Relief valve

How to read it

Relief and safety valves protect a system from overpressure. The DOE’s Mechanical Science handbook says they prevent equipment damage by relieving accidental over-pressurization, and that most open against the force of a compression spring. Kuphaldt’s Lessons In Industrial Instrumentation describes them as normally shut and opened by the process fluid’s own pressure, not by an outside signal or force, and compares them to self-resetting circuit breakers: they open to relieve pressure, then close again.

The symbol is an angle valve, one triangle on the line and one turned down at a right angle, with the spring drawn on the stem above. The DOE’s chart and Kuphaldt’s both draw the spring as short strokes across the stem.

Also searched as: pressure relief valve symbol, prv symbol, psv symbol, relief valve p&id symbol, relief valve symbol, safety valve symbol.

Relief valve or safety valve

The two names are sometimes used for the same thing, but the valves behave differently. In the DOE’s handbook, a relief valve opens gradually as pressure rises past its setpoint and only as far as needed, and a safety valve pops fully open as soon as its setting is reached. Relief valves are typically used for liquids such as water or oil, and safety valves for steam and other gases. Kuphaldt makes the same distinction under the names PRV and PSV.

Look-alikes

A control valve has an actuator on its stem and moves on a signal. Kuphaldt shows the rupture disk beside the relief valve as the other overpressure device on a P&ID. He compares it to an electrical fuse: it bursts at its set pressure and has to be replaced.