API 521 PSV sizing basis when using ARC valve

[wpm1]

http://www.eng-tips.com/viewthread.cfm?qid=82879

Have 2 x 100% multistage centrifugal pumps each with arc valves for min flow. Assuming operator error on both 2 x 100 pumps and blocked flow API states that no credit should be taken for instrumentation,should you assume both arc valves have also failed for the design basis of piping protection PSV?

It seems allot like multi-jeopardy to me. Also one consideration in my mind is the direct acting nature of the arc valves which makes their reliability much higher than a more indirect type of instrument.

 

[don1980]

That's a risk tolerance decision that you are empowered to make. If the ARC valve is properly maintained (periodically inspected and tested) then I would not assume that the ARC valve fails to work, when sizing a PSV on the downstream equipment.

 

[wpm1]

Thanks Don.

 

[georgeverghese]

If there was a sudden isolation of the discharge pipeline, there would be a finite control response time for each of the ARCs'. Hence the PSV sizing case should account for the interim time when both the ARCs' have yet to respond completely to the sudden rise in discharge pressure.