Hello,
First of all, get your hand on a PSV sizing software, I suggest Pentair (PRV2SIZE), It is free and can be used for preliminary calculation with good accuracy (I always double check with a handmade API 520 excel tool, and in general the value of both are equal or extremely close).
Second thing, use standard value for your gage pressure (14.7 psia), there is no reason to complicate things your own gage pressure (13.5) for PSV calculation because it is under standard atmospheric. The impact will only be a small to negligable increase of the capacity of your PSV, this approach is conservative.
Third thing, the capacity you see on the nameplate of the PSV is not an air equivalent rated capacity.
This capacity is called the "nameplate capacity" and its the capacity you will get with the PSV on the test bench with the test gas or liquid depending in the PSV trim (water, air or steam) at testing conditions.
The proper way of finding the rated capacity of any PSV is to use the equations provided in API 520 and calculate the capacity you will get according to your own process conditions.
In the case you provided, I made calculation with API 520 formula (homemade tool) and with PRV2SIZE to show nameplate and rated capacity :
L orifice
Set pressure : 720 psig
Relief pressure : 806.7 psia (10% overpressure)
Preliminary nameplate capacity (air at 60 F) : 41 154 SCFM (using preliminary discharge coefficient of 0.975) IN PRV2SIZE / API 520
Farris as discharge coefficient of 0.953 according to their documentation for this PSV, so their nameplate capacity would drop to 40 225 SCFM (41 154 / 0.975 X 0.953 )
Final nameplate capacity : 40 225 SCFM (you report stamped of 40220 SCFM)
Rated Capacity calculation using PRV2SIZE and API 520 :
Edit : MW :19.3
Ratio of specific heats : 1.66
Compressibility factor (Z) : 0.798
Relief temperature : 40.83 F
Coefficient of discharge (preliminary) : 0.975
Preliminary rated capacity : 186 024 lb / hr using PRV2SIZE - 185 996 using API 520 equations
Farris discharge coefficient : 0.953
Rated capacity (using API value) : 181 799 lb / hr
Since your required relief capacity is 146000 lb/hr (Edit : Updated with op value), A L orifice is the correct size for your PSV.
Moral of story : DO NOT CONVERT NAMEPLATE CAPACITY TO RATED CAPACITY, IT IS FAR MORE RELIABLE TO CALCULATE BOTH INDEPENDANTLY !
Nameplate cacacity should not be used for anything beside the testing of the PSV at the test bench.
Regards
Marc-Andre
Edited by Marc-Andre Leblanc, 05 August 2014 - 12:05 PM.