PSVs in a pressure declined field

[TzPetro]

I am operating the gas plant which fed by wells that were originally had STHP of 160bar. Currently the STHP is hardly 110bar. The PSVs on the plant were are set at 125bar to protect the pressure vessels. Since the wells can do only 110bar, PSVs with settings of 125bar are redundant! Do I need PSVs/overpressure protection in this situation; vessels operating pressures are way higher than the actual pressure incoming to the plant?

 

[Compositepro]

What if the vessel is shut-in and in full sun or there is a fire? Pressure vessels almost always have relief valves. It is cheap insurance against catastrophic failure.

 

[LittleInch]

Agree. Operating pressure is only one aspect. Fire and thermal expansion are the others.

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[1503-44]

Yeah, you don't really need them for operating pressure, if the pressure source cannot reach vessel's design pressure. As such if the vessels can be shut in, you could replace them with probably much smaller PSVs for thermal relief only. If it is also true that the vessel cannot be shut in, then PSVs would not be required at all. Maybe that could save some maintenance, regular testing and record keeping requirements, but ... you might could just leave them there too.

 

[LittleInch]

"vessels operating pressures are way higher than the actual pressure incoming to the plant?"

Do you mean vessel MAWP is way higher than.... ??

But if it isn't broken why mess with it?

You need to do this properly if you're going to change the fundamentals of the design basis, so a Management of Change Procedure, maybe a re-HAZOP, change the P&IDs, change the set points?, change the valves?

Seems a lot of hassle for little benefit.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

You could reduce process design pressure to 110barg through a formal plant design change notice, which may most likely require a HAZOP, but you still need the PSV at 110barg for fire case at the downstream vessels at the least. Process drawings and many other related critical design docs, including the Operating Manual, plant relief and depressure calcs and other docs, will need to be revised to avoid confusion and errors in the future. Also trip settings for PSHH may need to be revised. Gas blowby flows to vessels further downstream may also be affected.

 

[TzPetro]

Thank you all. Your contributions are really invaluable.
Responding to LittleInch,
"Do you mean vessel MAWP is way higher than.... ?" Current wells STHPs. So, no way flowing in the wells will ever overpressure the vessels.
It did not cross my mind on factors of fire and thermal expansion! Thanks.

 

[LittleInch]

it was that in the OP you had written "vessel operating pressure", not MAWP. Two different things.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.