never heard a mov on the downstream of drum psv (any psv)

To avoid potential high H2S content backflow from Blowdown Header, since PSVs are not considered TSO valves. PSV MOVs must be closed before top unheading; the interlock is with Vent valves, so it can be closed only when Vent valves are open, and vent valves can not be closed unless PSV MOVs are open.

What is the typiocal valve type for this service? Any particular design features?

below , but the material has to be inconel for the below

Generally there is a steam injection before and after the PSV, for line cleaning. The probability of the backflow from blowdown reach the drum is very low. Is the MOV really necessary ?

Sir, you’re right, however the low probability of ocurrence does not eliminate the hazard while the close MOV will.
Lucky

Also these valve can are included in the drum vent interlock (limit switch);for safety operationally these valves are been open and close with every operation of decoked

espana2005