Conversion between PSV nameplate capacity and Mass Flow rate

[chemter]

Hello.
In ASME Sec. VIII, Div. I, Appen. 11, equations are provided to convert the nameplate capacity of a safety valve from one medium to another. You can therefore arrange the equations, for example, to convert a nameplate capacity in SCFM of Air to LB/HR of air. I understand the methodology they are using here. I've also read several threads showing the conversion from SCFM of air to LB/HR of air (and vice versa) based on the volume the gas occupies at STP (specified at what ever conditions the user desires). My question is if the safety valve capacity is really in SCFM of Air at a specific temperature, why do you end up with two different results when you use the two equations above (assuming the same STP conditions are used with each)? I wouldn't expect an exact match, but I would think they would be close. Does it have to do with the actual flow through the PSV? What am I missing.
I'm aware this subject has been touched upon many times before, but searching through the threads I didn't see a conversation on my specific question. Thank you.

 

[Latexman]

chemter,

If you would reproduce the two equations and their nomenclature definitions here, I'm sure you will get a response. Use the Process TGML link for formatting information.

Good luck,
Latexman