PSV Inlet pressure drop question

[brownplayboy310]

If I'm installing a PSV on a steam header and have a CV Fail open case is the 3% rule calculated from the header to the inlet of the PSV or from the CV on the header?

 

[Latexman]

Per API-RP-520 Part 2 section 4.2.2
"When a pressure-relief valve is installed on a line directly
connected to a vessel, the total non-recoverable pressure loss
between the protected equipment and the pressure-relief
valve should not exceed 3 percent of the set pressure of the
valve except as permitted in 4.2.3 for pilot-operated pressure relief
valves. When a pressure-relief valve is installed on a
process line, the 3 percent limit should be applied to the sum
of the loss in the normally non-flowing pressure-relief valve
inlet pipe and the incremental pressure loss in the process line
caused by the flow through the pressure-relief valve."

Does this help?

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[MikeClay]

In general Latexman is correct. The only case that I would use the pressure drop from the header to PSV instead of the entire pressure drop would be the case where the header PSV is set to protect downstream equipment and the header piping is rated to a higher pressure. However, I would still check the pressure drop from the PCV to the PSV to determine if the flowing relief pressure could over pressure the header piping.

--Mike--