What Are Check Valves and How Do They Work in Pneumatic Systems?

A check valve (also called a non-return valve) is one of the simplest and most useful components in a pneumatic system. It allows compressed air to flow freely in one direction and blocks it in the reverse direction — requiring no electrical signal, no pilot pressure, just the differential pressure of the flow itself to operate. Understanding where and how to use check valves can solve a number of common pneumatic system problems.

How a Check Valve Works

Inside a pneumatic check valve is a poppet or ball seat held closed by a light spring. When air pressure on the inlet side exceeds the outlet side pressure plus the spring force (the cracking pressure), the poppet lifts off its seat and air flows through. When flow stops or reverses, the spring and back-pressure close the poppet immediately, blocking reverse flow.

The cracking pressure — the minimum pressure difference needed to open the valve — is typically 0.05 to 0.3 bar. Low cracking pressure is desirable in most applications to avoid unnecessary restriction; higher cracking pressure is used where a specific back-pressure is needed.

Common Applications of Check Valves

Receiver tank protection — a check valve on the compressor outlet prevents the compressed air in the receiver tank from back-feeding through the compressor when it stops. Without this, the compressor would have to restart against full tank pressure.

Pilot circuit isolation — a check valve in a pilot air supply line holds pilot pressure in a circuit when the main supply is interrupted, preventing accidental actuation during brief pressure dips.

Cylinder position holding — a pair of check valves fitted at the cylinder ports can lock the cylinder in position by trapping air in both ends of the bore when the control valve is in a blocked-centre position.

Speed controller check valve — the integrated check valve in a flow control fitting allows free flow in one direction while forcing exhaust flow through the needle orifice, enabling meter-out speed control.

Dual-pressure circuits — check valves allow two different supply pressures to be selected without one supply back-feeding into the other. A high-pressure supply and a low-pressure circuit can share a component if check valves prevent cross-contamination between sources.

Shuttle Valves: A Related Component

A shuttle valve is a variation on the check valve concept with two inlets and one outlet. A ball inside the body seals one inlet while the other is open. Whichever inlet has the higher pressure pushes the ball to seal the opposite inlet and flows to the outlet. This is used in fail-safe and redundant supply circuits where either of two supply sources should be able to pressurise a function, but they must not connect to each other.

Selecting a Check Valve for Pneumatics

Key parameters when selecting a pneumatic check valve:

• Port size — must match the pipe or tube connection (G1/8, G1/4, G3/8, G1/2 BSP are common)
• Cracking pressure — select the lowest cracking pressure that still provides reliable closing; high cracking pressure wastes energy and reduces available working pressure
• Kv rating — the open-state flow capacity; size to avoid restriction of the circuit
• Body material — brass for general use; stainless steel for corrosive environments
• Seal material — NBR for general compressed air; FKM for elevated temperatures or chemical compatibility

Browse our range of Parker pneumatic check valves and non-return valves including inline and push-in configurations for all standard port and tube sizes.

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