Modulating Pilot Operated Oversized PSV

[Sawsan311]

Dear All,

I would like to get your views on the use of modulating non flowing pilot operated PSVs and whether they can mitigate/eliminate the challenges associated with a PSV selected for an orifice size for a governing scenario significantly exceeding other applicable overpressure scenarios.

Considering that oversized PSV would experience a ''cyclic prone chattering'' driven by the high momentum pressure drop at the inlet of the PSV when it momentarily opens at its full bore orifice corresponding rated capacity (above 1/4 of its nozzle area as per API 520). However, do you agree that using a modulated pilot operated PSV (non flowing type) can eliminate/mitigate the issue of oversizing since:
- It will open in response to the required relieving rate needed.
- Non flowing type doesn't mandate fulfilling the 3% inlet pressure loss requirement (as per API 520 PII) and its remote sensing line can sufficiently have an area of 0.07 in2.

Thanks

Regards,

 

[RVAmeche]

Pilot PSVs have some advantages compared to direct spring operated PSVs, such as they can typically be a smaller/lighter device for a given capacity and maintaining tight seal.

If the service application doesn't result in plugging of the sensing lines or have issues w/ condensation, pilot reliefs can be a good choice.