LPG Hydrostatic Relief Valve 2

[Danybony]

Hello! I have a question about pressure relieving on a LPG line section and in particular on the hydrostatic relief valve that is usually placed downstream the lpg pump and that I see on filling skids. I have four shut-off valves that ,when operated, isolate a liquid volume of about 50 liters and the valve I'd put between them would be like one produced by Rego or similar. In case of emergency, can I vent the lpg to the atmosphere (as it happens with PSV on lpg tanks) or return it to the tank? Such valve could operate even in case of fire? NFPA 58 seems to allow the atmospheric discharge but I'm afraid it's quite dangerous! Any suggestion? Thanks

Daniele

 

[don1980]

There's generally not much fire risk from liquid relief valves in ambient thermal expansion services. That's because the amount of fluid released is tiny. But, there's always the possibility the valve could remain partly open due to debris in the seat, or it might open due to some unexpected scenario and remain open long enough to create a hazard.

Many thermal valves like this are routed to the atmosphere. A risk assessment is needed for each particular application. If the risks are intolerable, or you're not sure whether they're tolerable or not, then route the discharge back to containment. route it to the pump suction vessel, or some segment of piping that's always open and flowing.

 

[Danybony]

Thanks Don! Given that the pipe in which I'd return the discharge line of the valve is filled with liquid (it's the by-pass valve return line to the tank), would a pilot operated valve be a better choice? I don't know if the Rego valve would operate well with a liquid counterpressure!

 

[don1980]

You don't need to use a pilot PSV unless the backpressure is too high for a balanced PSV (balanced bellows or balanced piston PSV). Balanced PSVs can tolerate backpressure up to ~ 35-45% of set pressure. Pilot valves are generally not needed unless the backpressure exceeds the upper limit for balanced valves, or unless you're operating pressure is too close to the set pressure (unlike conventional PSVs, pilot valves remain tight as the pressure approaches the set pressure).

When you route a PSV anywhere other than to atmosphere, you generally want to try to use a balanced PSV. That's because the set pressure of a balanced PSV isn't affected by backpressure - as long as you don't exceed the max allowable backpressure for that valve (typically 35-45%). If your backpressure is > 30% of set pressure, check with the manufacturer to verify you're not exceeding the valve's upper limit.

For your application, I suggest you look at the Consolidated 19000 series valves. Those are balanced PSVs. Alternatively, you can check with Rego to see if they make a balanced valve - I don't think they do, but you'll need to verify that.

 

[Danybony]

Thanks for the info!!!!

Daniele