PSV Set Pressure
PSV Set Pressure
(OP)
Is there a limit on how much higher a PSV Set Pressure can be above the normal operating pressure of the equipment it is protecting? For example, an ASME Section VIII, Div. 1 pressure vessel has a MAWP of 14.4 Bar, normal operating pressure is 5 bar and the PSV set pressure is 13.9 bar. Is this acceptable? I know its acceptable as far as the MAWP and PSV Set Pressure but it seems like a large difference between operating pressure and the PSV set pressure. I could have sworn in the older ASME Codes there was a requirement regarding the difference between the normal operating pressure and the PSV Set Pressure but I can't find anything like that in the later Code editions.





RE: PSV Set Pressure
thread794-197791: Overpressurization of heat exchanger tube side
RE: PSV Set Pressure
I am concerned about the large difference between the Operating Pressure (5 bar) on the equipment and the PSV Set Pressure (13.9 bar).
RE: PSV Set Pressure
If the system was designed properly, then there shouldn't be any concern. If you want something operationally, put in a power operated relief valve.
I'd rather have margin between operating and design pressures. You don't want a small pressure transient turning itself into a really large one becuase the safety valve setpoint was reached.
Patricia Lougheed
******
Please see FAQ731-376: Eng-Tips.com Forum Policies: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.
RE: PSV Set Pressure
"We don't believe things because they are true, things are true because we believe them."
RE: PSV Set Pressure
RE: PSV Set Pressure
As for standards, each company have one. I don't remember if API STD 521 proposes such margins...
"We don't believe things because they are true, things are true because we believe them."
RE: PSV Set Pressure
"We don't believe things because they are true, things are true because we believe them."
RE: PSV Set Pressure
I do realize it is normal to have the shell side and tubeside MAWP's for a heat exchanger to be the same, but that's not the question.
RE: PSV Set Pressure
"We don't believe things because they are true, things are true because we believe them."
RE: PSV Set Pressure
RE: PSV Set Pressure
"We don't believe things because they are true, things are true because we believe them."
RE: PSV Set Pressure
subhen praharaj
RE: PSV Set Pressure
I would read ASME SEC VIII DIV.1 UG-134.
RE: PSV Set Pressure
I don't understand your responses. UG-125(c)refers to the PSV's having sufficient relieving capacity to prevent the pressure in the equipment from rising more than 10% or 3 psi, which ever is greater, above the MAWP. UG-134 refers to one PSV not having a set pressure above the equipment MAWP and any additional PSV can have set presures up to 105% of the MAWP.
Neither of these are relevant to my question of what would be the normal difference between the normal Operating Pressure of the equipment and the PSV set pressures. Is there a maximum difference between these two pressures?
RE: PSV Set Pressure
RE: PSV Set Pressure
My two cents;- it appears that your concern is really why the MAWP is so high, so that you need to set the PSV around that high allowable pressure. That's simply because of the vessel over-design. A good economical design would see the MAWP at the design pressure level or slightly higher. However, there might be other considerations which required additional strength to the vessel, thus increasing the MAWP. Either way, the high MAWP drove your PSV setting higher than you would expect, but as said above, that worked well by sizing a smaller (and cheaper) SRV.
Cheers,
gr2vessels
RE: PSV Set Pressure
RE: PSV Set Pressure
Any setpoint between 5 and 14.4 barg is "legal" and meets Code, and is thus deemed "good engineering practice". And the higher the PRV setting is, the smaller [cheaper] the PRV can be to provide the mandatory volume. If it was my vessel, I would have picked a setpoint very similar to 13.9