PSV Sizing

[klyons]

How do I determine if a PSV will still be at maximum relief capacity if reset? There are two sets of PSVs, ones currently set at 500psig and need to be reset 475psig and ones that are set from 1210 to 1290 psig and need to be reset to 1100psig. I also have a PSV that is set at 445 psig and one that is at 600 psig that need to be reset to 475psig and 1100psig respectively but I didn't know if they could be set higher than what it was rated for or if it just had to be replaced. The PSVs need to be able to relieve 6.2 mmcf.

 

[psafety]

You very well may be out of the range of the spring.

If you are in a VR state, a Code shop may be required to make the change.

 

[skearse]

For the actual sizing calcs you would need to re-validate the calculations based on the new relieving pressure, and then determine whether or not the orifice is adequate. For actual resetting (presuming the actual valve size is correct) then a VR or licensed valve shop would be able to reset the spring, and if the body size doesn't change, replace the orifice if possible.

 

[klyons]

I revalidated the calculations and got an orifice size of 0.510in^2 for the 475psig PSVs and 0.220in^2 for the 1100psig PSV's. The sizes for the exisiting PSVs vary from size 1 to 2, how do I determine if the exisiting PSVs are adequate? What I am asking is based on the size how do I determine what the orifice size is?

 

[TD2K]

Your data sheets should contain the orifice area either directly or the API orifice designation (D, E, F, etc). You calculate the required area using the API 520 formulas or vendor formulas, the required area needs to be less than the available area. The vendor should also be able to help you if you provide them with the serial number.

In the US you commonly can size a PSV using either the API orifice areas and API discharge coefficients or the ASME areas and ASME discharge coefficients (the ASME capacity is the valve's tested capacity less 10%). The two approaches will give you similar but not identical results. I've seen some significant differences in the smaller orifice sizes.