Thermal Relief of LPG 2

[gaterine]

I have a relief scenario regarding a blocked in LPG line at pressure so the contents are liquid. Expansion due to temperature changes are going to be large. To prevent release of LPG to atmosphere, the thermal relief has to be relieved back to the upstream end of the valve that is blocking it in. There is no flare available. Is it normal to use a pilot operated valve in these cicumstances? It seems extravagant!

Thanks

 

[TD2K]

I've seen conventional relief valves used in this service.

The main problem with conventional relief valves is that back pressures increase the effective set pressure for the relief valve and therefore, what scenarios do you consider and the resulting effects on the pressure relief. If you think you might or want to have a string of these valves relieving in series, the cascade effect becomes more of a problem. Some people have suggested setting them with a relatively small set pressure, 25 psi or so so they start to relieve as soon as a block valve is closed and any differential starts to build.

You can alternatively use balanced bellows if needed or pilot operated.

 

[jalvarez]

If the issue is only thermal reliefyou may use a pilot operated safety valve (the LPG must be clean, no unsaturated HC if possible, to avoid clogging the pilot), but you only need a simple SRV set as TDK suggest.
But a simple check valve around your valve could be the most simple solution if the section upstream the block valve is correctly protected.
See a related thread: thread798-96681, where guidoo suggest the mentioned solution.
Have a safe day
J.Alvarez

 

[gaterine]

Many thaks to you guys for your responses. Looks like we will have to go for a Pilot Valve. The total flow during normal operation is small and I am concerned the spring loaded check valce idea may result in loss of flow control. The saving grace is the pilot valve will be small.