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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Liquid system without pump

,

,

PS PDtn

HS

GRADE

Hv

HD

tn

PH PL

Source Destination


iifupstreamSSH PPPP ,,

onscontributidownstream

iifdownstreamSDtnL PPPP ,,

5, 10

gHHP VSupstreamS

5, 10

gHHP VSupstreamS

5, 10

gHHP VDtn

downstreamS

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• Liquid system with pump

PS PDtn

PH PL

GRADE

Hv

HD

tn

HS

HP

PP

Source Destination


iifPupstreamSSH PPHPPP ,,

valvetopumponscontributi

iifPf


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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Documents

CV sizing