actually my filled is instrument engg. & i give you ans related to PSV.. Basically three types of PSV convention /bellow /pilot how its select, if PSV outlet open to atmospher or Back pressure is constant that case we considered conventional psv, if back pressure is variable that or back pressure above 10% of set pressure then we go through bellow type construction. also if back pressure is 50% of set pressure or max operating pressure is 90% of set pressure in that case we go ahead through Pilot operated construction..for sizing calculation selection & instalation testing of psv we used API-520/521/526/527/528