hpcl project report
TRANSCRIPT
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
1. INTRODUCTION
Hindustan Petroleum Corporation Limited (HPCL) is a Global Fortune 500 company in
the Energy sector. HPCL has two refineries located in Mumbai (West Coast) and Visakh
(East Coast) with capacities of 5.5 MMTPA and 7.5 MMTPA respectively, churning out
a wide range of petroleum products. and over 300 grades of lubricants, specialties and
greases as per BIS standard.
HPCL has successfully contributed close to 20% of India's total refining requirements. Over the
years HPCL's capacity of production has expanded massively through various up gradation
initiatives.
The refineries, known for the full utilization of capacity and world class performance are
the foundations of HPCL's successful journey towards meeting India's energy requirements.
Hindustan Petroleum Corporation (HPCL) came into being in mid 1974 after take over and
merging of erstwhile Esso and Lube India undertakings.
Catlex was taken over by government of India in 1976 and subsequently merged with HPCL.
Hindustan Petroleum Corporation Limited thus emerged after merging Refining/Marketing
facilities of ESSO and CALTEX.
Hindustan Petroleum Corporation Limited today is the second largest integrated oil company in
India playing a significant role in the nation’s economic development and growth. against the
backdrop of economic liberalization,
HPCL is consistently improving its existence by strengthening its infrastructural facilities as
well as diversifying upstream and downstream into exploration and production and power and
petrochemicals and horizontally into LNG sector.
Page | 1DEPARTMENT OF ECE, GITAM UNIVERISTY
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HPCL produces the entire range of petroleum products and serves all sectors of the economy-
industry, agriculture, transport, domestic, public utilities and also major consumers like the
railways, power plants, defense, fertilizer plants, etc.;
Visakh refinery performance had been consistently “excellent” over the years. The major
performance indicators are crude thruput, total distillate, fuel and loss and implementation of
ENCON and environmental projects.
1.1 Origin and growth of HPCL-VR
Commissioned in 1957 as Catlex oil refinery India limited (CORIL). First oil refinery on the East
Coast and the major industry in the city of Visakhapatnam, Andhra Pradesh.
Installed capacity of 0.65 Million Metric Tones per Annum {MMTA} in 1957 for refining of
crude oil into petroleum products [13200 bbl/day]. CORIL was taken over by the government of
India and merged with HPCL in 1978.
1.2 Refinery Overview
Visakh refinery is fuels based refinery generating major products of mass consumption like
petrol, diesel and kerosene. Hence, crude meeting general purpose characteristics can be
processed with this refinery configuration. Visakh refinery can process crude from Prussian gulf
under non-bituminous category, bituminous crude (crude yielding bitumen, used for paving
road).
The crude processed at refinery include
Page | 2DEPARTMENT OF ECE, GITAM UNIVERISTY
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CRUDE COUNTRY
Kuwait Kuwait
Dubai UAE
Ummshaif UAE
Upper zakum UAE
Murban Saudi Arab
Arab medium Saudi Arab
Iran mix Iran
Lavan Blend Iran
Barash Lt Iran
Products And Treatment Facilities
S.NO Process unit Capacity (in MMPA)
1 CDU-I 1.5
2 CDU-II 3.0
3 CDU-III 3.0
4 BBU 0.225
5 VBU 1.0
6 FCCU-I(R) 0.95
7 FCCU-II 0.60
8 DHDS 1.8
9 PRU 0.1
Page | 3DEPARTMENT OF ECE, GITAM UNIVERISTY
Table-1 crude export countries
Table-2 Capacity of various processing units
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Legend:
CDU: Crude Distillation Unit FCCU : Fluidized Catalytic Cracking
DHDS: Diesel Hydro De-Sulphurization Unit VBU : Vis Breaker Unit
BBU : Bitumen Blowing Unit PRU : Propylene Recovery Unit
Products:
S.NO DAILY PRODUCTION CAPACITY(in tones)
1 Crude processing 22500
2 LPG 610
3 Propylene 100
4 Sulphur 17+65
5 Diesel 7800
6 Naphtha 2150
7 LSHS 1790
8 Fuel oil 3500
Page | 4DEPARTMENT OF ECE, GITAM UNIVERISTY
Table-3 Daily Production capacity of various products
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Treatment Units :
DHDS:Diesel Hydro De-Sulphurization Unit:1.8 MMPTA
LPG Amine Treatment Unit
LPG, ATF and Petrol Merox Units Amine Regeneration Unit
Environmental Control Facilities:
Sulphur Recovery Units: 3 no. [2 Locate Technology of Clauss process]
Sour water striping Units: 2 no.
Effluent Treatment Plants: 4 no.
CO Boilers: 2 no.
Page | 5DEPARTMENT OF ECE, GITAM UNIVERISTY
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2. CONFIGURATION OF REFINERY
Visakh Refinery is being operated under the following major units:
Process Units
Treating Units
Power and utilities
Oil movement and storage units
Environment related units
2.1 Process UnitThe Process unit consists of three units:
1. CDU 2.FCCU 3.PRU
Crude Distillation UnitCDU consists of two sections:
Atmos section
Vacuum section
Atmos section:
Crude oil is first preheated from 30-1250c and pressure about 10kg/cm2 enters the Desalter. The
salts from crude are removed in the desalter units. The desalted crude is then boosted to a
pressure of 30-35kg/cm2, pre-heated to around 3600c.
The oil is allowed into the flash zone of atmos distribution column and the product to stripper
with steam to strip off the lighter products. The over head-vapors of the atmos column are
condensed in a series of conductors and the liquid in the receiver.
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Heavy Naphtha, kerosene / ATF & Diesel product are withdrawn as side steams and stripped off
as lighter ends with supper heater MP steam in the respective strippers. The bottom stream in
atmos column is called RCO.
Products: Fuel Gas, LPG, Light-Naphtha, Heavy-Naphtha, kerosene, diesel & Reduced Crude
Oil.
Vacuum section:
Hot reduced crude oil from atmospheric column bottom is heated in a vacuum to 380oc and
introduced into the flash zone of vacuum column. The stop distillate out is withdrawn first.
The hydrocarbon vapors rising in the column are condensed into Heavy Vacuum Gas Oil
(HVGO) and Light Vacuum Gas Oil (LVGO). VGO is feed to FCCU as feed. The bottom
product of vacuum column is vacuum residue. The vacuum in the column is maintained by a
multistage ejector system.
Products: LVGO and the HVGO obtained are fed to FCCU, the combination of Short-Residue
and the slop cut forms the fuel oil which is consumed by the refinery.
Vis Breaking Unit (VBU)
Vacuum reside from either CDU I II or III or storage is received in visbreaking feed surge drum.
It operates at a pressure which is floating on main fractioning pressure visbreaking feed @ 5.0
kg/cm2g, 1200c – 1600c from surge drum is pumped by visbreaking feed charge pump which are
of screw type to a pressure of 7.6 kg/cm2g.
It is then heated in visbreaking tar exchange to 3200c by visbreaking crude is then routed to
heater through booster pumps @ 5.8kg/cm2g preheated visbreaker feed entries both passes of
visbreaker heater under individual pass.
Page | 7DEPARTMENT OF ECE, GITAM UNIVERISTY
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Control visbreaker heater is a two-pass single shell heater with bridge wall type configuration
turbulizing water (BFKL) is injected to both the passes at a point where visbreaking reaction
starts. Fuel gasses heat visbreaker feed to 4550c (4700c) Residual heat recovered by superheating
LP & MP steam. Gas oil quench works primarily by vaporization quench effluent entries main
fractionators @ 4250c and 7 kg/cm2g where it is separated into visbreaker tar or fuel oil as side
stream product and naphtha and gas as overhead product.
Page | 8DEPARTMENT OF ECE, GITAM UNIVERISTY
Fig-2.1 Crude distillation unit
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Bitumen Blowing Unit (BBU)
The unit normally receives hot vacuum residue directly from vacuum unit. The feed is cooled to
about 2320c in a stream generator before entering the bitumen converter. In bitumen converter
the vacuum residue is blown with air, since the reaction exothermic, the heat evolved has to be
removed.
This is done injected steam into the reactor at the top. Heat is recovered from bitumen leaving
converter bottom by generating steam and the bitumen is further trim cooled before sending to
storage.
The hydrocarbon vapors steam and unreacted air leave the converter top to water quench drum
where hydrocarbons are condensed along with some water. Hydrocarbon layer is sent to slop oil
whereas water sent to waste water treatment plant (WWTP) after separation of same in the
settler.
Fluidized Catalytic Cracking Unit (FCCU)
Vacuum Gas Oil from vacuum unit and recycle streams are pumped to raw oil furnace for
preheating the fresh feed.
This fresh feed is mixed with regenerated catalyst and enters the reactor at the base of riser
where they are vapourized and raised to the reactor temperature by the hot catalyst.
The mixture of oil, vapour and catalyst travels up the riser into reactor. The gas oil commences
to crack immediately when it contact the hot catalyst in the riser and continues until the oil
vapour is disengaged from the catalyst in the reactor.
The cracked products in vapour form continue through the reactor vapour line to fractionators.
The catalyst stripper surrounds the upper portion of the reactor passes around the reactor grid and
into the stripper, where if flows over baffles counter current to the rising stripping steam,
displaces oil vapours to the reactor.
Coke is deposited on the circulation catalyst in the reaction zone. The fuel gas leaving the top of
the regenerator goes to co-boilers where super heat is produced.
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The regenerated catalyst is recycled with the incoming feed to the reactor. Vapours from reactor
are sending to Fractionators section where they are fractionated into recycle gas oil which is
returned to the reactor and produces Clarified Oil, Cycle Oil, Motor Sprint (Petrol), and Gas
products.
This is achieved by first sending the reactor products to fractionators where recycled gas oil and
clarified oil are taken as bottom products, cycle oil as side draw off and unstabilised motor sprint
and gas as overhead products.
The overhead gas is compressed and liquefied and separated from the separator is scrubbed with
unstabilised motor sprint in an absorber to recover the C3 &C4 in it.
Page | 10DEPARTMENT OF ECE, GITAM UNIVERISTY
Fig 2.2 Fluidized Catalytic Cracking Unit
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The liquids form the separator and the absorbers are stripped off ethane and the gas stripped off
is recycled back to the gas compressor and liquefaction system to recover C3’s and C4’s carried
with the stripped gases. The liquid form stripper bottom is send to a Debutanizers where LPG is
taken as overhead product and stabilized MS as bottom product.
Products: Fuel Gas, Cracked LPG, Cracked Gasoline, Cracked Naphtha, Diesel component
[Light Cyclic Oil (LCO), Heavy Cyclic Oil (HCO), Clarified Oil, Low Sulphur Heavy Stock
(LSHS) Used as fuel for Industries and boilers from low sulphur crude processing. Also used in
Ships, Jute Batch Oil, Wash Oil-B, Propylene.
Propylene Recovery Unit (PRU )
The Propylene Recovery Unit is defined to recover Propylene from Cracked LPG, which is one
of the product streams of Fluidized Catalytic Cracking Unit (FCCU). Cracked LPG is a mixture
of Propane, Propylene, and Butane with some traces of C2 & C5 Hydrocarbons.
The unit is designed to process about 1,00,000TPA of cracked LPG produced at FCCU-I & II
and to recover 22,000TPA of Propylene. The process consists of four steps. In the first step, the
feed to unit i.e., Cr. LPG is prepared by draining out the traces of caustic carryover.
In the second step Cracked LPG is separated into C3’s and C4’s in a distillation column
consisting of 55 trays. C3’s being lighter is recovered from the column top.
In the third step, the C3’s are again separated into propylene and propane in the second
distillation column consisting of 98 trays. Propylene being lighter is recovered from the top of
the column. In the fourth step, the Propylene recovered is subjected to chemical treatment with a
mixture of Mono Ethanol Amine (MEA) and Caustic, then water washed and passed through a
mechanical coalesce to knock off moisture to meet the following specifications:
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Purity : 95 %
Water : NIL
Total Sulphur : 5 ppm
The bottom products of the first column consisting of Butane & Butylenes along the bottom
product of second column consisting Propane are routed to LPG Spheres. On special Propylene
is routed to its spheres and off- Special is routed to LPG spheres.
2.2 Treating Units
1. Merox Unit 2. Diesel Hydro De-Sulphurisation
Merox Unit
The LPG containing is treated here and the sulphur is removed from it. The Kerosene’s flash
point is increased in this unit and the sore water containing gas is treated here and the water is
recycled for usage.
LPG Merox Units
While separate facilities are provided for straight run and cracked LPG’S for extraction, a
common facility is provided for caustic generation. After Amine washing LPG enters the caustic
pre-wash tower, the purpose of which is to remove traces of hydrogen sulphide.
The LPG extractor which is perforated tray column. In this type of extraction column, caustic
soda containing dispersed Merox catalyst is would lead to caustic soda entertainment. The LPG
is introduced near the bottom of the column below the first perforated tray. LPG, with
mercaptans, is transferred to the caustic solution forming sodium mercaptides.
The LPG then goes in to the LPG settler. The spent caustic carried over from the LPG extractor
decants and treated LPG is recovered and sent to storage.
Page | 12DEPARTMENT OF ECE, GITAM UNIVERISTY
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Kerosene Merox
Kerosene after a caustic pre wash goes to the Merox reactor consists of a catalyst bed of
activated charcoal impregnated with Merox catalyst. Air is injected with the feed to the Merox
reactor.
Diesel Hydro De-Sulphurisation
Sulfur in Diesel enhances the pollution & contributes significantly to SOx in exhaust emissions.
It leads to corrosion and wear of engine systems.
In order to make eco-friendly diesel, it is desirable to remove impurities by treating the Diesel
streams at certain operating conditions in presence of catalyst and H2 through a process known as
DHDS. Straight run/Cracked diesel streams have certain inherent impurities viz Sulphur,
Oxygen, Olefins, metals etc. Quantity of these impurities depend on crude quality, generally
poorer the crude quality, higher the impurities.
With the implementation of Bharat Stage-II and Euro-III spec, it is mandatory to produce Diesel
with ultra low Sulphur content.
The Process Steps in this unit are:
Feed (Naphtha) Pre-desulphurization
Final desulphurization
Steam Naphtha Reforming
CO HT shift conversion
Final purification of H 2 (PSA)
To remove Sulphur from Naphtha, which is poison to reformer catalyst Naphtha and recycle H2
are heated and sent to Reactor where Sulphur compounds are converted to H2S over Cobalt-
Molybdenum based catalyst.
R-SH + H2 → R-H + H2S
Page | 13DEPARTMENT OF ECE, GITAM UNIVERISTY
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Sulphur reduction from 1000ppm to 10ppm. To reduce the sulphur content of Naphtha from 10
ppm to < 0.5 ppm., Naphtha and H2 are heated and processed in Reactor-II to convert S
compounds to H2S over Cobalt-Molybdenum based catalyst. The H2S removed from the
Reactor-II is absorbed in ZnO reactor.
ZnO + H2S → ZnS + H2O
De-Sulphurised Naphtha is mixed with steam and passed through a Nickel catalyst packed in
vertical narrow 108 tubes mounted in the reformer at high temperature
CnHm + nH2O → nCO + (2n+m)/2H2
CH4 + H2O → CO + 3H2 (endothermic)
C + H2O → CO + H2 (endothermic)
Shift: CO + H2O → CO2 + H2 (exothermic)
Process is endothermic and heat is supplied by fuel firing with 40 top-fired burners.
2.3 Power And Utilities:
Captive Power Plant (CPP)
Capacitive power plant meets the total power demand of the HPCL. This unit comprises for four
gas turbine generators (GTG), two with 9mw capacity (FRAME-3 Machines) each and two with
25mw capacity (FRAME-5 Machines) each.FRAME-3 Machine is a two shaft machine whereas
FRAME-5 Machine is a single shaft machine. HSD and Naphtha are used for the combustion of
gas turbine.
.Steam Generation Unit Steam Generation unit is sub divided in to two i.e. Power plant I& II. In these units the DM
water is converted in to steam by combusting the fuel oil in the presence of air in the boilers CO
produced in FCCU is brought in to CO2 for pollution in power plant II. The steam produced here
is utilized for unit purposes. It is Kg/cm2.
Page | 14DEPARTMENT OF ECE, GITAM UNIVERISTY
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Demineralization water unit
De-Mineralization is a process of removing mineral salts from water by using ion exchange
process. In this unit the raw water is treated & the PH value is maintained at 7 by making the free
from acids, bases, etc, and making it a neutral solution, to use in boilers.
2.4 Environment Related Plant
Effluent treatment plantThe waste water from every plant containing oil is separated and then reused. The remaining
water containing contaminants is neutralized and sent to the sea to control the environmental
pollution.
Sulphur Recovery Unit (SRU):
Sulphur Recovery Unit is designed to process and remove Hydrogen Sulphide (H2S) gas from
fuel gas (3386 - 8838) Nm3/hr, Sour water Stripper gas (38 -257) Nm3/hr and Amine Acid Gas
(1.6 - 26) Nm3/hr, the process is based on the modified Claus reaction.
H2S + 1/2 O2 → H2O + S
This reaction is accomplished by a solution called LO-CAT solution supplied by M/S ARI
Technologies Inc., USA.
All the three gas streams mentioned above are treated with LO-CAT solution. Due to wide
variation in the qualities the fuel gas is treated separately in an absorber column and the other
two streams are treated combined in the absorber section of the oxidizer vessel. In oxidizer
vessel, the spent LO-CAT solution is regeneration using air.
Page | 15DEPARTMENT OF ECE, GITAM UNIVERISTY
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The sulphur generated due to the above reaction remains finely suspended in the LO-CAT
solution. A slipstream form the oxidizer is routed to the sulphur removal system consisting of
mainly a vacuum belt filter, Sulphur Smelter and a molten sulphur storage tank. After removing
sulphur the balance LO-CAT solution routed back to the oxidizer. The treated fuel gas is then
routed to the Refinery Fuel Gas Header. The vent gases forms the oxidizers (free of H2S) are
then vented through a stack.
Page | 16DEPARTMENT OF ECE, GITAM UNIVERISTY
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3. STUDY OF ANALYZERS
The analyzers are mainly used as continuous ambient air monitoring systems
(CAAMS).This gives the analysis of various components in the air that are released
into the atmosphere during the crude extraction process.
For this purpose the software named “ENVIDAS”.This gives the concentration of
various components present in the atmosphere in the stipulated time.The time can be
set from 1 minute to 48 hours.
This CAAMS is used in getting the dust concentration in the plant.In total there are
three monitoring stations in and around the plant.This helps for knowing the amount
of NOx gases, SO2, hydrocarbons, co and co2 present in the pollutant air.
The information will be sent to the APPCS: Andhra Pradesh Pollution Control
Station.The APPCS gives the standards of all the gases to be present in the air.
According to the information obtained, the board will take the action on the
production from the plant.The CAAMS also measures the dust concentration in the
plant.
There are two channels for the measurement of suspended particle matter
1. SPM :Suspended Particle Matter
2. RSPM :Respirable Suspended Particle Matter
This SPM measures the dust particles up to 10ug.The RSPM measures particular matter
up to 25ug.
The HPCL plant uses BAM 1020 model for this purpose.
As per the time the software uploads the data and sends to the central monitoring station
and that will be forwarded to APPCS.
Page | 17DEPARTMENT OF ECE, GITAM UNIVERISTY
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3.1 BAM 1020 :
The main features of this BAM 1020 are
Low operating costs.
Automatic hourly span checks.
It can store upto 182 days of digital data in the internal storage.
Fast and easy audits using common tools.
Highly accurate, reliable and mechanically simple flow system.
Hourly filters advances minimize effects on volatile compounds..
Rugged anodized aluminum, stainless steel and baked enamel construction.
Data retrieval through RS232 ports using direct connection to pc, modem or digital data
collection systems.
PRINCIPLE :
The BAM1020 automatically collects, measures, records the air bourne particulates (in milligram
or micrograms per cubic meter) using the principle of beta ray attenuation.
Thousands of BAM 1020 are deployed all over the world now making it the most successful air
monitoring platforms in the world.
OPERATION:
Each hour, a small 14C (carbon-14) element emits a constant source of high-energy
electrons (known as beta rays) through a spot of clean filter tape.
These beta rays are detected and counted by a sensitive scintillation detector to
determine a zero reading.
Page | 18DEPARTMENT OF ECE, GITAM UNIVERISTY
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The BAM-1020 automatically advances this spot of tape to the sample nozzle, where a
vacuum pump then pulls a measured and controlled amount of dust-laden air through the
filter tape, loading it with ambient dust.
At the end of the hour this dirty spot is placed back between the beta source and the
detector thereby causing an attenuation of the beta ray signal which is used to determine
the mass of the particulate matter on the filter tape and the volumetric concentration of
particulate matter in ambient air.
These are used in measuring the amount of NOX, SO2, CO, HC that are present in the
atmosphere.
These levels are sent to the central board and from it to the A.P. pollution board.
Page | 19DEPARTMENT OF ECE, GITAM UNIVERISTY
Fig 3.1 BAM-1020 Analyzer Instrument
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This is the view of the BAM 1020.recorder,which shows the readings at that particular time.this
is interfaced with the atmosphere, and the readings are plotted down with the help of a recorder
which is as shown below.
Page | 20DEPARTMENT OF ECE, GITAM UNIVERISTY
Fig 3.2 Analyzer section in HPCL
Fig 3.3 Filter tapes inside BAM-1020
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Along with SPM and RSPM there are some gas analyzers like
1. NOX : Chemilumenescene analyzer
2. SO2 : Fluorescence analyzers
3. HC : Hydrocarbon analyzers
4. CO : Gas filter correlation analyzers
Also we find zero span analyzers, weather monitoring station, noise meter
The principles of each analyzer is
a) SPM and RSPM : loss of beta rays
b) NOx :loss of energy due to excitation
c) SO2 :loss of energy where UV rays act as a source
d) CO :M-R ratio
3.2 NOX ANALYSER: (RANGE BETWEEN (0-500ppm))
For the measurement of nitric oxides in the polluted air.
The principle of chemilumenescene is employed.
It means that emission of light during a chemical reaction that does not produce
significant quantities of heat.
The NOx reacts with the ozone, gets excited and return to low energy level with loss of
energy.
The change in the intensity of light energy gives the measurement of NOx
In this process a beta rays are emitted on to the air that is collected and the output of it is passed
through ozone layer.
The NOx present in the atmosphere reacts with the ozone and gets excited to the higher level.
After a certain amount of time this excited NOx gets back to the normal level.
The change in the intensity of light energy gives us the exact measurement of NOx present in the
atmosphere.
Page | 21DEPARTMENT OF ECE, GITAM UNIVERISTY
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3.3 SO2 ANLYSER: (RANGE BETWEEN (0-250ppm))
For the measurement of SO2 in the ambient air.
The principle is fluorescence.
Same as that of NOx, here the UV rays are used for the light radiation.
Even this follows the same process but instead of beta rays the UV rays are used in this process.
Followed by ozone layer. Due to the UV rays the SOx molecules in the air gets activated and
when passed through ozone get excited to higher energy levels. After some time they come back
to the original state.
The difference in the energy levels gives us the measurement of SOx present in the air.
3.4 HC ANALYSER: (RANGE BETWEEN (0-10ppm))
In this, for the measurement of HC, a continuous air supply is required.
Fuel is H2 and carrier gas is N2.
The principle is flame ionization detection.
The number of ions that pass through the flame and the output is a measure of
concentrated HC.
3.5CO ANALYSER: (RANGE BETWEEN (0-10ppm)) This consists of a wheel where in one half is filled with N2 and the other half is filled with
CO+N2.
When the beta rays is passed then N2 does not absorb the radiation where as the other half
having CO and N2, absorbs the light radiation due to the presence of CO.
Then the beta rays are imposed on the optical mirrors in order to adjust the intensity with
that of detectors.
Here the concentration is measured with the reference known as MR ratio which is
approximately 1.2.
Page | 22DEPARTMENT OF ECE, GITAM UNIVERISTY
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3.6 ZERO SPAN CALIBRATION:
This is used for calibrating the analyzers where for the zero.
Zero air modules are used in which the concentration of impurities is almost negligible.
Span measurement is taken by passing the air with maximum amount of impurities in it.
The calibration is done for every one month.
Page | 23DEPARTMENT OF ECE, GITAM UNIVERISTY
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4.PROGRAMMABLE LOGIC CONTROLLER
A Programmable Logic Controller, or PLC for short, is simply a special computer device
used for industrial control systems. It is a sequence controller, i.e it accepts inputs from switches
and sensors, evaluates these in accordance with a stored program, and generates outputs to
control machines and processes.
They are used in many industries such as oil refineries, manufacturing lines, conveyor
systems and so on. Where ever there is a need to control devices the PLC provides a flexible way
to "softwire" the components together.
It uses a programmable memory to store instructions and execute specific functions that
include ON/OFF control, timing, counting, sequencing, arithmetic, and data handling.
4.1 History
The early history of the PLC is fascinating. Imagine if you will a fifty foot long cabinet
filled with relays whose function in life is to control a machine. Wires run in and out of the
system as the relays click and clack to the logic.
Now imagine there is a problem or a small design change and you have to figure it all out on
paper and then shut down the machine, move some wires, add some relays, debug and do it all
over again.
Imagine the labor involved in the simplest of changes. This is the problem that faced the
engineers at the Hydra-matic division of GM motors in the late 1960's. Fortunately for them the
prospect of computer control was rapidly becoming a reality for large corporations as
themselves. So in 1968 the GM engineers developed design criteria for a "standard machine
controller". This early model simply had to replace relays but it also had to be:
A solid-state system that was flexible like a computer but priced competitively with a like
kind relay logic system.
Page | 24DEPARTMENT OF ECE, GITAM UNIVERISTY
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
Easily maintained and programmed in line with the all ready accepted relay ladder logic
way of doing things.
Work in an industrial environment with all the dirt, moisture, electromagnetism and
vibration.
Modular in form to allow easy exchange of components and expandability.
This was a tall order in 1968 but four companies took on the challenge.
1. Information Instruments, Inc. (fully owned by Allen-Bradley a year later).
2. Digital Equipment Corp. (DEC)
3. Century Detroit
4. Bedford Associates
Bedford Associates won the contract and quickly formed a new company around the technology
called MODICON after Modular Digital Control. By June of 1969 they were selling the first
viable Programmable Controller, the "084" which sold over one thousand units. These early
experiences gave birth to their next model the "184" in 1973 which set Modicon as the early
leader in programmable controllers. Not to be outdone, the powerhouse Allen-Bradley (all ready
known for its rheostats, relays and motor controls) purchased Information Instruments in 1969
and began development on this new technology.
The early models (PDQ-II and PMC) were deemed to be too large and complex. By 1971 Odo
Struger and Ernst Dummermuth had begun to develop a new concept known as the Bulletin 1774
PLC which would make them successful for years to come. Allen-Bradley termed their new
device the "Programmable Logic Controller" (patent #3,942,158) over the then accepted term
"Programmable Controller". The PLC terminology became the industry standard especially
when PC became associated with personal.
A PLC System
The basic units have a CPU (a computer processor) that is dedicated to run one program that
monitors a series of different inputs and logically manipulates the outputs for the desired
Page | 25DEPARTMENT OF ECE, GITAM UNIVERISTY
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
control. They are meant to be very flexible in how they can be programmed while also providing
the advantages of high reliability (no program crashes or mechanical failures), compact and
economical over traditional control systems.
Unlike a personal computer, though the PLC is designed to survive in a rugged industrial
atmosphere and to be very flexible in how it interfaces with inputs and outputs to the real world.
PLCs come in many shapes and sizes. They can be so small as to fit in a shirt pocket while more
involved controls systems require large PLC racks. Smaller PLCs (a.k.a. “bricks”) are typically
designed with fixed I/O points. The PLC’s used at HPCL are the ‘modular’ ones. It’s called
“modular” because the rack can accept many different types of I/O modules that simply slide
into the rack and plug in.
The components that make a PLC work can be divided into three core areas.
The power supply and rack
The central processing unit (CPU)
The input/output (I/O) section
Page | 26DEPARTMENT OF ECE, GITAM UNIVERISTY
Fig 4.1 A Basic PLC system
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
Power Supply and Racks:
The rack is the component that holds everything together. Depending on the needs of the control
system it can be ordered in different sizes to hold more modules. Like a human spine the rack
has a backplane at the rear which allows the cards to communicate with the CPU.
The power supply plugs into the rack as well and supplies a regulated DC power to other
modules that plug into the rack. The most popular power supplies work with 120 VAC or 24
VDC sources.
The CPU:
Page | 27DEPARTMENT OF ECE, GITAM UNIVERISTY
Fig 4.2 PLC Architecture
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
The brain of the whole PLC is the CPU module. This module typically lives in the slot beside
the power supply. Manufacturers offer different types of CPUs based on the complexity needed
for the system.
The CPU consists of a microprocessor, memory chip and other integrated circuits to control
logic, monitoring and communications.
The CPU has different operating modes. In programming mode it accepts the downloaded logic
from a PC. The CPU is then placed in run mode so that it can execute the program and operate
the process.
Since a PLC is a dedicated controller, it will only process this one program over and over again.
One cycle through the program is called a scan time and involves reading the inputs from the
other modules, executing the logic based on these inputs and then updated the outputs
accordingly.
The scan time happens very quickly (in the range of 1/1000th of a second). The memory in the
CPU stores the program while also holding the status of the I/O and providing a means to store
values.
I/O System:
The I/O system provides the physical connection between the equipment and the PLC. Opening
the doors on an I/O card reveals a terminal strip where the devices connect. There are many
different kinds of I/O cards which serve to condition the type of input or output so the CPU can
use it for its logic.
It's simply a matter of determining what inputs and outputs are needed, filling the rack with the
appropriate cards and then addressing them correctly in the CPUs program.
Input Module: These modules act as interface between real-time status of process variable
and the CPU.
Page | 28DEPARTMENT OF ECE, GITAM UNIVERISTY
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
Analog input module: Typical input to these modules is 4-20 mA, 0-10 V. For eg: Pressure,
Flow, Level Tx, RTD (Ohm), Thermocouple (mV)
Digital input module : Typical input to these modules is 24 V DC, 115 V AC and 230 V AC.
For eg: Switches, Pushbuttons, Relays, pump valve on off status.
Output Module: These modules act as link between the CPU and the output devices in the
field.
Analog output module : Typical output from these modules is 4-20 mA, 0-10V. For eg: Control
Valve, Speed, and Vibration
Digital output module: Typical output from these modules is 24 V DC, 115 V AC and 230 V
AC. For eg: Solenoid Valves, lamps, Actuators, dampers, Pump valve on off control.
PLC and PC are said to be similar in their physical construction but differ in their functions.
A PLC is specifically designed for harsh conditions with electrical noise, magnetic fields,
vibration, extreme temperatures or humidity. Common PCs are not designed for harsh
environments. Industrial PCs are available but cost more.
By design PLCs are friendlier to technicians since they are in ladder logic and have easy
connections. Operating systems like Windows are common. Connecting I/O to the PC is not
always as easy.
Page | 29DEPARTMENT OF ECE, GITAM UNIVERISTY
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
.
PLCs execute a single program in sequential order. They have better ability to handle events in
real time. PCs, by design, are meant to handle simultaneous tasks. They have difficulty handling
real time events.
4.2 Vendors of PLCs used in HPCL:1. ICS Triplex
2. Schneider Electric
3. Honeywell Authority India Limited (HAIL)
4. Modicon
5. GE FanucPage | 30
DEPARTMENT OF ECE, GITAM UNIVERISTY
Fig 4.3 A PLC System
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
Various PLCs used in Visakh Refinery:
S No PLC
SYSTEM
MODEL
LOCATIO
N UNITS BACKUP CONTENTS
1
SCHNEIDE
R QUANTUM CENTUM
OLD CO
BLR LADDER LOGIC
2
ALLEN
BRADLEY 2/5 CPP CPP HRSG1 SUPPLM FIRING
3
ALLEN
BRADLEY 2/5 CPP CPP HRSG2 SUPPLM FIRING
4
ALLEN
BRADLEY 5/20 PP1 F/R PP1 WIL8 LADDER LOGIC
5
ALLEN
BRADLEY 5/40 CENTUM CDU3
42F01/F02/46F01 LADDER
LOGIC
6
ALLEN
BRADLEY 5/60 LPG C/R LPG LPG LOGIC WITH MMI
7
ALLEN
BRADLEY SLC CDU-I CDU-I
2F04 SOOT BLOWERS
LADDER
8
ALLEN
BRADLEY SLC CENTUM DHDS-SRU 65K201A LADDER LOGIC
9
ALLEN
BRADLEY SLC CENTUM DHDS-SRU 65K201B LADDER LOGIC
10
ALLEN
BRADLEY SLC CENTUM DHDS-SRU 65K101B LADDER LOGIC
11
ALLEN
BRADLEY SLC CENTUM DHDS-SRU 65K101A LADDER LOGIC
12
ALLEN
BRADLEY SLC PP1 F/R PP1
STACK ANALYSERS
LADDER
13
ANSHUMA
N DMP DMP SILICA ANAL LADDER
14 GE-FANUC LM 90/30 CENTUM PP1 NCO BOILER LADDER
Page | 31DEPARTMENT OF ECE, GITAM UNIVERISTY
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
15 GE-FANUC LM 90/30 DMP C/R DMP2 DM2 LOGIC WITH HMI
16 GE-FANUC LM 90/30 DMP C/R DMP3 DM3 LOGIC WITH HMI
17 GE-FANUC LM 90/30 F&S BLDG F & S FIRE SIREN SYSTEM
18
ICS-
TRIPLEX
REGENT
PLUS+ CENTUM FCCU1(R)
MAB / CAB LADDER
LOGIC
19 MARK-V TMR CPP C/R CPP I-Station T1/T2 Backup
20 MARK-V TMR CPP C/R CPP I-Station T3 Backup
21 MARK-V TMR CPP C/R CPP I-Station T4 Backup
22 MODICON MICRO CPP CPP ANALYSERS LADDER
23 MODICON
TXS
QUANTUM CENTUM H2
PSA CONTROLS WITH
MMI
24 SIEMENS MICRO ETP2 F & S FIRE WATER SYSTEM
25 SIEMENS S5 CENTUM CDU2
11F01 SOOT BLOWERS
LADDER
26 SIEMENS S5-SIMATIC PP1 F/R PP1 BHPV BOILER LADDER
2
7 SIEMENS S7 FCCU1 FCCU1 CAT TIMER LADDER
28 THL 620-12 PP2 F/R WIL B BLR LADDER LOGIC
29 THL 620-16 CPP CPP HRSG3 S/B LADDER
30 THL 620-16 CPP CPP HRSG4 S/B LADDER
31 THL 620-35 CPP CPP HRSG4 SUPPLM FIRING
32 THL 620-35 CPP CPP HRSG3 SUPPLM FIRING
33 THL 620-35 CDU1 F/R 2F01/F02 LADDER LOGIC
34 THL 620-35 CENTUM
FCCU1(R)
WGC LADDER LOGIC
35 THL 620-35
FCCU2
FIELD FCCU2 MAB LADDER LOGIC
36 THL 620-35
FCCU2
FIELD FCCU2 WGC LADDER LOGIC
37 THL 620-35 CENTUM 2F04 LADDER LOGIC
Page | 32DEPARTMENT OF ECE, GITAM UNIVERISTY
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
WITH LMM
38 THL
620-35
WITH LMM CENTUM
FCCU1(R)
RR LADDER LOGIC
39 THL FSC CENTUM D-SRU FSC BACK UP
40 THL FSC CENTUM DHDS FSC BACK UP
41 THL FSC CENTUM H2 FSC BACK UP
42 THL FSC SOE CENTUM DHDS COFIGURATION BACKUP
43 THL
SMOKE
DET
SYSTEM F&S BLDG F&S
DEVELOPEMENT PC
BACKUP
4.3 Configuration of PLCs ALLEN BRADLEY PLC:
MODICON PLC:
Page | 33DEPARTMENT OF ECE, GITAM UNIVERISTY
Fig 4.4 Allen Bradley PLC
Fig 4.5 Modicon PLC
Table-4 List of Various PLCs used in Visakh refinary
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
SIEMENS PLC:
4.4 Applications of PLCs
Page | 34DEPARTMENT OF ECE, GITAM UNIVERISTY
Fig 4.6 Siemens PLC
Fig 4.5 Modicon PLC
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
PLCs are used in industries where there is a need for the scan time to be the minimum possible.
For emergency shutdown of furnaces, heaters, batch processing, motor valves, plant interlocks,
machine protection system, ESG of any furnaces, turbine control system, water treatment in de-
mineralized plants, fire water auto cutting systems, fire siren operation systems, etc.
Traditional application of PLCs:
Packaging
Bottling and canning
Material Handling
Power Generation
HVAC/Building control systems
Security Systems
Automated Assembly
Water Treatment
Food and Beverage
Chemicals
Pulp and Paper
Pharmaceuticals
Metals
In industry, there are many production tasks, which are of highly repetitive nature. Although
repetitive and monotonous, each stage needs careful attention of operator to ensure good
quality of final product.
Many a times, flow supervision of process causes high fatigue on operator, resulting in loss
of track of process control.
Sometimes, it is hazardous also as in case of potentially explosive chemical processes.
Under all the above conditions we can use PLCs effectively in totally eliminating the
possibilities of human error.
Page | 35DEPARTMENT OF ECE, GITAM UNIVERISTY
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
Some of the capabilities of PLCs are:
Logic control
PID control
Co-ordination and communication
Operator control
Signaling and listing etc.
4.5 Advantages of PLCs
Reduced space:
PLCs are fully solid state and hence extremely compact as compared to hardwired controller
wherein electromechanical devices are used.
Energy saving:
Average power consumption is just one tenth of power consumed by an equivalent relay logic
control.
Ease of Maintenance:
Modular replacement
Easy troubleshooting
Error diagnostics with programmer
Economical:
Considering one time investment PLC is most economical system
Cost of PLC recovers within a short period (low payback period)
Greater Life and Reliability:
Page | 36DEPARTMENT OF ECE, GITAM UNIVERISTY
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
Static devices, hence lesser number of moving parts, reduces wear and tear In the case of
hardwired logic the control, hardware is either electromechanical or pneumatic and therefore it is
prone to faults due to wear and tear of moving parts resulting in lesser ON TIME of the system.
Tremendous flexibility:
To implement changes in control logic, no rewiring is required. So, considerable time is
saved.
PLC can carry out complex functions such as generation of time delays, counting,
comparing, arithmetic operations etc.
Online as well as Offline programming is possible.
High processing speed and greater flexibility in processing in both analog and digital
signals.
Suitability for closed loop tasks with several loops and high sampling frequencies.
Shorter Project Time:
The hardwired control system can be constructed only after the task is defined. In PLC, however,
the construction of the controller and wiring are independent of control program definition. This
means that the total hardware is standard and desired control is achieved through program.
Easier Storage Archiving and Documentation:
This is due to its compatibility with PC/AT, Printer and Floppy Disk etc.
Page | 37DEPARTMENT OF ECE, GITAM UNIVERISTY
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
5.DISTRIBUTED CONTROL SYSTEM
Distributed Control System is a type of automated control system that is distributed throughout a
machine to provide instructions to different parts of the machine. Instead of having a centrally
located device controlling all machines, each section of a machine has its own computer that
controls the operation.
For instance, there may be one machine with a section that controls dry elements of cake frosting
and another section controlling the liquid elements, but each section is individually managed by
a DCS. A DCS is commonly used in manufacturing equipment and utilizes input and output
protocols to control the machine.
5.1 History
Early minicomputers were used in the control of industrial processes since the beginning of the
1960s. The IBM 1800, for example, was an early computer that had input/output hardware to
gather process signals in a plant for conversion from field contact levels (for digital points) and
analog signals to the digital domain.
The DCS was introduced in 1975. Both Honeywell and Japanese electrical engineering firm
Yokogawa introduced their own independently produced DCSs at roughly the same time, with
the TDC 2000 and CENTUM systems, respectively. US-based Bristol also introduced their UCS
3000 universal controller in 1975. In 1980, Bailey (now part of ABB) introduced the
NETWORK 90 system. Also in 1980, Fischer & Porter Company (now also part of ABB)
introduced DCI-4000 (DCI stands for Distributed Control Instrumentation).
The DCS largely came about due to the increased availability of microcomputers and the
proliferation of microprocessors in the world of process control. Computers had already been
Page | 38DEPARTMENT OF ECE, GITAM UNIVERISTY
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
applied to process automation for some time in the form of both Direct Digital Control (DDC)
and Set Point Control. In the early 1970s Taylor Instrument Company, (now part of ABB)
developed the 1010 system, Foxboro the FOX1 system and Bailey Controls the 1055 systems.
All of these were DDC applications implemented within mini-computers (DEC PDP 11, Varian
Data Machines, MODCOMP etc) and connected to proprietary Input/output hardware.
Sophisticated (for the time) continuous as well as batch control was implemented in this way.
A more conservative approach was Set Point Control, where process computers supervised
clusters of analog process controllers. A CRT-based workstation provided visibility into the
process using text and crude character graphics. Availability of a fully functional graphical user
interface was a way away.
5.2 The Hierarchy of DCS
Dedicated control system
Centralized computer control
Distributed control system
Dedicated control system:
As the name suggests, a computer is assigned to each process. However, this makes the system
bulky and costly. As there are a greater number of systems, there may be lack of coordination.
Page | 39DEPARTMENT OF ECE, GITAM UNIVERISTY
Fig 5.1 Dedicated control system
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
Centralized computer control :
This system uses a computer called Mainframe computer. There is said to be only one single
computer in the system, controlling all the functions.
The main disadvantage of this system is that as a single computer has to control the system, it is
costly. If there is a problem with any one loop, the total system gets smashed and identifying the
loop is also very difficult. As it needs to handle many processes, the speed decreases.
Programming is very difficult. The system is not reliable and accurate.
About fifty years back pneumatic system was used for process controls. The transmitters and
controllers were all pneumatic instruments operating on 3 to 15 psi air signals. The main
disadvantages of these pneumatic instruments were
Very slow, response
Highly maintenance oriented
Specialized skill required for maintenance
Page | 40DEPARTMENT OF ECE, GITAM UNIVERISTY
Fig 5.2 Centralized Control Network
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
Developments in the electronic field in 1970s led to the use of electronic instruments. Electronic
transmitters and electronic controllers came to be widely used in process control applications.
They were all analog instruments and 4 to 20 ma became the industry standard for
instrumentation signals. Towards 1980 remarkable progress was made in digital electronics.
The advent of microprocessor initiated a new era in the field of instrumentation for process
control. The existing process plant pneumatic and electronics instrumentation is getting replaced
with the microprocessor based distributed control system.
New plants are coming up only with the distributed digital control system (DCS). The benefits
that accrue from the introduction of the DCS in then old plants as well as a new plants are many
such as improved productivity, high amount of flexibility, advanced control and optimization,
quick start up of the plant, less maintenance on the instrumentation, MIS, etc.
Following is a brief description of the various components of the system.
5.3 Distributed Control Systems in HPCL
The basic functionality of the DCS is “The work is distributed depending upon the
functionality.” The DCS is said to have a layered structure.
Each layer corresponds to a group of group of functions to be performed on lower layer, on
getting some instructions from the higher layer and each layer can work independently.
Three companies provide HPCL with DCS. They are:
1. Honeywell Automation India Limited (HAIL)
2. Yokogawa India Limited (YIL)
3. Asian Brown Bravery (ABB)
At the HPCL Visakh Refinery,
The CDU-I, FCCU-I, DHDS and SRU are operated by using the Honeywell DCS. Page | 41
DEPARTMENT OF ECE, GITAM UNIVERISTY
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
The Power plants, CDU-II, FCCU-II, MEROX, SRU & PRU units are operated by using
the Yokogawa DCS.
The CDU-III and Oil Movement and Storage units are operated using ABB DCS.
6.DATA COMMUNICATION
6.1 SERIAL COMMUNICATION
In telecommunication and computer science, serial communication is the process of sending data
one bit at one time, sequentially, over a communication channel or computer bus.
This is in contrast to parallel communication, where several bits are sent together, on a link with
several parallel channels. Serial communication is used for all long-haul communication and
most computer networks, where the cost of cable and synchronization difficulties make parallel
communication impractical.
At shorter distances, serial computer buses are becoming more common because of a tipping
point where the disadvantages of parallel buses (clock skew, interconnect density) outweigh their
advantage of simplicity (no need for serializer and deserializer (SERDES))
Improved technology to ensure signal integrity and to transmit and receive at a sufficiently high
speed per lane have made serial links competitive. The migration from PCI to PCI Express is an
example.
Different Serial Communication Architectures:
RS 232
RS 422
Page | 42DEPARTMENT OF ECE, GITAM UNIVERISTY
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
RS 485
Ethernet
ModBus
RS 232
Definitely the most popular interface, also being one of the first. However, things may soon
change for obvious reasons. Any PC that is purchased will have one (and sometimes more) RS-
232 port.
Sometimes, they are simply referred to as SERIAL PORTS, however this may cause confusion
since there are other Serial interfaces available. RS-232 is widely used because it is so readily
available. You don't usually need to purchase an RS-232 port since it is available on any PC.
However, it does have some disadvantages. Here are a few:
Limited Distance - Cable lengths are limited to 50 ft or less. Many will claim to
go further, but this is not recommended, and is not part of the RS-232 specification.
Susceptible to Noise - RS-232 is single-ended, which means that the transmit and
receive lines are referenced to a common ground
Not Multi-drop - You can only connect one RS-232 device per port. There are
some devices designed to echo a command to a second unit of the same family of
products, but this is very rare. This means that if you have 3 meters to connect to a PC,
you will need 3 ports, or at least, an RS-232 multiplexor.
RS-485
RS-485 is very similar to RS-422. So much so that it often causes confusion. Both are multi-
drop, and both can communicate via very long distances, so then why choose one over the other?
Page | 43DEPARTMENT OF ECE, GITAM UNIVERISTY
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
First of all, RS-485 is generally a 2-wire system, although some manufacturers may specify 4-
wire RS-485, which is far less common and very similar to RS-422.
It is important that you identify which one is being employed when considering an instrument.
Here are some main differences between 2-wire RS-485 and RS-422:
RS-485 can have multiple Commanding Devices and multiple Listening Devices.
RS-422 can have only one Commander and multiple Listeners. For example, you can
connect one PC (the Commanding device) to 10 temperature controllers (listeners).
The PC can instruct any of the controllers to change setpoint, or to send a
temperature reading, but none of the controllers can command any of the other
controllers. With RS-485, you can have multiple PC's and multiple controllers on one
bus, so that one PC can send a command to change a setpoint,and another PC can send a
command to send back data, etc. Remember that all devices on the bus must have a
unique unit address, so that only the addressed unit will respond. (similar to RS-422)
RS-485 wiring is easier since you are only dealing with 2 wires instead of 4.
Programming RS-485 is more difficult, since you are sending and receiving on
the same two wires, you need to enable and disable the transmitter at the correct time so
that you may perform proper communications. Imagine sending a command $2SEND out
of the transmitter. If the transmitter is not turned off in time, then data being sent by
another device will be missed. If the transmitter is turned off too quickly, there is a
chance that part of the command $S2END will be truncated before it ever has a chance
finishing the transmission of the character bits.
When programming an RS-485 plug-in card, you would read the STATUS
REGISTER to determine if it is time to switch or not. Some cards, such as the OMG-
ULTRA-485 has an AUTO mode where it is intelligent enough to do this automatically,
making it transparent to the programmer. Since RS-422, and RS-232 for that matter, have
separate transmit and receive lines, they are easier to implement. Of course, there are Page | 44
DEPARTMENT OF ECE, GITAM UNIVERISTY
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
other matters to consider such as handshaking, but will not be covered in this brief
description.
6.2 PARLLEL COMMUNICATION:
In telecommunication and computer science, parallel communication is a method of sending
several data signals simultaneously over several parallel channels. It contrasts with serial
communication; this distinction is one way of characterizing a communications link.
The basic difference between a parallel and a serial communication channel is the number of
distinct wires or strands at the physical layer used for simultaneous transmission from a device.
Parallel communication implies more than one such wire/strand, in addition to a ground
connection. An 8-bit parallel channel transmits eight bits (or a byte) simultaneously. A serial
channel would transmit those bits one at a time. If both operated at the same clock speed, the
parallel channel would be eight times faster.
A parallel channel will generally have additional control signals such as a clock, to indicate that
the data is valid, and possibly other signals for handshaking and directional control of data
transmission.
Examples of parallel communication systems
Computer peripheral buses: ISA, ATA, SCSI, PCI and Front side bus, and the once-
ubiquitous IEEE-1284 / Centronics "printer port"
Laboratory Instrumentation bus IEEE-488
Comparison of Serial and Parallel Communication
Before the development of high-speed serial technologies, the choice of parallel links over serial
links was driven by these factors:
Speed: Superficially, the speed of a parallel data link is equal to the number of bits sent
at one time times the bit rate of each individual path; doubling the number of bits sent at Page | 45
DEPARTMENT OF ECE, GITAM UNIVERISTY
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
once doubles the data rate (see Parallel transmission). In practice, skew reduces the speed
of every link to the slowest of all of the links.
Cable length: Crosstalk creates interference between the parallel lines, and the effect
worsens with the length of the communication link. This places an upper limit on the
length of a parallel data connection that is usually shorter than a serial connection.
Complexity: Parallel data links are easily implemented in hardware, making them a
logical choice. Creating a parallel port in a computer system is relatively simple,
requiring only a latch to copy data onto a data bus.
In contrast, most serial communication must first be converted back into parallel form by
a universal asynchronous receiver/transmitter (UART) before they may be directly
connected to a data bus.
The decreasing cost of integrated circuits, combined with greater consumer demand for speed
and cable length, has led to parallel communication links becoming deprecated in favor of serial
links; for example, IEEE 1284 printer ports vs. USB, Advanced Technology Attachment vs.
Serial ATA, and SCSI vs. FireWire.
On the other hand, there has been a resurgence of parallel data links in RF communication.
Rather than transmitting one bit at a time (as in Morse code and BPSK), well-known techniques
such as PSM, PAM, and Multiple-input multiple-output communication send a few bits in
parallel. (Each such group of bits is called a "symbol").
Such techniques can be extended to send an entire byte at once (256-QAM). More recently
techniques such as OFDM have been used in Asymmetric Digital Subscriber Line to transmit
over 224 bits in parallel, and in DVB-T to transmit over 6048 bits in parallel.
Page | 46DEPARTMENT OF ECE, GITAM UNIVERISTY
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
6.3 Fiber Optic Communication:
Fiber-optic communication is a method of transmitting information from one place to another by
sending pulses of light through an optical fiber. The light forms an electromagnetic carrier wave
that is modulated to carry information.
First developed in the 1970s, fiber-optic communication systems have revolutionized the
telecommunications industry and have played a major role in the advent of the Information Age.
Because of its advantages over electrical transmission, optical fibers have largely replaced
copper wire communications in core networks in the developed world.
The process of communicating using fiber-optics involves the following basic steps: Creating the
optical signal involving the use of a transmitter, relaying the signal along the fiber, ensuring that
the signal does not become too distorted or weak, receiving the optical signal, and converting it
into an electrical signal.
Applications:
Optical fiber is used by many telecommunications companies to transmit telephone signals,
Internet communication, and cable television signals. Due to much lower attenuation and
interference, optical fiber has large advantages over existing copper wire in long-distance and
high-demand applications. However, infrastructure development within cities was relatively
difficult and time-consuming, and fiber-optic systems were complex and expensive to install and
operate. Due to these difficulties, fiber-optic communication systems have primarily been
installed in long-distance applications, where they can be used to their full transmission capacity,
offsetting the increased cost.
Transmitters: The most commonly-used optical transmitters are semiconductor devices such
as light-emitting diodes (LEDs) and laser diodes.
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A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
Receivers: The main component of an optical receiver is a photodetector, which converts light
into electricity using the photoelectric effect. The photodetector is typically a semiconductor-
based photodiode.
Fiber: An optical fiber consists of a core, cladding, and a buffer (a protective outer coating), in
which the cladding guides the light along the core by using the method of total internal
reflection.
The core and the cladding (which has a lower-refractive-index) are usually made of high-quality
silica glass, although they can both be made of plastic as well. Connecting two optical fibers is
done by fusion splicing or mechanical splicing and requires special skills and interconnection
technology due to the microscopic precision required to align the fiber cores.
Two main types of optical fiber used in fiber optic communications include multi-mode optical
fibers and single-mode optical fibers. A multi-mode optical fiber has a larger core (≥ 50
micrometres), allowing less precise, cheaper transmitters and receivers to connect to it as well as
cheaper connectors.
However, a multi-mode fiber introduces multimode distortion, which often limits the bandwidth
and length of the link. Furthermore, because of its higher dopant content, multimode fibers are
usually expensive and exhibit higher attenuation.
The core of a single-mode fiber is smaller (<10 micrometres) and requires more expensive
components and interconnection methods, but allows much longer, higher-performance links.
Comparison with Electric Transmission:
The choice between optical fiber and electrical (or copper) transmission for a particular system is
made based on a number of trade-offs. Optical fiber is generally chosen for systems requiring
higher bandwidth or spanning longer distances than electrical cabling can accommodate.
The main benefits of fiber are its exceptionally low loss, allowing long distances between
amplifiers or repeaters; and its inherently high data-carrying capacity, such that thousands of
electrical links would be required to replace a single high bandwidth fiber cable.
Another benefit of fibers is that even when run alongside each other for long distances, fiber
cables experience effectively no crosstalk, in contrast to some types of electrical transmission
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A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
lines. Fiber can be installed in areas with high electromagnetic interference (EMI),(along the
sides of utility lines, power-carrying lines, and railroad tracks). All-dielectric cables are also
ideal for areas of high lightning-strike incidence.
For comparison, while single-line, voice-grade copper systems longer than a couple of
kilometers require in-line signal repeaters for satisfactory performance; it is not unusual for
optical systems to go over 100 kilometers (60 miles), with no active or passive processing.
Single-mode fiber cables are commonly available in 12 km lengths, minimizing the number of
splices required over a long cable run. Multi-mode fiber is available in lengths up to 4 km,
although industrial standards only mandate 2 km unbroken runs. In short distance and relatively
low bandwidth applications, electrical transmission is often preferred.
6.4 ETHERNET:
Ethernet is a family of frame-based computer networking technologies for local area networks
(LANs). The name comes from the physical concept of the ether.
It defines a number of wiring and signaling standards for the Physical Layer of the OSI
networking model, through means of network access at the Media Access Control (MAC) /Data
Link Layer, and a common addressing format.
Ethernet is standardized as IEEE 802.3. The combination of the twisted pair versions of Ethernet
for connecting end systems to the network, along with the fiber optic versions for site backbones,
is the most widespread wired LAN technology.
It has been in use from around 1980 to the present, largely replacing competing LAN standards
such as token ring, FDDI, and ARCNET.
Ethernet was originally based on the idea of computers communicating over a shared coaxial
cable acting as a broadcast transmission medium. The methods used show some similarities to
radio systems, although there are fundamental differences, such as the fact that it is much easier
to detect collisions in a cable broadcast system than a radio broadcast.
The common cable providing the communication channel was likened to the ether and it was
from this reference that the name "Ethernet" was derived.
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A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
From this early and comparatively simple concept, Ethernet evolved into the complex
networking technology that today underlies most LANs. The coaxial cable was replaced with
point-to-point links connected by Ethernet hubs and/or switches to reduce installation costs,
increase reliability, and enable point-to-point management and troubleshooting
StarLAN was the first step in the evolution of Ethernet from a coaxial cable bus to a hub-
managed, twisted-pair network. The advent of twisted-pair wiring dramatically lowered
installation costs relative to competing technologies, including the older Ethernet technologies.
Above the physical layer, Ethernet stations communicate by sending each other data packets,
blocks of data that are individually sent and delivered. As with other IEEE 802 LANs, each
Ethernet station is given a single 48-bit MAC address, which is used to specify both the
destination and the source of each data packet.
Network interface cards (NICs) or chips normally do not accept packets addressed to other
Ethernet stations. Adapters generally come programmed with a globally unique address, but this
can be overridden, either to avoid an address change when an adapter is replaced, or to use
locally administered addresses.
Due to the ubiquity of Ethernet, the ever-decreasing cost of the hardware needed to support it,
and the reduced panel space needed by twisted pair Ethernet, most manufacturers now build the
functionality of an Ethernet card directly into PC motherboards, eliminating the need for
installation of a separate network card.
6.5 MODBUS:
Modbus is a serial communications protocol published by Modicon in 1979 for use with its
programmable logic controllers (PLCs). It has become a de facto standard communications
protocol in industry, and is now the most commonly available means of connecting industrial
electronic devices.
At HPCL-VR one of the most crucial uses of the Modbus is that it is used for interconnection
between PLCs and DCS. The main reasons for the extensive use of Modbus over other
communications protocols are:
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A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
It is openly published and royalty-free
Relatively easy industrial network to deploy
It moves raw bits or words without placing many restrictions on vendors
Modbus allows for communication between many devices connected to the same network, for
example a system that measures temperature and humidity and communicates the results to a
computer. Modbus is often used to connect a supervisory computer with a remote terminal unit
(RTU) in supervisory control and data acquisition (SCADA) systems.
Protocol Versions:
Versions of the Modbus protocol exist for serial port and for Ethernet and other networks that
support the Internet protocol suite.
Most Modbus devices communicate over a serial EIA-485 physical layer.
For serial connections, two variants exist, with different representations of numerical data and
slightly different protocol details. Modbus RTU is a compact, binary representation of the data.
Modbus ASCII is human readable, and more verbose.
Both of these variants use serial communication. The RTU format follows the commands/data
with a cyclic redundancy check checksum, while the ASCII format uses a longitudinal
redundancy check checksum. Nodes configured for the RTU variant will not communicate with
nodes set for ASCII, and the reverse.
For connections over TCP/IP, the more recent variant Modbus/TCP exists. It does not require a
checksum calculation.
Data model and function calls are identical for all three communication protocols; only the
encapsulation is different.
An extended version, Modbus Plus (Modbus+ or MB+), also exists, but remains proprietary to
Modicon. It requires a dedicated co-processor to handle fast HDLC-like token rotation. It uses
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DEPARTMENT OF ECE, GITAM UNIVERISTY
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
transition/edge triggered instead of voltage/level triggered. Special interfaces are required to
connect Modbus Plus to a computer, typically a card made for the ISA (SA85), PCI or PCMCIA
bus.
Implementations:
Almost all implementations have variations from the official standard. Different varieties may
not communicate correctly between different suppliers equipment. Some of the most common
variations are:
Data Types
Floating Point IEEE
32 bit integer
8 bit data
mixed data types
bit fields in integers
multipliers to change data to/from integer. 10, 100, 1000, 256 ...
Protocol extensions
16 bit slave addresses
32 bit data size (1 address = 32 bits of data returned.)
word swapped data
Limitations
Modbus was designed in the late 1970s to communicate to programmable logic
controllers, the number of data types is limited to those understood by PLCs at the time.
Large binary objects are not supported.
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A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
No standard way exists for a node to find the description of a data object, for example, to
determine if a register value represents a temperature between 30 and 175 degrees.
Since Modbus is a master/slave protocol, there is no way for a field device to "report by
exception" (except over Ethernet TCP/IP, called open-mbus)- the master node must
routinely poll each field device, and look for changes in the data. This consumes
bandwidth and network time in applications where bandwidth may be expensive, such as
over a low-bit-rate radio link.
Modbus is restricted to addressing 247 devices on one data link, which limits the number
of field devices that may be connected to a master station (once again Ethernet TCP/IP
proving the exception).
Modbus transmissions must be contiguous which limits the types of remote
communications devices to those that can buffer data to avoid gaps in the transmission.
Page | 53DEPARTMENT OF ECE, GITAM UNIVERISTY
A STUDY OF ANALYSERS, PROGRAMMABLE LOGIC CONTROLLERS, DISTRIBUTED CONTROL SYSTEMS AND DATA COMMUNICATION
BIOBLIOGRAPHY
1. Computer architecture and Organisation : Morris Mano Mc.Graw-Hill,Newyork, Second edition.
2. Advanced Microprocessors and pheripherals :A.K.Ray ,K M Bhurchandi,Mc.Graw-Hill Second edition.
3. IEEE standard Programmable Logic Interface Technology :The institute of Electrical and Electronics Engineers,1994.
4. BAM-1020 HPCL-VR user maintainance manual 3rd Edition.
5. Wireless Communication And Networks :Theodre Rappaport.Willey Publication
6. Abromovici.M.breuer and Freid Man ,F Data Communication systems and its testing. Indianapolis,Ind.Wiley-IEEE press,1994.
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