Gas valve calculations

[mechengr02]

I asked Nibco valve service department for source of gas flow equation that appears in one of their catalogs. Here is the equation: Q = 1360 Cv x sq. rt. of (delta-P x P1 / ST) where

Q = gas flow (Scfh)

S = specific gravity of gas

T = temperature (degrees R)

delta P = pressure drop across valve (psi)

P1 = upstream pressure (psia)

This equation appears on page 103 of their catalog C-BIV-0402, rev. 04-26-02, at the top of a flow data table for Nibco valves.

I have tried without success to find the relationship between Nibco's equation and derivations I have located in other sources, such as Crane's Technical Paper 410. Perhaps you can shed some light on this item for me. The reason for my interest is that I have utilized Nibco's equation to find gas flow under low pressures of 0 to 1 psi in assemblies of pipe, valves, and fittings for which I was able to determine Cv values. I want to make certain that my application doesn't run counter to some unwritten condition on its use.

Any direction you can provide would be appreciated.

 

[davefitz]

This looks like a choked flow equation, which implies the downstream presure is less than 50% of the upstream pressure.

From the ISA handbook of control valves, one can obtain a choked flow equation thru a valve as :

W= 63.4*0.667*Cv*SQRT{Pi*Fk*Xt/sv,i} where
W= flow, lb/hr
Cv= valve Cv
Pi= inlet pressure, psia
Fk = (Cp/Cv)/1.4 ( ratio of spec ht /1.4)
Xt= valve oblique shock wave coef, typ =0.75 for globe, =0.65 for angle, =0.15 for ball, =0.30 for butterfly.
sv,i= inlet spec volume, ft3/lbm

 

[jsummerfield]

Perhaps your application for the Nibco valve should be as a block instead of a control valve. Block valve makers are not likely to furnish the level of data expected from a control valve manufacturer.

John