Hello! Good day!

I have some questions...

I am doing PSV Sizing for a Shell Project. In their procedure, not all PRV sizes are available, say orifice G, J, M and N are not to be selected. In this case, I always choose the next higher size. For example, I have calculated am orifice area of 19.2cm2, orifice L is a little small so M would be enough. But since this size is not permitted to be used based on their procedure, P would be the proper size, skipping orifice sizes M and N. My questions are:

1. Isn't this relief valve oversized and there is a high risk that this will chatter?
2. I was thinking of converting it to a multiple relief valve so that I can have smaller standard orifice size that will give me closer area with my calculated value. But on the second thought, considering engineering economics, this would be more costly. Do you think changing it to multiple relief valve would be more appropriate?

Thanks!

You won't have "chatter" is the true sense, you will have cycling. Neither is a good thing but cycling is slower and usually doesn't cause damage to the valve.

I would ask why they skip these particular PSV sizes, if not for anything else but to learn and understand their reasoning. As engineers, we should not take things just on face value, we need to understand why and if not for what you are currently doing then for the future.

Look into the multiple relief valve system. Maybe it will be cost effective. Only by getting the number can you tell for sure. Selecting a grossly oversized PSV presents more problems than just cycling. How about sizing the downstream piping and equipment (if there is any)? Add this cost into your analysis as well.

Yes. As long as your inlet line lost based on rated flow is lower than 3% of set pressure (per API recomendation, your PSV probably will not chatter. This is general understanding. In addition, the gas PSV with pop action characteristic will furher minimise chattering. But the negative side, oversized PSV result extremely high rated flow and the pressure will drop so fast and touch the low pressure tripping point and eventually causing unit (or plant) shutdown...

SHELL DEP also limit PSV to "P" maximum...I guess it has to do with low allowable set pressure, extremely high PWL, etc. If you get the answer from your counter part, appreciate you can drop some note in this forum for the benefits of all...

JoeWong