138625544 Ch 5 Control Valves Ppt

7/28/2019 138625544 Ch 5 Control Valves Ppt

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Control Valves

Ted HuddlestonDept of Chemical EngineeringUniversity of South Alabama


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Fig C-7.2 page 747

Reciprocating StemSliding Stem

vp


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Control Valve ActionPage 201 - see Figure C-7.2 on page 747, In this figuresupply air enters the case above the diaphragm. For anair pressure of 3 psig, the diaphragm and valve stem

are at their topmost position and the valve is wide open.When the air pressure is 9 psig, the diaphragm andvalve stem are at their mid point and the valve is half open. When the air pressure is 15 psig, the diaphragmand valve stem are at their bottom position and the valveis closed.

This is an air-to-close (ATC) valve. If control air pressure were to fail, the spring would push the stem upand open the valve - called a fail-open (FO valve.


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ATO and ATC Actuators


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ATO and ATC Actuators


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Control Valve Actions

Which to select? ATC or ATO ?Ask Which is safer if there is a malfunctionwhich includes loss of control air? On fuel and steam valves, use FC.On cooling water, and inert gas blanket linesuse FO.On all valves perform a safety audit.


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P&ID Example


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Control Valve Characteristics

Describes how flow through valve varieswith stem position.Results from the shape of the valves seatand plug, which determines how valveresistance changes with stem position.Inherent valve characteristicInstalled valve characteristic


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Control Valve Action

Actuators are built to either open a valve with increasingair pressure (ATO) or to close a valve with increasing air

pressure (ATC).

See Figure C-8.1 page 750.


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Plugs and Seats

Parabolic Characteristic Quick-Opening Characteristic(Equal Percentage)


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Valve Equation

f

v

v G

p

Cf

f = liquid flow, U.S. gpm

D pv = pressure drop, psiGf = specific gravity of liquidCv = valve coefficientCv = C v(vp)

Eqn (5-2.1) page 203

LIQUID SERVICE


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Inherent Characteristic

constant)vp(Cf v

Valve is mounted on a test stand such that vp can be

changed, changing C v and f, but pressures areadjusted such that D pv is held constant.

f

vv G

p)vp(Cf


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Inherent Valve Characteristic

Curves

% vp

Cv,maxf max100

vmax

Cf 100f

f

0

constant)vp(Cf v

Page 211


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Inherent Valve Characteristic

CurvesLinear Characteristic:

Cv

= Cv,max

(vp) 0 < vp < 1

Equal Percentage Characteristic

Cv = C v,max a (vp-1) 0 < vp < 125 < a < 100

Page 211


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Equal Percentage Valves

Note the shape of the inherent characteristic.

When the valve is at vp = 0.2, flow is small, and the slopeD f/D vp is small.

When the valve is at vp = 0.8, flow is large, and theslope D f/D vp is large.


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The quantity

100vp

f f 100f vpf

represents the percent change of flow per unitchange in valve position.

The percentage change of flow per unit change

in valve position is the same or equal for all vp.


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constantCf 1)(vp

maxv,

)1)(ln(constantC 1)(vpmaxv, a

dvp

df


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)ln(

constantC)1)(ln(constantC1

1)(vpmaxv,

1)(vpmaxv,

a

a

dvpdf

f


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Installed Valve Characteristics

Section starts on page 212. See diagram on page 213. Acontrol valve is installed in series with a heat exchanger.Water is supplied from the left at a constant pressure, andexhausts to the atmosphere at the right. Valve positioncan vary, flow through the process can vary, pressure dropthrough the exchanger, through the pipe and fittings, and

through the valve can vary. But the total pressuredrop D p o is constant.


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Assume flow is fully turbulent. Pressure drop throughline, fittings, and heat exchanger is

2f LL f Gk p

Pressure drop across valve is obtained from valve equation

2

v

2

f v Cf

Gp


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The total pressure drop is

2f L2

v

o f Gk C1

p

Solving for the flow f yields


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f

o

2vL

v

Gp

Ck 1

Cf(vp)

Linear

=%1)(vp

maxv,v

maxv,v

CC(vp)CC


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What installed valve characteristic isdesirable?Flow rate should change by the same amount

per vp change throughout valve stroke.

Gain (flow rate change/vp change) should beconstant for all values of vp.This is a linear installed valve characteristic.


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Inherent vs Installed Characteristics

vp

flow

When a valve is installed in series with processequipment, the installed characteristic flow curve isshifted up and to the left of the inherent flow curve.

See Figure 5-2.7 on page 217.

Installed

Inherent


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So which to choose ?

When valve takes up most ( > 80 %) of the pressuredrop use a linear valve. (Ex: Valve on a fuel gasline between the header and the burner.)When valve is installed in series with other equipment and valve takes up 20 60% of the

pressure drop use =% valve.When valve takes up 60-80% of pressure drop let

boss decide.


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Valve Rangeability

f(0.05)f(0.95)

positionvalve5%atFlow

positionvalve95%atFlowtyRangeabili

Defined on page 212. Also known as the turn-downratio, it is the ratio of the maximum controllable flow tothe minimum controllable flow FOR THEINSTALLED VALVE. Flow f(vp) is computed usingEQN (5-2.14) on page 214.


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Control Valve C v

The size of a control valve is described by its C v.

f

vmaxv, G

pCf

The valve is fully open.

D pv = 1 psiG f = 1 (fluid is water)

f = C v,max gpm


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Control Valve C v

The C v of a control valve is the flow in gpm

of water that would flow through the fullyopen valve caused by a pressure drop of 1

psi.


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Control Valve Sizing

Linear Installed Characteristic

Small Pressure DropHigh RangeabilityOperates approximately half open at design

flow


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Method One: The Overcapacity M ethod in which the valve

is specified so that when the valve is fully open with specifiedD pv, the flow is double the design flow.

1) flow = 2 x Design Flow

2p

p4

p)2 ov

o


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4) From the manufacturers literature select thesmallest C v that exceeds the value of C v,max found in

part (3).

f

vmaxv,

Gp

FlowDesign2C)3


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Method Two: The Design F low M ethod in which the valve

will pass the design flow at valve position vp = 0.6 with thespecified D pv.

1) flow = Design Flow

2p

p4

p)2 ov

o


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f

vmaxv,

Gp

vp

FlowDesignC

)3 Valve Linear


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f

v1vpmaxv,

Gp

FlowDesignC

%)3 Valve


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4) From the manufacturers literature select the

smallest C v that exceeds the value of C v,max found in part (3).


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We have not considered cavitation, flashing, sonic flow, the pressure drop recovery factor, the Reynolds Number

recovery factor, piping geometry factor, noise reduction,and other important items. The equation used here is for liquid service. Equations for gases and vapors are morecomplicated.

When buying valves from a particular manufacturer use that manufacturers equationsand technical support.

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138625544 Ch 5 Control Valves Ppt