cv sizing
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Safety Moment
Put chairs back to its right place when leave your seat
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Control Valve Sizing
Hydraulics
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What is Control Valve Sizing???
It is a procedure by which the dynamics of a process system are matched to the performance characteristics of a valve.
This is to provide a control valve that will best meet the needs of managing flow within that process system.
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Basic information requirements for effective valve sizing
• For the system:- Pressure before and after the control valve, P Flow rate Process temperature Properties of media(viscosity, density, molecular
weight, vapor pressure)
• For the control valve:- Flow capacity (Cv) Fluid recovery factor(FL)
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– Definition: Kv : quantity of water (m3/h) between 5 and 40°C that will pass control valve at 1 bar pressure drop at a specified travel
– Definition Cv : quantity of water at = 60°F (USgal/min) that will pass control valve at 1 psi pressure drop at a specified travel
– Kv = 0.856 ∙ Cv
– SI :
– Eng:
– Cv generally used in our business
Valve Flow Coefficient
VVV P
K
0
V
VV PC
0
856.0
ΦV = volume flowΡ = densityρ0 = standard densityΔPV = pressure drop over valve
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CONTROL VALVESSizing
Valve Body size can be estimated from cV
D = diameter in mm
Cv = 12 25 mm or 1 inch valve full sizeCv = 27 38 mm or 1½ inch valve full sizeCv = 48 50 mm or 2 inch valve full sizeCv = 108 76 mm or 3 inch valve full sizeCv = 192 100 mm or 4 inch valve full size
124.25 VCD
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Fluid Recovery Factor
The fluid recovery factor is effectively an index of
Pressure recovery in a control valve.High FL value indicates low
pressure recovery. High FL value results in better
resistance to cavitation.
FL ~ (P1-P2)/(P1-PVC)
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Choked flow
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Cavitation and Flashing
Choked Flow
Cavitation
Flashing
( increase in pressure downstream of the valve)
( deccrease in pressure downstream of the valve)
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Methods to reduce the potential for cavitation
The value of p1 can be increased while keeping Δp the same by moving the control valve to a location further upstream, or to a location at a lower elevation.
The vapour pressure can be decreased by installing the valve where the liquid temperature is lower, such as the cool side of a heat exchanger.
A valve style with a higher value of FL can be selected.
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FL for different valve types
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What is critical flow?
It is an extension of cavitation in which it simply gets worse as the pressure drop increases to the point that changes (reductions) in downstream pressure no longer influences flow rate.
Generally, W = f( Pv) Non-critical flowWhereas in critical flow:-
,which is independent of PL
),( , propertiesvalvepropertiesfluidPfW H
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Flow is critical when:-
For incompressible fluid flow:-
For compressible fluid flow:-
vap
C
vapHLLHV P
PP
PFPPP 28.096.02
Tcc
HLHV xPPPP vp
4.1
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Importance of calculating appropriate Cv
If the calculated Cv is too small, the valve will be undersized and the process system may be starved for fluid. This also causes a higher pressure drop across the valve causing cavitation or flashing.
If Cv is too high, the valve will be too big leading to monetary waste and difficult machine manoeuvrability. A larger Cv can also be a problem for throttling because the flow cannot be effectively controlled at the openings.
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Flow Characteristics
Three basic flow characteristics available are:-
Linear:- Flow change is proportional to valve opening
Equal percentage:- Equal valve position changes give equal percentage flow changes
Quick opening:- Flow increases rapidly in a linear relationship with plug lift, reaching a max. value at a low lift
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Valve Characterization
Since it is desirable to achieve and maintain process stability, the proper inherent valve characteristic must be selected to compensate for process changes. The first step is to determine the controlled process variable. There are four main classes: Liquid level Pressure Flow Temperature.
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Controlled Variable:- Liquid Level
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Controlled Variable:- Pressure
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Controlled Variable:- Flow
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Controlled Variable:- Temperature
• Experience has shown that the best inherent characteristic, when temperature is the controlled variable, is equal-percentage valve.
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Basic Hydraulic Systems
Vapor \ gas system
PS PDtn
PH PLSource Destination
LHV PPP
onscontributiupstream
iifSH PPP ,
onscontributidownstream
iifDtnL PPP ,
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Basic Hydraulic Systems
Liquid system without pump
,
,
PS PDtn
HS
GRADE
Hv
HD
tn
PH PL
Source Destination
onscontributiupstream
iifupstreamSSH PPPP ,,
onscontributidownstream
iifdownstreamSDtnL PPPP ,,
5, 10
gHHP VSupstreamS
5, 10
gHHP VSupstreamS
5, 10
gHHP VDtn
downstreamS
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Basic Hydraulic Systems
• Liquid system with pump
PS PDtn
PH PL
GRADE
Hv
HD
tn
HS
HP
PP
Source Destination
onscontributiupstream
iifPupstreamSSH PPHPPP ,,
valvetopumponscontributi
iifPf
onscontributiupstream
iif PPP ,,,
5,
10gHH
PPHPP PSPfPSP
PSOPdynP PHPHP ,,
pumpexclvalvetopumpfromonscontributi
iifdynPpf
valvetopumponscontributi
iifpf
onscontributiupstreami
if PPPPPP
.
,,,,,,
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System performance – static only
Response to flow variations for static only system
PS PDtn
HS
GRADEH
v
HD
tn
PH PL
Source Destination
– No friction in lines– Constant source and destination pressures
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System performance – Static only
Response to flow variations for static only system
Flow
Pres
sure
Source Pressure
Destination Pressure
ControlValve∆P
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CONTROL VALVESSystem performance – with friction
Response to flow variations for system with friction
Flow
Pres
sure
Source Pressure
Destination Pressure
ControlValve∆P Downstream
Friction∆P
UpstreamFriction∆P
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