Pilot-operated relief valve

ISO 10628-2 (group 23, safety valves and fittings) / PIP PIC001 / API 520

Drawn as

An angle relief body with a piston chamber (a box) on top instead of a spring, and a small spring-loaded pilot beside it with a sensing line back to the inlet.

Meaning

Relief valve whose main piston is held shut by inlet pressure and released by a small pilot. Tight up to the set point, so used where operating pressure is close to set pressure (gas export, large flare loads).

Variants

The pilot can be drawn as a small PSV symbol or a box with "P"
Remote-sensing pilots have the sensing line run to the vessel, not the valve inlet