The typically lower rated outlet (there are PSVs with 150# inlet and 150# outlet) is also a factor in the relatively recent ASME Pressure Testing requirements for the components of the PSV Shell. Shell is a new term in ASME with respect to PSVs. It refers to the Body, Bonnet & Cap in most cases, but may also apply to the Nozzle in many cases and the disc in Open Bonnet Designs. The Pressure Test is 1.5 times the 100F Rating of the Outlet Flange, not the inlet. So, the Pressure Test (typically hydrostatic) of the Body, Bonnet & Cap need not meet the 300# or 600# inlet rating, just the 150# rating of the outlet.
JAC