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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Equal Percentage Valves
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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Equal Percentage Valves
constantCf 1)(vp
maxv,
)1)(ln(constantC 1)(vpmaxv, a
dvp
df
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Equal Percentage Valves
)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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Installed Valve Characteristics
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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Installed Valve Characteristics
The total pressure drop is
2f L2
v
o f Gk C1
p
Solving for the flow f yields
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Installed Valve Characteristics
f
o
2vL
v
Gp
Ck 1
Cf(vp)
Linear
=%1)(vp
maxv,v
maxv,v
CC(vp)CC
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Installed Valve Characteristics
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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Control Valve Sizing
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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Control Valve Sizing
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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Control Valve Sizing
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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Control Valve Sizing
f
vmaxv,
Gp
vp
FlowDesignC
)3 Valve Linear
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Control Valve Sizing
f
v1vpmaxv,
Gp
FlowDesignC
%)3 Valve
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Control Valve Sizing
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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Control Valve Sizing
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.