Cv Calcs & Choked Pressure

[MechEngNBallValves]

Hi all,

I'm developing software to be able to size our ball valves properly for compressible & incompressible flow. I've been searching for easy methods to calculate choked pressure drop (to compare it to actual pressure drop & determine if flow is choked) and I haven't had too much luck.

I have searched through these forums and have found a link: Link
which seems useful, but we do not have cavitation index the formula calls for.

I'm currently using this equation: FL^2 (P1 - FF * PV) FL - some kind of shape factor ~0.55-0.60 for ball valves (not sure where this came from), P1 - inlet pressure, FF in ISA S75.01-1985 Annex G as 0.96-0.28(vapor pressure/critical pressure)^1/2, PV - vapor pressure.

I need to know if this is OK by ISA standards. My company has been using this equation, but in order to use it in the software I need to prove that it is valid by some credible reference. I'm not entirely sure where the equation came from, but we have been using it for a while.

Is this equation OK or is there something else I should be using? Please assume I have little to no empirical data about our valves and am unable to attain data.

Thanks in advance,
K

 

[MechEngNBallValves]

EDIT: the above equation I described is supposed to be equal to choked pressure drop (ie deltaP_choked = FL^2...)

 

[davefitz]

Try the method defined in the "ISA handbook for control valves". That method implies you will need to characterize your ball valves for Cv and Xt, and the individual cases for choked flow will also vary according to the flowing gase's Cp/Cv ratio.