PSV vs Full Flow Relief Process 1

[Krausen]

Gentlemen - Just wanted to draw off others' expertise or experience on a facility process issue:

The office I work in recently had a debate over pressure settings on PSV thermal relief valves & full-flow process relief rupture pin valves. Without being too long-winded, basically some co-workers argue that the PSVs should be set at the highest pressure & the process relief valves set below them (assuming all components installed on same piping rated system - Class 150, 300, 600, etc.). Others vehemently claim that your process relief valves should be set at the highest pressure, meaning the PSVs would end up relieving volumes in the event of a pressure surge.

I can see both sides of this coin. I understand that full-flow process relief should be an infrequent event and can see how that would be your "last line of defense" and set at the highest pressure tolerable (typically a max of 10% over MOP). If you were to size your PSV lines/orifices small enough, it would not allow high volumes to relieves through them during a pressure surge event I suppose.

However, in a perfect world it would not be ideal to relieve any pressure surge volumes through PSVs set at lower points. I can also see how thermal pressurization of a specific isolated piping segment can be more of a "local" problem vs "global", thus allowing higher set pressures for certain segments to be protected.

I appreciate any comments or advice on this. Thanks.

 

[zdas04]

I dearly hate it when we try to replace "Engineering Judgement" with "Policy". There are specific cases where any of the scenarios you mention would be near-optimum. There are also [probably as many] cases where the same decision would be a near-disaster.

What is wrong with looking at a process and developing your list of credible scenarios and evaluating each for proper over pressure protection? If I have multiple credible scenarios with widely different credible maximum flow rates then I pick the overpressure device(s) that have: (1) an acceptable probability of preventing an exceedence; (2) the highest probability of reseating at the end of a transient; and (3) the lowest predicted relieved mass. If this ends up with a thermal relief set at 95% of MAWP, a smallish PSV set at MAWP, and a big PSV set at 105% then so be it. On the other hand, if my most likely credible scenario is also the biggest required flow rate then I may end up with just the big PSV. It should depend on a case by case Engineering analysis, not on some inflexible policy.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[TD2K]

Good post Dave. Krausen, code simply sets the maximum pressure PSVs can be set to, there's nothing say you can't set a PSV at a lower pressure if needed. Sounds like this is one of these cases where different settings are justified.