Question to a crygenic vessel

[wangzh2k]

Hi all,

I ran into a problem to a cryogenic storage tank which consists of internal vessel and external vessel. The internal vessel has internal pressure 150 psig and external pressure -15 psig due to vacuum between internal and external vessels.

Per UG-21 of ASME VIII-1 A06, the design pressure of internal vessel shall be 150+15 = 165 psig. After the calculation, the MAWP is conservatively set to 165 psig for the internal vessel. Per UG-134, the set pressure for pressure relief valve shall be more than 10% or 3 psig, whichever is greater, above the MAWP (165 psig). If that, the PSV will be unable to protect the interval vessel since internal pressure is limited in 150 psig.

If saying the MAWP is 150 psig to the internal vessel, the design pressure (165 psig) will be greater than its MAWP (150 psig). To my knowledge, the MAWP may be equal to design pressure or greater than the design pressure. Is it possible to a vessel design pressure is greater than its MAWP in a nameplate?

If the design pressure is 150 psig (internal pressure) and consider external pressure like wind or seismic loading, the MAWP pressure is 150 psig. The internal vessel will be protected by PSV setting pressure per UG-134. If that, how to interpret the design pressure definition in UG-21.

Any comments and instruction will be greatly appreciated.

By the way, I am a green bee in pressure vessel design.

 

[eliebl]

Be very careful when you discuss design pressure and MAWP. This is an area, at least in Alberta, where there have been many misconceptions. The design pressure is what is used to determine the thickness of each component. Design pressure could possible be higher at the bottom of a vessel than at the top as you would have to consider static head in a liquid filled vessel. The MAWP can only be gauged at the top of the vessel as that is where the PRV is to be located. In your case above the MAWP cannot be set to 165 PSI as this would exceed the design pressure of the inner vessel.

I have never understood how a Maximum Allowable Working Pressure can exceed what a person is DESIGNING the vessel for. Going backwards and calculating an MAWP you are essentially designing it for the MAWP.

Please re-read UG-134. I believe that the first line states that the set pressure cannot exceed the MAWP. There is a tolerance on actual relief pressure but the set pressure is fixed.

EJL

 

[wangzh2k]

eliebl,

Thanks for your instruction. I totally agree with your point. My understanding to the internal vessel, the design pressure shall be 150 psig and MAWP shall be 150 psig, set equal to the design pressure.

Since I am a new guy just involving cryogenic vessel design I am not confident to my judgment to the design data submitted from a cryogenic vessel fabricator. In their design data of the internal vessel, it was written as:

Design Pressure=165 psig
MAWP=150 psig


Again, thanks for your response