Pilot PSV Popping far below set pressure

[Focal]

Anyone have experience with Pilot PSVs popping significantly below set pressure? I have some in ethylene service that keep popping and not in the other systems. There appears to be some issue with Ethylene and the pilot mechanism. Any ideas on what could be the failure mechanism or how to investigate this would be appreciated.

 

[e43u8]

Focal,

It might the pilot valve being stuck open due to very low temperature conditions; hence the process pressure cannot be kept above the main valve to keep it closed even in low and normal pressure inside the relevant vessel/tank...

 

[The Obturator]

POSRV's require system pressure to remain tight. The area above the disc called the dome, requires system pressure to keep the main valve closed. Running at below normal operating pressure (anything below 90% of set pressure), will quite often make the main valve seats leak. Then again you have Ethylene service which can be tricky.

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[Focal]

Thanks for your replies e43u8 and avalveman. We have believed it to be low ambient temperatures because we had so many pops in the winter months. The summer was fine and we have heat tracing on the pilots now, but there have been pops in the evenings (Again lower temperatures). I can understand why an immediate change in temperature would cause a relief scenario, but when it's such a slow temperature drop over a duration of hours I would assume the pressure/temperature/volume changes in the pilot/dome would be recovered from the process feed before any imbalance the dome would cause an opening. I'm going to inspect the PSVs tomorrow.

 

[gerhardl]

I am sure you can find a commercial available PSV system with pilot fluid separated from the process fluid. This could perhaps help avoiding unwanted release. Although a high-cost solution this could be economic positive in the long run avoiding extra costs at shut-downs and maintenance.

Example:

http://www-eng.lbl.gov/~shuman/NEXT/MATERIALS&COMPONENTS/Pressure_vessels/pilot_relief_Sarasin.pdf

 

[georgeverghese]

Sounds like you've got a leak somewhere in the pilot mechanism - check if a non flowing type pilot valve helps.