A pressure relief valve or pressure safety valve in which the major relieving device or main valve is combined with and controlled by a self-actuated auxiliary pressure relief valve called a pilot.
The primary difference between a pilot-operated safety relief valve and a spring-loaded pressure relief valve is that the pilot-operated valve uses process pressure to keep the main valve closed instead of a spring.
Pilot PSV works on a very simple principle; Greater the area greater will be the force as below
P=F/A F=P*A at constant pressure greater area will have the greater force and this force difference causes the piston of the main valve to shut during normal operation.