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Anonymous Poster

Control Valve

06/08/2008 11:24 PM

Can any body please let me know how the rated Cv is calculated for any control valve ..While calculating it what is the test fluid taken by manufacturer .whether it is air or water is taken as test fluid?
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shogun7
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Join Date: Dec 2005
Posts: 92
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#1

Re: Control Valve

06/09/2008 2:27 AM

For liquids the flow coefficient - Cv - expresses the flow capacity in gallons per minute (GPM) of 60oF water with a pressure drop of 1 psi (lb/in2).

Cv = q (SG / dp)1/2 (1)

where

q = water flow (US gallons per minute)

SG = specific gravity (1 for water)

dp = pressure drop (psi)

So for 250 gpm the Cv =250 but if you have a 5# PD then the Cv = 112
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gsuhas
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#2

Re: Control Valve

06/09/2008 10:47 PM

The Cv can not be calculated. It is to me derived from the measurements.

You pass the flow of water through the control valve opened to required degree, control the flow using another valve, so that the delta P across the control valve is 1 psig. The flow passing is the Cv at the set opening of the control valve.

Or you can derive Cv by just passing the flow without any control. Measuring the deplta P across the valve and the flow rate. using the equations find out what will be the flow rate at 1 psig delta P.
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srini
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Location: Houston, Texas
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#3

Re: Control Valve

06/10/2008 10:38 AM

An alternate unit for valve capacity is Kv.This is arrived at from flow in M3/hr per 1.0 Bar pressure drop across the valve.

So Kv=0.865Cv

There is one more unit Av which I have never seen being used.

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shogun7
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#4

Re: Control Valve

06/10/2008 3:40 PM

The flow coefficient-Cv required for a specific application can be estimated by using specific formulas for the different fluids or gases. With the estimated Cv value - the correct valve can be selected from the manufacturers catalogues.
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