3 replies to this topic

[Sonny]

Hi,

Does any one have experience with relief valve in cryogenic vessel? This is the problem we are struggling with.

We have a vacuum jacketed liquid hydrogen storage vessel with a stamped MAWP of 77 psig with full vaccum. The vessel is limited by the thickness of the head with 92 psig design pressure (77 psig + 14.7 for vacuum) per UG-32 equation (1). Liquid is transferred out of the vessel by the use of a closed loop pressure build coil (vaporizer).

The relief devices for this vessel is a 4" x 6" spring actuated relief valve and an 8" rupture disc. Our calculations have shown that the relief valve capacity is more than sufficient to accommodate a complete failure of the pressure build coil (i.e. the vaporizer output at maximum capacity).

Per ASME Section VIII, the relief valve is set to the vessel MAWP. What should be the set burst pressure of the rupture disc? The rupture disc capacity was size to accommodate a complete failure of the vacuum system (i.e. unexpected source of external heat). Can the burst pressure be set to 92 psig without violating UG-134( ?

Have I included the necessary info above?

[pleckner]

Let's put it this way. If the rupture disk is sized to protect only against a fire scenario then the burst pressure may be set to 110% of the MAWP. This assumes that the PSV is designed to relieve all other non-fire scenarios.

The set pressure basis is the stamped MAWP.

But, setting the burst pressure of a rupture disk is not necessarily as straight forward as we would like or expect. I invite you to read my series on rupture disks found on this website to see what you need to consider in your design.

Any questions, just let us know.

[Sonny]

Thanks Phil,

But, is there anything in the ASME code that would allow us to say that the MAWP with full vacuum is 77 psig and the MAWP with a loss of vacuum is 91 psig (since the governing thickness can withstand 92 psig? Essentially assigning dual MAWP to the vessel.

One thing I fogot to mention earlier is that there is an additional vent valve with interlocks that open the vent valve at 76 psig. Can we take credit for this vent valve to justify having a second MAWP at 91 psig?

Not sure whether its my computer or your web page, but the "search " function doesn't work. I get a fatal error message when submitting a search.

And finally, can you provide a link to your rupture disk serires, I can't seem to find anyhting that will link me to it.

Thanks.

[pleckner]

Use this link to get to all of my aritcles on pressure relief. You will have to cut and paste it into your browser.

http://www.cheresour.../asiseeit.shtml

Another way is to go the main webpage (www.cheresources.com). Look at the menu options on the left. Go to "Free Aricles". (This will open a whole new world when you see what is available). Select "Pressure Relief Design".

Now to your questions.

The equipment owner must never assign MAWP, that can only be done by the authorized manufacturer and must be stamped in accordance with ASME. In some designs, a vessel has more than one MAWP stamped based on different corresponding temperatures. If your vessel has only one stamp value, that is what you must base all your calculations on. If you wanted it to be stamped for 92 psig, you should have either specified the design pressure that way or you should have asked the manufacturer to stamp the vessel with the true MAWP. In most cases, the true MAWP is greater than the specified design pressure (as you are apparantly seeing). But the manufacturer will only stamp the vessel with your specified design pressure unless you specifically ask them to stamp it with the true MAWP, and often this cost extra.

To determine relieving scenarios, you may consider instrumentation (and these should be approved and certified instrumented safety systems) but once you've identified the scenario as being credible, you can't take instrumentation credit when determining relieving rates.