Elimination of overpressure scenario on ASME pressure vessel

[met11]

Hi all,

I was hoping someone had some guidance on something that frequently comes up. Say you had an ASME VIII pressure vessel with a relief valve set at 150 psig. The vessel has an ASME relief valve which meets the requirements of ASME VIII including inlet/outlet piping requirements etc.

Now say there is a pump feeding into the vessel, and the pump is capable of generating 300 psig. However the pump has its own relief valve which is set at 150 psig, relieving back to the pump suction. However it is not an ASME relief valve. It is just a simple proportional relief valve sized for the capacity of the pump. The pressure the pump normally puts out is just the backpressure from the vessel plus whatever frictional losses are in the line.

My question is, there is the potential to block in the outlet of the vessel and have it become liquid-full if the level controls fail. Do you have to consider the case for the ASME relief valve on the vessel where the pump could overpressure the vessel? Or can you just assume that the proportional relief valve on the pump will do its job, and eliminate the scenario? Does the code allow you to do this? Seems like technically you're using a non-ASME valve to protect an ASME vessel against a certain scenario?

 

[e43u8]

met11,

It's better to consider possibility of liquid relief for the PSV on the pressure vessel. It might it has been certified for liquid as well as for vapor.
Then, to determine if mentioned PSV is certified for liquid too, refer to NB and look at NB-18, Pressure Relief Device Certifications.

 

[vladrath]

This type of issue is always a grey area. As others have said before you need to determine the risk associated with that pump relief valve failing. I agree with those who pointed out they may be failed for a prolonged period of time. Unless they fail open you can be sure that they are forgotten about.

So if this is a critical process for your plant I would not exclude it from sizing. If it is a auxiliary piece of equipment I would discuss this with stake holders and come to a decision as a group. You can be surprised by the insights brought on when you talk to the production folks instead of sitting at your desk looking at your P&IDs.

If your vessel is limited with nozzles, or has a large relief valve I would consider installing a 'real' PSV on the pump discharge. The PSV can return material either back to the suction side or back to the tank. These often will be fairly small and in addition to eliminating an over pressure scenario can help reduce the amount of material in the vessel should this over pressure scenario present itself.