PSV Set Pressure 10% Above Piping MAP?

[gocougs1]

Hello,

I seem to remember reading somewhere that PSVs may be set 10% above piping MAP, as ASME B31.1 allows up to 20% overpressure of flanged piping for short durations. Has anyone else read this somewhere or did I just imagine this? Anyone have a code reference on this?

I am interested in raising the PSV set pressure to 10% above the 150# flange rating as the PSV often operates at 95-99% of PSV set pressure.

 

[don1980]

For liquid thermal expansion, B31.3 allows the set pressure to be the lesser of the system hydrotest pressure or 120% of the design pressure. (Ref: B31.3 paragraph 322.6.3)

The relieving pressure can be as high as 133% of the piping design pressure, provided you don't exceed the hours limitation (no more than 10 hrs per incident and no more than 100 hrs/yr). (Ref: B31.3 paragraph 302.2.4)

Common question: Is the 33% accumulation allowance (paragraph 302.2.4) applicable to pressure relief events?
Answer: Yes, as long as the allowable hours are not exceeded. This is stated in an ASME B31.3 interpretation (Ref: ASME B31.3 Interpretation: 20-51).

 

[georgeverghese]

In most cases, these exceedences are permitted only for rare operating scenarios - even liquid hammer is typically required to be contained within the B31.3 normal design limits.

The fact that this PSV is triggered at MAWP " often " wouldnt permit the justification for this exceedence.