not worry about the others Typically the others are checking the system

and updating the current internal counter and timer values

Step 1-Check Input Status-First the PLC takes a look at each input to determine if it is on or

off In other words is the sensor connected to the first input on How about the second input

How about the third It records this data into its memory to be used during the next step

17

Step 2-Execute Program-Next the PLC executes your program one instruction at a time

Maybe your program said that if the first input was on then it should turn on the first output

Since it already knows which inputs are onoff from the previous step it will be able to decide

whether the first output should be turned on based on the state of the first input It will store

the execution results for use later during the next step

Step 3-Update Output Status-Finally the PLC updates the status of the outputs It updates the

outputs based on which inputs were on during the first step and the results of executing your

program during the second step Based on the example in step 2 it would now turn on the first

output because the first input was on and your program said to turn on the first output when

this condition is trueProcess Control and Automation Process Control

The process of recognizing the state of the process at all times analyze the information

according to the set rules and guidelines and accordingly actuate the control elements is

referred to as process control

RECOGNISING THE STATUS

In control of process all these actions can be taken manually with human involvement or in a

semiautomatic or fully automatic manner Automation -Automation is basically the

delegation of human control functions to technical equipment aimed towards achieving

- Higher-productivity

-Superior quality of end product

-Efficient usage of energy and raw materials

-Improved safety in working conditions etc

Methods adopted for Process Control and Automation

- Manual control

- Hard wired logic control

- Electronics control

-PLC control

- Manual Control

Hardwired Control

-This was considered to be the first step towards automation

- Here the contractor amp relays together with timers amp counters were used

Electronics Control

18

-With the advent of electronics the logic gates started replacing the relays amp auxiliary

contractors in the control circuits amp timers

- With changes the benefits are

1) Reduced space requirements

2) Energy saving

3) Less maintenance and hence greater reliability etc

-With electronics the implementation of changes in the control logic as well as reducing the

project lead-time was not possible

Programmable Logic Controller

- With microprocessor and associated peripherals chips the process of control and

automation went a radical change

- Instead of achieving the desired control or automation through physical wiring of control

devices in PLC it is through a program or software Thus these controllers are referred to as

programmable logic controllers

- The programmable controllers have experienced an unprecedented growth as universal

element It can be effectively used in applications ranging from simple control like replacing

small number relays to complex automation problem

25 PROGRAMMING THE PLC

Ladder Logic

Ladder logic is the main programming method used for PLCs The ladder logic has been

developed to mimic relay logic The decision to use the relay logic diagrams was a strategic

one By selecting ladder logic as the main programming method the amount of retraining

needed for engineers and trades people was greatly reduced

Modern control systems still include relays but these are rarely used for logic A relay is a

simple device that uses a magnetic field to control a switch as pictured in Fig When a

voltage is applied to the input coil the resulting current creates a magnetic field The

magnetic field pulls a metal switch (or reed) towards it and the contacts touch closing the

switch The contact that closes when the coil is energized is called normally open The

normally closed contacts touch when the input coil is not energized Relays are normally

drawn in schematic form using a circle to represent the input coil The output contacts are

shown with two parallel lines Normally open contacts are shown as two lines and will be

open (non-conducting) when the input is not energized Normally closed contacts are shown

with two lines with a diagonal line through them When the input coil is not energized the

19

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

42 RELAY

43 PRESSURE SENSOR

5 WIRELESS NETWORK

51 ENCODER

52 TRANSMITTER

53 RECEIVER

54 DECODER

6 POWER SUPPLY

61 RECTFIER

62 FILTER

63 VOLTAGE REGULATOR

7 CONTROL ACTION

71 BUZZER

72 BUFFER AMPLIFIER

73 SOLONIOD VALVE

REFERENCES

6

CHAPTER ndash 1

INTRODUCTION

7

1 INTRODUCTION

11 Oil gas and water are the natural resources that have been

the key of life and source of economy for most countries in the world

These natural resources are transported from its original plants or storage

through extended pipelines which spreads throughout the countries In

some countries pipelines also being used to supply gas directly to users

These pipelines serve as the backbone between producers and

consumers Maintaining pipelines is essential to sustain economic growth

political stability and also safety Delays in detecting leakage on pipeline

may lead to more serious matters such as fire and fatality Wireless

technologies have evolved so rapidly now-a-days and widely used in many

applications and services This mainly because it can remotely placed and

uses its own power source such as battery to power itself There are

number of technologies to monitor and protect pipelines Most of these

are designed specifically for detecting and locating pipeline leakages

These technologies allow a remote facility to detect and to report the

positions of any leakage Most of these solutions rely on the availability of

a network to transfer the information and report leakages Pipeline

monitoring systems have been using wireless devices as part of the

system communication and information transfer

One of the industries that have taken advantage of wireless

sensors is oil and gas industry Gas main distribution method is using

pipeline Monitoring of pipeline is very crucial because of its valuable

resources as well as for safety precautions Wireless sensors have been

widely used to monitor the health and the condition of the pipeline

Continuous monitoring of pipelines is necessary to ensure the safety

operation of pipelines transmission However several issues and

problems have been discussed related to the usage of wireless sensors in

pipeline monitoring system One of the issues is to detect leakage or

abnormality of the pipeline as early as possible particularly in gas

pipeline It is very important that we detect the signs of pipeline defects

as early as possible because if that allow it to go on it could cause some

8

serious problems later on such as endanger the environment and if the

pipeline near human population area it can be a disaster In the section

of pipeline

ruptures at 146 millions of cubic feet per day (MMCFD) of gas for pressure

of 1198 psi and 544deg Celsius was flowing through the 18-inch diameter

pipe and caused one life Structural defects on gas pipeline may caused

by several factors such as corrosion Several factors have been identified

that lead to the event of buried pipeline corrosion such as pipe coatings

soil conditions changing of temperature stresses pipe pressure and

cyclic loading effects

The objective of this project is to develop a method for early

detection of corrosion which the buried pipeline is exposed to changing

temperatures and gas flow pressures so that prompt actions can be done

to prevent any incidents or fatality It is a necessity to have system that

detects signs or abnormalities that will resulted to leakage event In a

pipeline monitoring and inspection system has several tasks to be

performed for natural gas pipelines environment to ensure the integrity of

the pipeline These include measuring pipe wall thickness measuring

velocity and flow of gas detecting gas contamination in pipeline and also

determining structural defects on pipes In gas pipelines commonly

placed in hazardous environmental such as deserts underwater and

buried deep These conditions can cause deterioration or even damage

due to corrosion erosion and fatigue Major problem is to detect corrosion

cracks

9

Fig of full circuit

10

RF Receiver Decoder PLC Load

Power Supply

12 BLOCK DIAGRAM

TRANSMITTER SECTION

RECEIVER SECTION

11

CHAPTER ndash 2

PROGRAMMABLE LOGIC CONTROLLER

12

2 PROGRAMMABLE LOGIC CONTROLLER

21 INTRODUCTION

Programmable logic controllers (PLCs) are members of the

computer family capable of storing instructions to control functions such as sequencing

timing and counting which control a machine or a process The PLC is composed of two

basic sections the Central Processing Unit (CPU) and the InputOutput (IO) interface

system The PLC measures input signals coming from a machine and through the internal

program provides output or control back to the machine Ladder logic is the

programming language used to represent electrical sequences of operation In hardwired

circuits the electrical wiring is connected from one device to another according to logic of

operation In a PLC the devices are connected to the input interface the outputs are

connected to the output interface and the actual wiring of the components is done

electronically inside the PLC using ladder logic This is known as soft wired PLC is a

device that is capable of being programmed to perform a controlling function Before the

advent of PLC the problem of industrial control was usually solved by relays or

hardwired solid-state logic blocks These are very flexible in design and easy for

maintenance personal to understand However they involved a vast amount of

interconnection For the wiring cost to be minimized relays and logic blocks had to be

kept together This led to development of control panel concept for larger and more

complex logic control system The PLC was first conceived by group of engineers from

hydramatic division of GM in 1968This was designed to provide flexibility in control

based on programming and executing logic instruction Adopting the ladder diagram

programming language simplifying maintenance and reducing the cost of spare parts

inventories realized major advantages

22PLC HISTORY

In the late 1960s PLCs were first introduced The primary reason for designing such a

device was eliminating the large cost involved in replacing the complicated relay based

machine control systems Bedford Associates (Bedford MA) proposed something called a

Modular Digital Controller (MODICON) to a major US car manufacturer Other companies

at the time proposed computer based schemes one of which was based upon the PDP-8 The

MODICON 084 brought the worlds first PLC into commercial production

13

When production requirements changed so did the control system This becomes very

expensive when the change is frequent Since relays are mechanical devices they also have a

limited lifetime which required strict adhesion to maintenance schedules Troubleshooting

was also quite tedious when so many relays are involved Now picture a machine control

panel that included many possibly hundreds or thousands of individual relays The size

could be mind boggling How about the complicated initial wiring of so many individual

devices These relays would be individually wired together in a manner that would yield the

desired outcome Were there problems You bet These new controllers also had to be

easily programmed by maintenance and plant engineers The lifetime had to be long and

programming changes easily performed They also had to survive the harsh industrial

environment Thats a lot to ask The answers were to use a programming technique most

people were already familiar with and replace mechanical parts with solid-state ones

In the mid70acircbdquocents the dominant PLC technologies were sequencer state-machines and the bit-

slice based CPU The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs

Conventional microprocessors lacked the power to quickly solve PLC logic in all but the

smallest PLCs As conventional microprocessors evolved larger and larger PLCs were being

based upon them However even today some are still based upon the 2903(ref A-Bs PLC-3)

Modicon has yet to build a faster PLC than their 984ABX which was based upon the 2901

Communications abilities began to appear in approximately 1973 The first such system was

Modicons Modbus The PLC could now talk to other PLCs and they could be far away from

the actual machine they were controlling They could also now be used to send and receive

varying voltages to allow them to enter the analog world Unfortunately the lack of

standardization coupled with continually changing technology has made PLC

communications a nightmare of incompatible protocols and physical networks Still it was a

great decade for the PLC The 80acircbdquocents saw an attempt to standardize communications with

General Motors manufacturing automation protocol(MAP) It was also a time for reducing

the size of the PLC and making them software programmable through symbolic programming

on personal computers instead of dedicated programming terminals or handheld

programmers Today the worlds smallest PLC is about the size of a single control relay

The 90acircbdquocents have seen a gradual reduction in the introduction of new protocols and the

modernization of the physical layers of some of the more popular protocols that survived the

1980s The latest standard (IEC 1131-3) has tried to merge plc programming languages

under one international standard We now have PLCs that are programmable in function

block diagrams instruction lists C and structured text all at the same time PCs are also

14

being used to replace PLCs in some applications The original company who commissioned

the MODICON 084 has actually switched to a PC based control system

23PLC HARDWARE

A programmable logic controller consists of the following components

Central Processing Unit (CPU) Memory Input modules Output modules and Power

supply A PLC hardware block diagram is shown in Figure The programming terminal in the

diagram is not a part of the PLC but it is essential to have a terminal for programming or

monitoring a PLC In the diagram the arrows between blocks indicate the information and

power-flowing-directions

Fig PLC-Hardware-Block-Diagram

CPU

Like other computerized devices there is a Central Processing Unit (CPU) in a PLC The

CPU which is the brain of a PLC does the following operations

Updating inputs and outputs This function allows a PLC to read the status of its input

terminals and energize or deenergize its output terminals

Performing logic and arithmetic operations A CPU conducts all the mathematic and logic

operations involved in a PLC

Communicating with memory The PLCacircbdquocents programs and data are stored in memory

When a PLC is operating its CPU may read or change the contents of memory locations

Scanning application programs An application program which is called a ladder logic

program is a set of instructions written by a PLC programmer The scanning function allows

the PLC to execute the application program as specified by the programmer

15

PROCESS

Programming Terminal

CPU MemoryPowerSupply

InputModule

OutputModule

InputDevices

OutputDevices

PLC

Communicating with a programming terminal The CPU transfers program and data

between itself and the programming terminal A PLC CPU is controlled by operating system

software The operating system software is a group of supervisory programs that are loaded

and stored permanently in the PLC memory by the PLC manufacturer

Memory

Memory is the component that stores information programs and data in a PLC The process

of putting new information into a memory location is called writing The process of retrieving

information from a memory location is called reading The common types of memory used in

PLCs are Read Only Memory (ROM) and Random Access Memory (RAM) A ROM

location can be read but not written ROM is used to store programs and data that should not

be altered For example the PLCs operating programs are stored in ROM

A RAM location can be read or written This means the information stored in a RAM

location can be retrieved andor altered Ladder logic programs are stored in RAM When a

new ladder logic program is loaded into a PLCs memory the old program that was stored in

the same locations is over-written and essentially erased The memory capacities of PLCs

vary Memory capacities are often expressed in terms of kilo-bytes (K) One byte is a group

of 8 bits One bit is a memory location that may store one binary number that has the value of

either 1 or 0 (Binary numbers are addressed in Module 2) 1K memory means that there are

1024 bytes of RAM 16K memory means there are 16 x 1024 =16384 bytes of RAM

Input modules and output modules

A PLC is a control device It takes information from inputs and makes decisions to

energize or de-energize outputs The decisions are made based on the statuses of inputs and

outputs and the ladder logic program that is being executed The input devices used with a

PLC include pushbuttons limit switches relay contacts photo sensors proximity switches

temperature sensors and the like These input devices can be AC (alternating current) or DC

(direct current) The input voltages can be high or low The input signals can be digital or

analog Differing inputs require different input modules An input module provides an

interface between input devices and a PLCs CPU which uses only a low DC voltage The

input moduleacircbdquocents function is to convert the input signals to DC voltages that are acceptable

to the CPU Standard discrete input modules include 24 V AC 48 V AC 120 V AC 220 V

AC 24 V DC 48 V DC 120 V DC 220 V DC and transistor-transistor logic (TTL) level

The devices controlled by a PLC include relays alarms solenoids fans lights and motor

starters These devices may require different levels of AC or DC voltages Since the signals

16

processed in a PLC are low DC voltages it is the function of the output module to convert

PLC control signals to the voltages required by the controlled circuits or devices Standard

discrete output modules include 24 V AC 48 V AC 120 V AC 220 V AC 24 V DC 48 V

DC 120 V DC 220 V DC and TTL level Power Supply -PLCs are powered by standard

commercial AC power lines However many PLC components such as the CPU and

memory utilize 5 volts or another level of DC power The PLC power supply converts AC

power into DC power to support those components of the PLC

Programming Terminal -A PLC requires a programming terminal and programming

software for operation The programming terminal can be a dedicated terminal or a generic

computer purchased anywhere The programming terminal is used for programming the PLC

and monitoring the PLCs operation It may also download a ladder logic program (the

sending of a program from the programming terminal to the PLC) or upload a ladder logic

program (the sending of a program from the PLC to the programming terminal) The terminal

uses programming software for programming and talking to a PLC

24 WORKING OF PLC

Bringing input signal status to the internal memory of CPU

The field signals are connected to the IP module At the output of IP module the field

status converted into the voltage level required by the CPU is always available

At the beginning of each cycle the CPU brings in all the field IP signals from IP module amp

stores into its internal memory called as PII meaning process image input

The programmable controller operates cyclically meaning when complete program has been

scanned it starts again at the beginning of the program

IOBUS

A PLC works by continually scanning a program We can think of this scan cycle as

consisting of 3 important steps There are typically more than 3 but we can focus on the

important parts and not worry about the others Typically the others are checking the system

and updating the current internal counter and timer values

Step 1-Check Input Status-First the PLC takes a look at each input to determine if it is on or

off In other words is the sensor connected to the first input on How about the second input

How about the third It records this data into its memory to be used during the next step

17

Step 2-Execute Program-Next the PLC executes your program one instruction at a time

Maybe your program said that if the first input was on then it should turn on the first output

Since it already knows which inputs are onoff from the previous step it will be able to decide

whether the first output should be turned on based on the state of the first input It will store

the execution results for use later during the next step

Step 3-Update Output Status-Finally the PLC updates the status of the outputs It updates the

outputs based on which inputs were on during the first step and the results of executing your

program during the second step Based on the example in step 2 it would now turn on the first

output because the first input was on and your program said to turn on the first output when

this condition is trueProcess Control and Automation Process Control

The process of recognizing the state of the process at all times analyze the information

according to the set rules and guidelines and accordingly actuate the control elements is

referred to as process control

RECOGNISING THE STATUS

In control of process all these actions can be taken manually with human involvement or in a

semiautomatic or fully automatic manner Automation -Automation is basically the

delegation of human control functions to technical equipment aimed towards achieving

- Higher-productivity

-Superior quality of end product

-Efficient usage of energy and raw materials

-Improved safety in working conditions etc

Methods adopted for Process Control and Automation

- Manual control

- Hard wired logic control

- Electronics control

-PLC control

- Manual Control

Hardwired Control

-This was considered to be the first step towards automation

- Here the contractor amp relays together with timers amp counters were used

Electronics Control

18

-With the advent of electronics the logic gates started replacing the relays amp auxiliary

contractors in the control circuits amp timers

- With changes the benefits are

1) Reduced space requirements

2) Energy saving

3) Less maintenance and hence greater reliability etc

-With electronics the implementation of changes in the control logic as well as reducing the

project lead-time was not possible

Programmable Logic Controller

- With microprocessor and associated peripherals chips the process of control and

automation went a radical change

- Instead of achieving the desired control or automation through physical wiring of control

devices in PLC it is through a program or software Thus these controllers are referred to as

programmable logic controllers

- The programmable controllers have experienced an unprecedented growth as universal

element It can be effectively used in applications ranging from simple control like replacing

small number relays to complex automation problem

25 PROGRAMMING THE PLC

Ladder Logic

Ladder logic is the main programming method used for PLCs The ladder logic has been

developed to mimic relay logic The decision to use the relay logic diagrams was a strategic

one By selecting ladder logic as the main programming method the amount of retraining

needed for engineers and trades people was greatly reduced

Modern control systems still include relays but these are rarely used for logic A relay is a

simple device that uses a magnetic field to control a switch as pictured in Fig When a

voltage is applied to the input coil the resulting current creates a magnetic field The

magnetic field pulls a metal switch (or reed) towards it and the contacts touch closing the

switch The contact that closes when the coil is energized is called normally open The

normally closed contacts touch when the input coil is not energized Relays are normally

drawn in schematic form using a circle to represent the input coil The output contacts are

shown with two parallel lines Normally open contacts are shown as two lines and will be

open (non-conducting) when the input is not energized Normally closed contacts are shown

with two lines with a diagonal line through them When the input coil is not energized the

19

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

CHAPTER ndash 1

INTRODUCTION

7

1 INTRODUCTION

11 Oil gas and water are the natural resources that have been

the key of life and source of economy for most countries in the world

These natural resources are transported from its original plants or storage

through extended pipelines which spreads throughout the countries In

some countries pipelines also being used to supply gas directly to users

These pipelines serve as the backbone between producers and

consumers Maintaining pipelines is essential to sustain economic growth

political stability and also safety Delays in detecting leakage on pipeline

may lead to more serious matters such as fire and fatality Wireless

technologies have evolved so rapidly now-a-days and widely used in many

applications and services This mainly because it can remotely placed and

uses its own power source such as battery to power itself There are

number of technologies to monitor and protect pipelines Most of these

are designed specifically for detecting and locating pipeline leakages

These technologies allow a remote facility to detect and to report the

positions of any leakage Most of these solutions rely on the availability of

a network to transfer the information and report leakages Pipeline

monitoring systems have been using wireless devices as part of the

system communication and information transfer

One of the industries that have taken advantage of wireless

sensors is oil and gas industry Gas main distribution method is using

pipeline Monitoring of pipeline is very crucial because of its valuable

resources as well as for safety precautions Wireless sensors have been

widely used to monitor the health and the condition of the pipeline

Continuous monitoring of pipelines is necessary to ensure the safety

operation of pipelines transmission However several issues and

problems have been discussed related to the usage of wireless sensors in

pipeline monitoring system One of the issues is to detect leakage or

abnormality of the pipeline as early as possible particularly in gas

pipeline It is very important that we detect the signs of pipeline defects

as early as possible because if that allow it to go on it could cause some

8

serious problems later on such as endanger the environment and if the

pipeline near human population area it can be a disaster In the section

of pipeline

ruptures at 146 millions of cubic feet per day (MMCFD) of gas for pressure

of 1198 psi and 544deg Celsius was flowing through the 18-inch diameter

pipe and caused one life Structural defects on gas pipeline may caused

by several factors such as corrosion Several factors have been identified

that lead to the event of buried pipeline corrosion such as pipe coatings

soil conditions changing of temperature stresses pipe pressure and

cyclic loading effects

The objective of this project is to develop a method for early

detection of corrosion which the buried pipeline is exposed to changing

temperatures and gas flow pressures so that prompt actions can be done

to prevent any incidents or fatality It is a necessity to have system that

detects signs or abnormalities that will resulted to leakage event In a

pipeline monitoring and inspection system has several tasks to be

performed for natural gas pipelines environment to ensure the integrity of

the pipeline These include measuring pipe wall thickness measuring

velocity and flow of gas detecting gas contamination in pipeline and also

determining structural defects on pipes In gas pipelines commonly

placed in hazardous environmental such as deserts underwater and

buried deep These conditions can cause deterioration or even damage

due to corrosion erosion and fatigue Major problem is to detect corrosion

cracks

9

Fig of full circuit

10

RF Receiver Decoder PLC Load

Power Supply

12 BLOCK DIAGRAM

TRANSMITTER SECTION

RECEIVER SECTION

11

CHAPTER ndash 2

PROGRAMMABLE LOGIC CONTROLLER

12

2 PROGRAMMABLE LOGIC CONTROLLER

21 INTRODUCTION

Programmable logic controllers (PLCs) are members of the

computer family capable of storing instructions to control functions such as sequencing

timing and counting which control a machine or a process The PLC is composed of two

basic sections the Central Processing Unit (CPU) and the InputOutput (IO) interface

system The PLC measures input signals coming from a machine and through the internal

program provides output or control back to the machine Ladder logic is the

programming language used to represent electrical sequences of operation In hardwired

circuits the electrical wiring is connected from one device to another according to logic of

operation In a PLC the devices are connected to the input interface the outputs are

connected to the output interface and the actual wiring of the components is done

electronically inside the PLC using ladder logic This is known as soft wired PLC is a

device that is capable of being programmed to perform a controlling function Before the

advent of PLC the problem of industrial control was usually solved by relays or

hardwired solid-state logic blocks These are very flexible in design and easy for

maintenance personal to understand However they involved a vast amount of

interconnection For the wiring cost to be minimized relays and logic blocks had to be

kept together This led to development of control panel concept for larger and more

complex logic control system The PLC was first conceived by group of engineers from

hydramatic division of GM in 1968This was designed to provide flexibility in control

based on programming and executing logic instruction Adopting the ladder diagram

programming language simplifying maintenance and reducing the cost of spare parts

inventories realized major advantages

22PLC HISTORY

In the late 1960s PLCs were first introduced The primary reason for designing such a

device was eliminating the large cost involved in replacing the complicated relay based

machine control systems Bedford Associates (Bedford MA) proposed something called a

Modular Digital Controller (MODICON) to a major US car manufacturer Other companies

at the time proposed computer based schemes one of which was based upon the PDP-8 The

MODICON 084 brought the worlds first PLC into commercial production

13

When production requirements changed so did the control system This becomes very

expensive when the change is frequent Since relays are mechanical devices they also have a

limited lifetime which required strict adhesion to maintenance schedules Troubleshooting

was also quite tedious when so many relays are involved Now picture a machine control

panel that included many possibly hundreds or thousands of individual relays The size

could be mind boggling How about the complicated initial wiring of so many individual

devices These relays would be individually wired together in a manner that would yield the

desired outcome Were there problems You bet These new controllers also had to be

easily programmed by maintenance and plant engineers The lifetime had to be long and

programming changes easily performed They also had to survive the harsh industrial

environment Thats a lot to ask The answers were to use a programming technique most

people were already familiar with and replace mechanical parts with solid-state ones

In the mid70acircbdquocents the dominant PLC technologies were sequencer state-machines and the bit-

slice based CPU The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs

Conventional microprocessors lacked the power to quickly solve PLC logic in all but the

smallest PLCs As conventional microprocessors evolved larger and larger PLCs were being

based upon them However even today some are still based upon the 2903(ref A-Bs PLC-3)

Modicon has yet to build a faster PLC than their 984ABX which was based upon the 2901

Communications abilities began to appear in approximately 1973 The first such system was

Modicons Modbus The PLC could now talk to other PLCs and they could be far away from

the actual machine they were controlling They could also now be used to send and receive

varying voltages to allow them to enter the analog world Unfortunately the lack of

standardization coupled with continually changing technology has made PLC

communications a nightmare of incompatible protocols and physical networks Still it was a

great decade for the PLC The 80acircbdquocents saw an attempt to standardize communications with

General Motors manufacturing automation protocol(MAP) It was also a time for reducing

the size of the PLC and making them software programmable through symbolic programming

on personal computers instead of dedicated programming terminals or handheld

programmers Today the worlds smallest PLC is about the size of a single control relay

The 90acircbdquocents have seen a gradual reduction in the introduction of new protocols and the

modernization of the physical layers of some of the more popular protocols that survived the

1980s The latest standard (IEC 1131-3) has tried to merge plc programming languages

under one international standard We now have PLCs that are programmable in function

block diagrams instruction lists C and structured text all at the same time PCs are also

14

being used to replace PLCs in some applications The original company who commissioned

the MODICON 084 has actually switched to a PC based control system

23PLC HARDWARE

A programmable logic controller consists of the following components

Central Processing Unit (CPU) Memory Input modules Output modules and Power

supply A PLC hardware block diagram is shown in Figure The programming terminal in the

diagram is not a part of the PLC but it is essential to have a terminal for programming or

monitoring a PLC In the diagram the arrows between blocks indicate the information and

power-flowing-directions

Fig PLC-Hardware-Block-Diagram

CPU

Like other computerized devices there is a Central Processing Unit (CPU) in a PLC The

CPU which is the brain of a PLC does the following operations

Updating inputs and outputs This function allows a PLC to read the status of its input

terminals and energize or deenergize its output terminals

Performing logic and arithmetic operations A CPU conducts all the mathematic and logic

operations involved in a PLC

Communicating with memory The PLCacircbdquocents programs and data are stored in memory

When a PLC is operating its CPU may read or change the contents of memory locations

Scanning application programs An application program which is called a ladder logic

program is a set of instructions written by a PLC programmer The scanning function allows

the PLC to execute the application program as specified by the programmer

15

PROCESS

Programming Terminal

CPU MemoryPowerSupply

InputModule

OutputModule

InputDevices

OutputDevices

PLC

Communicating with a programming terminal The CPU transfers program and data

between itself and the programming terminal A PLC CPU is controlled by operating system

software The operating system software is a group of supervisory programs that are loaded

and stored permanently in the PLC memory by the PLC manufacturer

Memory

Memory is the component that stores information programs and data in a PLC The process

of putting new information into a memory location is called writing The process of retrieving

information from a memory location is called reading The common types of memory used in

PLCs are Read Only Memory (ROM) and Random Access Memory (RAM) A ROM

location can be read but not written ROM is used to store programs and data that should not

be altered For example the PLCs operating programs are stored in ROM

A RAM location can be read or written This means the information stored in a RAM

location can be retrieved andor altered Ladder logic programs are stored in RAM When a

new ladder logic program is loaded into a PLCs memory the old program that was stored in

the same locations is over-written and essentially erased The memory capacities of PLCs

vary Memory capacities are often expressed in terms of kilo-bytes (K) One byte is a group

of 8 bits One bit is a memory location that may store one binary number that has the value of

either 1 or 0 (Binary numbers are addressed in Module 2) 1K memory means that there are

1024 bytes of RAM 16K memory means there are 16 x 1024 =16384 bytes of RAM

Input modules and output modules

A PLC is a control device It takes information from inputs and makes decisions to

energize or de-energize outputs The decisions are made based on the statuses of inputs and

outputs and the ladder logic program that is being executed The input devices used with a

PLC include pushbuttons limit switches relay contacts photo sensors proximity switches

temperature sensors and the like These input devices can be AC (alternating current) or DC

(direct current) The input voltages can be high or low The input signals can be digital or

analog Differing inputs require different input modules An input module provides an

interface between input devices and a PLCs CPU which uses only a low DC voltage The

input moduleacircbdquocents function is to convert the input signals to DC voltages that are acceptable

to the CPU Standard discrete input modules include 24 V AC 48 V AC 120 V AC 220 V

AC 24 V DC 48 V DC 120 V DC 220 V DC and transistor-transistor logic (TTL) level

The devices controlled by a PLC include relays alarms solenoids fans lights and motor

starters These devices may require different levels of AC or DC voltages Since the signals

16

processed in a PLC are low DC voltages it is the function of the output module to convert

PLC control signals to the voltages required by the controlled circuits or devices Standard

discrete output modules include 24 V AC 48 V AC 120 V AC 220 V AC 24 V DC 48 V

DC 120 V DC 220 V DC and TTL level Power Supply -PLCs are powered by standard

commercial AC power lines However many PLC components such as the CPU and

memory utilize 5 volts or another level of DC power The PLC power supply converts AC

power into DC power to support those components of the PLC

Programming Terminal -A PLC requires a programming terminal and programming

software for operation The programming terminal can be a dedicated terminal or a generic

computer purchased anywhere The programming terminal is used for programming the PLC

and monitoring the PLCs operation It may also download a ladder logic program (the

sending of a program from the programming terminal to the PLC) or upload a ladder logic

program (the sending of a program from the PLC to the programming terminal) The terminal

uses programming software for programming and talking to a PLC

24 WORKING OF PLC

Bringing input signal status to the internal memory of CPU

The field signals are connected to the IP module At the output of IP module the field

status converted into the voltage level required by the CPU is always available

At the beginning of each cycle the CPU brings in all the field IP signals from IP module amp

stores into its internal memory called as PII meaning process image input

The programmable controller operates cyclically meaning when complete program has been

scanned it starts again at the beginning of the program

IOBUS

A PLC works by continually scanning a program We can think of this scan cycle as

consisting of 3 important steps There are typically more than 3 but we can focus on the

important parts and not worry about the others Typically the others are checking the system

and updating the current internal counter and timer values

Step 1-Check Input Status-First the PLC takes a look at each input to determine if it is on or

off In other words is the sensor connected to the first input on How about the second input

How about the third It records this data into its memory to be used during the next step

17

Step 2-Execute Program-Next the PLC executes your program one instruction at a time

Maybe your program said that if the first input was on then it should turn on the first output

Since it already knows which inputs are onoff from the previous step it will be able to decide

whether the first output should be turned on based on the state of the first input It will store

the execution results for use later during the next step

Step 3-Update Output Status-Finally the PLC updates the status of the outputs It updates the

outputs based on which inputs were on during the first step and the results of executing your

program during the second step Based on the example in step 2 it would now turn on the first

output because the first input was on and your program said to turn on the first output when

this condition is trueProcess Control and Automation Process Control

The process of recognizing the state of the process at all times analyze the information

according to the set rules and guidelines and accordingly actuate the control elements is

referred to as process control

RECOGNISING THE STATUS

In control of process all these actions can be taken manually with human involvement or in a

semiautomatic or fully automatic manner Automation -Automation is basically the

delegation of human control functions to technical equipment aimed towards achieving

- Higher-productivity

-Superior quality of end product

-Efficient usage of energy and raw materials

-Improved safety in working conditions etc

Methods adopted for Process Control and Automation

- Manual control

- Hard wired logic control

- Electronics control

-PLC control

- Manual Control

Hardwired Control

-This was considered to be the first step towards automation

- Here the contractor amp relays together with timers amp counters were used

Electronics Control

18

-With the advent of electronics the logic gates started replacing the relays amp auxiliary

contractors in the control circuits amp timers

- With changes the benefits are

1) Reduced space requirements

2) Energy saving

3) Less maintenance and hence greater reliability etc

-With electronics the implementation of changes in the control logic as well as reducing the

project lead-time was not possible

Programmable Logic Controller

- With microprocessor and associated peripherals chips the process of control and

automation went a radical change

- Instead of achieving the desired control or automation through physical wiring of control

devices in PLC it is through a program or software Thus these controllers are referred to as

programmable logic controllers

- The programmable controllers have experienced an unprecedented growth as universal

element It can be effectively used in applications ranging from simple control like replacing

small number relays to complex automation problem

25 PROGRAMMING THE PLC

Ladder Logic

Ladder logic is the main programming method used for PLCs The ladder logic has been

developed to mimic relay logic The decision to use the relay logic diagrams was a strategic

one By selecting ladder logic as the main programming method the amount of retraining

needed for engineers and trades people was greatly reduced

Modern control systems still include relays but these are rarely used for logic A relay is a

simple device that uses a magnetic field to control a switch as pictured in Fig When a

voltage is applied to the input coil the resulting current creates a magnetic field The

magnetic field pulls a metal switch (or reed) towards it and the contacts touch closing the

switch The contact that closes when the coil is energized is called normally open The

normally closed contacts touch when the input coil is not energized Relays are normally

drawn in schematic form using a circle to represent the input coil The output contacts are

shown with two parallel lines Normally open contacts are shown as two lines and will be

open (non-conducting) when the input is not energized Normally closed contacts are shown

with two lines with a diagonal line through them When the input coil is not energized the

19

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

1 INTRODUCTION

11 Oil gas and water are the natural resources that have been

the key of life and source of economy for most countries in the world

These natural resources are transported from its original plants or storage

through extended pipelines which spreads throughout the countries In

some countries pipelines also being used to supply gas directly to users

These pipelines serve as the backbone between producers and

consumers Maintaining pipelines is essential to sustain economic growth

political stability and also safety Delays in detecting leakage on pipeline

may lead to more serious matters such as fire and fatality Wireless

technologies have evolved so rapidly now-a-days and widely used in many

applications and services This mainly because it can remotely placed and

uses its own power source such as battery to power itself There are

number of technologies to monitor and protect pipelines Most of these

are designed specifically for detecting and locating pipeline leakages

These technologies allow a remote facility to detect and to report the

positions of any leakage Most of these solutions rely on the availability of

a network to transfer the information and report leakages Pipeline

monitoring systems have been using wireless devices as part of the

system communication and information transfer

One of the industries that have taken advantage of wireless

sensors is oil and gas industry Gas main distribution method is using

pipeline Monitoring of pipeline is very crucial because of its valuable

resources as well as for safety precautions Wireless sensors have been

widely used to monitor the health and the condition of the pipeline

Continuous monitoring of pipelines is necessary to ensure the safety

operation of pipelines transmission However several issues and

problems have been discussed related to the usage of wireless sensors in

pipeline monitoring system One of the issues is to detect leakage or

abnormality of the pipeline as early as possible particularly in gas

pipeline It is very important that we detect the signs of pipeline defects

as early as possible because if that allow it to go on it could cause some

8

serious problems later on such as endanger the environment and if the

pipeline near human population area it can be a disaster In the section

of pipeline

ruptures at 146 millions of cubic feet per day (MMCFD) of gas for pressure

of 1198 psi and 544deg Celsius was flowing through the 18-inch diameter

pipe and caused one life Structural defects on gas pipeline may caused

by several factors such as corrosion Several factors have been identified

that lead to the event of buried pipeline corrosion such as pipe coatings

soil conditions changing of temperature stresses pipe pressure and

cyclic loading effects

The objective of this project is to develop a method for early

detection of corrosion which the buried pipeline is exposed to changing

temperatures and gas flow pressures so that prompt actions can be done

to prevent any incidents or fatality It is a necessity to have system that

detects signs or abnormalities that will resulted to leakage event In a

pipeline monitoring and inspection system has several tasks to be

performed for natural gas pipelines environment to ensure the integrity of

the pipeline These include measuring pipe wall thickness measuring

velocity and flow of gas detecting gas contamination in pipeline and also

determining structural defects on pipes In gas pipelines commonly

placed in hazardous environmental such as deserts underwater and

buried deep These conditions can cause deterioration or even damage

due to corrosion erosion and fatigue Major problem is to detect corrosion

cracks

9

Fig of full circuit

10

RF Receiver Decoder PLC Load

Power Supply

12 BLOCK DIAGRAM

TRANSMITTER SECTION

RECEIVER SECTION

11

CHAPTER ndash 2

PROGRAMMABLE LOGIC CONTROLLER

12

2 PROGRAMMABLE LOGIC CONTROLLER

21 INTRODUCTION

Programmable logic controllers (PLCs) are members of the

computer family capable of storing instructions to control functions such as sequencing

timing and counting which control a machine or a process The PLC is composed of two

basic sections the Central Processing Unit (CPU) and the InputOutput (IO) interface

system The PLC measures input signals coming from a machine and through the internal

program provides output or control back to the machine Ladder logic is the

programming language used to represent electrical sequences of operation In hardwired

circuits the electrical wiring is connected from one device to another according to logic of

operation In a PLC the devices are connected to the input interface the outputs are

connected to the output interface and the actual wiring of the components is done

electronically inside the PLC using ladder logic This is known as soft wired PLC is a

device that is capable of being programmed to perform a controlling function Before the

advent of PLC the problem of industrial control was usually solved by relays or

hardwired solid-state logic blocks These are very flexible in design and easy for

maintenance personal to understand However they involved a vast amount of

interconnection For the wiring cost to be minimized relays and logic blocks had to be

kept together This led to development of control panel concept for larger and more

complex logic control system The PLC was first conceived by group of engineers from

hydramatic division of GM in 1968This was designed to provide flexibility in control

based on programming and executing logic instruction Adopting the ladder diagram

programming language simplifying maintenance and reducing the cost of spare parts

inventories realized major advantages

22PLC HISTORY

In the late 1960s PLCs were first introduced The primary reason for designing such a

device was eliminating the large cost involved in replacing the complicated relay based

machine control systems Bedford Associates (Bedford MA) proposed something called a

Modular Digital Controller (MODICON) to a major US car manufacturer Other companies

at the time proposed computer based schemes one of which was based upon the PDP-8 The

MODICON 084 brought the worlds first PLC into commercial production

13

When production requirements changed so did the control system This becomes very

expensive when the change is frequent Since relays are mechanical devices they also have a

limited lifetime which required strict adhesion to maintenance schedules Troubleshooting

was also quite tedious when so many relays are involved Now picture a machine control

panel that included many possibly hundreds or thousands of individual relays The size

could be mind boggling How about the complicated initial wiring of so many individual

devices These relays would be individually wired together in a manner that would yield the

desired outcome Were there problems You bet These new controllers also had to be

easily programmed by maintenance and plant engineers The lifetime had to be long and

programming changes easily performed They also had to survive the harsh industrial

environment Thats a lot to ask The answers were to use a programming technique most

people were already familiar with and replace mechanical parts with solid-state ones

In the mid70acircbdquocents the dominant PLC technologies were sequencer state-machines and the bit-

slice based CPU The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs

Conventional microprocessors lacked the power to quickly solve PLC logic in all but the

smallest PLCs As conventional microprocessors evolved larger and larger PLCs were being

based upon them However even today some are still based upon the 2903(ref A-Bs PLC-3)

Modicon has yet to build a faster PLC than their 984ABX which was based upon the 2901

Communications abilities began to appear in approximately 1973 The first such system was

Modicons Modbus The PLC could now talk to other PLCs and they could be far away from

the actual machine they were controlling They could also now be used to send and receive

varying voltages to allow them to enter the analog world Unfortunately the lack of

standardization coupled with continually changing technology has made PLC

communications a nightmare of incompatible protocols and physical networks Still it was a

great decade for the PLC The 80acircbdquocents saw an attempt to standardize communications with

General Motors manufacturing automation protocol(MAP) It was also a time for reducing

the size of the PLC and making them software programmable through symbolic programming

on personal computers instead of dedicated programming terminals or handheld

programmers Today the worlds smallest PLC is about the size of a single control relay

The 90acircbdquocents have seen a gradual reduction in the introduction of new protocols and the

modernization of the physical layers of some of the more popular protocols that survived the

1980s The latest standard (IEC 1131-3) has tried to merge plc programming languages

under one international standard We now have PLCs that are programmable in function

block diagrams instruction lists C and structured text all at the same time PCs are also

14

being used to replace PLCs in some applications The original company who commissioned

the MODICON 084 has actually switched to a PC based control system

23PLC HARDWARE

A programmable logic controller consists of the following components

Central Processing Unit (CPU) Memory Input modules Output modules and Power

supply A PLC hardware block diagram is shown in Figure The programming terminal in the

diagram is not a part of the PLC but it is essential to have a terminal for programming or

monitoring a PLC In the diagram the arrows between blocks indicate the information and

power-flowing-directions

Fig PLC-Hardware-Block-Diagram

CPU

Like other computerized devices there is a Central Processing Unit (CPU) in a PLC The

CPU which is the brain of a PLC does the following operations

Updating inputs and outputs This function allows a PLC to read the status of its input

terminals and energize or deenergize its output terminals

Performing logic and arithmetic operations A CPU conducts all the mathematic and logic

operations involved in a PLC

Communicating with memory The PLCacircbdquocents programs and data are stored in memory

When a PLC is operating its CPU may read or change the contents of memory locations

Scanning application programs An application program which is called a ladder logic

program is a set of instructions written by a PLC programmer The scanning function allows

the PLC to execute the application program as specified by the programmer

15

PROCESS

Programming Terminal

CPU MemoryPowerSupply

InputModule

OutputModule

InputDevices

OutputDevices

PLC

Communicating with a programming terminal The CPU transfers program and data

between itself and the programming terminal A PLC CPU is controlled by operating system

software The operating system software is a group of supervisory programs that are loaded

and stored permanently in the PLC memory by the PLC manufacturer

Memory

Memory is the component that stores information programs and data in a PLC The process

of putting new information into a memory location is called writing The process of retrieving

information from a memory location is called reading The common types of memory used in

PLCs are Read Only Memory (ROM) and Random Access Memory (RAM) A ROM

location can be read but not written ROM is used to store programs and data that should not

be altered For example the PLCs operating programs are stored in ROM

A RAM location can be read or written This means the information stored in a RAM

location can be retrieved andor altered Ladder logic programs are stored in RAM When a

new ladder logic program is loaded into a PLCs memory the old program that was stored in

the same locations is over-written and essentially erased The memory capacities of PLCs

vary Memory capacities are often expressed in terms of kilo-bytes (K) One byte is a group

of 8 bits One bit is a memory location that may store one binary number that has the value of

either 1 or 0 (Binary numbers are addressed in Module 2) 1K memory means that there are

1024 bytes of RAM 16K memory means there are 16 x 1024 =16384 bytes of RAM

Input modules and output modules

A PLC is a control device It takes information from inputs and makes decisions to

energize or de-energize outputs The decisions are made based on the statuses of inputs and

outputs and the ladder logic program that is being executed The input devices used with a

PLC include pushbuttons limit switches relay contacts photo sensors proximity switches

temperature sensors and the like These input devices can be AC (alternating current) or DC

(direct current) The input voltages can be high or low The input signals can be digital or

analog Differing inputs require different input modules An input module provides an

interface between input devices and a PLCs CPU which uses only a low DC voltage The

input moduleacircbdquocents function is to convert the input signals to DC voltages that are acceptable

to the CPU Standard discrete input modules include 24 V AC 48 V AC 120 V AC 220 V

AC 24 V DC 48 V DC 120 V DC 220 V DC and transistor-transistor logic (TTL) level

The devices controlled by a PLC include relays alarms solenoids fans lights and motor

starters These devices may require different levels of AC or DC voltages Since the signals

16

processed in a PLC are low DC voltages it is the function of the output module to convert

PLC control signals to the voltages required by the controlled circuits or devices Standard

discrete output modules include 24 V AC 48 V AC 120 V AC 220 V AC 24 V DC 48 V

DC 120 V DC 220 V DC and TTL level Power Supply -PLCs are powered by standard

commercial AC power lines However many PLC components such as the CPU and

memory utilize 5 volts or another level of DC power The PLC power supply converts AC

power into DC power to support those components of the PLC

Programming Terminal -A PLC requires a programming terminal and programming

software for operation The programming terminal can be a dedicated terminal or a generic

computer purchased anywhere The programming terminal is used for programming the PLC

and monitoring the PLCs operation It may also download a ladder logic program (the

sending of a program from the programming terminal to the PLC) or upload a ladder logic

program (the sending of a program from the PLC to the programming terminal) The terminal

uses programming software for programming and talking to a PLC

24 WORKING OF PLC

Bringing input signal status to the internal memory of CPU

The field signals are connected to the IP module At the output of IP module the field

status converted into the voltage level required by the CPU is always available

At the beginning of each cycle the CPU brings in all the field IP signals from IP module amp

stores into its internal memory called as PII meaning process image input

The programmable controller operates cyclically meaning when complete program has been

scanned it starts again at the beginning of the program

IOBUS

A PLC works by continually scanning a program We can think of this scan cycle as

consisting of 3 important steps There are typically more than 3 but we can focus on the

important parts and not worry about the others Typically the others are checking the system

and updating the current internal counter and timer values

Step 1-Check Input Status-First the PLC takes a look at each input to determine if it is on or

off In other words is the sensor connected to the first input on How about the second input

How about the third It records this data into its memory to be used during the next step

17

Step 2-Execute Program-Next the PLC executes your program one instruction at a time

Maybe your program said that if the first input was on then it should turn on the first output

Since it already knows which inputs are onoff from the previous step it will be able to decide

whether the first output should be turned on based on the state of the first input It will store

the execution results for use later during the next step

Step 3-Update Output Status-Finally the PLC updates the status of the outputs It updates the

outputs based on which inputs were on during the first step and the results of executing your

program during the second step Based on the example in step 2 it would now turn on the first

output because the first input was on and your program said to turn on the first output when

this condition is trueProcess Control and Automation Process Control

The process of recognizing the state of the process at all times analyze the information

according to the set rules and guidelines and accordingly actuate the control elements is

referred to as process control

RECOGNISING THE STATUS

In control of process all these actions can be taken manually with human involvement or in a

semiautomatic or fully automatic manner Automation -Automation is basically the

delegation of human control functions to technical equipment aimed towards achieving

- Higher-productivity

-Superior quality of end product

-Efficient usage of energy and raw materials

-Improved safety in working conditions etc

Methods adopted for Process Control and Automation

- Manual control

- Hard wired logic control

- Electronics control

-PLC control

- Manual Control

Hardwired Control

-This was considered to be the first step towards automation

- Here the contractor amp relays together with timers amp counters were used

Electronics Control

18

-With the advent of electronics the logic gates started replacing the relays amp auxiliary

contractors in the control circuits amp timers

- With changes the benefits are

1) Reduced space requirements

2) Energy saving

3) Less maintenance and hence greater reliability etc

-With electronics the implementation of changes in the control logic as well as reducing the

project lead-time was not possible

Programmable Logic Controller

- With microprocessor and associated peripherals chips the process of control and

automation went a radical change

- Instead of achieving the desired control or automation through physical wiring of control

devices in PLC it is through a program or software Thus these controllers are referred to as

programmable logic controllers

- The programmable controllers have experienced an unprecedented growth as universal

element It can be effectively used in applications ranging from simple control like replacing

small number relays to complex automation problem

25 PROGRAMMING THE PLC

Ladder Logic

Ladder logic is the main programming method used for PLCs The ladder logic has been

developed to mimic relay logic The decision to use the relay logic diagrams was a strategic

one By selecting ladder logic as the main programming method the amount of retraining

needed for engineers and trades people was greatly reduced

Modern control systems still include relays but these are rarely used for logic A relay is a

simple device that uses a magnetic field to control a switch as pictured in Fig When a

voltage is applied to the input coil the resulting current creates a magnetic field The

magnetic field pulls a metal switch (or reed) towards it and the contacts touch closing the

switch The contact that closes when the coil is energized is called normally open The

normally closed contacts touch when the input coil is not energized Relays are normally

drawn in schematic form using a circle to represent the input coil The output contacts are

shown with two parallel lines Normally open contacts are shown as two lines and will be

open (non-conducting) when the input is not energized Normally closed contacts are shown

with two lines with a diagonal line through them When the input coil is not energized the

19

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

serious problems later on such as endanger the environment and if the

pipeline near human population area it can be a disaster In the section

of pipeline

ruptures at 146 millions of cubic feet per day (MMCFD) of gas for pressure

of 1198 psi and 544deg Celsius was flowing through the 18-inch diameter

pipe and caused one life Structural defects on gas pipeline may caused

by several factors such as corrosion Several factors have been identified

that lead to the event of buried pipeline corrosion such as pipe coatings

soil conditions changing of temperature stresses pipe pressure and

cyclic loading effects

The objective of this project is to develop a method for early

detection of corrosion which the buried pipeline is exposed to changing

temperatures and gas flow pressures so that prompt actions can be done

to prevent any incidents or fatality It is a necessity to have system that

detects signs or abnormalities that will resulted to leakage event In a

pipeline monitoring and inspection system has several tasks to be

performed for natural gas pipelines environment to ensure the integrity of

the pipeline These include measuring pipe wall thickness measuring

velocity and flow of gas detecting gas contamination in pipeline and also

determining structural defects on pipes In gas pipelines commonly

placed in hazardous environmental such as deserts underwater and

buried deep These conditions can cause deterioration or even damage

due to corrosion erosion and fatigue Major problem is to detect corrosion

cracks

9

Fig of full circuit

10

RF Receiver Decoder PLC Load

Power Supply

12 BLOCK DIAGRAM

TRANSMITTER SECTION

RECEIVER SECTION

11

CHAPTER ndash 2

PROGRAMMABLE LOGIC CONTROLLER

12

2 PROGRAMMABLE LOGIC CONTROLLER

21 INTRODUCTION

Programmable logic controllers (PLCs) are members of the

computer family capable of storing instructions to control functions such as sequencing

timing and counting which control a machine or a process The PLC is composed of two

basic sections the Central Processing Unit (CPU) and the InputOutput (IO) interface

system The PLC measures input signals coming from a machine and through the internal

program provides output or control back to the machine Ladder logic is the

programming language used to represent electrical sequences of operation In hardwired

circuits the electrical wiring is connected from one device to another according to logic of

operation In a PLC the devices are connected to the input interface the outputs are

connected to the output interface and the actual wiring of the components is done

electronically inside the PLC using ladder logic This is known as soft wired PLC is a

device that is capable of being programmed to perform a controlling function Before the

advent of PLC the problem of industrial control was usually solved by relays or

hardwired solid-state logic blocks These are very flexible in design and easy for

maintenance personal to understand However they involved a vast amount of

interconnection For the wiring cost to be minimized relays and logic blocks had to be

kept together This led to development of control panel concept for larger and more

complex logic control system The PLC was first conceived by group of engineers from

hydramatic division of GM in 1968This was designed to provide flexibility in control

based on programming and executing logic instruction Adopting the ladder diagram

programming language simplifying maintenance and reducing the cost of spare parts

inventories realized major advantages

22PLC HISTORY

In the late 1960s PLCs were first introduced The primary reason for designing such a

device was eliminating the large cost involved in replacing the complicated relay based

machine control systems Bedford Associates (Bedford MA) proposed something called a

Modular Digital Controller (MODICON) to a major US car manufacturer Other companies

at the time proposed computer based schemes one of which was based upon the PDP-8 The

MODICON 084 brought the worlds first PLC into commercial production

13

When production requirements changed so did the control system This becomes very

expensive when the change is frequent Since relays are mechanical devices they also have a

limited lifetime which required strict adhesion to maintenance schedules Troubleshooting

was also quite tedious when so many relays are involved Now picture a machine control

panel that included many possibly hundreds or thousands of individual relays The size

could be mind boggling How about the complicated initial wiring of so many individual

devices These relays would be individually wired together in a manner that would yield the

desired outcome Were there problems You bet These new controllers also had to be

easily programmed by maintenance and plant engineers The lifetime had to be long and

programming changes easily performed They also had to survive the harsh industrial

environment Thats a lot to ask The answers were to use a programming technique most

people were already familiar with and replace mechanical parts with solid-state ones

In the mid70acircbdquocents the dominant PLC technologies were sequencer state-machines and the bit-

slice based CPU The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs

Conventional microprocessors lacked the power to quickly solve PLC logic in all but the

smallest PLCs As conventional microprocessors evolved larger and larger PLCs were being

based upon them However even today some are still based upon the 2903(ref A-Bs PLC-3)

Modicon has yet to build a faster PLC than their 984ABX which was based upon the 2901

Communications abilities began to appear in approximately 1973 The first such system was

Modicons Modbus The PLC could now talk to other PLCs and they could be far away from

the actual machine they were controlling They could also now be used to send and receive

varying voltages to allow them to enter the analog world Unfortunately the lack of

standardization coupled with continually changing technology has made PLC

communications a nightmare of incompatible protocols and physical networks Still it was a

great decade for the PLC The 80acircbdquocents saw an attempt to standardize communications with

General Motors manufacturing automation protocol(MAP) It was also a time for reducing

the size of the PLC and making them software programmable through symbolic programming

on personal computers instead of dedicated programming terminals or handheld

programmers Today the worlds smallest PLC is about the size of a single control relay

The 90acircbdquocents have seen a gradual reduction in the introduction of new protocols and the

modernization of the physical layers of some of the more popular protocols that survived the

1980s The latest standard (IEC 1131-3) has tried to merge plc programming languages

under one international standard We now have PLCs that are programmable in function

block diagrams instruction lists C and structured text all at the same time PCs are also

14

being used to replace PLCs in some applications The original company who commissioned

the MODICON 084 has actually switched to a PC based control system

23PLC HARDWARE

A programmable logic controller consists of the following components

Central Processing Unit (CPU) Memory Input modules Output modules and Power

supply A PLC hardware block diagram is shown in Figure The programming terminal in the

diagram is not a part of the PLC but it is essential to have a terminal for programming or

monitoring a PLC In the diagram the arrows between blocks indicate the information and

power-flowing-directions

Fig PLC-Hardware-Block-Diagram

CPU

Like other computerized devices there is a Central Processing Unit (CPU) in a PLC The

CPU which is the brain of a PLC does the following operations

Updating inputs and outputs This function allows a PLC to read the status of its input

terminals and energize or deenergize its output terminals

Performing logic and arithmetic operations A CPU conducts all the mathematic and logic

operations involved in a PLC

Communicating with memory The PLCacircbdquocents programs and data are stored in memory

When a PLC is operating its CPU may read or change the contents of memory locations

Scanning application programs An application program which is called a ladder logic

program is a set of instructions written by a PLC programmer The scanning function allows

the PLC to execute the application program as specified by the programmer

15

PROCESS

Programming Terminal

CPU MemoryPowerSupply

InputModule

OutputModule

InputDevices

OutputDevices

PLC

Communicating with a programming terminal The CPU transfers program and data

between itself and the programming terminal A PLC CPU is controlled by operating system

software The operating system software is a group of supervisory programs that are loaded

and stored permanently in the PLC memory by the PLC manufacturer

Memory

Memory is the component that stores information programs and data in a PLC The process

of putting new information into a memory location is called writing The process of retrieving

information from a memory location is called reading The common types of memory used in

PLCs are Read Only Memory (ROM) and Random Access Memory (RAM) A ROM

location can be read but not written ROM is used to store programs and data that should not

be altered For example the PLCs operating programs are stored in ROM

A RAM location can be read or written This means the information stored in a RAM

location can be retrieved andor altered Ladder logic programs are stored in RAM When a

new ladder logic program is loaded into a PLCs memory the old program that was stored in

the same locations is over-written and essentially erased The memory capacities of PLCs

vary Memory capacities are often expressed in terms of kilo-bytes (K) One byte is a group

of 8 bits One bit is a memory location that may store one binary number that has the value of

either 1 or 0 (Binary numbers are addressed in Module 2) 1K memory means that there are

1024 bytes of RAM 16K memory means there are 16 x 1024 =16384 bytes of RAM

Input modules and output modules

A PLC is a control device It takes information from inputs and makes decisions to

energize or de-energize outputs The decisions are made based on the statuses of inputs and

outputs and the ladder logic program that is being executed The input devices used with a

PLC include pushbuttons limit switches relay contacts photo sensors proximity switches

temperature sensors and the like These input devices can be AC (alternating current) or DC

(direct current) The input voltages can be high or low The input signals can be digital or

analog Differing inputs require different input modules An input module provides an

interface between input devices and a PLCs CPU which uses only a low DC voltage The

input moduleacircbdquocents function is to convert the input signals to DC voltages that are acceptable

to the CPU Standard discrete input modules include 24 V AC 48 V AC 120 V AC 220 V

AC 24 V DC 48 V DC 120 V DC 220 V DC and transistor-transistor logic (TTL) level

The devices controlled by a PLC include relays alarms solenoids fans lights and motor

starters These devices may require different levels of AC or DC voltages Since the signals

16

processed in a PLC are low DC voltages it is the function of the output module to convert

PLC control signals to the voltages required by the controlled circuits or devices Standard

discrete output modules include 24 V AC 48 V AC 120 V AC 220 V AC 24 V DC 48 V

DC 120 V DC 220 V DC and TTL level Power Supply -PLCs are powered by standard

commercial AC power lines However many PLC components such as the CPU and

memory utilize 5 volts or another level of DC power The PLC power supply converts AC

power into DC power to support those components of the PLC

Programming Terminal -A PLC requires a programming terminal and programming

software for operation The programming terminal can be a dedicated terminal or a generic

computer purchased anywhere The programming terminal is used for programming the PLC

and monitoring the PLCs operation It may also download a ladder logic program (the

sending of a program from the programming terminal to the PLC) or upload a ladder logic

program (the sending of a program from the PLC to the programming terminal) The terminal

uses programming software for programming and talking to a PLC

24 WORKING OF PLC

Bringing input signal status to the internal memory of CPU

The field signals are connected to the IP module At the output of IP module the field

status converted into the voltage level required by the CPU is always available

At the beginning of each cycle the CPU brings in all the field IP signals from IP module amp

stores into its internal memory called as PII meaning process image input

The programmable controller operates cyclically meaning when complete program has been

scanned it starts again at the beginning of the program

IOBUS

A PLC works by continually scanning a program We can think of this scan cycle as

consisting of 3 important steps There are typically more than 3 but we can focus on the

important parts and not worry about the others Typically the others are checking the system

and updating the current internal counter and timer values

Step 1-Check Input Status-First the PLC takes a look at each input to determine if it is on or

off In other words is the sensor connected to the first input on How about the second input

How about the third It records this data into its memory to be used during the next step

17

Step 2-Execute Program-Next the PLC executes your program one instruction at a time

Maybe your program said that if the first input was on then it should turn on the first output

Since it already knows which inputs are onoff from the previous step it will be able to decide

whether the first output should be turned on based on the state of the first input It will store

the execution results for use later during the next step

Step 3-Update Output Status-Finally the PLC updates the status of the outputs It updates the

outputs based on which inputs were on during the first step and the results of executing your

program during the second step Based on the example in step 2 it would now turn on the first

output because the first input was on and your program said to turn on the first output when

this condition is trueProcess Control and Automation Process Control

The process of recognizing the state of the process at all times analyze the information

according to the set rules and guidelines and accordingly actuate the control elements is

referred to as process control

RECOGNISING THE STATUS

In control of process all these actions can be taken manually with human involvement or in a

semiautomatic or fully automatic manner Automation -Automation is basically the

delegation of human control functions to technical equipment aimed towards achieving

- Higher-productivity

-Superior quality of end product

-Efficient usage of energy and raw materials

-Improved safety in working conditions etc

Methods adopted for Process Control and Automation

- Manual control

- Hard wired logic control

- Electronics control

-PLC control

- Manual Control

Hardwired Control

-This was considered to be the first step towards automation

- Here the contractor amp relays together with timers amp counters were used

Electronics Control

18

-With the advent of electronics the logic gates started replacing the relays amp auxiliary

contractors in the control circuits amp timers

- With changes the benefits are

1) Reduced space requirements

2) Energy saving

3) Less maintenance and hence greater reliability etc

-With electronics the implementation of changes in the control logic as well as reducing the

project lead-time was not possible

Programmable Logic Controller

- With microprocessor and associated peripherals chips the process of control and

automation went a radical change

- Instead of achieving the desired control or automation through physical wiring of control

devices in PLC it is through a program or software Thus these controllers are referred to as

programmable logic controllers

- The programmable controllers have experienced an unprecedented growth as universal

element It can be effectively used in applications ranging from simple control like replacing

small number relays to complex automation problem

25 PROGRAMMING THE PLC

Ladder Logic

Ladder logic is the main programming method used for PLCs The ladder logic has been

developed to mimic relay logic The decision to use the relay logic diagrams was a strategic

one By selecting ladder logic as the main programming method the amount of retraining

needed for engineers and trades people was greatly reduced

Modern control systems still include relays but these are rarely used for logic A relay is a

simple device that uses a magnetic field to control a switch as pictured in Fig When a

voltage is applied to the input coil the resulting current creates a magnetic field The

magnetic field pulls a metal switch (or reed) towards it and the contacts touch closing the

switch The contact that closes when the coil is energized is called normally open The

normally closed contacts touch when the input coil is not energized Relays are normally

drawn in schematic form using a circle to represent the input coil The output contacts are

shown with two parallel lines Normally open contacts are shown as two lines and will be

open (non-conducting) when the input is not energized Normally closed contacts are shown

with two lines with a diagonal line through them When the input coil is not energized the

19

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

Fig of full circuit

10

RF Receiver Decoder PLC Load

Power Supply

12 BLOCK DIAGRAM

TRANSMITTER SECTION

RECEIVER SECTION

11

CHAPTER ndash 2

PROGRAMMABLE LOGIC CONTROLLER

12

2 PROGRAMMABLE LOGIC CONTROLLER

21 INTRODUCTION

Programmable logic controllers (PLCs) are members of the

computer family capable of storing instructions to control functions such as sequencing

timing and counting which control a machine or a process The PLC is composed of two

basic sections the Central Processing Unit (CPU) and the InputOutput (IO) interface

system The PLC measures input signals coming from a machine and through the internal

program provides output or control back to the machine Ladder logic is the

programming language used to represent electrical sequences of operation In hardwired

circuits the electrical wiring is connected from one device to another according to logic of

operation In a PLC the devices are connected to the input interface the outputs are

connected to the output interface and the actual wiring of the components is done

electronically inside the PLC using ladder logic This is known as soft wired PLC is a

device that is capable of being programmed to perform a controlling function Before the

advent of PLC the problem of industrial control was usually solved by relays or

hardwired solid-state logic blocks These are very flexible in design and easy for

maintenance personal to understand However they involved a vast amount of

interconnection For the wiring cost to be minimized relays and logic blocks had to be

kept together This led to development of control panel concept for larger and more

complex logic control system The PLC was first conceived by group of engineers from

hydramatic division of GM in 1968This was designed to provide flexibility in control

based on programming and executing logic instruction Adopting the ladder diagram

programming language simplifying maintenance and reducing the cost of spare parts

inventories realized major advantages

22PLC HISTORY

In the late 1960s PLCs were first introduced The primary reason for designing such a

device was eliminating the large cost involved in replacing the complicated relay based

machine control systems Bedford Associates (Bedford MA) proposed something called a

Modular Digital Controller (MODICON) to a major US car manufacturer Other companies

at the time proposed computer based schemes one of which was based upon the PDP-8 The

MODICON 084 brought the worlds first PLC into commercial production

13

When production requirements changed so did the control system This becomes very

expensive when the change is frequent Since relays are mechanical devices they also have a

limited lifetime which required strict adhesion to maintenance schedules Troubleshooting

was also quite tedious when so many relays are involved Now picture a machine control

panel that included many possibly hundreds or thousands of individual relays The size

could be mind boggling How about the complicated initial wiring of so many individual

devices These relays would be individually wired together in a manner that would yield the

desired outcome Were there problems You bet These new controllers also had to be

easily programmed by maintenance and plant engineers The lifetime had to be long and

programming changes easily performed They also had to survive the harsh industrial

environment Thats a lot to ask The answers were to use a programming technique most

people were already familiar with and replace mechanical parts with solid-state ones

In the mid70acircbdquocents the dominant PLC technologies were sequencer state-machines and the bit-

slice based CPU The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs

Conventional microprocessors lacked the power to quickly solve PLC logic in all but the

smallest PLCs As conventional microprocessors evolved larger and larger PLCs were being

based upon them However even today some are still based upon the 2903(ref A-Bs PLC-3)

Modicon has yet to build a faster PLC than their 984ABX which was based upon the 2901

Communications abilities began to appear in approximately 1973 The first such system was

Modicons Modbus The PLC could now talk to other PLCs and they could be far away from

the actual machine they were controlling They could also now be used to send and receive

varying voltages to allow them to enter the analog world Unfortunately the lack of

standardization coupled with continually changing technology has made PLC

communications a nightmare of incompatible protocols and physical networks Still it was a

great decade for the PLC The 80acircbdquocents saw an attempt to standardize communications with

General Motors manufacturing automation protocol(MAP) It was also a time for reducing

the size of the PLC and making them software programmable through symbolic programming

on personal computers instead of dedicated programming terminals or handheld

programmers Today the worlds smallest PLC is about the size of a single control relay

The 90acircbdquocents have seen a gradual reduction in the introduction of new protocols and the

modernization of the physical layers of some of the more popular protocols that survived the

1980s The latest standard (IEC 1131-3) has tried to merge plc programming languages

under one international standard We now have PLCs that are programmable in function

block diagrams instruction lists C and structured text all at the same time PCs are also

14

being used to replace PLCs in some applications The original company who commissioned

the MODICON 084 has actually switched to a PC based control system

23PLC HARDWARE

A programmable logic controller consists of the following components

Central Processing Unit (CPU) Memory Input modules Output modules and Power

supply A PLC hardware block diagram is shown in Figure The programming terminal in the

diagram is not a part of the PLC but it is essential to have a terminal for programming or

monitoring a PLC In the diagram the arrows between blocks indicate the information and

power-flowing-directions

Fig PLC-Hardware-Block-Diagram

CPU

Like other computerized devices there is a Central Processing Unit (CPU) in a PLC The

CPU which is the brain of a PLC does the following operations

Updating inputs and outputs This function allows a PLC to read the status of its input

terminals and energize or deenergize its output terminals

Performing logic and arithmetic operations A CPU conducts all the mathematic and logic

operations involved in a PLC

Communicating with memory The PLCacircbdquocents programs and data are stored in memory

When a PLC is operating its CPU may read or change the contents of memory locations

Scanning application programs An application program which is called a ladder logic

program is a set of instructions written by a PLC programmer The scanning function allows

the PLC to execute the application program as specified by the programmer

15

PROCESS

Programming Terminal

CPU MemoryPowerSupply

InputModule

OutputModule

InputDevices

OutputDevices

PLC

Communicating with a programming terminal The CPU transfers program and data

between itself and the programming terminal A PLC CPU is controlled by operating system

software The operating system software is a group of supervisory programs that are loaded

and stored permanently in the PLC memory by the PLC manufacturer

Memory

Memory is the component that stores information programs and data in a PLC The process

of putting new information into a memory location is called writing The process of retrieving

information from a memory location is called reading The common types of memory used in

PLCs are Read Only Memory (ROM) and Random Access Memory (RAM) A ROM

location can be read but not written ROM is used to store programs and data that should not

be altered For example the PLCs operating programs are stored in ROM

A RAM location can be read or written This means the information stored in a RAM

location can be retrieved andor altered Ladder logic programs are stored in RAM When a

new ladder logic program is loaded into a PLCs memory the old program that was stored in

the same locations is over-written and essentially erased The memory capacities of PLCs

vary Memory capacities are often expressed in terms of kilo-bytes (K) One byte is a group

of 8 bits One bit is a memory location that may store one binary number that has the value of

either 1 or 0 (Binary numbers are addressed in Module 2) 1K memory means that there are

1024 bytes of RAM 16K memory means there are 16 x 1024 =16384 bytes of RAM

Input modules and output modules

A PLC is a control device It takes information from inputs and makes decisions to

energize or de-energize outputs The decisions are made based on the statuses of inputs and

outputs and the ladder logic program that is being executed The input devices used with a

PLC include pushbuttons limit switches relay contacts photo sensors proximity switches

temperature sensors and the like These input devices can be AC (alternating current) or DC

(direct current) The input voltages can be high or low The input signals can be digital or

analog Differing inputs require different input modules An input module provides an

interface between input devices and a PLCs CPU which uses only a low DC voltage The

input moduleacircbdquocents function is to convert the input signals to DC voltages that are acceptable

to the CPU Standard discrete input modules include 24 V AC 48 V AC 120 V AC 220 V

AC 24 V DC 48 V DC 120 V DC 220 V DC and transistor-transistor logic (TTL) level

The devices controlled by a PLC include relays alarms solenoids fans lights and motor

starters These devices may require different levels of AC or DC voltages Since the signals

16

processed in a PLC are low DC voltages it is the function of the output module to convert

PLC control signals to the voltages required by the controlled circuits or devices Standard

discrete output modules include 24 V AC 48 V AC 120 V AC 220 V AC 24 V DC 48 V

DC 120 V DC 220 V DC and TTL level Power Supply -PLCs are powered by standard

commercial AC power lines However many PLC components such as the CPU and

memory utilize 5 volts or another level of DC power The PLC power supply converts AC

power into DC power to support those components of the PLC

Programming Terminal -A PLC requires a programming terminal and programming

software for operation The programming terminal can be a dedicated terminal or a generic

computer purchased anywhere The programming terminal is used for programming the PLC

and monitoring the PLCs operation It may also download a ladder logic program (the

sending of a program from the programming terminal to the PLC) or upload a ladder logic

program (the sending of a program from the PLC to the programming terminal) The terminal

uses programming software for programming and talking to a PLC

24 WORKING OF PLC

Bringing input signal status to the internal memory of CPU

The field signals are connected to the IP module At the output of IP module the field

status converted into the voltage level required by the CPU is always available

At the beginning of each cycle the CPU brings in all the field IP signals from IP module amp

stores into its internal memory called as PII meaning process image input

The programmable controller operates cyclically meaning when complete program has been

scanned it starts again at the beginning of the program

IOBUS

A PLC works by continually scanning a program We can think of this scan cycle as

consisting of 3 important steps There are typically more than 3 but we can focus on the

important parts and not worry about the others Typically the others are checking the system

and updating the current internal counter and timer values

Step 1-Check Input Status-First the PLC takes a look at each input to determine if it is on or

off In other words is the sensor connected to the first input on How about the second input

How about the third It records this data into its memory to be used during the next step

17

Step 2-Execute Program-Next the PLC executes your program one instruction at a time

Maybe your program said that if the first input was on then it should turn on the first output

Since it already knows which inputs are onoff from the previous step it will be able to decide

whether the first output should be turned on based on the state of the first input It will store

the execution results for use later during the next step

Step 3-Update Output Status-Finally the PLC updates the status of the outputs It updates the

outputs based on which inputs were on during the first step and the results of executing your

program during the second step Based on the example in step 2 it would now turn on the first

output because the first input was on and your program said to turn on the first output when

this condition is trueProcess Control and Automation Process Control

The process of recognizing the state of the process at all times analyze the information

according to the set rules and guidelines and accordingly actuate the control elements is

referred to as process control

RECOGNISING THE STATUS

In control of process all these actions can be taken manually with human involvement or in a

semiautomatic or fully automatic manner Automation -Automation is basically the

delegation of human control functions to technical equipment aimed towards achieving

- Higher-productivity

-Superior quality of end product

-Efficient usage of energy and raw materials

-Improved safety in working conditions etc

Methods adopted for Process Control and Automation

- Manual control

- Hard wired logic control

- Electronics control

-PLC control

- Manual Control

Hardwired Control

-This was considered to be the first step towards automation

- Here the contractor amp relays together with timers amp counters were used

Electronics Control

18

-With the advent of electronics the logic gates started replacing the relays amp auxiliary

contractors in the control circuits amp timers

- With changes the benefits are

1) Reduced space requirements

2) Energy saving

3) Less maintenance and hence greater reliability etc

-With electronics the implementation of changes in the control logic as well as reducing the

project lead-time was not possible

Programmable Logic Controller

- With microprocessor and associated peripherals chips the process of control and

automation went a radical change

- Instead of achieving the desired control or automation through physical wiring of control

devices in PLC it is through a program or software Thus these controllers are referred to as

programmable logic controllers

- The programmable controllers have experienced an unprecedented growth as universal

element It can be effectively used in applications ranging from simple control like replacing

small number relays to complex automation problem

25 PROGRAMMING THE PLC

Ladder Logic

Ladder logic is the main programming method used for PLCs The ladder logic has been

developed to mimic relay logic The decision to use the relay logic diagrams was a strategic

one By selecting ladder logic as the main programming method the amount of retraining

needed for engineers and trades people was greatly reduced

Modern control systems still include relays but these are rarely used for logic A relay is a

simple device that uses a magnetic field to control a switch as pictured in Fig When a

voltage is applied to the input coil the resulting current creates a magnetic field The

magnetic field pulls a metal switch (or reed) towards it and the contacts touch closing the

switch The contact that closes when the coil is energized is called normally open The

normally closed contacts touch when the input coil is not energized Relays are normally

drawn in schematic form using a circle to represent the input coil The output contacts are

shown with two parallel lines Normally open contacts are shown as two lines and will be

open (non-conducting) when the input is not energized Normally closed contacts are shown

with two lines with a diagonal line through them When the input coil is not energized the

19

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

RF Receiver Decoder PLC Load

Power Supply

12 BLOCK DIAGRAM

TRANSMITTER SECTION

RECEIVER SECTION

11

CHAPTER ndash 2

PROGRAMMABLE LOGIC CONTROLLER

12

2 PROGRAMMABLE LOGIC CONTROLLER

21 INTRODUCTION

Programmable logic controllers (PLCs) are members of the

computer family capable of storing instructions to control functions such as sequencing

timing and counting which control a machine or a process The PLC is composed of two

basic sections the Central Processing Unit (CPU) and the InputOutput (IO) interface

system The PLC measures input signals coming from a machine and through the internal

program provides output or control back to the machine Ladder logic is the

programming language used to represent electrical sequences of operation In hardwired

circuits the electrical wiring is connected from one device to another according to logic of

operation In a PLC the devices are connected to the input interface the outputs are

connected to the output interface and the actual wiring of the components is done

electronically inside the PLC using ladder logic This is known as soft wired PLC is a

device that is capable of being programmed to perform a controlling function Before the

advent of PLC the problem of industrial control was usually solved by relays or

hardwired solid-state logic blocks These are very flexible in design and easy for

maintenance personal to understand However they involved a vast amount of

interconnection For the wiring cost to be minimized relays and logic blocks had to be

kept together This led to development of control panel concept for larger and more

complex logic control system The PLC was first conceived by group of engineers from

hydramatic division of GM in 1968This was designed to provide flexibility in control

based on programming and executing logic instruction Adopting the ladder diagram

programming language simplifying maintenance and reducing the cost of spare parts

inventories realized major advantages

22PLC HISTORY

In the late 1960s PLCs were first introduced The primary reason for designing such a

device was eliminating the large cost involved in replacing the complicated relay based

machine control systems Bedford Associates (Bedford MA) proposed something called a

Modular Digital Controller (MODICON) to a major US car manufacturer Other companies

at the time proposed computer based schemes one of which was based upon the PDP-8 The

MODICON 084 brought the worlds first PLC into commercial production

13

When production requirements changed so did the control system This becomes very

expensive when the change is frequent Since relays are mechanical devices they also have a

limited lifetime which required strict adhesion to maintenance schedules Troubleshooting

was also quite tedious when so many relays are involved Now picture a machine control

panel that included many possibly hundreds or thousands of individual relays The size

could be mind boggling How about the complicated initial wiring of so many individual

devices These relays would be individually wired together in a manner that would yield the

desired outcome Were there problems You bet These new controllers also had to be

easily programmed by maintenance and plant engineers The lifetime had to be long and

programming changes easily performed They also had to survive the harsh industrial

environment Thats a lot to ask The answers were to use a programming technique most

people were already familiar with and replace mechanical parts with solid-state ones

In the mid70acircbdquocents the dominant PLC technologies were sequencer state-machines and the bit-

slice based CPU The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs

Conventional microprocessors lacked the power to quickly solve PLC logic in all but the

smallest PLCs As conventional microprocessors evolved larger and larger PLCs were being

based upon them However even today some are still based upon the 2903(ref A-Bs PLC-3)

Modicon has yet to build a faster PLC than their 984ABX which was based upon the 2901

Communications abilities began to appear in approximately 1973 The first such system was

Modicons Modbus The PLC could now talk to other PLCs and they could be far away from

the actual machine they were controlling They could also now be used to send and receive

varying voltages to allow them to enter the analog world Unfortunately the lack of

standardization coupled with continually changing technology has made PLC

communications a nightmare of incompatible protocols and physical networks Still it was a

great decade for the PLC The 80acircbdquocents saw an attempt to standardize communications with

General Motors manufacturing automation protocol(MAP) It was also a time for reducing

the size of the PLC and making them software programmable through symbolic programming

on personal computers instead of dedicated programming terminals or handheld

programmers Today the worlds smallest PLC is about the size of a single control relay

The 90acircbdquocents have seen a gradual reduction in the introduction of new protocols and the

modernization of the physical layers of some of the more popular protocols that survived the

1980s The latest standard (IEC 1131-3) has tried to merge plc programming languages

under one international standard We now have PLCs that are programmable in function

block diagrams instruction lists C and structured text all at the same time PCs are also

14

being used to replace PLCs in some applications The original company who commissioned

the MODICON 084 has actually switched to a PC based control system

23PLC HARDWARE

A programmable logic controller consists of the following components

Central Processing Unit (CPU) Memory Input modules Output modules and Power

supply A PLC hardware block diagram is shown in Figure The programming terminal in the

diagram is not a part of the PLC but it is essential to have a terminal for programming or

monitoring a PLC In the diagram the arrows between blocks indicate the information and

power-flowing-directions

Fig PLC-Hardware-Block-Diagram

CPU

Like other computerized devices there is a Central Processing Unit (CPU) in a PLC The

CPU which is the brain of a PLC does the following operations

Updating inputs and outputs This function allows a PLC to read the status of its input

terminals and energize or deenergize its output terminals

Performing logic and arithmetic operations A CPU conducts all the mathematic and logic

operations involved in a PLC

Communicating with memory The PLCacircbdquocents programs and data are stored in memory

When a PLC is operating its CPU may read or change the contents of memory locations

Scanning application programs An application program which is called a ladder logic

program is a set of instructions written by a PLC programmer The scanning function allows

the PLC to execute the application program as specified by the programmer

15

PROCESS

Programming Terminal

CPU MemoryPowerSupply

InputModule

OutputModule

InputDevices

OutputDevices

PLC

Communicating with a programming terminal The CPU transfers program and data

between itself and the programming terminal A PLC CPU is controlled by operating system

software The operating system software is a group of supervisory programs that are loaded

and stored permanently in the PLC memory by the PLC manufacturer

Memory

Memory is the component that stores information programs and data in a PLC The process

of putting new information into a memory location is called writing The process of retrieving

information from a memory location is called reading The common types of memory used in

PLCs are Read Only Memory (ROM) and Random Access Memory (RAM) A ROM

location can be read but not written ROM is used to store programs and data that should not

be altered For example the PLCs operating programs are stored in ROM

A RAM location can be read or written This means the information stored in a RAM

location can be retrieved andor altered Ladder logic programs are stored in RAM When a

new ladder logic program is loaded into a PLCs memory the old program that was stored in

the same locations is over-written and essentially erased The memory capacities of PLCs

vary Memory capacities are often expressed in terms of kilo-bytes (K) One byte is a group

of 8 bits One bit is a memory location that may store one binary number that has the value of

either 1 or 0 (Binary numbers are addressed in Module 2) 1K memory means that there are

1024 bytes of RAM 16K memory means there are 16 x 1024 =16384 bytes of RAM

Input modules and output modules

A PLC is a control device It takes information from inputs and makes decisions to

energize or de-energize outputs The decisions are made based on the statuses of inputs and

outputs and the ladder logic program that is being executed The input devices used with a

PLC include pushbuttons limit switches relay contacts photo sensors proximity switches

temperature sensors and the like These input devices can be AC (alternating current) or DC

(direct current) The input voltages can be high or low The input signals can be digital or

analog Differing inputs require different input modules An input module provides an

interface between input devices and a PLCs CPU which uses only a low DC voltage The

input moduleacircbdquocents function is to convert the input signals to DC voltages that are acceptable

to the CPU Standard discrete input modules include 24 V AC 48 V AC 120 V AC 220 V

AC 24 V DC 48 V DC 120 V DC 220 V DC and transistor-transistor logic (TTL) level

The devices controlled by a PLC include relays alarms solenoids fans lights and motor

starters These devices may require different levels of AC or DC voltages Since the signals

16

processed in a PLC are low DC voltages it is the function of the output module to convert

PLC control signals to the voltages required by the controlled circuits or devices Standard

discrete output modules include 24 V AC 48 V AC 120 V AC 220 V AC 24 V DC 48 V

DC 120 V DC 220 V DC and TTL level Power Supply -PLCs are powered by standard

commercial AC power lines However many PLC components such as the CPU and

memory utilize 5 volts or another level of DC power The PLC power supply converts AC

power into DC power to support those components of the PLC

Programming Terminal -A PLC requires a programming terminal and programming

software for operation The programming terminal can be a dedicated terminal or a generic

computer purchased anywhere The programming terminal is used for programming the PLC

and monitoring the PLCs operation It may also download a ladder logic program (the

sending of a program from the programming terminal to the PLC) or upload a ladder logic

program (the sending of a program from the PLC to the programming terminal) The terminal

uses programming software for programming and talking to a PLC

24 WORKING OF PLC

Bringing input signal status to the internal memory of CPU

The field signals are connected to the IP module At the output of IP module the field

status converted into the voltage level required by the CPU is always available

At the beginning of each cycle the CPU brings in all the field IP signals from IP module amp

stores into its internal memory called as PII meaning process image input

The programmable controller operates cyclically meaning when complete program has been

scanned it starts again at the beginning of the program

IOBUS

A PLC works by continually scanning a program We can think of this scan cycle as

consisting of 3 important steps There are typically more than 3 but we can focus on the

important parts and not worry about the others Typically the others are checking the system

and updating the current internal counter and timer values

Step 1-Check Input Status-First the PLC takes a look at each input to determine if it is on or

off In other words is the sensor connected to the first input on How about the second input

How about the third It records this data into its memory to be used during the next step

17

Step 2-Execute Program-Next the PLC executes your program one instruction at a time

Maybe your program said that if the first input was on then it should turn on the first output

Since it already knows which inputs are onoff from the previous step it will be able to decide

whether the first output should be turned on based on the state of the first input It will store

the execution results for use later during the next step

Step 3-Update Output Status-Finally the PLC updates the status of the outputs It updates the

outputs based on which inputs were on during the first step and the results of executing your

program during the second step Based on the example in step 2 it would now turn on the first

output because the first input was on and your program said to turn on the first output when

this condition is trueProcess Control and Automation Process Control

The process of recognizing the state of the process at all times analyze the information

according to the set rules and guidelines and accordingly actuate the control elements is

referred to as process control

RECOGNISING THE STATUS

In control of process all these actions can be taken manually with human involvement or in a

semiautomatic or fully automatic manner Automation -Automation is basically the

delegation of human control functions to technical equipment aimed towards achieving

- Higher-productivity

-Superior quality of end product

-Efficient usage of energy and raw materials

-Improved safety in working conditions etc

Methods adopted for Process Control and Automation

- Manual control

- Hard wired logic control

- Electronics control

-PLC control

- Manual Control

Hardwired Control

-This was considered to be the first step towards automation

- Here the contractor amp relays together with timers amp counters were used

Electronics Control

18

-With the advent of electronics the logic gates started replacing the relays amp auxiliary

contractors in the control circuits amp timers

- With changes the benefits are

1) Reduced space requirements

2) Energy saving

3) Less maintenance and hence greater reliability etc

-With electronics the implementation of changes in the control logic as well as reducing the

project lead-time was not possible

Programmable Logic Controller

- With microprocessor and associated peripherals chips the process of control and

automation went a radical change

- Instead of achieving the desired control or automation through physical wiring of control

devices in PLC it is through a program or software Thus these controllers are referred to as

programmable logic controllers

- The programmable controllers have experienced an unprecedented growth as universal

element It can be effectively used in applications ranging from simple control like replacing

small number relays to complex automation problem

25 PROGRAMMING THE PLC

Ladder Logic

Ladder logic is the main programming method used for PLCs The ladder logic has been

developed to mimic relay logic The decision to use the relay logic diagrams was a strategic

one By selecting ladder logic as the main programming method the amount of retraining

needed for engineers and trades people was greatly reduced

Modern control systems still include relays but these are rarely used for logic A relay is a

simple device that uses a magnetic field to control a switch as pictured in Fig When a

voltage is applied to the input coil the resulting current creates a magnetic field The

magnetic field pulls a metal switch (or reed) towards it and the contacts touch closing the

switch The contact that closes when the coil is energized is called normally open The

normally closed contacts touch when the input coil is not energized Relays are normally

drawn in schematic form using a circle to represent the input coil The output contacts are

shown with two parallel lines Normally open contacts are shown as two lines and will be

open (non-conducting) when the input is not energized Normally closed contacts are shown

with two lines with a diagonal line through them When the input coil is not energized the

19

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

CHAPTER ndash 2

PROGRAMMABLE LOGIC CONTROLLER

12

2 PROGRAMMABLE LOGIC CONTROLLER

21 INTRODUCTION

Programmable logic controllers (PLCs) are members of the

computer family capable of storing instructions to control functions such as sequencing

timing and counting which control a machine or a process The PLC is composed of two

basic sections the Central Processing Unit (CPU) and the InputOutput (IO) interface

system The PLC measures input signals coming from a machine and through the internal

program provides output or control back to the machine Ladder logic is the

programming language used to represent electrical sequences of operation In hardwired

circuits the electrical wiring is connected from one device to another according to logic of

operation In a PLC the devices are connected to the input interface the outputs are

connected to the output interface and the actual wiring of the components is done

electronically inside the PLC using ladder logic This is known as soft wired PLC is a

device that is capable of being programmed to perform a controlling function Before the

advent of PLC the problem of industrial control was usually solved by relays or

hardwired solid-state logic blocks These are very flexible in design and easy for

maintenance personal to understand However they involved a vast amount of

interconnection For the wiring cost to be minimized relays and logic blocks had to be

kept together This led to development of control panel concept for larger and more

complex logic control system The PLC was first conceived by group of engineers from

hydramatic division of GM in 1968This was designed to provide flexibility in control

based on programming and executing logic instruction Adopting the ladder diagram

programming language simplifying maintenance and reducing the cost of spare parts

inventories realized major advantages

22PLC HISTORY

In the late 1960s PLCs were first introduced The primary reason for designing such a

device was eliminating the large cost involved in replacing the complicated relay based

machine control systems Bedford Associates (Bedford MA) proposed something called a

Modular Digital Controller (MODICON) to a major US car manufacturer Other companies

at the time proposed computer based schemes one of which was based upon the PDP-8 The

MODICON 084 brought the worlds first PLC into commercial production

13

When production requirements changed so did the control system This becomes very

expensive when the change is frequent Since relays are mechanical devices they also have a

limited lifetime which required strict adhesion to maintenance schedules Troubleshooting

was also quite tedious when so many relays are involved Now picture a machine control

panel that included many possibly hundreds or thousands of individual relays The size

could be mind boggling How about the complicated initial wiring of so many individual

devices These relays would be individually wired together in a manner that would yield the

desired outcome Were there problems You bet These new controllers also had to be

easily programmed by maintenance and plant engineers The lifetime had to be long and

programming changes easily performed They also had to survive the harsh industrial

environment Thats a lot to ask The answers were to use a programming technique most

people were already familiar with and replace mechanical parts with solid-state ones

In the mid70acircbdquocents the dominant PLC technologies were sequencer state-machines and the bit-

slice based CPU The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs

Conventional microprocessors lacked the power to quickly solve PLC logic in all but the

smallest PLCs As conventional microprocessors evolved larger and larger PLCs were being

based upon them However even today some are still based upon the 2903(ref A-Bs PLC-3)

Modicon has yet to build a faster PLC than their 984ABX which was based upon the 2901

Communications abilities began to appear in approximately 1973 The first such system was

Modicons Modbus The PLC could now talk to other PLCs and they could be far away from

the actual machine they were controlling They could also now be used to send and receive

varying voltages to allow them to enter the analog world Unfortunately the lack of

standardization coupled with continually changing technology has made PLC

communications a nightmare of incompatible protocols and physical networks Still it was a

great decade for the PLC The 80acircbdquocents saw an attempt to standardize communications with

General Motors manufacturing automation protocol(MAP) It was also a time for reducing

the size of the PLC and making them software programmable through symbolic programming

on personal computers instead of dedicated programming terminals or handheld

programmers Today the worlds smallest PLC is about the size of a single control relay

The 90acircbdquocents have seen a gradual reduction in the introduction of new protocols and the

modernization of the physical layers of some of the more popular protocols that survived the

1980s The latest standard (IEC 1131-3) has tried to merge plc programming languages

under one international standard We now have PLCs that are programmable in function

block diagrams instruction lists C and structured text all at the same time PCs are also

14

being used to replace PLCs in some applications The original company who commissioned

the MODICON 084 has actually switched to a PC based control system

23PLC HARDWARE

A programmable logic controller consists of the following components

Central Processing Unit (CPU) Memory Input modules Output modules and Power

supply A PLC hardware block diagram is shown in Figure The programming terminal in the

diagram is not a part of the PLC but it is essential to have a terminal for programming or

monitoring a PLC In the diagram the arrows between blocks indicate the information and

power-flowing-directions

Fig PLC-Hardware-Block-Diagram

CPU

Like other computerized devices there is a Central Processing Unit (CPU) in a PLC The

CPU which is the brain of a PLC does the following operations

Updating inputs and outputs This function allows a PLC to read the status of its input

terminals and energize or deenergize its output terminals

Performing logic and arithmetic operations A CPU conducts all the mathematic and logic

operations involved in a PLC

Communicating with memory The PLCacircbdquocents programs and data are stored in memory

When a PLC is operating its CPU may read or change the contents of memory locations

Scanning application programs An application program which is called a ladder logic

program is a set of instructions written by a PLC programmer The scanning function allows

the PLC to execute the application program as specified by the programmer

15

PROCESS

Programming Terminal

CPU MemoryPowerSupply

InputModule

OutputModule

InputDevices

OutputDevices

PLC

Communicating with a programming terminal The CPU transfers program and data

between itself and the programming terminal A PLC CPU is controlled by operating system

software The operating system software is a group of supervisory programs that are loaded

and stored permanently in the PLC memory by the PLC manufacturer

Memory

Memory is the component that stores information programs and data in a PLC The process

of putting new information into a memory location is called writing The process of retrieving

information from a memory location is called reading The common types of memory used in

PLCs are Read Only Memory (ROM) and Random Access Memory (RAM) A ROM

location can be read but not written ROM is used to store programs and data that should not

be altered For example the PLCs operating programs are stored in ROM

A RAM location can be read or written This means the information stored in a RAM

location can be retrieved andor altered Ladder logic programs are stored in RAM When a

new ladder logic program is loaded into a PLCs memory the old program that was stored in

the same locations is over-written and essentially erased The memory capacities of PLCs

vary Memory capacities are often expressed in terms of kilo-bytes (K) One byte is a group

of 8 bits One bit is a memory location that may store one binary number that has the value of

either 1 or 0 (Binary numbers are addressed in Module 2) 1K memory means that there are

1024 bytes of RAM 16K memory means there are 16 x 1024 =16384 bytes of RAM

Input modules and output modules

A PLC is a control device It takes information from inputs and makes decisions to

energize or de-energize outputs The decisions are made based on the statuses of inputs and

outputs and the ladder logic program that is being executed The input devices used with a

PLC include pushbuttons limit switches relay contacts photo sensors proximity switches

temperature sensors and the like These input devices can be AC (alternating current) or DC

(direct current) The input voltages can be high or low The input signals can be digital or

analog Differing inputs require different input modules An input module provides an

interface between input devices and a PLCs CPU which uses only a low DC voltage The

input moduleacircbdquocents function is to convert the input signals to DC voltages that are acceptable

to the CPU Standard discrete input modules include 24 V AC 48 V AC 120 V AC 220 V

AC 24 V DC 48 V DC 120 V DC 220 V DC and transistor-transistor logic (TTL) level

The devices controlled by a PLC include relays alarms solenoids fans lights and motor

starters These devices may require different levels of AC or DC voltages Since the signals

16

processed in a PLC are low DC voltages it is the function of the output module to convert

PLC control signals to the voltages required by the controlled circuits or devices Standard

discrete output modules include 24 V AC 48 V AC 120 V AC 220 V AC 24 V DC 48 V

DC 120 V DC 220 V DC and TTL level Power Supply -PLCs are powered by standard

commercial AC power lines However many PLC components such as the CPU and

memory utilize 5 volts or another level of DC power The PLC power supply converts AC

power into DC power to support those components of the PLC

Programming Terminal -A PLC requires a programming terminal and programming

software for operation The programming terminal can be a dedicated terminal or a generic

computer purchased anywhere The programming terminal is used for programming the PLC

and monitoring the PLCs operation It may also download a ladder logic program (the

sending of a program from the programming terminal to the PLC) or upload a ladder logic

program (the sending of a program from the PLC to the programming terminal) The terminal

uses programming software for programming and talking to a PLC

24 WORKING OF PLC

Bringing input signal status to the internal memory of CPU

The field signals are connected to the IP module At the output of IP module the field

status converted into the voltage level required by the CPU is always available

At the beginning of each cycle the CPU brings in all the field IP signals from IP module amp

stores into its internal memory called as PII meaning process image input

The programmable controller operates cyclically meaning when complete program has been

scanned it starts again at the beginning of the program

IOBUS

A PLC works by continually scanning a program We can think of this scan cycle as

consisting of 3 important steps There are typically more than 3 but we can focus on the

important parts and not worry about the others Typically the others are checking the system

and updating the current internal counter and timer values

Step 1-Check Input Status-First the PLC takes a look at each input to determine if it is on or

off In other words is the sensor connected to the first input on How about the second input

How about the third It records this data into its memory to be used during the next step

17

Step 2-Execute Program-Next the PLC executes your program one instruction at a time

Maybe your program said that if the first input was on then it should turn on the first output

Since it already knows which inputs are onoff from the previous step it will be able to decide

whether the first output should be turned on based on the state of the first input It will store

the execution results for use later during the next step

Step 3-Update Output Status-Finally the PLC updates the status of the outputs It updates the

outputs based on which inputs were on during the first step and the results of executing your

program during the second step Based on the example in step 2 it would now turn on the first

output because the first input was on and your program said to turn on the first output when

this condition is trueProcess Control and Automation Process Control

The process of recognizing the state of the process at all times analyze the information

according to the set rules and guidelines and accordingly actuate the control elements is

referred to as process control

RECOGNISING THE STATUS

In control of process all these actions can be taken manually with human involvement or in a

semiautomatic or fully automatic manner Automation -Automation is basically the

delegation of human control functions to technical equipment aimed towards achieving

- Higher-productivity

-Superior quality of end product

-Efficient usage of energy and raw materials

-Improved safety in working conditions etc

Methods adopted for Process Control and Automation

- Manual control

- Hard wired logic control

- Electronics control

-PLC control

- Manual Control

Hardwired Control

-This was considered to be the first step towards automation

- Here the contractor amp relays together with timers amp counters were used

Electronics Control

18

-With the advent of electronics the logic gates started replacing the relays amp auxiliary

contractors in the control circuits amp timers

- With changes the benefits are

1) Reduced space requirements

2) Energy saving

3) Less maintenance and hence greater reliability etc

-With electronics the implementation of changes in the control logic as well as reducing the

project lead-time was not possible

Programmable Logic Controller

- With microprocessor and associated peripherals chips the process of control and

automation went a radical change

- Instead of achieving the desired control or automation through physical wiring of control

devices in PLC it is through a program or software Thus these controllers are referred to as

programmable logic controllers

- The programmable controllers have experienced an unprecedented growth as universal

element It can be effectively used in applications ranging from simple control like replacing

small number relays to complex automation problem

25 PROGRAMMING THE PLC

Ladder Logic

Ladder logic is the main programming method used for PLCs The ladder logic has been

developed to mimic relay logic The decision to use the relay logic diagrams was a strategic

one By selecting ladder logic as the main programming method the amount of retraining

needed for engineers and trades people was greatly reduced

Modern control systems still include relays but these are rarely used for logic A relay is a

simple device that uses a magnetic field to control a switch as pictured in Fig When a

voltage is applied to the input coil the resulting current creates a magnetic field The

magnetic field pulls a metal switch (or reed) towards it and the contacts touch closing the

switch The contact that closes when the coil is energized is called normally open The

normally closed contacts touch when the input coil is not energized Relays are normally

drawn in schematic form using a circle to represent the input coil The output contacts are

shown with two parallel lines Normally open contacts are shown as two lines and will be

open (non-conducting) when the input is not energized Normally closed contacts are shown

with two lines with a diagonal line through them When the input coil is not energized the

19

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

2 PROGRAMMABLE LOGIC CONTROLLER

21 INTRODUCTION

Programmable logic controllers (PLCs) are members of the

computer family capable of storing instructions to control functions such as sequencing

timing and counting which control a machine or a process The PLC is composed of two

basic sections the Central Processing Unit (CPU) and the InputOutput (IO) interface

system The PLC measures input signals coming from a machine and through the internal

program provides output or control back to the machine Ladder logic is the

programming language used to represent electrical sequences of operation In hardwired

circuits the electrical wiring is connected from one device to another according to logic of

operation In a PLC the devices are connected to the input interface the outputs are

connected to the output interface and the actual wiring of the components is done

electronically inside the PLC using ladder logic This is known as soft wired PLC is a

device that is capable of being programmed to perform a controlling function Before the

advent of PLC the problem of industrial control was usually solved by relays or

hardwired solid-state logic blocks These are very flexible in design and easy for

maintenance personal to understand However they involved a vast amount of

interconnection For the wiring cost to be minimized relays and logic blocks had to be

kept together This led to development of control panel concept for larger and more

complex logic control system The PLC was first conceived by group of engineers from

hydramatic division of GM in 1968This was designed to provide flexibility in control

based on programming and executing logic instruction Adopting the ladder diagram

programming language simplifying maintenance and reducing the cost of spare parts

inventories realized major advantages

22PLC HISTORY

In the late 1960s PLCs were first introduced The primary reason for designing such a

device was eliminating the large cost involved in replacing the complicated relay based

machine control systems Bedford Associates (Bedford MA) proposed something called a

Modular Digital Controller (MODICON) to a major US car manufacturer Other companies

at the time proposed computer based schemes one of which was based upon the PDP-8 The

MODICON 084 brought the worlds first PLC into commercial production

13

When production requirements changed so did the control system This becomes very

expensive when the change is frequent Since relays are mechanical devices they also have a

limited lifetime which required strict adhesion to maintenance schedules Troubleshooting

was also quite tedious when so many relays are involved Now picture a machine control

panel that included many possibly hundreds or thousands of individual relays The size

could be mind boggling How about the complicated initial wiring of so many individual

devices These relays would be individually wired together in a manner that would yield the

desired outcome Were there problems You bet These new controllers also had to be

easily programmed by maintenance and plant engineers The lifetime had to be long and

programming changes easily performed They also had to survive the harsh industrial

environment Thats a lot to ask The answers were to use a programming technique most

people were already familiar with and replace mechanical parts with solid-state ones

In the mid70acircbdquocents the dominant PLC technologies were sequencer state-machines and the bit-

slice based CPU The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs

Conventional microprocessors lacked the power to quickly solve PLC logic in all but the

smallest PLCs As conventional microprocessors evolved larger and larger PLCs were being

based upon them However even today some are still based upon the 2903(ref A-Bs PLC-3)

Modicon has yet to build a faster PLC than their 984ABX which was based upon the 2901

Communications abilities began to appear in approximately 1973 The first such system was

Modicons Modbus The PLC could now talk to other PLCs and they could be far away from

the actual machine they were controlling They could also now be used to send and receive

varying voltages to allow them to enter the analog world Unfortunately the lack of

standardization coupled with continually changing technology has made PLC

communications a nightmare of incompatible protocols and physical networks Still it was a

great decade for the PLC The 80acircbdquocents saw an attempt to standardize communications with

General Motors manufacturing automation protocol(MAP) It was also a time for reducing

the size of the PLC and making them software programmable through symbolic programming

on personal computers instead of dedicated programming terminals or handheld

programmers Today the worlds smallest PLC is about the size of a single control relay

The 90acircbdquocents have seen a gradual reduction in the introduction of new protocols and the

modernization of the physical layers of some of the more popular protocols that survived the

1980s The latest standard (IEC 1131-3) has tried to merge plc programming languages

under one international standard We now have PLCs that are programmable in function

block diagrams instruction lists C and structured text all at the same time PCs are also

14

being used to replace PLCs in some applications The original company who commissioned

the MODICON 084 has actually switched to a PC based control system

23PLC HARDWARE

A programmable logic controller consists of the following components

Central Processing Unit (CPU) Memory Input modules Output modules and Power

supply A PLC hardware block diagram is shown in Figure The programming terminal in the

diagram is not a part of the PLC but it is essential to have a terminal for programming or

monitoring a PLC In the diagram the arrows between blocks indicate the information and

power-flowing-directions

Fig PLC-Hardware-Block-Diagram

CPU

Like other computerized devices there is a Central Processing Unit (CPU) in a PLC The

CPU which is the brain of a PLC does the following operations

Updating inputs and outputs This function allows a PLC to read the status of its input

terminals and energize or deenergize its output terminals

Performing logic and arithmetic operations A CPU conducts all the mathematic and logic

operations involved in a PLC

Communicating with memory The PLCacircbdquocents programs and data are stored in memory

When a PLC is operating its CPU may read or change the contents of memory locations

Scanning application programs An application program which is called a ladder logic

program is a set of instructions written by a PLC programmer The scanning function allows

the PLC to execute the application program as specified by the programmer

15

PROCESS

Programming Terminal

CPU MemoryPowerSupply

InputModule

OutputModule

InputDevices

OutputDevices

PLC

Communicating with a programming terminal The CPU transfers program and data

between itself and the programming terminal A PLC CPU is controlled by operating system

software The operating system software is a group of supervisory programs that are loaded

and stored permanently in the PLC memory by the PLC manufacturer

Memory

Memory is the component that stores information programs and data in a PLC The process

of putting new information into a memory location is called writing The process of retrieving

information from a memory location is called reading The common types of memory used in

PLCs are Read Only Memory (ROM) and Random Access Memory (RAM) A ROM

location can be read but not written ROM is used to store programs and data that should not

be altered For example the PLCs operating programs are stored in ROM

A RAM location can be read or written This means the information stored in a RAM

location can be retrieved andor altered Ladder logic programs are stored in RAM When a

new ladder logic program is loaded into a PLCs memory the old program that was stored in

the same locations is over-written and essentially erased The memory capacities of PLCs

vary Memory capacities are often expressed in terms of kilo-bytes (K) One byte is a group

of 8 bits One bit is a memory location that may store one binary number that has the value of

either 1 or 0 (Binary numbers are addressed in Module 2) 1K memory means that there are

1024 bytes of RAM 16K memory means there are 16 x 1024 =16384 bytes of RAM

Input modules and output modules

A PLC is a control device It takes information from inputs and makes decisions to

energize or de-energize outputs The decisions are made based on the statuses of inputs and

outputs and the ladder logic program that is being executed The input devices used with a

PLC include pushbuttons limit switches relay contacts photo sensors proximity switches

temperature sensors and the like These input devices can be AC (alternating current) or DC

(direct current) The input voltages can be high or low The input signals can be digital or

analog Differing inputs require different input modules An input module provides an

interface between input devices and a PLCs CPU which uses only a low DC voltage The

input moduleacircbdquocents function is to convert the input signals to DC voltages that are acceptable

to the CPU Standard discrete input modules include 24 V AC 48 V AC 120 V AC 220 V

AC 24 V DC 48 V DC 120 V DC 220 V DC and transistor-transistor logic (TTL) level

The devices controlled by a PLC include relays alarms solenoids fans lights and motor

starters These devices may require different levels of AC or DC voltages Since the signals

16

processed in a PLC are low DC voltages it is the function of the output module to convert

PLC control signals to the voltages required by the controlled circuits or devices Standard

discrete output modules include 24 V AC 48 V AC 120 V AC 220 V AC 24 V DC 48 V

DC 120 V DC 220 V DC and TTL level Power Supply -PLCs are powered by standard

commercial AC power lines However many PLC components such as the CPU and

memory utilize 5 volts or another level of DC power The PLC power supply converts AC

power into DC power to support those components of the PLC

Programming Terminal -A PLC requires a programming terminal and programming

software for operation The programming terminal can be a dedicated terminal or a generic

computer purchased anywhere The programming terminal is used for programming the PLC

and monitoring the PLCs operation It may also download a ladder logic program (the

sending of a program from the programming terminal to the PLC) or upload a ladder logic

program (the sending of a program from the PLC to the programming terminal) The terminal

uses programming software for programming and talking to a PLC

24 WORKING OF PLC

Bringing input signal status to the internal memory of CPU

The field signals are connected to the IP module At the output of IP module the field

status converted into the voltage level required by the CPU is always available

At the beginning of each cycle the CPU brings in all the field IP signals from IP module amp

stores into its internal memory called as PII meaning process image input

The programmable controller operates cyclically meaning when complete program has been

scanned it starts again at the beginning of the program

IOBUS

A PLC works by continually scanning a program We can think of this scan cycle as

consisting of 3 important steps There are typically more than 3 but we can focus on the

important parts and not worry about the others Typically the others are checking the system

and updating the current internal counter and timer values

Step 1-Check Input Status-First the PLC takes a look at each input to determine if it is on or

off In other words is the sensor connected to the first input on How about the second input

How about the third It records this data into its memory to be used during the next step

17

Step 2-Execute Program-Next the PLC executes your program one instruction at a time

Maybe your program said that if the first input was on then it should turn on the first output

Since it already knows which inputs are onoff from the previous step it will be able to decide

whether the first output should be turned on based on the state of the first input It will store

the execution results for use later during the next step

Step 3-Update Output Status-Finally the PLC updates the status of the outputs It updates the

outputs based on which inputs were on during the first step and the results of executing your

program during the second step Based on the example in step 2 it would now turn on the first

output because the first input was on and your program said to turn on the first output when

this condition is trueProcess Control and Automation Process Control

The process of recognizing the state of the process at all times analyze the information

according to the set rules and guidelines and accordingly actuate the control elements is

referred to as process control

RECOGNISING THE STATUS

In control of process all these actions can be taken manually with human involvement or in a

semiautomatic or fully automatic manner Automation -Automation is basically the

delegation of human control functions to technical equipment aimed towards achieving

- Higher-productivity

-Superior quality of end product

-Efficient usage of energy and raw materials

-Improved safety in working conditions etc

Methods adopted for Process Control and Automation

- Manual control

- Hard wired logic control

- Electronics control

-PLC control

- Manual Control

Hardwired Control

-This was considered to be the first step towards automation

- Here the contractor amp relays together with timers amp counters were used

Electronics Control

18

-With the advent of electronics the logic gates started replacing the relays amp auxiliary

contractors in the control circuits amp timers

- With changes the benefits are

1) Reduced space requirements

2) Energy saving

3) Less maintenance and hence greater reliability etc

-With electronics the implementation of changes in the control logic as well as reducing the

project lead-time was not possible

Programmable Logic Controller

- With microprocessor and associated peripherals chips the process of control and

automation went a radical change

- Instead of achieving the desired control or automation through physical wiring of control

devices in PLC it is through a program or software Thus these controllers are referred to as

programmable logic controllers

- The programmable controllers have experienced an unprecedented growth as universal

element It can be effectively used in applications ranging from simple control like replacing

small number relays to complex automation problem

25 PROGRAMMING THE PLC

Ladder Logic

Ladder logic is the main programming method used for PLCs The ladder logic has been

developed to mimic relay logic The decision to use the relay logic diagrams was a strategic

one By selecting ladder logic as the main programming method the amount of retraining

needed for engineers and trades people was greatly reduced

Modern control systems still include relays but these are rarely used for logic A relay is a

simple device that uses a magnetic field to control a switch as pictured in Fig When a

voltage is applied to the input coil the resulting current creates a magnetic field The

magnetic field pulls a metal switch (or reed) towards it and the contacts touch closing the

switch The contact that closes when the coil is energized is called normally open The

normally closed contacts touch when the input coil is not energized Relays are normally

drawn in schematic form using a circle to represent the input coil The output contacts are

shown with two parallel lines Normally open contacts are shown as two lines and will be

open (non-conducting) when the input is not energized Normally closed contacts are shown

with two lines with a diagonal line through them When the input coil is not energized the

19

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

When production requirements changed so did the control system This becomes very

expensive when the change is frequent Since relays are mechanical devices they also have a

limited lifetime which required strict adhesion to maintenance schedules Troubleshooting

was also quite tedious when so many relays are involved Now picture a machine control

panel that included many possibly hundreds or thousands of individual relays The size

could be mind boggling How about the complicated initial wiring of so many individual

devices These relays would be individually wired together in a manner that would yield the

desired outcome Were there problems You bet These new controllers also had to be

easily programmed by maintenance and plant engineers The lifetime had to be long and

programming changes easily performed They also had to survive the harsh industrial

environment Thats a lot to ask The answers were to use a programming technique most

people were already familiar with and replace mechanical parts with solid-state ones

In the mid70acircbdquocents the dominant PLC technologies were sequencer state-machines and the bit-

slice based CPU The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs

Conventional microprocessors lacked the power to quickly solve PLC logic in all but the

smallest PLCs As conventional microprocessors evolved larger and larger PLCs were being

based upon them However even today some are still based upon the 2903(ref A-Bs PLC-3)

Modicon has yet to build a faster PLC than their 984ABX which was based upon the 2901

Communications abilities began to appear in approximately 1973 The first such system was

Modicons Modbus The PLC could now talk to other PLCs and they could be far away from

the actual machine they were controlling They could also now be used to send and receive

varying voltages to allow them to enter the analog world Unfortunately the lack of

standardization coupled with continually changing technology has made PLC

communications a nightmare of incompatible protocols and physical networks Still it was a

great decade for the PLC The 80acircbdquocents saw an attempt to standardize communications with

General Motors manufacturing automation protocol(MAP) It was also a time for reducing

the size of the PLC and making them software programmable through symbolic programming

on personal computers instead of dedicated programming terminals or handheld

programmers Today the worlds smallest PLC is about the size of a single control relay

The 90acircbdquocents have seen a gradual reduction in the introduction of new protocols and the

modernization of the physical layers of some of the more popular protocols that survived the

1980s The latest standard (IEC 1131-3) has tried to merge plc programming languages

under one international standard We now have PLCs that are programmable in function

block diagrams instruction lists C and structured text all at the same time PCs are also

14

being used to replace PLCs in some applications The original company who commissioned

the MODICON 084 has actually switched to a PC based control system

23PLC HARDWARE

A programmable logic controller consists of the following components

Central Processing Unit (CPU) Memory Input modules Output modules and Power

supply A PLC hardware block diagram is shown in Figure The programming terminal in the

diagram is not a part of the PLC but it is essential to have a terminal for programming or

monitoring a PLC In the diagram the arrows between blocks indicate the information and

power-flowing-directions

Fig PLC-Hardware-Block-Diagram

CPU

Like other computerized devices there is a Central Processing Unit (CPU) in a PLC The

CPU which is the brain of a PLC does the following operations

Updating inputs and outputs This function allows a PLC to read the status of its input

terminals and energize or deenergize its output terminals

Performing logic and arithmetic operations A CPU conducts all the mathematic and logic

operations involved in a PLC

Communicating with memory The PLCacircbdquocents programs and data are stored in memory

When a PLC is operating its CPU may read or change the contents of memory locations

Scanning application programs An application program which is called a ladder logic

program is a set of instructions written by a PLC programmer The scanning function allows

the PLC to execute the application program as specified by the programmer

15

PROCESS

Programming Terminal

CPU MemoryPowerSupply

InputModule

OutputModule

InputDevices

OutputDevices

PLC

Communicating with a programming terminal The CPU transfers program and data

between itself and the programming terminal A PLC CPU is controlled by operating system

software The operating system software is a group of supervisory programs that are loaded

and stored permanently in the PLC memory by the PLC manufacturer

Memory

Memory is the component that stores information programs and data in a PLC The process

of putting new information into a memory location is called writing The process of retrieving

information from a memory location is called reading The common types of memory used in

PLCs are Read Only Memory (ROM) and Random Access Memory (RAM) A ROM

location can be read but not written ROM is used to store programs and data that should not

be altered For example the PLCs operating programs are stored in ROM

A RAM location can be read or written This means the information stored in a RAM

location can be retrieved andor altered Ladder logic programs are stored in RAM When a

new ladder logic program is loaded into a PLCs memory the old program that was stored in

the same locations is over-written and essentially erased The memory capacities of PLCs

vary Memory capacities are often expressed in terms of kilo-bytes (K) One byte is a group

of 8 bits One bit is a memory location that may store one binary number that has the value of

either 1 or 0 (Binary numbers are addressed in Module 2) 1K memory means that there are

1024 bytes of RAM 16K memory means there are 16 x 1024 =16384 bytes of RAM

Input modules and output modules

A PLC is a control device It takes information from inputs and makes decisions to

energize or de-energize outputs The decisions are made based on the statuses of inputs and

outputs and the ladder logic program that is being executed The input devices used with a

PLC include pushbuttons limit switches relay contacts photo sensors proximity switches

temperature sensors and the like These input devices can be AC (alternating current) or DC

(direct current) The input voltages can be high or low The input signals can be digital or

analog Differing inputs require different input modules An input module provides an

interface between input devices and a PLCs CPU which uses only a low DC voltage The

input moduleacircbdquocents function is to convert the input signals to DC voltages that are acceptable

to the CPU Standard discrete input modules include 24 V AC 48 V AC 120 V AC 220 V

AC 24 V DC 48 V DC 120 V DC 220 V DC and transistor-transistor logic (TTL) level

The devices controlled by a PLC include relays alarms solenoids fans lights and motor

starters These devices may require different levels of AC or DC voltages Since the signals

16

processed in a PLC are low DC voltages it is the function of the output module to convert

PLC control signals to the voltages required by the controlled circuits or devices Standard

discrete output modules include 24 V AC 48 V AC 120 V AC 220 V AC 24 V DC 48 V

DC 120 V DC 220 V DC and TTL level Power Supply -PLCs are powered by standard

commercial AC power lines However many PLC components such as the CPU and

memory utilize 5 volts or another level of DC power The PLC power supply converts AC

power into DC power to support those components of the PLC

Programming Terminal -A PLC requires a programming terminal and programming

software for operation The programming terminal can be a dedicated terminal or a generic

computer purchased anywhere The programming terminal is used for programming the PLC

and monitoring the PLCs operation It may also download a ladder logic program (the

sending of a program from the programming terminal to the PLC) or upload a ladder logic

program (the sending of a program from the PLC to the programming terminal) The terminal

uses programming software for programming and talking to a PLC

24 WORKING OF PLC

Bringing input signal status to the internal memory of CPU

The field signals are connected to the IP module At the output of IP module the field

status converted into the voltage level required by the CPU is always available

At the beginning of each cycle the CPU brings in all the field IP signals from IP module amp

stores into its internal memory called as PII meaning process image input

The programmable controller operates cyclically meaning when complete program has been

scanned it starts again at the beginning of the program

IOBUS

A PLC works by continually scanning a program We can think of this scan cycle as

consisting of 3 important steps There are typically more than 3 but we can focus on the

important parts and not worry about the others Typically the others are checking the system

and updating the current internal counter and timer values

Step 1-Check Input Status-First the PLC takes a look at each input to determine if it is on or

off In other words is the sensor connected to the first input on How about the second input

How about the third It records this data into its memory to be used during the next step

17

Step 2-Execute Program-Next the PLC executes your program one instruction at a time

Maybe your program said that if the first input was on then it should turn on the first output

Since it already knows which inputs are onoff from the previous step it will be able to decide

whether the first output should be turned on based on the state of the first input It will store

the execution results for use later during the next step

Step 3-Update Output Status-Finally the PLC updates the status of the outputs It updates the

outputs based on which inputs were on during the first step and the results of executing your

program during the second step Based on the example in step 2 it would now turn on the first

output because the first input was on and your program said to turn on the first output when

this condition is trueProcess Control and Automation Process Control

The process of recognizing the state of the process at all times analyze the information

according to the set rules and guidelines and accordingly actuate the control elements is

referred to as process control

RECOGNISING THE STATUS

In control of process all these actions can be taken manually with human involvement or in a

semiautomatic or fully automatic manner Automation -Automation is basically the

delegation of human control functions to technical equipment aimed towards achieving

- Higher-productivity

-Superior quality of end product

-Efficient usage of energy and raw materials

-Improved safety in working conditions etc

Methods adopted for Process Control and Automation

- Manual control

- Hard wired logic control

- Electronics control

-PLC control

- Manual Control

Hardwired Control

-This was considered to be the first step towards automation

- Here the contractor amp relays together with timers amp counters were used

Electronics Control

18

-With the advent of electronics the logic gates started replacing the relays amp auxiliary

contractors in the control circuits amp timers

- With changes the benefits are

1) Reduced space requirements

2) Energy saving

3) Less maintenance and hence greater reliability etc

-With electronics the implementation of changes in the control logic as well as reducing the

project lead-time was not possible

Programmable Logic Controller

- With microprocessor and associated peripherals chips the process of control and

automation went a radical change

- Instead of achieving the desired control or automation through physical wiring of control

devices in PLC it is through a program or software Thus these controllers are referred to as

programmable logic controllers

- The programmable controllers have experienced an unprecedented growth as universal

element It can be effectively used in applications ranging from simple control like replacing

small number relays to complex automation problem

25 PROGRAMMING THE PLC

Ladder Logic

Ladder logic is the main programming method used for PLCs The ladder logic has been

developed to mimic relay logic The decision to use the relay logic diagrams was a strategic

one By selecting ladder logic as the main programming method the amount of retraining

needed for engineers and trades people was greatly reduced

Modern control systems still include relays but these are rarely used for logic A relay is a

simple device that uses a magnetic field to control a switch as pictured in Fig When a

voltage is applied to the input coil the resulting current creates a magnetic field The

magnetic field pulls a metal switch (or reed) towards it and the contacts touch closing the

switch The contact that closes when the coil is energized is called normally open The

normally closed contacts touch when the input coil is not energized Relays are normally

drawn in schematic form using a circle to represent the input coil The output contacts are

shown with two parallel lines Normally open contacts are shown as two lines and will be

open (non-conducting) when the input is not energized Normally closed contacts are shown

with two lines with a diagonal line through them When the input coil is not energized the

19

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

being used to replace PLCs in some applications The original company who commissioned

the MODICON 084 has actually switched to a PC based control system

23PLC HARDWARE

A programmable logic controller consists of the following components

Central Processing Unit (CPU) Memory Input modules Output modules and Power

supply A PLC hardware block diagram is shown in Figure The programming terminal in the

diagram is not a part of the PLC but it is essential to have a terminal for programming or

monitoring a PLC In the diagram the arrows between blocks indicate the information and

power-flowing-directions

Fig PLC-Hardware-Block-Diagram

CPU

Like other computerized devices there is a Central Processing Unit (CPU) in a PLC The

CPU which is the brain of a PLC does the following operations

Updating inputs and outputs This function allows a PLC to read the status of its input

terminals and energize or deenergize its output terminals

Performing logic and arithmetic operations A CPU conducts all the mathematic and logic

operations involved in a PLC

Communicating with memory The PLCacircbdquocents programs and data are stored in memory

When a PLC is operating its CPU may read or change the contents of memory locations

Scanning application programs An application program which is called a ladder logic

program is a set of instructions written by a PLC programmer The scanning function allows

the PLC to execute the application program as specified by the programmer

15

PROCESS

Programming Terminal

CPU MemoryPowerSupply

InputModule

OutputModule

InputDevices

OutputDevices

PLC

Communicating with a programming terminal The CPU transfers program and data

between itself and the programming terminal A PLC CPU is controlled by operating system

software The operating system software is a group of supervisory programs that are loaded

and stored permanently in the PLC memory by the PLC manufacturer

Memory

Memory is the component that stores information programs and data in a PLC The process

of putting new information into a memory location is called writing The process of retrieving

information from a memory location is called reading The common types of memory used in

PLCs are Read Only Memory (ROM) and Random Access Memory (RAM) A ROM

location can be read but not written ROM is used to store programs and data that should not

be altered For example the PLCs operating programs are stored in ROM

A RAM location can be read or written This means the information stored in a RAM

location can be retrieved andor altered Ladder logic programs are stored in RAM When a

new ladder logic program is loaded into a PLCs memory the old program that was stored in

the same locations is over-written and essentially erased The memory capacities of PLCs

vary Memory capacities are often expressed in terms of kilo-bytes (K) One byte is a group

of 8 bits One bit is a memory location that may store one binary number that has the value of

either 1 or 0 (Binary numbers are addressed in Module 2) 1K memory means that there are

1024 bytes of RAM 16K memory means there are 16 x 1024 =16384 bytes of RAM

Input modules and output modules

A PLC is a control device It takes information from inputs and makes decisions to

energize or de-energize outputs The decisions are made based on the statuses of inputs and

outputs and the ladder logic program that is being executed The input devices used with a

PLC include pushbuttons limit switches relay contacts photo sensors proximity switches

temperature sensors and the like These input devices can be AC (alternating current) or DC

(direct current) The input voltages can be high or low The input signals can be digital or

analog Differing inputs require different input modules An input module provides an

interface between input devices and a PLCs CPU which uses only a low DC voltage The

input moduleacircbdquocents function is to convert the input signals to DC voltages that are acceptable

to the CPU Standard discrete input modules include 24 V AC 48 V AC 120 V AC 220 V

AC 24 V DC 48 V DC 120 V DC 220 V DC and transistor-transistor logic (TTL) level

The devices controlled by a PLC include relays alarms solenoids fans lights and motor

starters These devices may require different levels of AC or DC voltages Since the signals

16

processed in a PLC are low DC voltages it is the function of the output module to convert

PLC control signals to the voltages required by the controlled circuits or devices Standard

discrete output modules include 24 V AC 48 V AC 120 V AC 220 V AC 24 V DC 48 V

DC 120 V DC 220 V DC and TTL level Power Supply -PLCs are powered by standard

commercial AC power lines However many PLC components such as the CPU and

memory utilize 5 volts or another level of DC power The PLC power supply converts AC

power into DC power to support those components of the PLC

Programming Terminal -A PLC requires a programming terminal and programming

software for operation The programming terminal can be a dedicated terminal or a generic

computer purchased anywhere The programming terminal is used for programming the PLC

and monitoring the PLCs operation It may also download a ladder logic program (the

sending of a program from the programming terminal to the PLC) or upload a ladder logic

program (the sending of a program from the PLC to the programming terminal) The terminal

uses programming software for programming and talking to a PLC

24 WORKING OF PLC

Bringing input signal status to the internal memory of CPU

The field signals are connected to the IP module At the output of IP module the field

status converted into the voltage level required by the CPU is always available

At the beginning of each cycle the CPU brings in all the field IP signals from IP module amp

stores into its internal memory called as PII meaning process image input

The programmable controller operates cyclically meaning when complete program has been

scanned it starts again at the beginning of the program

IOBUS

A PLC works by continually scanning a program We can think of this scan cycle as

consisting of 3 important steps There are typically more than 3 but we can focus on the

important parts and not worry about the others Typically the others are checking the system

and updating the current internal counter and timer values

Step 1-Check Input Status-First the PLC takes a look at each input to determine if it is on or

off In other words is the sensor connected to the first input on How about the second input

How about the third It records this data into its memory to be used during the next step

17

Step 2-Execute Program-Next the PLC executes your program one instruction at a time

Maybe your program said that if the first input was on then it should turn on the first output

Since it already knows which inputs are onoff from the previous step it will be able to decide

whether the first output should be turned on based on the state of the first input It will store

the execution results for use later during the next step

Step 3-Update Output Status-Finally the PLC updates the status of the outputs It updates the

outputs based on which inputs were on during the first step and the results of executing your

program during the second step Based on the example in step 2 it would now turn on the first

output because the first input was on and your program said to turn on the first output when

this condition is trueProcess Control and Automation Process Control

The process of recognizing the state of the process at all times analyze the information

according to the set rules and guidelines and accordingly actuate the control elements is

referred to as process control

RECOGNISING THE STATUS

In control of process all these actions can be taken manually with human involvement or in a

semiautomatic or fully automatic manner Automation -Automation is basically the

delegation of human control functions to technical equipment aimed towards achieving

- Higher-productivity

-Superior quality of end product

-Efficient usage of energy and raw materials

-Improved safety in working conditions etc

Methods adopted for Process Control and Automation

- Manual control

- Hard wired logic control

- Electronics control

-PLC control

- Manual Control

Hardwired Control

-This was considered to be the first step towards automation

- Here the contractor amp relays together with timers amp counters were used

Electronics Control

18

-With the advent of electronics the logic gates started replacing the relays amp auxiliary

contractors in the control circuits amp timers

- With changes the benefits are

1) Reduced space requirements

2) Energy saving

3) Less maintenance and hence greater reliability etc

-With electronics the implementation of changes in the control logic as well as reducing the

project lead-time was not possible

Programmable Logic Controller

- With microprocessor and associated peripherals chips the process of control and

automation went a radical change

- Instead of achieving the desired control or automation through physical wiring of control

devices in PLC it is through a program or software Thus these controllers are referred to as

programmable logic controllers

- The programmable controllers have experienced an unprecedented growth as universal

element It can be effectively used in applications ranging from simple control like replacing

small number relays to complex automation problem

25 PROGRAMMING THE PLC

Ladder Logic

Ladder logic is the main programming method used for PLCs The ladder logic has been

developed to mimic relay logic The decision to use the relay logic diagrams was a strategic

one By selecting ladder logic as the main programming method the amount of retraining

needed for engineers and trades people was greatly reduced

Modern control systems still include relays but these are rarely used for logic A relay is a

simple device that uses a magnetic field to control a switch as pictured in Fig When a

voltage is applied to the input coil the resulting current creates a magnetic field The

magnetic field pulls a metal switch (or reed) towards it and the contacts touch closing the

switch The contact that closes when the coil is energized is called normally open The

normally closed contacts touch when the input coil is not energized Relays are normally

drawn in schematic form using a circle to represent the input coil The output contacts are

shown with two parallel lines Normally open contacts are shown as two lines and will be

open (non-conducting) when the input is not energized Normally closed contacts are shown

with two lines with a diagonal line through them When the input coil is not energized the

19

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

Communicating with a programming terminal The CPU transfers program and data

between itself and the programming terminal A PLC CPU is controlled by operating system

software The operating system software is a group of supervisory programs that are loaded

and stored permanently in the PLC memory by the PLC manufacturer

Memory

Memory is the component that stores information programs and data in a PLC The process

of putting new information into a memory location is called writing The process of retrieving

information from a memory location is called reading The common types of memory used in

PLCs are Read Only Memory (ROM) and Random Access Memory (RAM) A ROM

location can be read but not written ROM is used to store programs and data that should not

be altered For example the PLCs operating programs are stored in ROM

A RAM location can be read or written This means the information stored in a RAM

location can be retrieved andor altered Ladder logic programs are stored in RAM When a

new ladder logic program is loaded into a PLCs memory the old program that was stored in

the same locations is over-written and essentially erased The memory capacities of PLCs

vary Memory capacities are often expressed in terms of kilo-bytes (K) One byte is a group

of 8 bits One bit is a memory location that may store one binary number that has the value of

either 1 or 0 (Binary numbers are addressed in Module 2) 1K memory means that there are

1024 bytes of RAM 16K memory means there are 16 x 1024 =16384 bytes of RAM

Input modules and output modules

A PLC is a control device It takes information from inputs and makes decisions to

energize or de-energize outputs The decisions are made based on the statuses of inputs and

outputs and the ladder logic program that is being executed The input devices used with a

PLC include pushbuttons limit switches relay contacts photo sensors proximity switches

temperature sensors and the like These input devices can be AC (alternating current) or DC

(direct current) The input voltages can be high or low The input signals can be digital or

analog Differing inputs require different input modules An input module provides an

interface between input devices and a PLCs CPU which uses only a low DC voltage The

input moduleacircbdquocents function is to convert the input signals to DC voltages that are acceptable

to the CPU Standard discrete input modules include 24 V AC 48 V AC 120 V AC 220 V

AC 24 V DC 48 V DC 120 V DC 220 V DC and transistor-transistor logic (TTL) level

The devices controlled by a PLC include relays alarms solenoids fans lights and motor

starters These devices may require different levels of AC or DC voltages Since the signals

16

processed in a PLC are low DC voltages it is the function of the output module to convert

PLC control signals to the voltages required by the controlled circuits or devices Standard

discrete output modules include 24 V AC 48 V AC 120 V AC 220 V AC 24 V DC 48 V

DC 120 V DC 220 V DC and TTL level Power Supply -PLCs are powered by standard

commercial AC power lines However many PLC components such as the CPU and

memory utilize 5 volts or another level of DC power The PLC power supply converts AC

power into DC power to support those components of the PLC

Programming Terminal -A PLC requires a programming terminal and programming

software for operation The programming terminal can be a dedicated terminal or a generic

computer purchased anywhere The programming terminal is used for programming the PLC

and monitoring the PLCs operation It may also download a ladder logic program (the

sending of a program from the programming terminal to the PLC) or upload a ladder logic

program (the sending of a program from the PLC to the programming terminal) The terminal

uses programming software for programming and talking to a PLC

24 WORKING OF PLC

Bringing input signal status to the internal memory of CPU

The field signals are connected to the IP module At the output of IP module the field

status converted into the voltage level required by the CPU is always available

At the beginning of each cycle the CPU brings in all the field IP signals from IP module amp

stores into its internal memory called as PII meaning process image input

The programmable controller operates cyclically meaning when complete program has been

scanned it starts again at the beginning of the program

IOBUS

A PLC works by continually scanning a program We can think of this scan cycle as

consisting of 3 important steps There are typically more than 3 but we can focus on the

important parts and not worry about the others Typically the others are checking the system

and updating the current internal counter and timer values

Step 1-Check Input Status-First the PLC takes a look at each input to determine if it is on or

off In other words is the sensor connected to the first input on How about the second input

How about the third It records this data into its memory to be used during the next step

17

Step 2-Execute Program-Next the PLC executes your program one instruction at a time

Maybe your program said that if the first input was on then it should turn on the first output

Since it already knows which inputs are onoff from the previous step it will be able to decide

whether the first output should be turned on based on the state of the first input It will store

the execution results for use later during the next step

Step 3-Update Output Status-Finally the PLC updates the status of the outputs It updates the

outputs based on which inputs were on during the first step and the results of executing your

program during the second step Based on the example in step 2 it would now turn on the first

output because the first input was on and your program said to turn on the first output when

this condition is trueProcess Control and Automation Process Control

The process of recognizing the state of the process at all times analyze the information

according to the set rules and guidelines and accordingly actuate the control elements is

referred to as process control

RECOGNISING THE STATUS

In control of process all these actions can be taken manually with human involvement or in a

semiautomatic or fully automatic manner Automation -Automation is basically the

delegation of human control functions to technical equipment aimed towards achieving

- Higher-productivity

-Superior quality of end product

-Efficient usage of energy and raw materials

-Improved safety in working conditions etc

Methods adopted for Process Control and Automation

- Manual control

- Hard wired logic control

- Electronics control

-PLC control

- Manual Control

Hardwired Control

-This was considered to be the first step towards automation

- Here the contractor amp relays together with timers amp counters were used

Electronics Control

18

-With the advent of electronics the logic gates started replacing the relays amp auxiliary

contractors in the control circuits amp timers

- With changes the benefits are

1) Reduced space requirements

2) Energy saving

3) Less maintenance and hence greater reliability etc

-With electronics the implementation of changes in the control logic as well as reducing the

project lead-time was not possible

Programmable Logic Controller

- With microprocessor and associated peripherals chips the process of control and

automation went a radical change

- Instead of achieving the desired control or automation through physical wiring of control

devices in PLC it is through a program or software Thus these controllers are referred to as

programmable logic controllers

- The programmable controllers have experienced an unprecedented growth as universal

element It can be effectively used in applications ranging from simple control like replacing

small number relays to complex automation problem

25 PROGRAMMING THE PLC

Ladder Logic

Ladder logic is the main programming method used for PLCs The ladder logic has been

developed to mimic relay logic The decision to use the relay logic diagrams was a strategic

one By selecting ladder logic as the main programming method the amount of retraining

needed for engineers and trades people was greatly reduced

Modern control systems still include relays but these are rarely used for logic A relay is a

simple device that uses a magnetic field to control a switch as pictured in Fig When a

voltage is applied to the input coil the resulting current creates a magnetic field The

magnetic field pulls a metal switch (or reed) towards it and the contacts touch closing the

switch The contact that closes when the coil is energized is called normally open The

normally closed contacts touch when the input coil is not energized Relays are normally

drawn in schematic form using a circle to represent the input coil The output contacts are

shown with two parallel lines Normally open contacts are shown as two lines and will be

open (non-conducting) when the input is not energized Normally closed contacts are shown

with two lines with a diagonal line through them When the input coil is not energized the

19

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

processed in a PLC are low DC voltages it is the function of the output module to convert

PLC control signals to the voltages required by the controlled circuits or devices Standard

discrete output modules include 24 V AC 48 V AC 120 V AC 220 V AC 24 V DC 48 V

DC 120 V DC 220 V DC and TTL level Power Supply -PLCs are powered by standard

commercial AC power lines However many PLC components such as the CPU and

memory utilize 5 volts or another level of DC power The PLC power supply converts AC

power into DC power to support those components of the PLC

Programming Terminal -A PLC requires a programming terminal and programming

software for operation The programming terminal can be a dedicated terminal or a generic

computer purchased anywhere The programming terminal is used for programming the PLC

and monitoring the PLCs operation It may also download a ladder logic program (the

sending of a program from the programming terminal to the PLC) or upload a ladder logic

program (the sending of a program from the PLC to the programming terminal) The terminal

uses programming software for programming and talking to a PLC

24 WORKING OF PLC

Bringing input signal status to the internal memory of CPU

The field signals are connected to the IP module At the output of IP module the field

status converted into the voltage level required by the CPU is always available

At the beginning of each cycle the CPU brings in all the field IP signals from IP module amp

stores into its internal memory called as PII meaning process image input

The programmable controller operates cyclically meaning when complete program has been

scanned it starts again at the beginning of the program

IOBUS

A PLC works by continually scanning a program We can think of this scan cycle as

consisting of 3 important steps There are typically more than 3 but we can focus on the

important parts and not worry about the others Typically the others are checking the system

and updating the current internal counter and timer values

Step 1-Check Input Status-First the PLC takes a look at each input to determine if it is on or

off In other words is the sensor connected to the first input on How about the second input

How about the third It records this data into its memory to be used during the next step

17

Step 2-Execute Program-Next the PLC executes your program one instruction at a time

Maybe your program said that if the first input was on then it should turn on the first output

Since it already knows which inputs are onoff from the previous step it will be able to decide

whether the first output should be turned on based on the state of the first input It will store

the execution results for use later during the next step

Step 3-Update Output Status-Finally the PLC updates the status of the outputs It updates the

outputs based on which inputs were on during the first step and the results of executing your

program during the second step Based on the example in step 2 it would now turn on the first

output because the first input was on and your program said to turn on the first output when

this condition is trueProcess Control and Automation Process Control

The process of recognizing the state of the process at all times analyze the information

according to the set rules and guidelines and accordingly actuate the control elements is

referred to as process control

RECOGNISING THE STATUS

In control of process all these actions can be taken manually with human involvement or in a

semiautomatic or fully automatic manner Automation -Automation is basically the

delegation of human control functions to technical equipment aimed towards achieving

- Higher-productivity

-Superior quality of end product

-Efficient usage of energy and raw materials

-Improved safety in working conditions etc

Methods adopted for Process Control and Automation

- Manual control

- Hard wired logic control

- Electronics control

-PLC control

- Manual Control

Hardwired Control

-This was considered to be the first step towards automation

- Here the contractor amp relays together with timers amp counters were used

Electronics Control

18

-With the advent of electronics the logic gates started replacing the relays amp auxiliary

contractors in the control circuits amp timers

- With changes the benefits are

1) Reduced space requirements

2) Energy saving

3) Less maintenance and hence greater reliability etc

-With electronics the implementation of changes in the control logic as well as reducing the

project lead-time was not possible

Programmable Logic Controller

- With microprocessor and associated peripherals chips the process of control and

automation went a radical change

- Instead of achieving the desired control or automation through physical wiring of control

devices in PLC it is through a program or software Thus these controllers are referred to as

programmable logic controllers

- The programmable controllers have experienced an unprecedented growth as universal

element It can be effectively used in applications ranging from simple control like replacing

small number relays to complex automation problem

25 PROGRAMMING THE PLC

Ladder Logic

Ladder logic is the main programming method used for PLCs The ladder logic has been

developed to mimic relay logic The decision to use the relay logic diagrams was a strategic

one By selecting ladder logic as the main programming method the amount of retraining

needed for engineers and trades people was greatly reduced

Modern control systems still include relays but these are rarely used for logic A relay is a

simple device that uses a magnetic field to control a switch as pictured in Fig When a

voltage is applied to the input coil the resulting current creates a magnetic field The

magnetic field pulls a metal switch (or reed) towards it and the contacts touch closing the

switch The contact that closes when the coil is energized is called normally open The

normally closed contacts touch when the input coil is not energized Relays are normally

drawn in schematic form using a circle to represent the input coil The output contacts are

shown with two parallel lines Normally open contacts are shown as two lines and will be

open (non-conducting) when the input is not energized Normally closed contacts are shown

with two lines with a diagonal line through them When the input coil is not energized the

19

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

Step 2-Execute Program-Next the PLC executes your program one instruction at a time

Maybe your program said that if the first input was on then it should turn on the first output

Since it already knows which inputs are onoff from the previous step it will be able to decide

whether the first output should be turned on based on the state of the first input It will store

the execution results for use later during the next step

Step 3-Update Output Status-Finally the PLC updates the status of the outputs It updates the

outputs based on which inputs were on during the first step and the results of executing your

program during the second step Based on the example in step 2 it would now turn on the first

output because the first input was on and your program said to turn on the first output when

this condition is trueProcess Control and Automation Process Control

The process of recognizing the state of the process at all times analyze the information

according to the set rules and guidelines and accordingly actuate the control elements is

referred to as process control

RECOGNISING THE STATUS

In control of process all these actions can be taken manually with human involvement or in a

semiautomatic or fully automatic manner Automation -Automation is basically the

delegation of human control functions to technical equipment aimed towards achieving

- Higher-productivity

-Superior quality of end product

-Efficient usage of energy and raw materials

-Improved safety in working conditions etc

Methods adopted for Process Control and Automation

- Manual control

- Hard wired logic control

- Electronics control

-PLC control

- Manual Control

Hardwired Control

-This was considered to be the first step towards automation

- Here the contractor amp relays together with timers amp counters were used

Electronics Control

18

-With the advent of electronics the logic gates started replacing the relays amp auxiliary

contractors in the control circuits amp timers

- With changes the benefits are

1) Reduced space requirements

2) Energy saving

3) Less maintenance and hence greater reliability etc

-With electronics the implementation of changes in the control logic as well as reducing the

project lead-time was not possible

Programmable Logic Controller

- With microprocessor and associated peripherals chips the process of control and

automation went a radical change

- Instead of achieving the desired control or automation through physical wiring of control

devices in PLC it is through a program or software Thus these controllers are referred to as

programmable logic controllers

- The programmable controllers have experienced an unprecedented growth as universal

element It can be effectively used in applications ranging from simple control like replacing

small number relays to complex automation problem

25 PROGRAMMING THE PLC

Ladder Logic

Ladder logic is the main programming method used for PLCs The ladder logic has been

developed to mimic relay logic The decision to use the relay logic diagrams was a strategic

one By selecting ladder logic as the main programming method the amount of retraining

needed for engineers and trades people was greatly reduced

Modern control systems still include relays but these are rarely used for logic A relay is a

simple device that uses a magnetic field to control a switch as pictured in Fig When a

voltage is applied to the input coil the resulting current creates a magnetic field The

magnetic field pulls a metal switch (or reed) towards it and the contacts touch closing the

switch The contact that closes when the coil is energized is called normally open The

normally closed contacts touch when the input coil is not energized Relays are normally

drawn in schematic form using a circle to represent the input coil The output contacts are

shown with two parallel lines Normally open contacts are shown as two lines and will be

open (non-conducting) when the input is not energized Normally closed contacts are shown

with two lines with a diagonal line through them When the input coil is not energized the

19

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

-With the advent of electronics the logic gates started replacing the relays amp auxiliary

contractors in the control circuits amp timers

- With changes the benefits are

1) Reduced space requirements

2) Energy saving

3) Less maintenance and hence greater reliability etc

-With electronics the implementation of changes in the control logic as well as reducing the

project lead-time was not possible

Programmable Logic Controller

- With microprocessor and associated peripherals chips the process of control and

automation went a radical change

- Instead of achieving the desired control or automation through physical wiring of control

devices in PLC it is through a program or software Thus these controllers are referred to as

programmable logic controllers

- The programmable controllers have experienced an unprecedented growth as universal

element It can be effectively used in applications ranging from simple control like replacing

small number relays to complex automation problem

25 PROGRAMMING THE PLC

Ladder Logic

Ladder logic is the main programming method used for PLCs The ladder logic has been

developed to mimic relay logic The decision to use the relay logic diagrams was a strategic

one By selecting ladder logic as the main programming method the amount of retraining

needed for engineers and trades people was greatly reduced

Modern control systems still include relays but these are rarely used for logic A relay is a

simple device that uses a magnetic field to control a switch as pictured in Fig When a

voltage is applied to the input coil the resulting current creates a magnetic field The

magnetic field pulls a metal switch (or reed) towards it and the contacts touch closing the

switch The contact that closes when the coil is energized is called normally open The

normally closed contacts touch when the input coil is not energized Relays are normally

drawn in schematic form using a circle to represent the input coil The output contacts are

shown with two parallel lines Normally open contacts are shown as two lines and will be

open (non-conducting) when the input is not energized Normally closed contacts are shown

with two lines with a diagonal line through them When the input coil is not energized the

19

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

normally closed contacts will be closed (conducting)

Fig Simple Relay Layouts and Schematics

Relays are used to let one power source close a switch for another (often high current) power

source while keeping them isolated An example of a relay in a simple control application is

shown in Figure In this system the first relay on the left is used as normally closed and will

allow current to flow until a voltage is applied to the input A The second relay is normally

open and will not allow current to flow until a voltage is applied to the input B If current is

flowing through the first two relays then current will flow through the coil in the third relay

and close the switch for output C This circuit would normally be drawn in the ladder logic

form This can be read logically as C will be on if A is off and B is on

Fig A Simple Relay Controller

The example in Figure does not show the entire control system but only the logic When we

consider a PLC there are inputs outputs and the logic Figure 4 shows a more complete

representation of the PLC Here there are two inputs from push buttons We can imagine the

inputs as activating 24V DC relay coils in the PLC This in turn drives an output relay that

switches 115V AC that will turn on a light Note in actual PLCs inputs are never relays but

outputs are often relays The ladder logic in the PLC is actually a computer program that the

20

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

user can enter and change Notice that both of the input push buttons are normally open but

the ladder logic inside the PLC has one normally open contact and one normally closed

contact Do not think that the ladder logic in the PLC needs to match the inputs or outputs

Many beginners will get caught trying to make the ladder logic match the input types

Fig A Simple Ladder Logic Diagram

There are other methods for programming PLCs One of the earliest techniques involved

mnemonic instructions These instructions can be derived directly from the ladder logic

diagrams and entered into the PLC through a simple programming terminal An example of

mnemonics is shown in Figure In this example the instructions are read one line at a time

from top to bottom The first line 00000 has the instruction LDN (input load and not) for

input A This will examine the input to the PLC and if it is off it will remember a 1 (or true)

if it is on it will remember a 0 (or false) The next line uses an LD (input load) statement to

look at the input If the input is off it remembers a 0 if the input is on it remembers a 1 (note

this is the reverse of the LDN) The AND statement recalls the last two numbers remembered

and if they are both true the result is a 1 otherwise the result is a 0 This result now replaces

the two numbers that were recalled and there is only one number remembered The process

is repeated for lines 00003 and 00004 but when these are done there are now three numbers

remembered The oldest number is from the AND the newer numbers are from the two LD

instructions The AND in line 00005 combines the results from the last LD instructions and

now there are two numbers remembered The OR instruction takes the two numbers now

remaining and if either one is a 1 the result is a 1 otherwise the result is a 0 This result

replaces the two numbers and there is now a single number there The last instruction is the

21

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

ST (store output) that will look at the last value stored and if it is 1 the output will be turned

on if it is 0 the output will be turned off

PLC Structure

PLC Divided into 4 parts IO Modules CPU Memory and Programming Terminal It operates by examining the input signals from a process and carrying out logic instructions and Producing output signals to drive process equipment The Standard interfaces built-in to PLC directly connected to process actuators amp transducers without the need for intermediate circuitry or relays It requires short installation amp commissioning times and it has Specific features for industrial control

1 Noise immune equipment2 Modular plug-in construction3 Standard IO connections amp signal levels4 Easily understood programming language5 Ease of programming amp reprogramming in-plant6 Capable of communicating with other PLCs computers amp intelligent devices7 Competitive in both cost amp space occupied with relay amp solid-state logic systems

Features

The main difference from other computers is that PLCs are armored for severe

condition (dust moisture heat cold etc) and have the facility for extensive

inputoutput (IO) arrangements

These connect the PLC to sensors and actuators

PLCs read limit switches analog process variables (such as temperature and

pressure) and the positions of complex positioning systems Some even use machine

vision

On the actuator side PLCs operate electric motors pneumatic or hydraulic cylinders

magnetic relays or solenoids or analog outputs

The inputoutput arrangements may be built into a simple PLC or the PLC may have

external IO modules attached to a computer network that plugs into the PLC

22

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

Fig Of PLC

ADVANTAGE OF PLC

Cost effective for controlling complex system

Smaller physical size than hard-wired solutions

Flexible and can be reapplied to control other systems quickly

PLCs have integrated diagnostics and override functions

Computational abilities allow more sophisticated control

Diagnostics are centrally available

Troubleshooting aids make programming easier and

reduce downtime

Applications can be immediately documented

Applications can be duplicated faster and less

expensively

Reliable components make these likely operate for

several years successfully

Communication is possibilities

23

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

DISADVANTAGE OF PLC

Programmable controllers are not equipped with enough memory to store big amounts of

data

In this field the communication system need to be more developed

24

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

CHAPTER ndash 3

CORROSION

25

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

31 INTRODUCTIONDefination Corrosion is the deterioration of materials by chemical interaction with their environment The term corrosion is sometimes also applied to the degradation of plastics concrete and wood but generally refers to metals

OR

The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties

Corrosion in environment

32 TYPES OF CORROSION

Underground corrosion

26

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

Buried gas or water supply pipes can suffer severe corrosion which is not detected until an actual leakage occurs by which time considerable damage may be doneElectronic components

In electronic equipment it is very important that there should be no raised resistance at low current connections Corrosion products can cause such damage and can also have sufficient conductance to cause short circuits These resistors form part of a radar installation

Corrosion influenced by flow

The cast iron pump impeller shown here suffered attack when acid accidentally entered the water that was being pumped The high velocities in the pump accentuated the corrosion damage

27

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

Corrosion in aircraft

The lower edge of this aircraft skin panel has suffered corrosion due to leakage and spillage from a wash basin in the toilet Any failure of a structural component of an aircraft can lead to the most serious resultsCorrosion at sea

Sea water is a highly corrosive electrolyte towards mild steel This ship has suffered severe damage in the areas which are most buffeted by waves where the protective coating of paint has been largely removed by mechanical actionldquoCorrosionrdquo of plastics

Not only metals suffer ldquocorrosionrdquo effects This dished end of a vessel is made of glass fibre reinforced PVC Due to internal stresses and an aggressive environment it has suffered ldquoenvironmental stress crackingrdquo Galvanic corrosion

28

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

This rainwater guttering is made of aluminium and would normally resist corrosion well Someone tied a copper aerial wire around it and the localised bimetallic cell led to a ldquoknife-cutrdquo effect

33 EFFECT OF CORROSION

bull Reduced Strength

bull Downtime of equipment

bull Escape of fluids

bull Lost surface properties

bull Reduced value of goods

The consequences of corrosion are many and varied and the effects of these on the safe reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small

Losses due to Corrosion

34 DISASTERS DUE TO CORROSION

29

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

Aloha Incident

198819-year old Boeing 737 operated by Aloha Airlines lost a major portion of the upper fuselage in full flight at 24000 ft

Bhopal Accident

In the early morning of December 3 1984 water inadvertently entered the methylisocyanate storage tank where gt40 metric tons of methylisocyanate were being stored The addition of water to the tank caused a runaway chemical reaction resulting in a rapid rise in pressure and temperature The heat generated by the reaction the presence of higher than normal concentrations of chloroform and the presence of an iron catalyst produced by the corrosion of the stainless steel tank wall resulted in a reaction of such momentum that gases formed could not be contained by safety systemsConsequently methylisocyanate and other reaction products in liquid and vapor form escaped from the plant into the surrounding areas There was no warning for people surrounding the plant since the emergency sirens had been switched off The effect on the people living in the shanty settlements just over the fence was immediate and devastating Many died in their beds others staggered from their homes blinded and choking to die in the street It has been estimated that at least 3000 people died as a result of this accident while figures for the number of people injured currently range from 200000 to 600000 with an estimated 500000 typically quoted

30

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

Carlsbad Pipeline Explosion

At 526 am on August 19 2000 a 75-cm diameter natural gas transmission pipeline operated by El Paso Natural Gas Company (EPNG) ruptured adjacent to the Pecos River near Carlsbad New Mexico The released gas ignited and burned for 55 min Twelve persons who were camping under a concrete-decked steel bridge that supported the pipeline across the river were killed and their three vehicles destroyed Two nearby steel suspension bridges for gas pipelines crossing the river were extensively damaged with $1 million in property and other damages or losses The force of the rupture and the violent ignition of the escaping gas created a 16-m wide crater 34 m along the pipe A 15-m section of the pipe was ejected from the crater in three pieces measuring sim1 6 and 8 m in length The largest piece was found 90 m northwest of the crater in the direction of the suspension bridges Investigators visually examined the pipeline that remained in the crater as well as the three ejected pieces All three ejected pieces showed evidence of internal corrosion damage but one of the pieces showed significantly more corrosion damage than the other two Pits were visible on the inside surface of this piece and at various locations the pipe wall evidenced significant thinningInterconnecting pits were observed on the inside of the pipe in the ruptured area Typically these pits showed the striations and undercutting features that are often associated with microbial corrosion A pit profile showed that chloride concentration in the pits increased steadily from top to bottom Increased chloride concentration can result from certain types of microbial activity All four types of microbes (sulfate reducing acid-producing general aerobic and anaerobic) were observed in samples collected from two pit areas in the piece of line where internal corrosion was discovered after the accident

35COROSSION PROTECTION

31

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

Barrier Protection

1048707 Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate

Cathodic Protection

1048707 Employs protecting one metal by connecting it to another metal that is more anodic according to the galvanic series

Corrosion Resistant Materials

1048707 Materials inherently resistant to corrosion in certain

32

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

CHAPTER ndash 4

SENSORS

33

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

41THE PRESSURE SENSOR

The piezoresistive pressure sensor or silicon cell ndash

This type of pressure sensor consists of a micro-machined silicon diaphragm with piezoresistive strain gauges diffused into it fused to a silicon or glass backplate

The resistors have a value of approx 35 kOhm Pressure induced strain increases the value of the radial resistors (r) and decreases the value of the resistors (t) transverse to the radius This resistance change can be high as 30

The resistors are connected as a Wheatstone Bridge the output of which is directly proportional to the pressure

Leadouts from the bridge

34

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

1) Gold or aluminium wires are welded to the aluminium contacts on the chip and to the glass feed-through pins of the header

2) TAB (Tape Automated Bonding) The contacts on the chip have a gold dot

A pretinned felxible printed circuit is directly soldered to these gold dots and the other end to a PC-board or the header

In the first method the sensor must be fixed on the header The TAB printed circuit however holds the sensor in place itself

Fig Pressure sensor

42Corrosion sensor

In corrosion sensor to detect the corrosion The Electric field signature method(EFSM) is used This is a method developed by CorrOcean ASA from the commonly used principle of electrical resistance (ER) determination applied for corrosion monitoring of steel pipe

A current is impressed through the object and the potential drop between several electrodes fixed directly to the outer surface eg on a spool piece in a pipe system is measured Changes in the geometry in the form of cracks general corrosion erosion corrosion or pitting will impair the potential field in the metal These measurements are compared to previous measurements and the development of corrosion or cracks can be recorded A computer usually treats the results before they are presented

35

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

Fig Of corrosion sensor

RELAY

A relay is an electrically operated switch Many relays use an electromagnet to operate a switching mechanism mechanically but other operating principles are also used Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits) or where several circuits must be controlled by one signal The first relays were used in long distance telegraph circuits repeating the signal coming in from one circuit and re-transmitting it to another Relays were used extensively in telephone exchanges and early computers to perform logical operations

A type of relay that can handle the high power required to directly drive an electric motor is called a contactor Solid-state relays control power circuits with no moving parts instead using a semiconductor device to perform switching Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays

CONSTRUCTION

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core an iron yoke which provides a low reluctance path for magnetic flux a movable iron armature and one or more sets of contacts (there are two in the relay pictured) The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts It is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit In this condition one of the two sets of contacts in the relay pictured is closed and the other set is open Other relays may have more or fewer sets of contacts depending on their function The relay in the picture also has a wire connecting the armature to the yoke This ensures continuity of the circuit between the moving contacts on the armature and the circuit track on the printed circuit board (PCB) via the yoke which is soldered to the PCB

36

WORKING

When an electric current is passed through the coil it generates a magnetic field that attracts the armature and the consequent movement of the movable contact(s) either makes or breaks (depending upon construction) a connection with a fixed contact If the set of contacts was closed when the relay was de-energized then the movement opens the contacts and breaks the connection and vice versa if the contacts were open When the current to the coil is switched off the armature is returned by a force approximately half as strong as the magnetic force to its relaxed position Usually this force is provided by a spring but gravity is also used commonly in industrial motor starters Most relays are manufactured to operate quickly In a low-voltage application this reduces noise in a high voltage or current application it reduces arcing

When the coil is energized with direct current a diode is often placed across the coil to dissipate the energy from the collapsing magnetic field at deactivation which would otherwise generate a voltage spike dangerous to semiconductor circuit components Some automotive relays include a diode inside the relay case Alternatively a contact protection network consisting of a capacitor and resistor in series (snubber circuit) may absorb the surge If the coil is designed to be energized with alternating current (AC) a small copper shading ring can be crimped to the end of the solenoid creating a small out-of-phase current which increases the minimum pull on the armature during the AC cycle A solid-state relay uses a thyristor or other solid-state switching device activated by the control signal to switch the controlled load instead of a solenoid An optocoupler (a light-emitting diode (LED) coupled with a photo transistor) can be used to isolate control and controlled circuits

CIRCUIT DIAGRAM

37

CHAPTER ndash 5

WIRELESS NETWORK

38

51 ENCODER(HT12E)

Features

1 Operating voltage24V~12V for the HT12E

2 Low power and high noise immunity CMOS technology

3 Minimum transmission wordrsquos of 4 words for the HT12E

4Built-in oscillator needs only 5 resistor

5Data code has positive polarity

6 Minimal external component of HT12E 18-pin DIP20-pin SOP package

Description

1The 2^12 encoders are a series of CMOS LSIs for remote control system

applications

2They are capable of encoding information which consists of N address bits and

12N

data bits

3 Each addressdata input can be set to one of the two logic states

4The programmed addressesdata are transmitted together with the header bits via

an

RF transmission medium

5Transmission is enabled by applying a low signal to the TE pin

52 RF TRANSMITTER (TWS-434A)

39

1The transmitter output is up to 8mW at 43392MHz with a range of

approximately few meters

2 It accepts both linear and digital inputs

3It can operate from 15 to 12 Volts-DC

4It is approximately the size of a standard postage stamp

Figof RF Transmitter

Connection of ENCODER amp RF TRANSMITTER

53 RF RECEIVER(RWS-434)

1 It also operates at 43392MHz and has a sensitivity of 3uV

2 It operates from 45 to 55 volts-DC and It has both linear and digital outputs

40

Fig Of HT12D Decoder

54DECODER(HT12D)

Features

1Operating voltage 24V~12V

2 Low power and high noise immunity

3 CMOS technology

4 Low standby current

5 Capable of decoding 12 bits of information

6 Binary address setting

7 Received codes are checked 3 times

8 AddressData number combination

9 8 address bits and 4 data bits

10 Built-in oscillator needs only 5 resistor

11 Valid transmission indicator

12Easy interface with an RF transmission medium

13 Minimal external components

14 Pair with Holteks 212 series of encoders18-pin DIP 20-pin SOP package

41

Fig Of HT12D Decoder

Description

1 2^12 decoders are a series of CMOS LSIs for remote control system applications

2 The decoders receive serial addresses and data from a programmed 2^12 series of

encoders that are transmitted by a carrier using an RF transmission medium

3 They compare the serial input data three times continuously with their local

addresses 4 If no error or unmatched codes are found the input data codes are

decoded and then transferred to the output pins

5 The VT pin also goes high to indicate a valid transmission

6The 2^12 series of decoders are capable of decoding informations that consist of N

bits of address and 12-N bits of data

Connection of RF receiver amp Decoder

42

43

CHAPTER ndash 6

POWER SUPPLY

44

61RECTIFIER

1A rectifier is a device which offers a low resistance to the current in one direction

and a high resistance in the opposite direction

2Such a device is capable of converting AC voltage into a pulsating DC voltage

3The rectifier employs one or more diodes It may be either a vacuum diode or a

semiconductor diode

4There are three types

1 Half wave rectifier

2 Full wave rectifier

3 Bridge rectifier

Bridge rectifier

1Bridge rectifier is a full wave rectifier It consists of four diodes arranged in the

form

of a bridge

2 It utilizes the advantages of the full wave rectifier and at the same time it

eliminates

the need for a centre tapped transformer

3The supply input and the rectified output are the two diagonally opposite

terminals of the bridge

4During the positive half cycle the secondary terminal A is positive with respect to

terminal B

5 Now the diodes D1 and D3 are forward biased and hence do not conduct

6The current flows from terminal A to terminal B through D1 load resistance RL and

the diode D3 and then through the secondary of the transformer

7During the negative half cycle terminal B is positive with respect to point A

8 Now diodes D2 and D4 are forward biased and hence conduct

9Diode D1 and D3 are reversed biased and hence do not conduct

10The current flows from terminal B to terminal A through diode D2 the load

resistance

RL and diode D4 and then through the secondary of the transformer

11On both positive and negative half cycles of the AC input the current flows

through

45

the load resistance RL in the same direction

12The polarity of the voltage developed across RL is such that the end connected to

the

junction of the diodes D1 and D2 will be positive

Fig Of Bridge rectifier

62FILTER

1Output from the rectifier unit having harmonic contents so we can provided the

filter circuit filter circuit is used to reduce the harmonics

2Here we can use the electrolytic capacitor

3This eliminates the harmonics from both voltage and current signals

Fig of filter

63VOLTAGE REGULATOR

1 Voltage regulator is used to maintain the constant voltage with the variation of the

supply voltage and the load current

2 When specifying individual ICs within this family the xx is replaced with a two-

digit number which indicates the output voltage the particular device is designed to

provide (for example the 7805 has a 5 volt output while the 7812 produces 12 volts)

3 The 78xx line are positive voltage regulators meaning that they are designed to

produce a voltage that is positive relative to a common ground

46

Fig Of voltage regulator

Circuit diagram of Power supply of RF recevier

-+

D 1

W 01G

2

1

3

4

TX1

V 1

C 1470uF

U 1

7805

1 3V IN VO U T

C 147uF

C 10 1uF

R 1

330

D 2

LN 211W P

12

J P 1

12

12

2309V

230V

Operation

1 Initially 230 V AC supply is reduced to (0-9V) with the help of a step down

transformer having a capacity of 500mA

2 Since the input voltage to the regulator IC should be more than its output voltage

transformer secondary voltage is 9V

47

3 This low voltage is rectified with the help of bridge rectifier The ripples are

minimized with the help of capacitor filter to get a smooth DC supply The rating of

the chosen capacitor filter is 1000microF

4 The regulated DC voltage is obtained by using a regulator IC 7805 In the case of IC

7805 the unregulated DC voltage is applied to Pin 1 and the output is taken at Pin 3

and Pin 2 is grounded

5 Another capacitor filter of rating 10microF is connected at the output of regulator IC to

eliminate the voltage oscillations at the output due to the large voltage oscillations at

the input of the regulator

48

CHAPTER ndash 7

CONTROL ACTION

49

71 BUZZER

A buzzer or beeper is an audio signaling device Typical uses of buzzers and beepers include

alarms timers and confirmation of user input such as a mouse click or keystroke

TYPES OF BUZZER

1 Mechanical

2 Electromechanical

3 Piezoelectric

1 Mechanical

A joy buzzer is an example of a purely mechanical buzzer

2 Electromechanical

Early devices were based on an electromechanical system identical to an electric bell

without the metal gong Similarly a relay may be connected to interrupt its own actuating

current causing the contacts to buzz Often these units were anchored to a wall or ceiling to

use it as a sounding board The word buzzer comes from the rasping noise that

electromechanical buzzers made

3 piezoelectric

A piezoelectric element may be driven by an oscillating electronic circuit or other

audio signal source driven with a piezoelectric audio amplifier Sounds commonly used to

indicate that a button has been pressed are a click a ring or a beep

Fig Of buzzers

50

51 ENCODER(HT12E)

Features

1 Operating voltage24V~12V for the HT12E

2 Low power and high noise immunity CMOS technology

3 Minimum transmission wordrsquos of 4 words for the HT12E

4Built-in oscillator needs only 5 resistor

5Data code has positive polarity

6 Minimal external component of HT12E 18-pin DIP20-pin SOP package

Description

1The 2^12 encoders are a series of CMOS LSIs for remote control system

applications

2They are capable of encoding information which consists of N address bits and

12N

data bits

3 Each addressdata input can be set to one of the two logic states

4The programmed addressesdata are transmitted together with the header bits via

an

RF transmission medium

5Transmission is enabled by applying a low signal to the TE pin

52 RF TRANSMITTER (TWS-434A)

39

1The transmitter output is up to 8mW at 43392MHz with a range of

approximately few meters

2 It accepts both linear and digital inputs

3It can operate from 15 to 12 Volts-DC

4It is approximately the size of a standard postage stamp

Figof RF Transmitter

Connection of ENCODER amp RF TRANSMITTER

53 RF RECEIVER(RWS-434)

1 It also operates at 43392MHz and has a sensitivity of 3uV

2 It operates from 45 to 55 volts-DC and It has both linear and digital outputs

40

Fig Of HT12D Decoder

54DECODER(HT12D)

Features

1Operating voltage 24V~12V

2 Low power and high noise immunity

3 CMOS technology

4 Low standby current

5 Capable of decoding 12 bits of information

6 Binary address setting

7 Received codes are checked 3 times

8 AddressData number combination

9 8 address bits and 4 data bits

10 Built-in oscillator needs only 5 resistor

11 Valid transmission indicator

12Easy interface with an RF transmission medium

13 Minimal external components

14 Pair with Holteks 212 series of encoders18-pin DIP 20-pin SOP package

41

Fig Of HT12D Decoder

Description

1 2^12 decoders are a series of CMOS LSIs for remote control system applications

2 The decoders receive serial addresses and data from a programmed 2^12 series of

encoders that are transmitted by a carrier using an RF transmission medium

3 They compare the serial input data three times continuously with their local

addresses 4 If no error or unmatched codes are found the input data codes are

decoded and then transferred to the output pins

5 The VT pin also goes high to indicate a valid transmission

6The 2^12 series of decoders are capable of decoding informations that consist of N

bits of address and 12-N bits of data

Connection of RF receiver amp Decoder

42

43

CHAPTER ndash 6

POWER SUPPLY

44

61RECTIFIER

1A rectifier is a device which offers a low resistance to the current in one direction

and a high resistance in the opposite direction

2Such a device is capable of converting AC voltage into a pulsating DC voltage

3The rectifier employs one or more diodes It may be either a vacuum diode or a

semiconductor diode

4There are three types

1 Half wave rectifier

2 Full wave rectifier

3 Bridge rectifier

Bridge rectifier

1Bridge rectifier is a full wave rectifier It consists of four diodes arranged in the

form

of a bridge

2 It utilizes the advantages of the full wave rectifier and at the same time it

eliminates

the need for a centre tapped transformer

3The supply input and the rectified output are the two diagonally opposite

terminals of the bridge

4During the positive half cycle the secondary terminal A is positive with respect to

terminal B

5 Now the diodes D1 and D3 are forward biased and hence do not conduct

6The current flows from terminal A to terminal B through D1 load resistance RL and

the diode D3 and then through the secondary of the transformer

7During the negative half cycle terminal B is positive with respect to point A

8 Now diodes D2 and D4 are forward biased and hence conduct

9Diode D1 and D3 are reversed biased and hence do not conduct

10The current flows from terminal B to terminal A through diode D2 the load

resistance

RL and diode D4 and then through the secondary of the transformer

11On both positive and negative half cycles of the AC input the current flows

through

45

the load resistance RL in the same direction

12The polarity of the voltage developed across RL is such that the end connected to

the

junction of the diodes D1 and D2 will be positive

Fig Of Bridge rectifier

62FILTER

1Output from the rectifier unit having harmonic contents so we can provided the

filter circuit filter circuit is used to reduce the harmonics

2Here we can use the electrolytic capacitor

3This eliminates the harmonics from both voltage and current signals

Fig of filter

63VOLTAGE REGULATOR

1 Voltage regulator is used to maintain the constant voltage with the variation of the

supply voltage and the load current

2 When specifying individual ICs within this family the xx is replaced with a two-

digit number which indicates the output voltage the particular device is designed to

provide (for example the 7805 has a 5 volt output while the 7812 produces 12 volts)

3 The 78xx line are positive voltage regulators meaning that they are designed to

produce a voltage that is positive relative to a common ground

46

Fig Of voltage regulator

Circuit diagram of Power supply of RF recevier

-+

D 1

W 01G

2

1

3

4

TX1

V 1

C 1470uF

U 1

7805

1 3V IN VO U T

C 147uF

C 10 1uF

R 1

330

D 2

LN 211W P

12

J P 1

12

12

2309V

230V

Operation

1 Initially 230 V AC supply is reduced to (0-9V) with the help of a step down

transformer having a capacity of 500mA

2 Since the input voltage to the regulator IC should be more than its output voltage

transformer secondary voltage is 9V

47

3 This low voltage is rectified with the help of bridge rectifier The ripples are

minimized with the help of capacitor filter to get a smooth DC supply The rating of

the chosen capacitor filter is 1000microF

4 The regulated DC voltage is obtained by using a regulator IC 7805 In the case of IC

7805 the unregulated DC voltage is applied to Pin 1 and the output is taken at Pin 3

and Pin 2 is grounded

5 Another capacitor filter of rating 10microF is connected at the output of regulator IC to

eliminate the voltage oscillations at the output due to the large voltage oscillations at

the input of the regulator

48

CHAPTER ndash 7

CONTROL ACTION

49

71 BUZZER

A buzzer or beeper is an audio signaling device Typical uses of buzzers and beepers include

alarms timers and confirmation of user input such as a mouse click or keystroke

TYPES OF BUZZER

1 Mechanical

2 Electromechanical

3 Piezoelectric

1 Mechanical

A joy buzzer is an example of a purely mechanical buzzer

2 Electromechanical

Early devices were based on an electromechanical system identical to an electric bell

without the metal gong Similarly a relay may be connected to interrupt its own actuating

current causing the contacts to buzz Often these units were anchored to a wall or ceiling to

use it as a sounding board The word buzzer comes from the rasping noise that

electromechanical buzzers made

3 piezoelectric

A piezoelectric element may be driven by an oscillating electronic circuit or other

audio signal source driven with a piezoelectric audio amplifier Sounds commonly used to

indicate that a button has been pressed are a click a ring or a beep

Fig Of buzzers

50

1The transmitter output is up to 8mW at 43392MHz with a range of

approximately few meters

2 It accepts both linear and digital inputs

3It can operate from 15 to 12 Volts-DC

4It is approximately the size of a standard postage stamp

Figof RF Transmitter

Connection of ENCODER amp RF TRANSMITTER

53 RF RECEIVER(RWS-434)

1 It also operates at 43392MHz and has a sensitivity of 3uV

2 It operates from 45 to 55 volts-DC and It has both linear and digital outputs

40

Fig Of HT12D Decoder

54DECODER(HT12D)

Features

1Operating voltage 24V~12V

2 Low power and high noise immunity

3 CMOS technology

4 Low standby current

5 Capable of decoding 12 bits of information

6 Binary address setting

7 Received codes are checked 3 times

8 AddressData number combination

9 8 address bits and 4 data bits

10 Built-in oscillator needs only 5 resistor

11 Valid transmission indicator

12Easy interface with an RF transmission medium

13 Minimal external components

14 Pair with Holteks 212 series of encoders18-pin DIP 20-pin SOP package

41

Fig Of HT12D Decoder

Description

1 2^12 decoders are a series of CMOS LSIs for remote control system applications

2 The decoders receive serial addresses and data from a programmed 2^12 series of

encoders that are transmitted by a carrier using an RF transmission medium

3 They compare the serial input data three times continuously with their local

addresses 4 If no error or unmatched codes are found the input data codes are

decoded and then transferred to the output pins

5 The VT pin also goes high to indicate a valid transmission

6The 2^12 series of decoders are capable of decoding informations that consist of N

bits of address and 12-N bits of data

Connection of RF receiver amp Decoder

42

43

CHAPTER ndash 6

POWER SUPPLY

44

61RECTIFIER

1A rectifier is a device which offers a low resistance to the current in one direction

and a high resistance in the opposite direction

2Such a device is capable of converting AC voltage into a pulsating DC voltage

3The rectifier employs one or more diodes It may be either a vacuum diode or a

semiconductor diode

4There are three types

1 Half wave rectifier

2 Full wave rectifier

3 Bridge rectifier

Bridge rectifier

1Bridge rectifier is a full wave rectifier It consists of four diodes arranged in the

form

of a bridge

2 It utilizes the advantages of the full wave rectifier and at the same time it

eliminates

the need for a centre tapped transformer

3The supply input and the rectified output are the two diagonally opposite

terminals of the bridge

4During the positive half cycle the secondary terminal A is positive with respect to

terminal B

5 Now the diodes D1 and D3 are forward biased and hence do not conduct

6The current flows from terminal A to terminal B through D1 load resistance RL and

the diode D3 and then through the secondary of the transformer

7During the negative half cycle terminal B is positive with respect to point A

8 Now diodes D2 and D4 are forward biased and hence conduct

9Diode D1 and D3 are reversed biased and hence do not conduct

10The current flows from terminal B to terminal A through diode D2 the load

resistance

RL and diode D4 and then through the secondary of the transformer

11On both positive and negative half cycles of the AC input the current flows

through

45

the load resistance RL in the same direction

12The polarity of the voltage developed across RL is such that the end connected to

the

junction of the diodes D1 and D2 will be positive

Fig Of Bridge rectifier

62FILTER

1Output from the rectifier unit having harmonic contents so we can provided the

filter circuit filter circuit is used to reduce the harmonics

2Here we can use the electrolytic capacitor

3This eliminates the harmonics from both voltage and current signals

Fig of filter

63VOLTAGE REGULATOR

1 Voltage regulator is used to maintain the constant voltage with the variation of the

supply voltage and the load current

2 When specifying individual ICs within this family the xx is replaced with a two-

digit number which indicates the output voltage the particular device is designed to

provide (for example the 7805 has a 5 volt output while the 7812 produces 12 volts)

3 The 78xx line are positive voltage regulators meaning that they are designed to

produce a voltage that is positive relative to a common ground

46

Fig Of voltage regulator

Circuit diagram of Power supply of RF recevier

-+

D 1

W 01G

2

1

3

4

TX1

V 1

C 1470uF

U 1

7805

1 3V IN VO U T

C 147uF

C 10 1uF

R 1

330

D 2

LN 211W P

12

J P 1

12

12

2309V

230V

Operation

1 Initially 230 V AC supply is reduced to (0-9V) with the help of a step down

transformer having a capacity of 500mA

2 Since the input voltage to the regulator IC should be more than its output voltage

transformer secondary voltage is 9V

47

3 This low voltage is rectified with the help of bridge rectifier The ripples are

minimized with the help of capacitor filter to get a smooth DC supply The rating of

the chosen capacitor filter is 1000microF

4 The regulated DC voltage is obtained by using a regulator IC 7805 In the case of IC

7805 the unregulated DC voltage is applied to Pin 1 and the output is taken at Pin 3

and Pin 2 is grounded

5 Another capacitor filter of rating 10microF is connected at the output of regulator IC to

eliminate the voltage oscillations at the output due to the large voltage oscillations at

the input of the regulator

48

CHAPTER ndash 7

CONTROL ACTION

49

71 BUZZER

A buzzer or beeper is an audio signaling device Typical uses of buzzers and beepers include

alarms timers and confirmation of user input such as a mouse click or keystroke

TYPES OF BUZZER

1 Mechanical

2 Electromechanical

3 Piezoelectric

1 Mechanical

A joy buzzer is an example of a purely mechanical buzzer

2 Electromechanical

Early devices were based on an electromechanical system identical to an electric bell

without the metal gong Similarly a relay may be connected to interrupt its own actuating

current causing the contacts to buzz Often these units were anchored to a wall or ceiling to

use it as a sounding board The word buzzer comes from the rasping noise that

electromechanical buzzers made

3 piezoelectric

A piezoelectric element may be driven by an oscillating electronic circuit or other

audio signal source driven with a piezoelectric audio amplifier Sounds commonly used to

indicate that a button has been pressed are a click a ring or a beep

Fig Of buzzers

50

Fig Of HT12D Decoder

54DECODER(HT12D)

Features

1Operating voltage 24V~12V

2 Low power and high noise immunity

3 CMOS technology

4 Low standby current

5 Capable of decoding 12 bits of information

6 Binary address setting

7 Received codes are checked 3 times

8 AddressData number combination

9 8 address bits and 4 data bits

10 Built-in oscillator needs only 5 resistor

11 Valid transmission indicator

12Easy interface with an RF transmission medium

13 Minimal external components

14 Pair with Holteks 212 series of encoders18-pin DIP 20-pin SOP package

41

Fig Of HT12D Decoder

Description

1 2^12 decoders are a series of CMOS LSIs for remote control system applications

2 The decoders receive serial addresses and data from a programmed 2^12 series of

encoders that are transmitted by a carrier using an RF transmission medium

3 They compare the serial input data three times continuously with their local

addresses 4 If no error or unmatched codes are found the input data codes are

decoded and then transferred to the output pins

5 The VT pin also goes high to indicate a valid transmission

6The 2^12 series of decoders are capable of decoding informations that consist of N

bits of address and 12-N bits of data

Connection of RF receiver amp Decoder

42

43

CHAPTER ndash 6

POWER SUPPLY

44

61RECTIFIER

1A rectifier is a device which offers a low resistance to the current in one direction

and a high resistance in the opposite direction

2Such a device is capable of converting AC voltage into a pulsating DC voltage

3The rectifier employs one or more diodes It may be either a vacuum diode or a

semiconductor diode

4There are three types

1 Half wave rectifier

2 Full wave rectifier

3 Bridge rectifier

Bridge rectifier

1Bridge rectifier is a full wave rectifier It consists of four diodes arranged in the

form

of a bridge

2 It utilizes the advantages of the full wave rectifier and at the same time it

eliminates

the need for a centre tapped transformer

3The supply input and the rectified output are the two diagonally opposite

terminals of the bridge

4During the positive half cycle the secondary terminal A is positive with respect to

terminal B

5 Now the diodes D1 and D3 are forward biased and hence do not conduct

6The current flows from terminal A to terminal B through D1 load resistance RL and

the diode D3 and then through the secondary of the transformer

7During the negative half cycle terminal B is positive with respect to point A

8 Now diodes D2 and D4 are forward biased and hence conduct

9Diode D1 and D3 are reversed biased and hence do not conduct

10The current flows from terminal B to terminal A through diode D2 the load

resistance

RL and diode D4 and then through the secondary of the transformer

11On both positive and negative half cycles of the AC input the current flows

through

45

the load resistance RL in the same direction

12The polarity of the voltage developed across RL is such that the end connected to

the

junction of the diodes D1 and D2 will be positive

Fig Of Bridge rectifier

62FILTER

1Output from the rectifier unit having harmonic contents so we can provided the

filter circuit filter circuit is used to reduce the harmonics

2Here we can use the electrolytic capacitor

3This eliminates the harmonics from both voltage and current signals

Fig of filter

63VOLTAGE REGULATOR

1 Voltage regulator is used to maintain the constant voltage with the variation of the

supply voltage and the load current

2 When specifying individual ICs within this family the xx is replaced with a two-

digit number which indicates the output voltage the particular device is designed to

provide (for example the 7805 has a 5 volt output while the 7812 produces 12 volts)

3 The 78xx line are positive voltage regulators meaning that they are designed to

produce a voltage that is positive relative to a common ground

46

Fig Of voltage regulator

Circuit diagram of Power supply of RF recevier

-+

D 1

W 01G

2

1

3

4

TX1

V 1

C 1470uF

U 1

7805

1 3V IN VO U T

C 147uF

C 10 1uF

R 1

330

D 2

LN 211W P

12

J P 1

12

12

2309V

230V

Operation

1 Initially 230 V AC supply is reduced to (0-9V) with the help of a step down

transformer having a capacity of 500mA

2 Since the input voltage to the regulator IC should be more than its output voltage

transformer secondary voltage is 9V

47

3 This low voltage is rectified with the help of bridge rectifier The ripples are

minimized with the help of capacitor filter to get a smooth DC supply The rating of

the chosen capacitor filter is 1000microF

4 The regulated DC voltage is obtained by using a regulator IC 7805 In the case of IC

7805 the unregulated DC voltage is applied to Pin 1 and the output is taken at Pin 3

and Pin 2 is grounded

5 Another capacitor filter of rating 10microF is connected at the output of regulator IC to

eliminate the voltage oscillations at the output due to the large voltage oscillations at

the input of the regulator

48

CHAPTER ndash 7

CONTROL ACTION

49

71 BUZZER

A buzzer or beeper is an audio signaling device Typical uses of buzzers and beepers include

alarms timers and confirmation of user input such as a mouse click or keystroke

TYPES OF BUZZER

1 Mechanical

2 Electromechanical

3 Piezoelectric

1 Mechanical

A joy buzzer is an example of a purely mechanical buzzer

2 Electromechanical

Early devices were based on an electromechanical system identical to an electric bell

without the metal gong Similarly a relay may be connected to interrupt its own actuating

current causing the contacts to buzz Often these units were anchored to a wall or ceiling to

use it as a sounding board The word buzzer comes from the rasping noise that

electromechanical buzzers made

3 piezoelectric

A piezoelectric element may be driven by an oscillating electronic circuit or other

audio signal source driven with a piezoelectric audio amplifier Sounds commonly used to

indicate that a button has been pressed are a click a ring or a beep

Fig Of buzzers

50

Fig Of HT12D Decoder

Description

1 2^12 decoders are a series of CMOS LSIs for remote control system applications

2 The decoders receive serial addresses and data from a programmed 2^12 series of

encoders that are transmitted by a carrier using an RF transmission medium

3 They compare the serial input data three times continuously with their local

addresses 4 If no error or unmatched codes are found the input data codes are

decoded and then transferred to the output pins

5 The VT pin also goes high to indicate a valid transmission

6The 2^12 series of decoders are capable of decoding informations that consist of N

bits of address and 12-N bits of data

Connection of RF receiver amp Decoder

42

43

CHAPTER ndash 6

POWER SUPPLY

44

61RECTIFIER

1A rectifier is a device which offers a low resistance to the current in one direction

and a high resistance in the opposite direction

2Such a device is capable of converting AC voltage into a pulsating DC voltage

3The rectifier employs one or more diodes It may be either a vacuum diode or a

semiconductor diode

4There are three types

1 Half wave rectifier

2 Full wave rectifier

3 Bridge rectifier

Bridge rectifier

1Bridge rectifier is a full wave rectifier It consists of four diodes arranged in the

form

of a bridge

2 It utilizes the advantages of the full wave rectifier and at the same time it

eliminates

the need for a centre tapped transformer

3The supply input and the rectified output are the two diagonally opposite

terminals of the bridge

4During the positive half cycle the secondary terminal A is positive with respect to

terminal B

5 Now the diodes D1 and D3 are forward biased and hence do not conduct

6The current flows from terminal A to terminal B through D1 load resistance RL and

the diode D3 and then through the secondary of the transformer

7During the negative half cycle terminal B is positive with respect to point A

8 Now diodes D2 and D4 are forward biased and hence conduct

9Diode D1 and D3 are reversed biased and hence do not conduct

10The current flows from terminal B to terminal A through diode D2 the load

resistance

RL and diode D4 and then through the secondary of the transformer

11On both positive and negative half cycles of the AC input the current flows

through

45

the load resistance RL in the same direction

12The polarity of the voltage developed across RL is such that the end connected to

the

junction of the diodes D1 and D2 will be positive

Fig Of Bridge rectifier

62FILTER

1Output from the rectifier unit having harmonic contents so we can provided the

filter circuit filter circuit is used to reduce the harmonics

2Here we can use the electrolytic capacitor

3This eliminates the harmonics from both voltage and current signals

Fig of filter

63VOLTAGE REGULATOR

1 Voltage regulator is used to maintain the constant voltage with the variation of the

supply voltage and the load current

2 When specifying individual ICs within this family the xx is replaced with a two-

digit number which indicates the output voltage the particular device is designed to

provide (for example the 7805 has a 5 volt output while the 7812 produces 12 volts)

3 The 78xx line are positive voltage regulators meaning that they are designed to

produce a voltage that is positive relative to a common ground

46

Fig Of voltage regulator

Circuit diagram of Power supply of RF recevier

-+

D 1

W 01G

2

1

3

4

TX1

V 1

C 1470uF

U 1

7805

1 3V IN VO U T

C 147uF

C 10 1uF

R 1

330

D 2

LN 211W P

12

J P 1

12

12

2309V

230V

Operation

1 Initially 230 V AC supply is reduced to (0-9V) with the help of a step down

transformer having a capacity of 500mA

2 Since the input voltage to the regulator IC should be more than its output voltage

transformer secondary voltage is 9V

47

3 This low voltage is rectified with the help of bridge rectifier The ripples are

minimized with the help of capacitor filter to get a smooth DC supply The rating of

the chosen capacitor filter is 1000microF

4 The regulated DC voltage is obtained by using a regulator IC 7805 In the case of IC

7805 the unregulated DC voltage is applied to Pin 1 and the output is taken at Pin 3

and Pin 2 is grounded

5 Another capacitor filter of rating 10microF is connected at the output of regulator IC to

eliminate the voltage oscillations at the output due to the large voltage oscillations at

the input of the regulator

48

CHAPTER ndash 7

CONTROL ACTION

49

71 BUZZER

A buzzer or beeper is an audio signaling device Typical uses of buzzers and beepers include

alarms timers and confirmation of user input such as a mouse click or keystroke

TYPES OF BUZZER

1 Mechanical

2 Electromechanical

3 Piezoelectric

1 Mechanical

A joy buzzer is an example of a purely mechanical buzzer

2 Electromechanical

Early devices were based on an electromechanical system identical to an electric bell

without the metal gong Similarly a relay may be connected to interrupt its own actuating

current causing the contacts to buzz Often these units were anchored to a wall or ceiling to

use it as a sounding board The word buzzer comes from the rasping noise that

electromechanical buzzers made

3 piezoelectric

A piezoelectric element may be driven by an oscillating electronic circuit or other

audio signal source driven with a piezoelectric audio amplifier Sounds commonly used to

indicate that a button has been pressed are a click a ring or a beep

Fig Of buzzers

50

43

CHAPTER ndash 6

POWER SUPPLY

44

61RECTIFIER

1A rectifier is a device which offers a low resistance to the current in one direction

and a high resistance in the opposite direction

2Such a device is capable of converting AC voltage into a pulsating DC voltage

3The rectifier employs one or more diodes It may be either a vacuum diode or a

semiconductor diode

4There are three types

1 Half wave rectifier

2 Full wave rectifier

3 Bridge rectifier

Bridge rectifier

1Bridge rectifier is a full wave rectifier It consists of four diodes arranged in the

form

of a bridge

2 It utilizes the advantages of the full wave rectifier and at the same time it

eliminates

the need for a centre tapped transformer

3The supply input and the rectified output are the two diagonally opposite

terminals of the bridge

4During the positive half cycle the secondary terminal A is positive with respect to

terminal B

5 Now the diodes D1 and D3 are forward biased and hence do not conduct

6The current flows from terminal A to terminal B through D1 load resistance RL and

the diode D3 and then through the secondary of the transformer

7During the negative half cycle terminal B is positive with respect to point A

8 Now diodes D2 and D4 are forward biased and hence conduct

9Diode D1 and D3 are reversed biased and hence do not conduct

10The current flows from terminal B to terminal A through diode D2 the load

resistance

RL and diode D4 and then through the secondary of the transformer

11On both positive and negative half cycles of the AC input the current flows

through

45

the load resistance RL in the same direction

12The polarity of the voltage developed across RL is such that the end connected to

the

junction of the diodes D1 and D2 will be positive

Fig Of Bridge rectifier

62FILTER

1Output from the rectifier unit having harmonic contents so we can provided the

filter circuit filter circuit is used to reduce the harmonics

2Here we can use the electrolytic capacitor

3This eliminates the harmonics from both voltage and current signals

Fig of filter

63VOLTAGE REGULATOR

1 Voltage regulator is used to maintain the constant voltage with the variation of the

supply voltage and the load current

2 When specifying individual ICs within this family the xx is replaced with a two-

digit number which indicates the output voltage the particular device is designed to

provide (for example the 7805 has a 5 volt output while the 7812 produces 12 volts)

3 The 78xx line are positive voltage regulators meaning that they are designed to

produce a voltage that is positive relative to a common ground

46

Fig Of voltage regulator

Circuit diagram of Power supply of RF recevier

-+

D 1

W 01G

2

1

3

4

TX1

V 1

C 1470uF

U 1

7805

1 3V IN VO U T

C 147uF

C 10 1uF

R 1

330

D 2

LN 211W P

12

J P 1

12

12

2309V

230V

Operation

1 Initially 230 V AC supply is reduced to (0-9V) with the help of a step down

transformer having a capacity of 500mA

2 Since the input voltage to the regulator IC should be more than its output voltage

transformer secondary voltage is 9V

47

3 This low voltage is rectified with the help of bridge rectifier The ripples are

minimized with the help of capacitor filter to get a smooth DC supply The rating of

the chosen capacitor filter is 1000microF

4 The regulated DC voltage is obtained by using a regulator IC 7805 In the case of IC

7805 the unregulated DC voltage is applied to Pin 1 and the output is taken at Pin 3

and Pin 2 is grounded

5 Another capacitor filter of rating 10microF is connected at the output of regulator IC to

eliminate the voltage oscillations at the output due to the large voltage oscillations at

the input of the regulator

48

CHAPTER ndash 7

CONTROL ACTION

49

71 BUZZER

A buzzer or beeper is an audio signaling device Typical uses of buzzers and beepers include

alarms timers and confirmation of user input such as a mouse click or keystroke

TYPES OF BUZZER

1 Mechanical

2 Electromechanical

3 Piezoelectric

1 Mechanical

A joy buzzer is an example of a purely mechanical buzzer

2 Electromechanical

Early devices were based on an electromechanical system identical to an electric bell

without the metal gong Similarly a relay may be connected to interrupt its own actuating

current causing the contacts to buzz Often these units were anchored to a wall or ceiling to

use it as a sounding board The word buzzer comes from the rasping noise that

electromechanical buzzers made

3 piezoelectric

A piezoelectric element may be driven by an oscillating electronic circuit or other

audio signal source driven with a piezoelectric audio amplifier Sounds commonly used to

indicate that a button has been pressed are a click a ring or a beep

Fig Of buzzers

50

CHAPTER ndash 6

POWER SUPPLY

44

61RECTIFIER

1A rectifier is a device which offers a low resistance to the current in one direction

and a high resistance in the opposite direction

2Such a device is capable of converting AC voltage into a pulsating DC voltage

3The rectifier employs one or more diodes It may be either a vacuum diode or a

semiconductor diode

4There are three types

1 Half wave rectifier

2 Full wave rectifier

3 Bridge rectifier

Bridge rectifier

1Bridge rectifier is a full wave rectifier It consists of four diodes arranged in the

form

of a bridge

2 It utilizes the advantages of the full wave rectifier and at the same time it

eliminates

the need for a centre tapped transformer

3The supply input and the rectified output are the two diagonally opposite

terminals of the bridge

4During the positive half cycle the secondary terminal A is positive with respect to

terminal B

5 Now the diodes D1 and D3 are forward biased and hence do not conduct

6The current flows from terminal A to terminal B through D1 load resistance RL and

the diode D3 and then through the secondary of the transformer

7During the negative half cycle terminal B is positive with respect to point A

8 Now diodes D2 and D4 are forward biased and hence conduct

9Diode D1 and D3 are reversed biased and hence do not conduct

10The current flows from terminal B to terminal A through diode D2 the load

resistance

RL and diode D4 and then through the secondary of the transformer

11On both positive and negative half cycles of the AC input the current flows

through

45

the load resistance RL in the same direction

12The polarity of the voltage developed across RL is such that the end connected to

the

junction of the diodes D1 and D2 will be positive

Fig Of Bridge rectifier

62FILTER

1Output from the rectifier unit having harmonic contents so we can provided the

filter circuit filter circuit is used to reduce the harmonics

2Here we can use the electrolytic capacitor

3This eliminates the harmonics from both voltage and current signals

Fig of filter

63VOLTAGE REGULATOR

1 Voltage regulator is used to maintain the constant voltage with the variation of the

supply voltage and the load current

2 When specifying individual ICs within this family the xx is replaced with a two-

digit number which indicates the output voltage the particular device is designed to

provide (for example the 7805 has a 5 volt output while the 7812 produces 12 volts)

3 The 78xx line are positive voltage regulators meaning that they are designed to

produce a voltage that is positive relative to a common ground

46

Fig Of voltage regulator

Circuit diagram of Power supply of RF recevier

-+

D 1

W 01G

2

1

3

4

TX1

V 1

C 1470uF

U 1

7805

1 3V IN VO U T

C 147uF

C 10 1uF

R 1

330

D 2

LN 211W P

12

J P 1

12

12

2309V

230V

Operation

1 Initially 230 V AC supply is reduced to (0-9V) with the help of a step down

transformer having a capacity of 500mA

2 Since the input voltage to the regulator IC should be more than its output voltage

transformer secondary voltage is 9V

47

3 This low voltage is rectified with the help of bridge rectifier The ripples are

minimized with the help of capacitor filter to get a smooth DC supply The rating of

the chosen capacitor filter is 1000microF

4 The regulated DC voltage is obtained by using a regulator IC 7805 In the case of IC

7805 the unregulated DC voltage is applied to Pin 1 and the output is taken at Pin 3

and Pin 2 is grounded

5 Another capacitor filter of rating 10microF is connected at the output of regulator IC to

eliminate the voltage oscillations at the output due to the large voltage oscillations at

the input of the regulator

48

CHAPTER ndash 7

CONTROL ACTION

49

71 BUZZER

A buzzer or beeper is an audio signaling device Typical uses of buzzers and beepers include

alarms timers and confirmation of user input such as a mouse click or keystroke

TYPES OF BUZZER

1 Mechanical

2 Electromechanical

3 Piezoelectric

1 Mechanical

A joy buzzer is an example of a purely mechanical buzzer

2 Electromechanical

Early devices were based on an electromechanical system identical to an electric bell

without the metal gong Similarly a relay may be connected to interrupt its own actuating

current causing the contacts to buzz Often these units were anchored to a wall or ceiling to

use it as a sounding board The word buzzer comes from the rasping noise that

electromechanical buzzers made

3 piezoelectric

A piezoelectric element may be driven by an oscillating electronic circuit or other

audio signal source driven with a piezoelectric audio amplifier Sounds commonly used to

indicate that a button has been pressed are a click a ring or a beep

Fig Of buzzers

50

61RECTIFIER

1A rectifier is a device which offers a low resistance to the current in one direction

and a high resistance in the opposite direction

2Such a device is capable of converting AC voltage into a pulsating DC voltage

3The rectifier employs one or more diodes It may be either a vacuum diode or a

semiconductor diode

4There are three types

1 Half wave rectifier

2 Full wave rectifier

3 Bridge rectifier

Bridge rectifier

1Bridge rectifier is a full wave rectifier It consists of four diodes arranged in the

form

of a bridge

2 It utilizes the advantages of the full wave rectifier and at the same time it

eliminates

the need for a centre tapped transformer

3The supply input and the rectified output are the two diagonally opposite

terminals of the bridge

4During the positive half cycle the secondary terminal A is positive with respect to

terminal B

5 Now the diodes D1 and D3 are forward biased and hence do not conduct

6The current flows from terminal A to terminal B through D1 load resistance RL and

the diode D3 and then through the secondary of the transformer

7During the negative half cycle terminal B is positive with respect to point A

8 Now diodes D2 and D4 are forward biased and hence conduct

9Diode D1 and D3 are reversed biased and hence do not conduct

10The current flows from terminal B to terminal A through diode D2 the load

resistance

RL and diode D4 and then through the secondary of the transformer

11On both positive and negative half cycles of the AC input the current flows

through

45

the load resistance RL in the same direction

12The polarity of the voltage developed across RL is such that the end connected to

the

junction of the diodes D1 and D2 will be positive

Fig Of Bridge rectifier

62FILTER

1Output from the rectifier unit having harmonic contents so we can provided the

filter circuit filter circuit is used to reduce the harmonics

2Here we can use the electrolytic capacitor

3This eliminates the harmonics from both voltage and current signals

Fig of filter

63VOLTAGE REGULATOR

1 Voltage regulator is used to maintain the constant voltage with the variation of the

supply voltage and the load current

2 When specifying individual ICs within this family the xx is replaced with a two-

digit number which indicates the output voltage the particular device is designed to

provide (for example the 7805 has a 5 volt output while the 7812 produces 12 volts)

3 The 78xx line are positive voltage regulators meaning that they are designed to

produce a voltage that is positive relative to a common ground

46

Fig Of voltage regulator

Circuit diagram of Power supply of RF recevier

-+

D 1

W 01G

2

1

3

4

TX1

V 1

C 1470uF

U 1

7805

1 3V IN VO U T

C 147uF

C 10 1uF

R 1

330

D 2

LN 211W P

12

J P 1

12

12

2309V

230V

Operation

1 Initially 230 V AC supply is reduced to (0-9V) with the help of a step down

transformer having a capacity of 500mA

2 Since the input voltage to the regulator IC should be more than its output voltage

transformer secondary voltage is 9V

47

3 This low voltage is rectified with the help of bridge rectifier The ripples are

minimized with the help of capacitor filter to get a smooth DC supply The rating of

the chosen capacitor filter is 1000microF

4 The regulated DC voltage is obtained by using a regulator IC 7805 In the case of IC

7805 the unregulated DC voltage is applied to Pin 1 and the output is taken at Pin 3

and Pin 2 is grounded

5 Another capacitor filter of rating 10microF is connected at the output of regulator IC to

eliminate the voltage oscillations at the output due to the large voltage oscillations at

the input of the regulator

48

CHAPTER ndash 7

CONTROL ACTION

49

71 BUZZER

A buzzer or beeper is an audio signaling device Typical uses of buzzers and beepers include

alarms timers and confirmation of user input such as a mouse click or keystroke

TYPES OF BUZZER

1 Mechanical

2 Electromechanical

3 Piezoelectric

1 Mechanical

A joy buzzer is an example of a purely mechanical buzzer

2 Electromechanical

Early devices were based on an electromechanical system identical to an electric bell

without the metal gong Similarly a relay may be connected to interrupt its own actuating

current causing the contacts to buzz Often these units were anchored to a wall or ceiling to

use it as a sounding board The word buzzer comes from the rasping noise that

electromechanical buzzers made

3 piezoelectric

A piezoelectric element may be driven by an oscillating electronic circuit or other

audio signal source driven with a piezoelectric audio amplifier Sounds commonly used to

indicate that a button has been pressed are a click a ring or a beep

Fig Of buzzers

50

the load resistance RL in the same direction

12The polarity of the voltage developed across RL is such that the end connected to

the

junction of the diodes D1 and D2 will be positive

Fig Of Bridge rectifier

62FILTER

1Output from the rectifier unit having harmonic contents so we can provided the

filter circuit filter circuit is used to reduce the harmonics

2Here we can use the electrolytic capacitor

3This eliminates the harmonics from both voltage and current signals

Fig of filter

63VOLTAGE REGULATOR

1 Voltage regulator is used to maintain the constant voltage with the variation of the

supply voltage and the load current

2 When specifying individual ICs within this family the xx is replaced with a two-

digit number which indicates the output voltage the particular device is designed to

provide (for example the 7805 has a 5 volt output while the 7812 produces 12 volts)

3 The 78xx line are positive voltage regulators meaning that they are designed to

produce a voltage that is positive relative to a common ground

46

Fig Of voltage regulator

Circuit diagram of Power supply of RF recevier

-+

D 1

W 01G

2

1

3

4

TX1

V 1

C 1470uF

U 1

7805

1 3V IN VO U T

C 147uF

C 10 1uF

R 1

330

D 2

LN 211W P

12

J P 1

12

12

2309V

230V

Operation

1 Initially 230 V AC supply is reduced to (0-9V) with the help of a step down

transformer having a capacity of 500mA

2 Since the input voltage to the regulator IC should be more than its output voltage

transformer secondary voltage is 9V

47

3 This low voltage is rectified with the help of bridge rectifier The ripples are

minimized with the help of capacitor filter to get a smooth DC supply The rating of

the chosen capacitor filter is 1000microF

4 The regulated DC voltage is obtained by using a regulator IC 7805 In the case of IC

7805 the unregulated DC voltage is applied to Pin 1 and the output is taken at Pin 3

and Pin 2 is grounded

5 Another capacitor filter of rating 10microF is connected at the output of regulator IC to

eliminate the voltage oscillations at the output due to the large voltage oscillations at

the input of the regulator

48

CHAPTER ndash 7

CONTROL ACTION

49

71 BUZZER

A buzzer or beeper is an audio signaling device Typical uses of buzzers and beepers include

alarms timers and confirmation of user input such as a mouse click or keystroke

TYPES OF BUZZER

1 Mechanical

2 Electromechanical

3 Piezoelectric

1 Mechanical

A joy buzzer is an example of a purely mechanical buzzer

2 Electromechanical

Early devices were based on an electromechanical system identical to an electric bell

without the metal gong Similarly a relay may be connected to interrupt its own actuating

current causing the contacts to buzz Often these units were anchored to a wall or ceiling to

use it as a sounding board The word buzzer comes from the rasping noise that

electromechanical buzzers made

3 piezoelectric

A piezoelectric element may be driven by an oscillating electronic circuit or other

audio signal source driven with a piezoelectric audio amplifier Sounds commonly used to

indicate that a button has been pressed are a click a ring or a beep

Fig Of buzzers

50

Fig Of voltage regulator

Circuit diagram of Power supply of RF recevier

-+

D 1

W 01G

2

1

3

4

TX1

V 1

C 1470uF

U 1

7805

1 3V IN VO U T

C 147uF

C 10 1uF

R 1

330

D 2

LN 211W P

12

J P 1

12

12

2309V

230V

Operation

1 Initially 230 V AC supply is reduced to (0-9V) with the help of a step down

transformer having a capacity of 500mA

2 Since the input voltage to the regulator IC should be more than its output voltage

transformer secondary voltage is 9V

47

3 This low voltage is rectified with the help of bridge rectifier The ripples are

minimized with the help of capacitor filter to get a smooth DC supply The rating of

the chosen capacitor filter is 1000microF

4 The regulated DC voltage is obtained by using a regulator IC 7805 In the case of IC

7805 the unregulated DC voltage is applied to Pin 1 and the output is taken at Pin 3

and Pin 2 is grounded

5 Another capacitor filter of rating 10microF is connected at the output of regulator IC to

eliminate the voltage oscillations at the output due to the large voltage oscillations at

the input of the regulator

48

CHAPTER ndash 7

CONTROL ACTION

49

71 BUZZER

A buzzer or beeper is an audio signaling device Typical uses of buzzers and beepers include

alarms timers and confirmation of user input such as a mouse click or keystroke

TYPES OF BUZZER

1 Mechanical

2 Electromechanical

3 Piezoelectric

1 Mechanical

A joy buzzer is an example of a purely mechanical buzzer

2 Electromechanical

Early devices were based on an electromechanical system identical to an electric bell

without the metal gong Similarly a relay may be connected to interrupt its own actuating

current causing the contacts to buzz Often these units were anchored to a wall or ceiling to

use it as a sounding board The word buzzer comes from the rasping noise that

electromechanical buzzers made

3 piezoelectric

A piezoelectric element may be driven by an oscillating electronic circuit or other

audio signal source driven with a piezoelectric audio amplifier Sounds commonly used to

indicate that a button has been pressed are a click a ring or a beep

Fig Of buzzers

50

3 This low voltage is rectified with the help of bridge rectifier The ripples are

minimized with the help of capacitor filter to get a smooth DC supply The rating of

the chosen capacitor filter is 1000microF

4 The regulated DC voltage is obtained by using a regulator IC 7805 In the case of IC

7805 the unregulated DC voltage is applied to Pin 1 and the output is taken at Pin 3

and Pin 2 is grounded

5 Another capacitor filter of rating 10microF is connected at the output of regulator IC to

eliminate the voltage oscillations at the output due to the large voltage oscillations at

the input of the regulator

48

CHAPTER ndash 7

CONTROL ACTION

49

71 BUZZER

A buzzer or beeper is an audio signaling device Typical uses of buzzers and beepers include

alarms timers and confirmation of user input such as a mouse click or keystroke

TYPES OF BUZZER

1 Mechanical

2 Electromechanical

3 Piezoelectric

1 Mechanical

A joy buzzer is an example of a purely mechanical buzzer

2 Electromechanical

Early devices were based on an electromechanical system identical to an electric bell

without the metal gong Similarly a relay may be connected to interrupt its own actuating

current causing the contacts to buzz Often these units were anchored to a wall or ceiling to

use it as a sounding board The word buzzer comes from the rasping noise that

electromechanical buzzers made

3 piezoelectric

A piezoelectric element may be driven by an oscillating electronic circuit or other

audio signal source driven with a piezoelectric audio amplifier Sounds commonly used to

indicate that a button has been pressed are a click a ring or a beep

Fig Of buzzers

50

CHAPTER ndash 7

CONTROL ACTION

49

71 BUZZER

A buzzer or beeper is an audio signaling device Typical uses of buzzers and beepers include

alarms timers and confirmation of user input such as a mouse click or keystroke

TYPES OF BUZZER

1 Mechanical

2 Electromechanical

3 Piezoelectric

1 Mechanical

A joy buzzer is an example of a purely mechanical buzzer

2 Electromechanical

Early devices were based on an electromechanical system identical to an electric bell

without the metal gong Similarly a relay may be connected to interrupt its own actuating

current causing the contacts to buzz Often these units were anchored to a wall or ceiling to

use it as a sounding board The word buzzer comes from the rasping noise that

electromechanical buzzers made

3 piezoelectric

A piezoelectric element may be driven by an oscillating electronic circuit or other

audio signal source driven with a piezoelectric audio amplifier Sounds commonly used to

indicate that a button has been pressed are a click a ring or a beep

Fig Of buzzers

50

71 BUZZER

A buzzer or beeper is an audio signaling device Typical uses of buzzers and beepers include

alarms timers and confirmation of user input such as a mouse click or keystroke

TYPES OF BUZZER

1 Mechanical

2 Electromechanical

3 Piezoelectric

1 Mechanical

A joy buzzer is an example of a purely mechanical buzzer

2 Electromechanical

Early devices were based on an electromechanical system identical to an electric bell

without the metal gong Similarly a relay may be connected to interrupt its own actuating

current causing the contacts to buzz Often these units were anchored to a wall or ceiling to

use it as a sounding board The word buzzer comes from the rasping noise that

electromechanical buzzers made

3 piezoelectric

A piezoelectric element may be driven by an oscillating electronic circuit or other

audio signal source driven with a piezoelectric audio amplifier Sounds commonly used to

indicate that a button has been pressed are a click a ring or a beep

Fig Of buzzers

50

Circuit diagram of buzzer

Circuit diagram of electronic buzzer

51

Fig Of electronic buzzer( PS1420P02CT)

FEATURES

bull Low frequency tone(2kHz)bull Suitable for automatic radial taping machine(15mm-pitch)

SPECIFICATIONS AND CHARACTERISTICS

1 Sound pressure70dBA10cm min[at 2kHz 5V0-P rectangular wavemeasuring temperature 25plusmn5degChumidity 60plusmn10]

2Temperature range Operating ndash20 to +70degC Storage ndash30 to +80degC

3Maximum input voltage 30V0-P max [without DC bias]

4Minimum delivery unit 1750 pieces [350 pieces1 reeltimes5 reels]

52

72 SOLONIOD VALVE

A solenoid valve is an electromechanical valve for use with liquid or gas The valve is controlled by an electric current through a solenoid in the case of a two-port valve the flow is switched on or off in the case of a three-port valve the outflow is switched between the two outlet ports Multiple solenoid valves can be placed together on a manifold

Solenoid valves are the most frequently used control elements in fluidics Their tasks are to shut off release dose distribute or mix fluids They are found in many application areas Solenoids offer fast and safe switching high reliability long service life good medium compatibility of the materials used low control power and compact design

Besides the plunger-type actuator which is used most frequently pivoted-armature actuators and rocker actuators are also used

Construction

A solenoid valve has two main parts the solenoid and the valve The solenoid converts electrical energy into mechanical energy which in turn opens or closes the valve mechanically A direct acting valve has only a small flow circuit shown within section E of this diagram (this section is mentioned below as a pilot valve) This diaphragm piloted valve multiplies this small flow by using it to control the flow through a much larger orifice

Solenoid valves may use metal seals or rubber seals and may also have electrical interfaces to allow for easy control A spring may be used to hold the valve opened or closed while the valve is not activated

Fig Of SOLONIOD VALVE

53

Fig Of electronic buzzer( PS1420P02CT)

FEATURES

bull Low frequency tone(2kHz)bull Suitable for automatic radial taping machine(15mm-pitch)

SPECIFICATIONS AND CHARACTERISTICS

1 Sound pressure70dBA10cm min[at 2kHz 5V0-P rectangular wavemeasuring temperature 25plusmn5degChumidity 60plusmn10]

2Temperature range Operating ndash20 to +70degC Storage ndash30 to +80degC

3Maximum input voltage 30V0-P max [without DC bias]

4Minimum delivery unit 1750 pieces [350 pieces1 reeltimes5 reels]

52

72 SOLONIOD VALVE

A solenoid valve is an electromechanical valve for use with liquid or gas The valve is controlled by an electric current through a solenoid in the case of a two-port valve the flow is switched on or off in the case of a three-port valve the outflow is switched between the two outlet ports Multiple solenoid valves can be placed together on a manifold

Solenoid valves are the most frequently used control elements in fluidics Their tasks are to shut off release dose distribute or mix fluids They are found in many application areas Solenoids offer fast and safe switching high reliability long service life good medium compatibility of the materials used low control power and compact design

Besides the plunger-type actuator which is used most frequently pivoted-armature actuators and rocker actuators are also used

Construction

A solenoid valve has two main parts the solenoid and the valve The solenoid converts electrical energy into mechanical energy which in turn opens or closes the valve mechanically A direct acting valve has only a small flow circuit shown within section E of this diagram (this section is mentioned below as a pilot valve) This diaphragm piloted valve multiplies this small flow by using it to control the flow through a much larger orifice

Solenoid valves may use metal seals or rubber seals and may also have electrical interfaces to allow for easy control A spring may be used to hold the valve opened or closed while the valve is not activated

Fig Of SOLONIOD VALVE

53

72 SOLONIOD VALVE

A solenoid valve is an electromechanical valve for use with liquid or gas The valve is controlled by an electric current through a solenoid in the case of a two-port valve the flow is switched on or off in the case of a three-port valve the outflow is switched between the two outlet ports Multiple solenoid valves can be placed together on a manifold

Solenoid valves are the most frequently used control elements in fluidics Their tasks are to shut off release dose distribute or mix fluids They are found in many application areas Solenoids offer fast and safe switching high reliability long service life good medium compatibility of the materials used low control power and compact design

Besides the plunger-type actuator which is used most frequently pivoted-armature actuators and rocker actuators are also used

Construction

A solenoid valve has two main parts the solenoid and the valve The solenoid converts electrical energy into mechanical energy which in turn opens or closes the valve mechanically A direct acting valve has only a small flow circuit shown within section E of this diagram (this section is mentioned below as a pilot valve) This diaphragm piloted valve multiplies this small flow by using it to control the flow through a much larger orifice

Solenoid valves may use metal seals or rubber seals and may also have electrical interfaces to allow for easy control A spring may be used to hold the valve opened or closed while the valve is not activated

Fig Of SOLONIOD VALVE

53

LIST OF REFERENCES

1 Bonny BN Umeadi KG Jones The Development of An Intelligent

Sensor For The Monitoring of Pipeline System Integrity nanomindidccom

Oil and Gas 2008

2 Richard Kluth Jerry Worsley Digital Pipeline Leak Detection Using

Fiber-optic Distributed Sensing (DTS) Westwick-Farrow Publishing

Whatrsquos New in Process Technology April 2008

3 I Jawhar N Mohamed and K Shuaib Reliability Challenges and

Enhancement Approaches for Pipeline Sensor and Actor Networks In

proceeding of The International Conference on Wireless Networks (ICWN

2008) Las Vegas Nevada USA July 2008

4 Odusina J Akingbola and D Mannel Software-Based Pipeline Leak

Detection Advanced Chemical Engineering Design CHE 4273Department

of Chemical Engineering and Materials Science University of Oklahoma

May 2 2008

5 Yumei Wen Ping Li Jin Yang Zhangmin Zhou Adaptive Leak Detection

and Location in Underground Buried Pipelines International Journal 0f

Information Acquisition vol1 no3 pp269-277 2004

6 Yuanwei Jin Ali Eydgahi Monitoring of Distributed Pipeline Systems by

Wireless Sensor Networks In proceeding of The 2008 IAJC-IJME

International Conference 2008

7 Daniele Inaudi Riccardo Belli Detection and Localization of Micro-

Leakages Using Distributed Fiber Optic Sensing In proceeding of The 7th

International Pipeline Conference Calgary CanadaOctober 2008

8 RC Tennyson WD Morison T Cherpillod Monitoring Pipeline

Integrity Using Fiber Optic Sensors In proceeding of Corrosion 2005

Conference Warsaw Poland June 2005

9 RK Ginzel WW Kanters Pipeline Corrosion and Cracking and The

Associated Calibration Considerations for Same Side Sizing Applications

e-Journal of Nondestructive Testing e-Journal of Nondestructive Testing

Vol 7 No 7 July 2002

10 F Hassan J Iqbal F Ahmed Stress corrosion failure of high-pressure gas

pipeline Journal of Engineering Failure Analysis Volume 14 pp 801-809

54

November 2006

11 Ahmad Taufik Nathanel Nainggolan Gas Flow Modelling and Simulation

To Predict Corrosion Attack On Pipeline In Proceeding of INDOPIPE

2006 Conference amp Exhibition The Ritz-Carlton Hotel Jakarta Indonesia

30 May-1 June 2006

12 Digby D Macdonald Effect of pressure on the rate of corrosion of metals

in high sub-critical and supercritical aqueous systems Journal of

Supercritical Fluids Volume 30 pp 375ndash382 September 2003

55

56

57

58

November 2006

11 Ahmad Taufik Nathanel Nainggolan Gas Flow Modelling and Simulation

To Predict Corrosion Attack On Pipeline In Proceeding of INDOPIPE

2006 Conference amp Exhibition The Ritz-Carlton Hotel Jakarta Indonesia

30 May-1 June 2006

12 Digby D Macdonald Effect of pressure on the rate of corrosion of metals

in high sub-critical and supercritical aqueous systems Journal of

Supercritical Fluids Volume 30 pp 375ndash382 September 2003

55

56

57

58

56

57

58

57

58

58