Pressure Relief Valves (PRV) operate by process pressure. The seat to seal interface is a highly polished disc forced onto a seat by a spring with a known tension.
Adjustment of the spring tension dictates the pressure at which the valve will relieve the line pressure i.e. the higher the spring tension, the higher the pressure required to lift the disc off the seat.
Some pressure relief valves have a manual over-ride hand lever which can be utilised when the user requires to relieve / dump excess pressure, but where the maximum allowable process pressure has not yet been reached or exceeded.
source: score-europe