2 replies to this topic

[Roark]

Dear all,

I am working on an LNG regasification project, and the typical of the relief valves we are using dictates a minimum of 600mm (in vertical rise) with cold conservation insulation. After those 600 mm it dictates a minimum of 300 mm up to the branch of the bypass line, or if there is no such bypass line, min 300 mm up to the first elbow.

I am wondering what is the reason of this minimum distances. Any feedback/explanation/digression on this will be appreciated.

Regards

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Edited by Roark, 07 March 2012 - 10:53 AM.

[Art Montemayor]

Roark:

That specification is to form a "Gas Trap" - a zone of dead, static gas that will be formed there because of conduction heat transfer and remain as a static gas layer close to the ambient temperature (as it approaches the walls of the cold box & insulation.

This is required because if you don't form a gas trap, the piping will get cold with the cryogenic fluid and will form layers of cold, water ice when it contacts the ambient air outside the cold box. This ice formation can (& will) clog up and envelope the PSV, not allowing it to work.

I not only have done this technique, but I also usually employ a "pig tail" configuration to ensure that a static gas trap is formed between the external PSV and the cold cryogenic fluid inside the cold box.

[Roark]

Art,

Thanks a lot for your explanation: as useful as usual!

After reading your answer, my understanding is as follows: the relief valve needs to have a "gas trap" between its inlet flange and the pipe/vessel it is protecting. Since LNG (or LIN) are cryogenic liquids, the uninsulated section of pipe (from the limit of the insulation to the inlet flange of the RV) allows the ingress of heat from the environment, which keeps the gas immediately upstream of the inlet flange at near ambient conditions. Plus, the indication of a minimum of 300 mm of uninsulated pipe from the limit of insulation to the branch of the bypass (or the RV inlet block valve) is required in order to prevent ice formation around the valve, which could prevent it from closing/opening when/if required.

Finally, the insulated 600 mm vertical rise segment is required to maintain a temperature gradient between the "gas trap" at ambient temperature and the cryogenic liquid. This gradient is caused by heat transfer by conduction in the fluid within the pipe. Hence, at some point of this 600 mm segment there will be a LNG/NG interface, at the same pressure of the vessel/pipe protected by the RV, and the insulation prevents excessive ice formation around the pipe.

Hence, knowing the rationale behind this arrangement/typical drawing, I can confirm to one of our piping designers that he can include a horizontal segment of pipe in the first insulated segment, provided that the vertical rise is at least 600 mm. In other words, "vertical rise" does not mean "strictly vertical rise" (i.e. always vertically rising), but that it can have a horizontal (actually, slightly sloped) segment.

Phew, that last paragraph was a pain to write. Apologies: English is not my native tongue.