PSV RBI - Dangerous Fail - Open Above Set Pressure

[Carlos121]

Regarding PRD failure mode, can anyone please explain why Open Above Set Pressure (OASP) of a PRD is classified as Dangerous fail in API 581 section 7.1.3 when test point opens above 1.3 times of set pressure?
But in ASME Section VIII, Division 1, Para UG 125©(3) stipulates that the PRD shall prevent the pressure from rising more than 10% above the MAWP.
Which one is right for assessment of a PSV using RBI? 1.1 times of MAWP or 1.3 times?

 

[RVAmeche]

I'm not familiar with API 581, but you're correct in ASME typically requiring pressure from rising more than 10% above the MAWP; the exception is an external fire scenario.

I think what that section is saying is if a PRD has opened and pressure has continued to build to 1.3x MAWP, the relief device is undersized or otherwise not sufficient since it should stop pressure from getting above 1.1x MAWP typically.

 

[MortenA]

Strictly speaking what API 520 specifies is that the capacity of the valve must be met at max. 10% over the set p for a conventional spring loaded PSV. This makes sense since the spring will only start to lift at the set-p and will require a higher force (dP in oreder to open more - and will only then let more flow through. So vessel standard (in Europe e.g. PED) allows a 10% incidental over prerssure w.o. damage to the vessel.

So if you calculated too low a relief flow your valve will require a higher dP to flow this volume - and you might have a problem!

API 581 is about preventive maintenance. I dont know this standard - but its not for designing relief systems!

Best regards, Morten

 

[don1980]

ASME Sec VIII explains the PRD design requirements which are necessary for compliance with that code. To satisfy ASME Sec VIII, the PRD must open and limit the pressure to 1.1MAWP for a non-fire case, and 1.21MAWP for a fire case.

API 581 is about risk management (how to assess risks) rather than about how to design for code compliance. Section 7 in API 581 explains how to evaluate risks associated with PRDs, including different modes of PRD failure such as FTO (failure to open). A PRD is labelled FTO if it doesn’t open by 1.3MAWP. A PRD that doesn’t open until 1.3MAWP is obviously not compliant with the conservative design requirements of ASME Sec VIII, but from a risk management perspective it’s not a serious safety risk.