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PSV set pressure on ASME B31.3 piping

PSV set pressure on ASME B31.3 piping

PSV set pressure on ASME B31.3 piping

(OP)
I’ve just moved to a refinery (part of an international group) and am currently overseeing the piping designs for modifications to the plant. The refinery piping is designed and constructed in accordance with ASME B31.3 but I keep seeing the situation where PSVs are set at 10% above piping MAWP. My understanding of ASME B31.3 is that the PSV is to be set at 110% of the design pressure (reference to ASME VIII but with ‘design pressure’ substituted in for ‘MAWP’). I’m seeing this trend on lines installed recently too, so changes in Code can’t be the reason. Am I missing something?

And, for my info, can anyone explain the rationale in differing approaches between ASME VIII Div 1 and ASME B31.3, where ASME VIII Div 1 requires the PSV to set at vessel MAWP whereas ASME B31.3 requires the PSV be set at piping design pressure?

Thanks!

RE: PSV set pressure on ASME B31.3 piping

Search the Pressure Vessel forum. That topic has been discussed in detail a number of times over there.
Personally I believe that it is better to have separate PSVs on vessels and piping, each protecting what they do best in the best manner to do it. Relying on one PSV to protect both can lead to gaps in the required protection level in one or the other system.

They are different, because the level of risk of a small diameter pipe bursting vs a large diameter pressure vessel exploding can be at the very least 1 order of magnitude greater.

Independent events are seldomly independent.

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