We had a post a while back asking for clarification about MAWP and design pressure. I scanned this from C.Parry's 'Relief System Handbook' It may be of use.

Click to view attachment

Yep, pretty much taken right out of API RP 520.

Overpressure is 21% (not 25% as indicated in the document)

May be 25% is overpressure value relevant to liquid relief from TSV.

21% accumulation is applicable to fire case...

Hi,

The API fire case is 21% and prehaps it has 3% for the line ?? not sure there

By the way in other standards such as the UK, the fire case has lower number ~ 16% if I recall correctly.

Actually the API is a guide not standard but its widely regarded

cheers,

SM

The figure from Parry shows a special case where the relief valve set pressure is below the design pressure of the system.

Recent issuance of API (e.g 521) has been labled as standard.

Base on API,

Inlet line pressure drop shall be less than 3% of set pressure based on rated flow of selected PSV.

Maximum allowable pressure accumulation shall be 121% of set pressure for fire contingency.

Recent release 2007 plus addendum May 2008 has changed from RP to STD for API 521. Nevertheless, API 520 still remain as RP.

My 2-cents:

Some publications are labeled as "Recommended Practices" or "Standards" [for example, API publications], some are called Codes (for example ASME VIII Div 1 & 2). Codes use such words as shall. Standards use words like should. Codes are respected more in the court of law. So there is not much difference between RPs and STDs: they are not law and only provide guidelines that help reaching legal requirements of codes.