Check valve

How to read it

A check valve works by itself. The DOE’s Mechanical Science handbook says check valves are designed to prevent the reversal of flow: the pressure of the fluid moving through opens the valve, and a reversal closes it, by the weight of the mechanism, by back pressure, by a spring or by a combination of these. The handbook lists swing, tilting-disk, piston, butterfly and stop check valves. Kuphaldt’s Lessons In Industrial Instrumentation gives a typical use: a check valve guarding a purge line against reverse flow.

Both the DOE’s chart and Kuphaldt’s draw a check valve as two short uprights across the line joined by a diagonal stroke, with a small arrow above showing the direction of flow. The DOE’s chart gives versions with a small dot on the diagonal as alternatives.

Also searched as: check valve p&id symbol, check valve symbol, non-return valve symbol.

Look-alikes

In the DOE’s chart, a pair of triangles with one triangle filled and an arrow under it is not a plain check valve but an alternative symbol for a stop check valve, which the DOE’s handbook describes as a lift check valve combined with a globe valve. Kuphaldt’s chart has a ball check valve too, drawn as a small circle against a point on the line.