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Final Control Elements

Introduction:• Final control element receives the signal from

the controller and implements the manipulation required to control the process.

• In industry, the final control element takes many forms. It can be a heating coil in the case of temperature control of a furnace etc.

• In chemical engineering applications, the manipulation is usually flow (liquids, gases or vapors) through control valves.

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Flow Manipulation through Control Valve

Current signal from thecontroller

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Principle of I / P (Current to Pressure) Converter

Signal to thecontrol valve

Signal fromthe controller

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What are the Components of a Control Valve?

Top portion is called as actuator

Bottom portionis valve body

Diaphragm

Calibrated spring

Stem

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Types of Control Valves based on Valve Action

• The valve is direct acting because the air pressure signal is directly applied on the top of the diaphragm.• The valve is air - to- close type because when the air pressure signal increases from 3 to 15 psi (20 to100kPa) the valve closes.

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Direct acting Air-to-Open Type Valve

• The valve is direct acting because the air pressure signal is directly applied on the top of the diaphragm.• The valve is air - to- open type because when the air pressure signal increases from 3 to 15 psi (20 to100kPa) the valve opens.

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Reverse acting Air-to-Close Type Valve

• The valve is reverse acting because the air pressure signal is applied at the bottom of the diaphragm.• The valve is air - to- close type because when the air pressure signal increases from 3 to 15 psi (20 to100kPa) the valve closes.

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Reverse acting Air-to-Open Type Valve• The valve is reverse acting because the air pressure signal is applied at the bottom of the diaphragm.• The valve is air - to- open type because when the air pressure signal increases from 3 to 15 psi (20 to100kPa) the valve opens.

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Types of Control Valves based on Valve Characteristics

S / Smax

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Quick Opening Control Valve

• This valve provides a large change in flow rate for a small change in valve position.

• This valve is used for ON / OFF applications (also called as two position control system).

• The valve moves quickly from open to close position or vice versa and hence makes the corrective action immediately.

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The Linear Control Valve

• The linear characteristic can be described by

S Smax

Q

Qmax

0 0

• The valve is used where the flow has to be linear with the controller signal

Flow

rate

Stem position

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Hysteresis in Linear Control Valves is eliminated by Valve Positioner

• The valve provides two different flow rates while the stem position is increasing and decreasing

• Hysteresis is caused by the friction in the stem movement

Valvepositioner

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An Equal Percentage Control Valve

• Rangeabilty:

R=50Qmin

Qmax

Smax0

Q

S

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Correct Estimation of ‘R’ from Plant Data

Semilog Plot:

Log

(Q/Q

max

)

(S-Smax) / Smax

Slope is Log R

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Control Valve Sizing

• Control valve sizing means determining the correct size of the control valve for specific installation.

• First we find the Valve Coefficient CV.• CV is defined as the number of US gallons of

water per minute that will flow through a fully open valve with a pressure drop of 1 psi.

• For example, a control valve with CV of 5 will pass 5 gal / min of water when the valve is fully open and the pressure drop is 1 psi.

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To find the Valve Size:

QLP1 P2

QL= Maximum liquid flow rate in m3 / min.

P1 = Valve inlet pressure, kPa

P2 = Valve outlet pressure, kPa

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Step1: Find CV from the Formula

CV = Valve coefficientSG = Specific gravity of the liquid flowing (density

of liquid divided by the density of water)K = 1.4 * 10-3ΔP = P1-P2 = Pressure drop across the valve

Note: Different formulae are given in the literature for gases, vapors and steam

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Step 2: Use the following Table to find the Valve Size in mm

CV Valve Size

CV Valve Size

0.3 7 108 76

3 13 174 102

14 25 400 152

35 38 725 203

55 50 1100 254

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Dynamic Characteristics of Control Valves (Speed of Response)

3 psi

15 psi

Time, secFl

ow

ΔX

ΔY

ΔX=Step change in input air pressure signal

ΔY=Transient response in the flow

Time, sec

X

Y

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Control Valves usually show First-order Response

X Y

X = Input air pressure signal, kPaY = Response in the flow, m3 / secGV = Transfer function of the control valveKV = Gain of the control Valve,

( m3 / sec) / kPaτ = Time constant of the control valve,

sec

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Conclusions

• List out various factors based on which you select a control valve.

• What is the source of hysteresis in a control valve and how do you overcome it?

• What are the applications of the three different control valves mentioned in this notes.