Check Valve Symbol
Definition: The Check Valve symbol represents a one-way (non-return) valve — standardised in ISO 1219-1 for hydraulic and pneumatic circuit diagrams — that allows fluid or gas to flow in only one direction from the In port to the Out port, automatically preventing backflow when the downstream pressure exceeds the upstream pressure.
Also known as: check valve, non-return valve, one-way valve, NRV, backflow preventer, clack valve, ball check valve, swing check valve.
What the Check Valve symbol means
The Check Valve symbol marks a one-way flow restriction in a pneumatic or hydraulic circuit. The In pin is the inlet (upstream) connection and the Out pin is the outlet (downstream) connection. When the inlet pressure exceeds the outlet pressure by at least the cracking pressure of the valve (typically a few kPa for pneumatic, or 0.03–0.35 bar for hydraulic), the valve opens and allows forward flow. When outlet pressure equals or exceeds inlet pressure, the valve closes automatically, blocking reverse flow.
In fluid power and process piping drawings the check valve symbol communicates the direction of permitted flow and protects pumps, compressors, and other components from reverse flow damage. In electrical diagrams representing fluid systems (electro-pneumatic, electro-hydraulic, process control), the check valve symbol appears alongside electrical actuators to show the complete system.
How to identify the Check Valve symbol
The Check Valve symbol is drawn as a circle containing a triangular arrowhead pointing in the direction of permitted flow, with a vertical bar (the valve seat/closure element) across the downstream side of the triangle. The arrow points from In (left/upstream) to Out (right/downstream). In ISO 1219-1 notation the symbol appears as a filled triangle (indicating flow direction) with a short bar perpendicular to the flow axis at the outlet side.
Function in a circuit
A check valve is a passive two-port device that opens under forward pressure differential (when Pinlet > Poutlet + cracking pressure) and closes under reverse pressure differential. It protects upstream equipment (pumps, compressors) from reverse-flow damage, prevents siphoning, maintains pressure in downstream circuits when the pump stops, and isolates circuit segments. In pneumatic circuits it also acts as a memory element by trapping compressed air to hold actuator positions.
Standards: IEC vs ANSI
| IEC 60617 | ISO 1219-1 (Fluid power systems and components — Graphical symbols and circuit diagrams) defines the check valve symbol for hydraulic and pneumatic circuits. IEC 61511 (Safety instrumented systems) covers check valves in process safety applications. ISO 4126 covers safety valves; ISO 6953 covers pneumatic valves. |
|---|---|
| ANSI/IEEE 315 | ANSI/ASME Y32.2.3 defines fluid-power graphic symbols including check valves in the US. ANSI B31.1 and B31.3 govern piping systems where check valves are used. NFPA T3.25.1 (pneumatic systems) and NFPA T2.6.1 (hydraulic systems) provide US fluid-power component specifications. |
| Key difference | ISO 1219-1 (IEC-aligned) and ANSI/ASME Y32.2.3 use nearly identical check valve symbols — a circle with a triangle and bar. Minor differences exist in the arrowhead style and the position of the line symbol versus the circle: ISO uses a circle enclosing the triangle; some ANSI drawings show the triangle without a circle. Both clearly indicate the direction of permitted flow. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
Cracking pressure: 0.01–0.35 bar (pneumatic); 0.03–0.5 bar (hydraulic). Operating pressure: up to 350 bar (hydraulic, high-pressure systems); up to 16 bar (pneumatic). Flow rates: 0.1 l/min to thousands of l/min depending on valve size. Sizes: 1/8" BSP / NPT to DN 300 and larger. Media: compressed air, nitrogen, hydraulic oil, water, process fluids (with appropriate materials). Temperature: −40 °C to +200 °C depending on seals.
Where the Check Valve symbol is used
- Pneumatic cylinders and actuators — preventing air from backflowing into the compressor when the system is depressurised
- Hydraulic power units — protecting the hydraulic pump from reverse rotation due to backflow when the pump stops
- Process piping and water systems — non-return valves on pump outlets preventing backflow and pump reverse rotation
- Boiler and steam systems — check valves on feedwater lines preventing hot boiler water from siphoning back into feedwater pumps
- Compressed air supply lines — check valves at the air receiver outlet maintaining pressure when the compressor is off
- Fuel systems — check valves in fuel lines preventing fuel backflow and fuel system priming loss
- HVAC refrigeration circuits — check valves in refrigerant lines on tandem compressor systems
Example
In a pneumatic single-acting cylinder circuit, a check valve symbol is placed in the supply line between the air source and the 3/2 directional valve; the In pin connects to the air supply and the Out pin connects to the directional valve inlet, preventing the cylinder from slowly depressurising when the directional valve is shifted to exhaust, which would allow reverse flow through the supply line if the compressor pressure drops.
Key facts
- A check valve (non-return valve) is a passive two-port valve that allows flow only from In to Out, automatically closing when outlet pressure exceeds inlet pressure, standardised in ISO 1219-1.
- The symbol has two pins: In (inlet / upstream) and Out (outlet / downstream); the flow direction is indicated by the triangular arrow in the symbol.
- The cracking pressure is the minimum pressure differential required to open the valve; below this differential the valve stays closed even in the forward-flow direction.
- Check valves are passive — they require no actuation signal or control power; they open and close automatically in response to the pressure differential across them.
- In hydraulic systems, check valves protect pumps from reverse rotation damage; in pneumatic systems they prevent system depressurisation when the compressor is off.
- ISO 1219-1 is the international standard defining check valve symbols for fluid power circuit diagrams; ANSI/ASME Y32.2.3 is the North American equivalent.
- Pilot-operated check valves have an additional pilot port that can force the valve open regardless of pressure differential, allowing controlled reverse flow in cylinder locking and regenerative circuits.
Frequently asked questions
What does the check valve symbol mean in a circuit diagram?
The check valve symbol represents a one-way valve that allows fluid or gas to flow only from the In port to the Out port. It closes automatically when the downstream pressure exceeds the upstream pressure, preventing backflow. The symbol indicates both the valve's location in the fluid circuit and the permitted direction of flow (shown by the arrowhead).
What does the check valve symbol look like?
The check valve symbol is a circle containing a filled triangle pointing in the direction of permitted flow, with a short bar at the downstream apex of the triangle representing the valve seat. The In pin is on the upstream (inlet) side and the Out pin is on the downstream (outlet) side. The direction of the arrow tells the reader which way flow is permitted.
What is the difference between a check valve and a one-way valve?
A check valve and a one-way valve (also called a non-return valve or NRV) are the same device — the terms are used interchangeably. 'Check valve' is common in North American plumbing and HVAC; 'non-return valve' (NRV) is more common in European process and hydraulic engineering. Both refer to a passive valve that permits flow in one direction and blocks reverse flow.
What standard defines the check valve symbol?
ISO 1219-1 defines the graphical symbols for fluid power systems and components, including the check valve symbol, for use in hydraulic and pneumatic circuit diagrams. In North America, ANSI/ASME Y32.2.3 defines equivalent symbols for fluid power diagrams. The symbols in both standards are nearly identical.
What is cracking pressure in a check valve?
The cracking pressure is the minimum pressure differential between the inlet (In) and outlet (Out) at which the valve begins to open and allow forward flow. Below the cracking pressure, the valve remains closed even in the permitted flow direction. Typical cracking pressures are 0.01–0.35 bar for pneumatic check valves and 0.03–0.5 bar for hydraulic check valves.
Why are check valves used in pump systems?
Check valves on pump outlets prevent backflow when the pump stops. Without a check valve, the downstream system pressure can drive flow backward through the stopped pump, potentially spinning the pump impeller in reverse (which can damage the pump seal and motor), siphoning the suction line dry, and causing water hammer when the pump restarts. The check valve traps the downstream pressure and prevents reverse rotation.
What is a pilot-operated check valve?
A pilot-operated check valve has an additional pilot pressure port that, when pressurised, mechanically forces the check valve open regardless of the pressure differential across the main ports. This allows controlled reverse flow through the valve — for example, to retract a double-acting cylinder that is being held in position by a locked check valve. Pilot-operated check valves are used in hydraulic cylinder locking, counterbalance, and load-holding circuits.
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