Relief capacity conversion

[JBombard]

I am looking to size a PRV for a propane cylinder and have run into a snag that my simple mind can't seem to get past. The NFPA code calculation gives an answer in lb/hr but the relief valves are certified in SCFM of free air. I can get to SCFM of propane without issue but how does one compare propane to air?

Thank you for any insight.

Jason

 

[psafety]

SCFM GAS = [(lb/hr)/(4.5*SpecGrav)]

 

[EGT01]

ASME Section VIII, Division 1, Appendix 11 explains how to perform capacity conversions for pressure safety valves. If you don't have the ASME code you can find an excerpt in the Crosby Engineering Handbook...

http://www.tycovalves-na.com/ld/CROMC-0296-US.pdf

Also, don't overlook contacting a local valve supplier. I'm sure they would be happy to assist.

 

[ggordil]

I use the following

SCFM (air)= (flow x 13.36)/(SG x (1/SG)^.5)

where

flow= flow rate of propane in lb/minute
and SG= specific gravity of propane

 

[ARenko]

density of gas = density of air x specific gravity of gas

 

[skearse]

If it's a PRV I'd go with the method in Appendix 11, since it's what the vendors will have to go by to certify the capacity of the valve (provided it's an ASME UV valve).