Difference between Pressure Safety Valve (PSV) and Pressure Relief Valve (PRV):-
Although freely used interchangeably, these terms differ in the following aspect,
Pressure Relief Valve is the term used to describe relief devices on a liquid-filled vessel. For such a valve the opening is proportional to the increase in the vessel pressure. Hence the opening of the valve is not sudden, but gradual if the pressure is increased gradually.
Pressure Safety Valves is the term used to describe the relief device on a compressible fluid or gas-filled vessel. For such a valve the opening is sudden. When the set pressure of the valve is reached, the valve opens almost fully. Thus, the Pressure Safety Valve safe a pressurized loop or equipment.
This means that although the PRV and the PSV are one and the same, their working principles are different.