why 21% for fire case

[sbade1234]

why 21% over pressure for fire case only & 10% for others???

 

[lizking]

I believe it's because a fire case is such a rare circumstance and even rarer that the RV discharges. Thus ASME allows 10% over the 10% = 21%.

 

[MortenA]

not true.

This has been discussed many times, however:

You have to consider that the RV is "the last defence". The main protection agains overpressure should be deluge and/or blow down.

The main purpose of the RV is to prevent escalation of the accident and risk to firefighting people. The vessel is considered "lost". When you dont have to consider that the vessel will go back into serve right after the overpressure scenario then using the design margin "a little more" is OK.

Best regards

Morten

 

[MortenA]

lizking: Going over the post again I do, however, think that your 10% over 10% may be the reason for 21% excactly - but somebody with a better understanding of vessel design than me may correct me here.
Best regards

Morten