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As required by NFPA and API standards, LPG vessels in bulk terminals require check valves on all inlet piping to prevent release of LPG in the event of piping rupture. This presents a problem for relief devices which need to recirculate back to the vessels. In our situation, we have a positive displacement pump taking suction from such a storage vessel. In order to prevent overpressure from a blocked in pump, a PSV is required. Since there is no flare system available, and LPG cannot be discharged to atmosphere, the PSV discharge must be routed back to the suction vessel. This presents a problem since the PSV discharge line into the vessel will contain a check valve. Is this an acceptable practice? It has always been my understanding that obstructions of this type are not permissible. Has anyone encountered this situation before?
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Check Valves In Psv Discharge Piping
Started by Guest_Nathan Latour_*, Feb 16 2006 09:16 AM
2 replies to this topic
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#2
djack77494
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Posted 17 February 2006 - 09:15 AM
Nathan,
I've not worked on an LPG storage/pumping facility design, but I have worked on the design for another liquified gas - ammonia. There are similar requirements for check valves or excess flow valves in all nozzles at the storage tank. Every line segment that can be blocked in must be provided with a relief path. However, we use tiny (I think they were 1/2") thermal relief valves for such segments. Discharge was to atmosphere, and I wouldn't consider ammonia to be any more safe than propane or butane. I think this is a normal and acceptable practice. What are your alternaltives? If you're at a simple site just storing and moving liquified gases, you probably don't have any sort of flare system. If the tank is not refrigerated (and I think few are), then it is at a relatively high pressure. So that's not a suitable destination for relief discharge. What's left? Sorry I don't have a more definitive answer.
I'll be monitoring this thread with interest.
Doug
I've not worked on an LPG storage/pumping facility design, but I have worked on the design for another liquified gas - ammonia. There are similar requirements for check valves or excess flow valves in all nozzles at the storage tank. Every line segment that can be blocked in must be provided with a relief path. However, we use tiny (I think they were 1/2") thermal relief valves for such segments. Discharge was to atmosphere, and I wouldn't consider ammonia to be any more safe than propane or butane. I think this is a normal and acceptable practice. What are your alternaltives? If you're at a simple site just storing and moving liquified gases, you probably don't have any sort of flare system. If the tank is not refrigerated (and I think few are), then it is at a relatively high pressure. So that's not a suitable destination for relief discharge. What's left? Sorry I don't have a more definitive answer.
I'll be monitoring this thread with interest.
Doug
#3
pleckner
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Posted 17 February 2006 - 12:49 PM
I also do not have direct experience in LPG so I'll just comment based on my "other" experiences.
I've been doing some digging and have not found any mention in the codes or standards that do not allow the use of a check valve in this application.
By itself, a check valve is not necessary a restriction to flow in the correct direction. As long as the purchased item is reliable and can only fail in the opened position, you should be fine.
Remember, the whole point in relief piping is not to adversely affect the operation of the relief valve. My only concern would be if the check valve locked up in some partially opened position and thereby increased the backpressure on the relief valve. I would definitely check with manufacturers on their experience with this application.
Remember all, the manufacturer or vendor rep doesn't bite. They live to get a contact and will gladely answer your questions...for free.
I've been doing some digging and have not found any mention in the codes or standards that do not allow the use of a check valve in this application.
By itself, a check valve is not necessary a restriction to flow in the correct direction. As long as the purchased item is reliable and can only fail in the opened position, you should be fine.
Remember, the whole point in relief piping is not to adversely affect the operation of the relief valve. My only concern would be if the check valve locked up in some partially opened position and thereby increased the backpressure on the relief valve. I would definitely check with manufacturers on their experience with this application.
Remember all, the manufacturer or vendor rep doesn't bite. They live to get a contact and will gladely answer your questions...for free.
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