Process Control 1

Suez University

Faculty Of Petroleum & Mining

Engineering

Prepared by/ Student/ Mohamed Salah abou El_hamed

Department/ Petroleum Refining

Year/ Fourth

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Measuring principle

The measuring system operates on the principle of transit time difference. In this measurement method, acoustic (ultrasonic) signals are transmitted between two sensors. The signals are sent in both directions, i.e. the sensor in question works as both a sound transmitter and a sound receiver.

As the propagation velocity of the waves is less when the waves travel against the direction of flow than along the direction of flow, a transit time difference occurs. This transit time difference is directl proportional to the flow velocity.

Principle of the transit time difference measurement method

a Sensor

b Sensor

Q Volume flow

v Flow velocity (v &∆t )

∆t Transit time difference (∆t = ta – tb)

A Pipe cross-sectional area

The measuring system calculates the volume flow of the fluid from the

measured transit time difference and the pipe cross-sectional area. In addition to

measuring the transit time difference, the system simultaneously

measures the sound velocity of the fluid. This additional measured variable can

be used to distinguish different fluids or as a measure of product quality.

The measuring device can be configured onsite to suit the specific

application using Quick Setup menus.

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Pressure

Temperature

flow

level

Concentration

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Control System Components A control system is comprised of the following components:

1. Primary elements (or sensors/transmitters)

2. Controllers

3. Final control elements (usually control valves)

4. Processes

illustrates a level control system and its components. The level in the tank

is read by a level sensor device, which transmits the information on to the

controller. The controller compares the level reading with the desired level or

set point and then computes a corrective action. The controller output adjusts the

control valve, referred to as the final control element. The valve percent opening

has been adjusted to correct for any deviations from the set point.

Primary Elements

Primary elements, also known as sensors/transmitters, are the instruments

used to measure variables in a process such as temperature and pressure. A full

listing of the types of primary elements available on the market would be very

long, but these sensor types can be broadly classified into groups including the

following:

1. Pressure and level

2. Temperature

3. Flow rate and total flow

4. Quality or analysis instruments (e.g. electrolytic conductivity, pH, Pion)

5. Transducers (working with the above or as individual units)

Some specific examples of instruments from the more common groups listed

above willbe examined, including pressure, level, temperature and flow.

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Pressure Measurement

There are numerous types of primary elements used for measuring pressure

that could

be studied,These include manometers, Bourdon tubes and differential pressure

(DP) cells.

Manometers.

Manometers are simple rugged.

cheap and give reliable static measurements.

very popular as calibration devices for pressure measurement.

The working concept of a manometer is simple.

where

ρ A fluid with a known density

P1–P2 is used to measure the pressure difference between two points

H is the height difference in the fluid level

The Bourdon Tube Pressure Gauge

The Bourdon tube pressure gauge, named

after Eugene Bourdon (ca. 1852) and shown

in Figure, is probably the most common

gauge used in industry. Figure illustrates

the Bourdon tube pressure gauge. The

essential feature of the Bourdon tube is its

oval-shaped cross section. The operating

principle behind the gauge is that when

pressure is applied to the inside of the tube

the tip is moved outward. This pulls up the

link and causes the quadrant to move the

pinion to which the pointer is attached. The

resultant movement is indicated on a dial.

A hairspring is also included (not shown) to

take up any backlash that exists between

quadrant and pinion; this has no effect on

calibration.

The accuracy of the gauge is ±0.5% of

full range for commercial models. Generally,

the normal working pressure will be specified as 60% of the full scale.

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The Differential Pressure Cell

The DP cell is considered by many as the start of modern-day automation.

The DP cell was developed at the outbreak of World War II by Foxboro in

Massachusetts, USA, on a government grant provided that it was not patented.

The idea was that competition would bring down the price of the instrument.

DP cells allow remote transmission to central control rooms where a small

number of operators can control large, complex plants.

For example, a typical petroleum refinery processing around 80 000

barrels/day

(530 m3/h) might have 2000 DP cells throughout the refinery.

Seal systems can be used to enhance the usefulness of the DP cell by

facilitating pressure measurement for many temperature ranges (−73–427◦C).They serve to protect the transmitter from the process fluid, using a hydraulic

system to conduct the pressure from the process fluid to the transmitter. Only

the seal’s diaphragm contacts the process fluid, and a capillary or tube of fluid

transfers the process pressure from the diaphragm to the transmitter. Before a

seal is installed consider ambient conditions, such as temperature, which may

introduce errors.

Some of the major benefits of DP cells are that their maintenance is

practically zero and no mercury is used in the operation of the transducer.

1. The Pneumatic DP Cell

Pressure is applied to the opposite sides of a silicone-filled twin diaphragm

capsule.

The pressure difference applies a force at the lower end of the force bar, which

is balanced through a simple lever system consisting of the force bar and baffle.

This force exerted by the capsule is opposed through the lever system by the

feedback bellows. The result is a 3 psi (or 20 kPa if calibrated in SI units) to 15

psi (or 100 kPa if calibrated in SI units) signal proportional to the DP.

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2. Modern DP Cells

E-type electronic transducers, strain gauges, capacitive cell transducers and

most recently digital electronics have replaced the pneumatic-type DP cell.

photo of a modern DP cell The features of the modern electronic DP cell,

such as the Rosemount Model 3051 or Honeywell’s ‘smart’ transmitter, include

remote range change, diagnostics that indicate the location and type of any

system faults, easy self-calibration, local digital display, reporting and

interrogation functions and local and remote reporting. The modern DP cell can

also be directly connected to a process computer and has the ability to

communicate with the computer indicating problem analysis that is then

displayed on the computer screen.

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Temperature Measurement

Temperature measurement can be accomplished using several types of

sensing mechanisms. Temperature measurement systems generally consist of a

sensor, a transmitter, an external power supply (for some types of systems), and

the wiring that connects these components.

The temperature measurement sensors most commonly used in engineering

applications are thermocouples, resistance temperature detectors (RTD’s), and

infrared (IR) thermometers; these devices are described in detail in the

following paragraphs. Integrated circuit (IC) temperature transducers and

thermistors also are commonly used but have more limitations than

thermocouples, RTD’s, and IR thermometers. measuring devices.

Other types of temperature sensors include bimetallic devices, fluid

expansion devices, and change-of-state devices. Bimetallic temperature sensors

relate temperature to the difference in thermal expansion between two bonded

strips of different metals.

Fluid expansion devices, such as the common thermometer, measure

temperature as a function of the thermal expansion of mercury or organic liquid,

such as alcohol. Change-of-state temperature sensors change appearance when a

specific temperature is reached. One major drawback of these types of sensors is

that they do not readily lend themselves to automatically recording temperatures

on a continuous or periodic basis.

1* Thermocouples

Due to their simplicity, reliability, and relatively low cost, thermocouples are

widely used. They are self-powered, eliminating the need for a separate power

supply to the sensor.

Thermocouples are fairly durable when they are appropriately chosen for a

given application.Thermocouples also can be used in high-temperature

applications, such as incinerators.

Measurement Principle and Description of Sensor

A thermocouple is a type of temperature transducer that operates on the

principle that dissimilar conductive materials generate current when joined (the

Seebeck effect). Such a device is made by joining two wires made of different

metals (or alloys) together at one end, generating a voltage eAB when heated, as

shown schematically in Figure.

The generated voltage is proportional to the difference between the temperatures

of the measured point and an experimentally determined reference point (block

temperature) and is also dependent on the materials used. A basic temperature

monitoring system using a thermocouple is made up of the thermocouple,

connectors, extension wires, isothermal block (also called temperature blocks,

terminal blocks, or zone boxes), and a voltmeter or transmitter.

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This schematic is for a type J iron (Fe)-constantin (Cu-Ni) thermocouple. As

the thermocouple junction point (J1) is heated or cooled, the resulting voltage

can be measured using a potentiometer or digital voltmeter (DVM), which is

calibrated to read in degrees of temperature. In practice, a programmed indicator

or a combination indicator/controller is used to convert the signal from voltage

to temperature using the appropriate equation for the particular thermocouple

materials and compensation for voltage generated at terminal connection points

(J3) and (J4).

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2*Resistance Thermometer Detectors (RTDs)

RTDs are made of either metal or semiconductor materials as resistive

elements that may be classed as follows :

1. Wire wound – range 240–260◦C, accuracy 0.75%

2. Photo etched – range 200–300◦C, accuracy 0.5%

3. Thermistor beads – range 0–400◦C, accuracy 0.5%

An example is the platinum RTD, which is the most accurate thermometer in

the world.

RTDs exhibit a highly linear and stable resistance versus temperature

relationship. However, resistance thermometers all suffer from a self-heating

effect that must be allowed for, and I2R must be kept below 20 mW, where I is

defined as the electrical current and R is the resistance.

When compared to thermocouples, RTDs have higher accuracy, better

linearity and longterm stability, do not require cold junction compensation or

extension lead wires and are less susceptible to noise. However, they have a

lower maximum temperature limit and are slower in response time in

applications without a thermal well (a protective well filled with conductive

material in which the sensor is placed).

Selecting Temperature Sensors

Getting the right operating data is crucial in selecting the proper sensor. A

good article on selecting the right sensor is by Johnson.

a selection of thermocouples, RTDs and temperature accessories, such as

thermal wells, that are typically available from instrument suppliers (in this case

Emerson Process Management). a picture of a typical temperature sensor and

transmitter assembly.

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Flow Measurement

Flow measurement techniques can be divided into the following categories :

1. Obstruction-type meters, such as

(a) Orifice plates

(b) Flow nozzles

(c) Venturi tubes

(d) Pitot tubes

(e) Dall tubes

(f) Combinations of (a) to (e)

(g) Elbow and target meters

3. Rotational or turbine meters

4. Variable area meters/rotameters

5. Ultrasonic and thermal-type meters

5. Square root extractors for obstruction-type meters

6. Quantity or total flowmeters, such as

(a) Positive displacement

(b) Sliding vane

(c) Bellows type

(d) Nutating disc

(e) Rotating piston

(f) Turbine type

7. Magnetic flowmeters

8. Vortex meters

9. Mass flowmeters, such as

(a) Coriolis effect flowmeters

(b) Thermal dispersion flowmeters

Selection of a flowmeter

is based on obtaining the optimum measuring accuracy at the minimum

price. It should be noted that flowmeters may use up a substantial amount of

energy, especially when used in low pressure vapour service. Therefore they

should only be provided when necessary.

There are many factors to consider when selecting a flowmeter, including

properties of the fluid being measured such as viscosity, and performance

requirements such as response time and accuracy. Ambient temperature effects,

vibration effects and ease of maintenance should also be compared when

selecting a flowmeter. For a more thorough presentation on the selection of

flowmeters, refer to the article by Parker .

Orifice plates and magnetic flowmeters will be discussed in detail since they

are two of the most common types found in the fluid-processing industry.

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1. Orifice Plates

The concentric orifice plate is the least expensive and the simplest of the head

meters. The orifice plate is a primary device that constricts the flow of a fluid to

produce a DP across the plate. The result is proportional to the square of the

flow. a typical thin-plate orifice meter.

An orifice plate usually produces a larger overall pressure loss than other

primary devices. A practical advantage of the orifice plate is that cost does not

increase significantly with pipe size. They are used widely in industrial

applications where line pressure losses and pumping costs are not critical.

The thin concentric orifice plate can be used with clean homogenous

fluids,which include liquids, vapours or gases, whose viscosity does not exceed

65 cP at 15◦C. In general the Reynolds number (Re) should not exceed 10 000.

The plate thickness should be 1.5–3.0 mm or, in certain applications, up to 4.5

mm .

Many variations for orifice plates have been suggested, especially during the

1950s when oil companies and universities in North America and Europe

sponsored numerous PhD studies on orifice plates. Of these only a few have

survived, which were the ones that incorporated cheaply some of the features of

the more expensive devices. Figure 2.15 shows some of these designs.Other

designs that are utilized include eccentric and segmental orifice configurations.

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2. Magnetic Flowmeters

Themagnetic flowmeter is a device that measures flow using amagnetic field,

as implied by the name. The working relationship for magnetic flowmeters is

based on Faraday’s law (see Equation), which states that a voltage will be

induced in a conductor moving through a magnetic field:

In Equation

E is the generated emf. B is the magnetic field strength

D is the pipe diameter. V is the average velocity of the fluid

k is a constant of proportionality.

3. Flow nozzles.

The flow nozzle is similar to the venturi tube in that it has a throat; the

primary difference is that the flow nozzle does not include a long converging

cone and diffuser. Flow nozzles are generally selected for high temperature,

pressure, and velocity applications (e.g., measuring steam flow).

Flow nozzles, which can be used to measure fluid flow in pipes with

diameters of approximately 7.6 to 61 cm (3 to 24 in.).

have the following advantages:

1. Net pressure loss is less than for an orifice plate (although the net pressure

loss is much greater than the loss associated with venturi tubes), and

2. Can be used in fluids containing solids that settle.

Flow nozzles have the following disadvantages:

1. More expensive than orifice plates.

2. Limited to moderate pipe sizes.

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4. Venturi tubes

The venturi tube consists of a converging cone, venturi throat, and diffuser.

The inlet section to the venturi tube consist of a converging cone that has an

included angle of roughly 21 degrees. The converging cone is joined by a

smooth curve to a short cylindrical section called the venturi throat. Another

smooth curve joins the throat to the diffuser, which consists of a cone with an

included angle of roughly 7 to 8 degrees. The diffuser recovers most of the

pressure normally lost by an orifice plate.

The venturi tube can be used to measure fluid flow in pipes with diameters

of approximately 5 to 120 centimeters (cm) (2 to 48 inches [in]).

The venturi has the following advantages over the orifice plate:

1. Handles more flow while imposing less permanent pressur loss

approximately 60 percent greater flow capacity.

2. Can be used with fluids containing a higher percentage of entrained solids.

3. Has greater accuracy over a wider flow rate range.

.

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Level Measurement

Level measurement is the determination of the location of the interface

between two fluids which separate by gravity, with respect to a fixed plane. The

most common level measurement is between a liquid and a gas.

Methods of level measurement include the following:

1. Float actuated devices, such as

(a) Chain or tape float gauge

(b) Lever and shaft mechanisms

(c) Magnetically coupled devices

2. Pressure/head devices, that is, DP cells or manometers:

(a)Bubble tube systems

(b)Electrical methods

3. Thermal methods

4. Sonic methods

5. Radar methods

6. Nuclear methods

7. Weight methods

It is extremely important that vessels are well protected from an overflow

condition. An overflowing vessel may have severe safety consequences,

impacting nearby employees, the environment and the surrounding community.

Some vessels require low-level protection to operate safely. Ideally, each vessel

should have a visual indication for the operator, an alarm point and a transmitted

level indicator.

Factors affecting the choice of levelmeasurement include corrosive process

fluids (requiring exotic materials), viscous process fluids which may cause

blockages, hazardous atmospheres, sanitary requirements, density changes,

dielectric and moisture changes and the required degree of accuracy and

durability.

Pressure/head devices such as the DP cell are the most popular of all level

measurements devices. TheDPcell can often be usedwheremanometers are

impracticable and floatswould cause problems. The DP cell requires a constant

product density for accurate measurement of level or a way of compensating for

density fluctuations. Figure 2.8 demonstrates a typical set-up for level

measurement using a Rosemount Model 3051SMV level controller, which is

essentially a combined DP cell and proportional controller.

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Ultrasonic Methods

Ultrasonic refers to sound of such high frequency that it is undetectable to

the human ear. Frequencies used in level measurement range from 30 kHz to

the megahertz range. A transducer sends pulses of ultrasonic sound to the

surface of the liquid to be measured.

The liquid surface reflects these pulses and the distance from transducer to

the liquid level is calculated. This calculation is based on the speed of the

signal and the time elapsed between the sending and receiving of the ultrasonic

sound signal .

Ultrasonics can be top or bottom mounted. Although a top-mounted device

is easier to service, mists, vapours and internal ladders and agitators may cause

erroneous readings.

Bottom-mounted devices must be calibrated to the density of the measured

fluid; however, bubbles and solids in the liquid may skew their reading.

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Concentration Measurement

Introduction

Many of the laws of optics were discovered or rediscovered in the period

called the Renaissance. Isaac Newton studied the properties of prisms and their

ability to separate white light into what we now call the visible spectrum and

also prepared lenses to use in telescopes. Laws of optics such as the law of

reflection,

Chromatography

Chromatography is a technique for separating chemical substances that relies

on differences in partitioning behaviour between a flowing mobile phase and a

stationary phase to separate the components in a mixture.

The sample is carried by a moving gas stream through a tube packed with a

finely divided solid or may be coated with a film of a liquid. Because of its

simplicity, sensitivity, and effectiveness in separating components of mixtures,

gas chromatography is one of the most important tools in chemistry. It is widely

used for quantitative and qualitative analysis of mixtures, for the purification of

compounds, and for the determination of such thermochemical constants as

heats of solution and vaporization, vapour pressure and activity coefficients. Gas

chromatography is also used to monitor industrial processes automatically: gas

streams are analyzed periodically and manual or automatic responses are made

to counteract undesirable variations.

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Many routine analyses are performed rapidly in environmental and other

fields. For example, many countries have fixed moniotor points to continuously

measure the emission levels of for instance nitrogen dioxides, carbon dioxide

and carbon monoxide. Gas chromatography is also useful in the analysis of

pharmaceutical products, alcohol in blood, essential oils and food products.

The method consists of, first, introducing the test mixture or sample into a

stream of an inert gas, commonly helium or argon, that acts as carrier. Liquid

samples are vaporized before injection into the carrier stream. The gas stream is

passed through the packed column, through which the components of the sample

move at velocities that are influenced by the degree of interaction of each

constituent with the stationary nonvolatile phase. The substances having the

greater interaction with the stationary phase are retarded to a greater extent and

consequently separate from those with smaller interaction. As the components

elute from the column they can be quantified by a detector and/or collected for

further analysis.

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Carrier gas; D: Detector gas; M: Make up gas

Two types of gas chromatography are encountered: gas-solid

chromatography (GSC) and gas-liquid chromatography (GLC). Gas-solid

chromatography is based upon a solid stationary phase on which retention of

analytes is the consequence of physical adsorption. Gas-liquid chromatography

is useful for separating ions or molecules that are dissolved in a solvent. If the

sample solution is in contact with a second solid or liquid phase, the different

solutes will interact with the other phase to differing degrees due to differences

in adsorption, ion-exchange, partitioning or size. These differences allow the

mixture components to be separated from each other by using these differences

to determine the transit time of the solutes through a column.

Gas Chromatography - Carrier gas

The choice of carrier gas depends on the type of detector that is used and the

components that are to be determined. Carrier gases for chromatographs must be

of high purity and chemically inert towards the sample e.g., helium (He), argon

(Ar), nitrogen (N2), carbon dioxide (CO2) and hydrogen (H2). The carrier gas

system can contain a molecular sieve to remove water or other impurities.

Sample injection system

The most common injection systems for introduction of gas samples are the

gas sampling valve and injection with a syringe.

Direct injection with syringe

Both gaseous and liquid samples can be injected with a syringe. In the

simplest form the sample is first injected into a heated chamber where it is

vaporized before it is transferred to the column.

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When packed columns are used, the first part of the column often serves as

injection chamber, separately heated to an appropriate temperature. For capillary

columns a separate injection chamber is used from which only a small part of

the vaporized/gaseous sample is transferred to the column, so called split-

injection. This is necessary in order not to overload the column in regard to the

sample volume .

When trace amounts can be found in the sample, so called on-column-

injection can be used for capillary-GC. The liquid sample is injected directly

into the column with a syringe. The solvent is thereafter allowed to evaporate

and a concentration of the sample components takes place. If the sample is

gaseous the concentration is achieved by so called cryo focusing. The sample

components are concentrated and separated from the matrix by condensation in

a cold-trap before the chromatographic separation.

Injection with valve/sample loop

Loop-injection is often used in process control, where gaseous or liquid

samples continuously flow through the sample loop. The sample loop is filled in

off-line position with a syringe or an automatic pump.

Thereafter the loop is connected in series with the column and the sample is

transferred by the mobile phase. Sometimes a concentration step is necessary.