Hallo,
If the valve design is not subject for design such as e.g. API 600, API 602, FCI 70-2 (control valve) and many others. Then usually it is subject for PED aka body test only (read: 1.5x Maximum Design pressure as per ASME 16.34 (if flange) or PN rating), and not necessarily subject for seat test.
Note: API valve codes usually refer to ISO 5208 or API 598 as pressure test standard of which both mentioned seat test.
Pressure reducing valve usually for utilities piping e.g. steam, nitrogen, air instrument, seal oil?, water, etc. and not for process piping. If it is for process then it would be considered as control valve, hence FCI 70-2 might be applicable.
But of course, criticality is for end user to decide. if deemed critical and isolation capability is required, you can consider this as needle or globe valve, of which seat test is required. Thus refer to 1.1xMAWP (ASME 16.34) or 1.1xPN (this I am not really sure since pressure testing have been many many years ago for me).
40 Psig considered as PN8 (around 116 Psi) or class 150 that is for you to check.
With leakage rate of zero shall seat is soft material and rate B if metal seated.
Seat pressure test is seating capability of obturator against seat when spindle is fully energized, therefore it is always about high (design) pressure side.
For example, check valve is seat tested from downstream pressure side, assuming there will be maximum pressure built up from that side. Given the upstream side is atm.
So in your case downstream pressure is not considered.
Assuming you are the one who's doing the repair, and it is repaired for some reason, then doing seat test would be an additional value. I would do it if I were you.
Just my two cents.
Regards,
D
