When to use required relief capacity for PSV piping delta P cal

[carol2005]

Some PSV sizing programs provide a feature to calculate PSV inlet and outlet piping pressure loss by using the process required relief rate instead of PSV actual relief capacity. I know ASME VIII Div I appendix M gives recommendation of 3% and 10% limitation for compressible fluid inlet/outlet piping pressure loss with conventional PSV. But I couldn't find clear guidiline about imcompressible fluid. Can I assume:
For compressible fluid, i.e. vapor or gas or two-phase relief, actual PSV capacity should be applied for piping delta P calculation.
For imcompressible fluid, i.e. pure liquid relief without flashing at tailpipe, the process required relief rate can be used for piping calculation, provide the PSV is ASME compliant, UV stamped, liquid trim PSV. By saying this, I assume the PSV will open proportional to process relief rate.
Please advise me if this is not correct.

 

[CMA010]

I never heard of a different approach for incompressible, compressible or two-phase.

Inlet losses: rated capacity
Outlet losses: rated capacity
Flare headers: relief loads

See API RP 520-II.

 

[JoeWong88]

carol2005,

PSV type (modulating and pop-acting) subject to flowing phase (incompressible, compressible or two-phase).

Capacity to be considered for tail pipe, lateral & main header subject to PSV type. Please refer to 7.2.1, table 12.

For tail pipe, please consider rated flow.

 

[JoeWong88]

carol2005,

To be exact, API Std 521 - ISO 23251 - Ed 5 - Jan 2007 - Pressure-relieving and Depressuring Systems, section 7.2.1, table 12.

For inlet line loss, same principle per table 12 should be applied per API Std 521 i.e. relieving flow for modulating type PSV. On the other hand, API RP 520 PII - Ed 5 Aug 2003 - Sizing, Selection & Installation of Pressure-Relieving Devices in Refinery, section 4.2.3.1 requested rated flow for modulating type...In order to handle the discrepancy, API Std 521, section 7.3.1.2. has some description on the issue. Conservative approach is to consider rated flow.