Pilot-operated relief valve

In the PORV, the valve is held shut by piping a small amount of the fluid to the rear of the sealing disk, with the pressure balanced on either side.

A separate actuator on the piping releases pressure in the line if it crosses a threshold.

[1] The pressure is supplied from the upstream side (the system being protected) to the dome often by a small pilot tube.

The downstream side is the pipe or open air where the PORV directs its exhaust.

The piston is then free to open and the main valve exhausts the system fluid.