Tek-Tips is the largest IT community on the Internet today!

Members share and learn making Tek-Tips Forums the best source of peer-reviewed technical information on the Internet!

  • Congratulations TugboatEng on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

Gas valve calculations

Status
Not open for further replies.

mechengr02

Mechanical
Joined
Feb 15, 2005
Messages
1
Location
US
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.
 
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
 
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
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor

Back
Top