I am doing a sizing calculation for a PSV required on a filter separator vessel. The PSV has to be sized for fire case.
I have sized the PSV as per API 521 for vapor flow and obtained a relieving flow of 260kg/h and an orifice area of 0.017in2.
This area is much smaller than the smallest API designated orifice 0.11in2 (orifice D). Can I choose orifice D for this purpose or is it possible to have a smaller valve custom designed by valve suppliers? Is the API code dictating to use orifce D as the smallest orifice to be used?
I believe if the calculated area is less than 60% of selected orifice, the valve will chatter.
Please let me know your comments.
Regards,
Kashyap.