Pressure safety valve (spring loaded)

ISO 10628-2 (group 23, safety valves and fittings) / PIP PIC001 / API 520 / ISA 5.1

Drawn as

An angle body with the inlet from below and the outlet to the side, and a zig-zag spring on top capped by a short bar. A PSV bubble with the loop number usually sits beside it (not drawn here), with the set pressure and the orifice size (e.g. "3L4").

Meaning

Last-line overpressure protection. It lifts at the set pressure and relieves to flare, vent or atmosphere. Inlet size, orifice letter and outlet size read "3L4" = 3" inlet, L orifice, 4" outlet. TSVs are small thermal-relief valves on blocked-in liquid lines.

Variants

Bellows-balanced PSVs have a small bellows drawn under the spring
Some legends draw a straight-through body with a spring
PSVs often sit between LO/CSO isolation valves, with a spare PSV on a changeover valve