Check Valve Symbol (P&ID): Diagram & How to Read It

The Check Valve Symbol on a P&ID, Explained

On a piping and instrumentation diagram (P&ID), a check valve is drawn as a valve body with a marker that shows it only allows flow one way. The key to reading it is simple: the symbol always points in the direction flow is permitted, and closes against flow from the other side. Below are the standard symbols, what each one means, and a clean reference you can save.

General (one-way)Flow opens the disc; reverse flow seats it.Swing checkHinged disc swings open, drops closed.Ball checkBall lifts off the seat with forward flow.Orange = permitted flow direction.
Standard check valve symbols. Styling varies slightly between ISA-5.1 and ISO 10628, but the disc, ball, or flapper always seats against reverse flow, and the open side faces the permitted flow direction.

How to read a check valve symbol

Find the moving element (the disc, ball, or flapper) and the seat it rests against. Flow is allowed from the open side toward the seat, and blocked in the reverse direction. Many drawings add a flow arrow next to the valve to remove any doubt. If you are placing the valve, that arrow must match the direction you want protected against backflow.

The symbols by type

General / one-way check valve. A disc or tongue drawn seating against a bar. The generic symbol when the specific internal type is not called out.

Swing check. A hinged disc swings open with forward flow and falls closed when flow stops. Drawn with the disc on a hinge pin.

Ball check. A ball lifts off its seat under forward flow and drops back to seal against reverse flow.

Spring / inline check. A spring holds a poppet or ball closed until forward pressure opens it. On a P&ID it usually shares the ball or general symbol; the spring is noted in the valve tag rather than drawn.

Why flow direction matters

Installing a check valve backward is one of the most common field errors, and it either blocks the line entirely or leaves the system unprotected against backflow. The symbol on the drawing and the arrow cast into the physical valve body must agree with the actual flow. For the install rules, see Check Valve Installation: Flow Direction & Orientation.

Choosing the actual valve

Once you know the type your drawing calls for, match it to the right hardware. See Check Valve Types Explained, use the check valve selection guide, or go straight to swing, spring, inline, wafer, or Y-pattern check valves.

Free to reference. If this diagram helps, a link back to this page is appreciated.