Pig Launcher PSV Sizing 1

[Donkin]

Hi

I have been looking into the sizing of an Offshore Pig Launcher PSV. API 521 indicates that I should size using 100% liquid inventory as if the vessel contains gas the metal temperature will exceed strength of the vessel and cause and explosion.

The Client wants the PSV sized for gas is this normal and if so which part of API do I use?

Thanks in Advance

 

[zdas04]

The question as always is credible scenarios. Is there a credible scenario that could cause overpressure in the launcher when the launcher is gas filled? The answer to that one depends on your system. If you have a 10,000 psig wellhead pressure and a 1440 psig MAWP, then a choke failing open is credible and you have to size your PSV for the flow rate through the port on the choke with 8560 psid.

Often, there would be intervening protected equipment (separators, dehy's, treaters, etc.) whose PSV would have to fail to operate before you could exceed the MAWP of the launcher so there is often no credible scenario (i.e., a scenario that does not require multiple unrelated failures simultaneously) for the launcher to be overpressured while gas filled. There is generally a credible scenario for a launcher or receiver to end up isolated while liquid filled (launchers often stretch credibility a bit, but receivers ending up liquid full are pretty common) and you only need thermal relief which is usually a very small PSV.

The answer to your client is a credible scenario evaluation which has been required by API for over 25 years and the Oil & Gas Industry is just beginning to discover.


David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"It is always a poor idea to ask your Bridge Club for medical advice or a collection of geek engineers for legal advice"

 

[Donkin]

Thanks for the reply, the launcher is only filled with gas for twenty minutes and then the pig is released and the depressurisation takes twenty minutes.

If you have a jet fire when there are ten guys launching the pig the PSV is the last of your problems.

 

[dcasto]

why have a psv? Just keep a 1/4" path open from the launcher to a protected system.

 

[zdas04]

"Just" is such an easy word. If you design a 1/4" path from the barrel back to the system, it has to have a valve so you can blow the barrel down to load or remove a pig. Is it credible for a pig to come in and fill the barrel with liquid and for someone to isolate the barrel, but not have time to mess with the pig today and he doesn't open the 1/4 inch valve? I think it probably is. That means you have a credible overpressure scenario (thermal) and need overpressure protection.

David

 

[dcasto]

the 1/4 valve has a car seal on it. We even posted a sign at the gate and on the pig lauchers that warn of such a situation. The DOT agreed with us and we removed all the fire reliefs on pig lanuchers, valve bodies, and above ground piping. This was back in the late 1980's. Exxon followed our lead and today most companies are accepting this practice.

 

[dcasto]

An remember, that a relief must be tested and inspected either annually or twice annually.

 

[zdas04]

No question about the whole "PSV's are a pain" thing. I just went through a "credible scenario" analysis on over 500 PSV's so that I could sign off on their capacity and set point. The scars are fresh.

I feel that any scenario that only requires one person to skip one procedural step (or to stop a procedure early) is credible. Even if Exxon, BP, Conoco, and the DOT signed off on replacing barrel thermal reliefs (I'm not even talking about fire case here) with 1/4 inch lines and valves, I wouldn't stamp it with my stamp for receivers.

Launchers are kind of a different basket of fish. It is hard to come up with a credible scenario that leaves them liquid full and isolated. Every scenario that I can come up with that leaves a launcher barrel liquid full and isolated requires too many unrelated things to go wrong simultaneously (like Clancy's The Sum of All Fears where 7 totally unrelated things have to go wrong on the same Sunday, and they do until Jack Ryan saves the world yet again).

David