Inlet/Outlet Piping to PSV Pressure Drop

[RJB32482]

Hello,
I am looking at sizing PSV inlet and discharge piping for the following 100% Vapor scenario:
Sizing Case: Fire
MAWP: 120 psig
Overpressure: 21%
Relief Material: Air

For a total vapor case, I looked at API 521 Section 5.4.1.3.2 to size the discharge piping (assuming isothermal flow conditions). Is there a recommended approach for sizing the inlet piping to satisfy the 3% of set pressure recommendation for vapor flow? Could equation 21 of the same section by calculating the inlet Mach number by equation 23 (replacing P2 with P1 and P1 is the Set pressure of the relief valve)?

Thanks.

 

[pleckner]

You don't need to use anything more than the non-compressible pressure loss calculation. The reason is that you are sizing the line for only a 3% pressure loss. The reason you use compressible specific calculations is that in gas flow, the density change is large with big pressure drops. In small pressure drops (less than 10% of upstream pressure), the non-compressible calculation is adequate.