# [APD] RE: Flow Coefficient vs. flow rate

```I'm not sure if I completely understand your question (as I think you need
to know some more variables) but the volumetric flow rate version of the
ANSI/ISA/IEC valve sizing equation is the following:

Cv = q /(Nt*Fp*P1*Y*sqrt(x/Gg*T1*Z))

where:
Cv=Valve sizing coefficient
q=Volume rate of flow
Nt=Numerical constant for different systems of units
Fp=Piping geometry factor
P1=Upstream absolute static pressure
Y=Expansion factor (ratio of flow coefficient for a gas to that for a liquid
at the same Reynolds Number)
x=Ratio of pressure drop to upstream absolute static pressure (deltaP/P1)
Gg=Gas specific gravity
T1=Absolute upstream temperature
Z=Compressibility factor

Email me off list to go over your question in more detail.

Regards,
Aaron in Calgary

Douglas Guynn wrote:

> I have a question for all of you engineering types out there.
>
> Does anyone know the formula that will answer the following question:
>
> If a valve with a max flow coefficient (Cv) will flow 0.040
> std. L/min of
> air with a 10 psig pressure drop, how would I calculate how
> much air the
> valve would flow with a Cv of 0.005, all other variables the same?
>
> Can you use the formula to account for changes in inlet pressure?
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