Yes- the discharge can and usually does have a lower flange rating than the inlet.

Conventional relief valves cannot have a back-pressure greater than 10% of their set pressure, or else a different design of device must be specified to be safe. Accordingly, it makes no sense for the discharge piping or the valve's discharge flange to be designed for the full inlet pressure.

and thats why if out have block valves downstream your PSV's these should be LO and/or interlocked with the spare so that you only will be able to close the BV once the upstream valve is closed.

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

The most known standard for inlet x outlet size and flange ratings/configurations is API-526. This defines the limits of pressure for size of valve against a set of temperatures. Note also that as the SRV size increases the pressure limit falls down. This is because of SRV design defined back in the 1940's or so where high pressures where not considered at such sizes. Nowadays we have pilot operated large size high pressure designs as well as manufacturers other designs exceeding stated API-526 limits. The outlet flange is almost always lower in pressure rating since the pressure drop from the inlet nozzle (smaller than the inlet flange size), is something like 90 % less than the set pressure. This is of course not allowing for back pressure. And back pressure can be higher than 10 % (of set) if it is constant (in the case of a conventional design - spring setting = set - constant back press).

Per ISO, only the term Safety Valve is used for all overpressure eventualities regardless of design.