External cavity relief

It is needed in that situation but other types of positive isolation valves also need it.

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but why would any other configuration that utilizes SPE need an external relief ?

I didn't say that. If you've got a SPE then you should be ok. But if you've got say a split whee gate valve then you also need one.

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Also: If you get a response it's polite to respond to it.

Got it! i was only referring to trunnions. thank you for confirming my thoughts.

what applications may require DPE x DPE ?

Really just any application where you would want increase reliability of sealing. It's somewhat of a trade off though. You could go for DPE seats, have two seats to achieve sealing in either direction, and then have an increased probability of atmospheric leakage if you have tubing/threaded connections for your body relief. If you have SPE seats then you only have a single seal in either direction but have inherent cavity overpressure protection. DPE x DPE seats seem better regarding their ability to seal although if you never verify that both or just one seat is working you may go years with only one seal working until the other one fails and you finally notice. If there's a damage mechanism for one seal then it'll likely affect both.

It's really just pro/cons between the two and they're only really comparable if you maintain, monitor, and test them on some frequency. The DPE x DPE arrangement, if you properly maintain them properly, lets you know that a single seal has failed, hopefully some time before the other one does so you can address it before both seals are shot. You can also get SPE x DPE if you want preferential, dual sealing in one direction while having inherent cavity overpressure protection.

Thanks,
Ehzin