08. Control & Instrumentation

8/3/2019 08. Control & Instrumentation

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Control&

Instrumentation


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Instrumentation and control

Instrumentation and control system is designed for minimum local manning and operator attention.

The basic need of the instrumentation us for safeoperation and efficient functioning of the plant.

This can be achieved by installing optimum number measurement, indicating, transmitting and controllinginstruments for all required parameters of the plants(e.g. pressure, level, temperature, flow etc.)


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BASIC COMPONENTS OF ACONTROL SYSTEM

S ensor Also often called as Primary Element.Acquires information about the status of the process variables.Typical examples:RTD/thermocouples (for temperature measurements),

Capacitance type cells (for liquid level /draft /pressuremeasurements), etc.

Controller The Brain or Heart Of the control system (the decision maker).

It is the hardware element with Built-in capacity for performing theonly task requiring some forms of Intelligence.Typical examples:Electronic controllers, digital computers used as controller.


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Transmitter S econdary Element.Responsible of passing the information acquired by the sensor tocontroller and sending the controller decision to the final controlelement.Measurement and control signals may be transmitted or aselectrical signals.Typical examples:Electrical transmitters.

Final control elementHave the task of actually implementing the control commandissued by the controller on the process.Typical examples:Control valve, variable speed motors, electric motors etc.


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The importance of these components is that they performthe three basic operations that must be present in everycontrol system:1. Measurement:Measuring the variable to be controlled is usually done by

the combination of sensor and transmitter.2 . Decision:Based on the measurements and the set point, thecontroller must then decide what to do to maintain thevariable at its desired value.3 . Action:

As the result of the controller's decision, the system mustthen take an action. This is usually accomplished by thefinal control element.


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CONTROL SYSTEM

I/O CARDS

PROCESSOR

I/O CARDS

OPERATOR STATIONS

ENGG

STATION


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Basic Parameters

Measurement & Control of followingParameters are essential for Instrumentation

PressureTemperatureFlowLevel


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TAPPING POINT S

GASES

STEAMGASES

LIQUIDS


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PressurePressure measurements are one of the most common measurements

required in the boiler .These range from very low -1 500 mmwcDraft to very high steam pressure 1 50 kg/cm 2 .That includedifferent type of media like steam ,water, fuel oil, air, gas eachwith varying degree of accuracy and reliability.Pressure varies depending on altitude above sea level, weather pressure fronts and other conditions. The measure of pressure is,therefore, relative and pressure measurements are stated aseither gauge or absolute. A gauge pressure device will indicatezero pressure when bled down to atmospheric pressure (i.e.,gauge pressure is referenced to atmospheric pressure). Gaugepressure is denoted by a (g) at the end of the pressure unit [e.g.,kPa (g)].Absolute pressure includes the effect of atmospheric pressure

with the gauge pressure. It is denoted by an (a) at the end of thepressure unit [e.g., kPa (a)]. An absolute pressure indicator wouldindicate atmospheric pressure when completely vented down toatmosphere - it would not indicate scale zero.Absolute Pressure = Gauge Pressure + Atmospheric PressureThe majority of pressure measurements in a plant are gauge.


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Pressure


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Pressure

Pressure measuring devicesThe common pressure measuring devicesused in IJT boilers areDiaphragm, capsule gauges for lowpressure measurementBourdon tube gauges for medium and highpressureS mart type Pressure Transmitter withrequired range


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Pressure


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Pressure

Installation:Pressure Gauges are installed on site usingaccessories as follows:S yphonsS unbber S eal2 way manifoldThese are protective devices for pressuremeasuring instrument from surges & pulsations.These devices used as per applicationrequirement


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Pressure Gauges (Normal Installation)


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Pressure Gauges along with S yphon


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Pressure Gauges along with S unbber:


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Pressure Transmitter (Normal Installation)


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Temperature Measurement

Temperature is measured by following:Temperature Gauge:

1. Mercury inS

teel type2 . Bi-Metallic3 . Gas Filled

Temperature S ensors:1. RTD2 . Thermocouple


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Temperature Measurement

Installation:Temperature Gauges / S ensors are

installed on site using thermowell.

It is a protective devices for temperature

measuring instrument from damages.


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Temperature Gauge / ELEMENT(Normal Installation)


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Temperature Transmitter (Normal Installation)


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Flow Measurement

Installation:Flow sensors are installed in line of the fluid to create DP &hook-ed up with transmitter for measuring DifferentialPressure, which corresponds to flow:

Q = K1* (P/T ) * D P W here,DP = differential pressure across the flow elementP = Main Steam Pressure

T = Main steam temperatureK1 = ConstantQ = Compensated flow


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Flow Transmitter (S TEAM)


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Flow Transmitter (AIR)


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Drum Level

STEAM

W ATER

H: Drum Center to CenterDistance

h: Height of water insidepressurized drum

HP LP

CondensatePot withconstant Headas a wet Leg

D1 = W ater Density Inside DrumD2 = W ater Density in Leg

HP LP = (P+hD1g) (P+HD2g)Diff. Pr., P= hD1g HD2g = hD1 HD2( W here P,h and H are measured in mm of water)(D1 & D2 are in gm/cc)Hence, h = P + HD2

D1


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Drum Level

D rum Level Control MethodS ingle ElementThree Element

Shrink &

Swell EffectS hrink : When S team Flow Decreases

Drum Level DecreasesS well : When S team Flow Increases

Drum Level Increases

Basic ElementsDrum LevelS team FlowFeed Water Flow


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Drum Level


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Transmitter

Typical Basic Operation of Transmitter

Sensing Element + Transducer =Transmitter unit

SENSINGELEMENT INDICATION TRANSDUCER


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Transmitter

Transmitters are generally use is of 2 wire type.This is the most widely used method for transmitter connections .There are three basic

elements in this loop, namely Power S

upply ,transmitter and the receiving instrument .Theyare connected in series and the transmitter actsas a current regulator in the series circuit . Thecurrent in the series circuit changes with respectto change in process parameter. This simplifiescabling and reduces erection and cable cost.


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Transmitter

Normal Pressure Transmitter


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Transmitter

Differential Pressure Transmitter


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TROUBLE S HOOTING OFTRAN S MITTER

I. POTENTIAL SOURCELOOP WIRING

II . HIGH OUTPUTPOTENTIAL S OURCE :1.PRIMARY ELEMENT2 .IMPUL S E PIPING3 .TRAN S MITTER ELECTRONIC S4 .S EN S ING ELEMENT


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III . LOW OUTPUT OR NO OUTPUTPOTENTIAL S OURCE :

1. PRIMARY ELEMENT2 . LOOP WIRING3 . IMPUL S E PIPING

4 . S EN S ING ELEMENT


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IV .TRANSMITTER D OES NOTCHARACTERIZE PROPERLY

POTENTIAL S OURCE :1.PRE SS URE S OURCE /CORRECTION2 .mA METER3 .POWER S UPPLY4 . TRAN S MITTER ELECTRONIC S5 . S EN S ING ELEMENT


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Control Valve

Being the Final Control Element in a system is not aneasy job.To start with, Control Valves are blamed for any and all

problems that crop up in the process.Control Valve are subjected to corrosion, high velocity,cavitations, flashing liquids, cryogenic temperatures,abrasion, high temperatures and thermal shock.Control Valve are expected not only to throttle alongthrough all this, but most likely, you are also being askedto act as a block valve and shut off tight .


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Control Valve Classification

Rotary Motion

Linear Motion

Butterfly

Eccentric Plug

Ball

Swing Through

Lined

EccentricSegmented

Full

V-Notch

Globe

Diaphragm

Pinch or Clamp

Globe

Angle

3-Way

Single Seated

Double Seated

Split body

Control

Valve


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Control Valve Flow Characteristics


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Control ValveLeakage Classifications

Class I:Identical to Class II, III, and IV in construction and design intent, but noactual shop test is made.

Class II:Intended for double-port or balanced singe-port valves with a metal pistonring seal and metal-to-metal seats. Air or water at 45 to 60 psig is the testfluid. Allowable leakage is 0 .5% of the rated full open capacity.

Class III:Intended for the same types of valves as in Class II.

Allowable leakage is limited to 0 .1 % of rated valve capacity.

Class IV:Intended for single-port and balanced single-port valves with extra-tightpiston seals and metal-to-metal seats. Leakage rate is limited to 0 .0 1% of rated valve capacity.


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Control ValveLeakage Classifications

Class V:Intended for the same types of valves as Class IV.The test fluid is water at 1 00 psig or operating pressure.Leakage allowed is limited to 5 X 10 ml per minute per

inch of orifice diameter per psi differential.Class VI:Intended for resilient-seating valves.The test fluid is air or nitrogen. Pressure is the lesser of 50 psig or operating pressure.The leakage limit depends on valve size and rangesfrom 0 .1 5 to 6 .75 ml per minute for valve sizes 1through 8 inches.


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Basic Control Loops in BOILER

Pre ssu re Cont rolF W h ead er p ressu reF u rnac e D raftCombustion Cont rol (M ain St em Pre ssu re)Soot Blow er

D e-aerato r

Leve l cont rolD rum Leve l

o Singl e El em ento Th ree El em ent

D e-aerato rCBD

Te mp eratu re Cont rolSup er H eat ed St eam

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08. Control & Instrumentation