ASME code and LPG pressure vessels

[garfio]

Dear All:
I posted this topic in the Safety Relief Valve Engineering Forum but did not get any response. I hope people in this Forum could help me out.

ASME Section VIII has an exception to the rule of installing relief valves with an overpressure of 10%: UG-125 3), allows in some cases (for products like LPG and in some specific situations), to install relief valves with overpressure of 20%. One of the conditions required is that the vessel should not have “permanent supply connection”. It is clear that this is the case with all commercial tanks with LPG filled by tank trucks, however, in the case of vessels installed terminals that receive LPG through pipelines, does the multiple block valves normally installed between vessels and pipelines count to be considered as “not permanently connected”? If this is not allowed, then all vessels in LPG terminals would need valves set at 10% plus valves for fire case set at 21%.

Anybody with experience in LPG storage/terminals has an idea about what is the interpretation of the Code and what is the normal practice in these cases?

Thanks

 

[gr2vessels]

The interconnecting piping between LPG tanks and the rest of refinery equipment cannot be regarded as non-permanent connections;- Shell DEP specifically noted the significant volume of LPG trapped in the piping upstream and downstream of any isolation valve, which makes the both type of relief valves mandatory. Similar wording is for BP specification for the LPG storage, including the off-spec LPG tanks.
I guess this clarifies the relief valves requirement.
cheers,
gr2vessels

 

[tigny]

Hello,

maybe you can find some clarification in NFPA 59A Standrad for the production, storage, and handling of liquefied natural gas (LNG).

It is stated about piping, that "9.8.3 A thermal expansion relief valve shall be installed to prevent overpressure in any section of a liquid or cold vapor pipeline that can be isolated by valves."

yours,

 

[garfio]

Thanks for the answers.

Tigny: My question was related to the pressure relief of the vessels, not the lines. They will be uner NFPA 58 instead.

I would like to correct my original post: When I mention "valves set at 10%' and "valves set at 21% I was meaning valves sized with overpressue of 10% and 21% respectively.

 

[simplemath]

Garfio:

gr2vessels made his point. I think 10% overpressure for single PRV and 16% for multi for your case.

Most of time , the case study will lead to the fire exposure case and 21% overpressure.

Thanks.

 

[freddmech]

Hello:

Large(ASME Stamped)LPG Storage vessels require that relieve valves are set at MAWP, as per ASME Section VIII & NFPA 58. The 10% overpressure allowance is for full dicharge capacity. In other words, it will start to open a MAWP and will release its full capacity at 1.1MAWP. Regarding the hydrostatic relief valves, they are installed on liquid piping when the pipe section can be isolated between valves. The seting wiil be depend on the pipe rating, and are normally between 375 and 400 PSI.

Hope this helps.