A pneumatic solenoid valve is used to direct or stop the flow of compressed air in a pneumatic system. Sometimes called a directional control valve, they are used in automation and control applications to actuate tools, cylinders and larger industrial valves.

What are the different types of solenoid valve?

As we shall see, solenoid valves can be split into the following broad categories: direct acting or solenoid piloted. Solenoid piloted valves can be further divided into internally or externally piloted valves, and are sometimes referred to as servo-assisted solenoid valves.

Direct acting

In the case of direct acting solenoid valves, the force generated by the solenoid must be greater than the force exerted by the pressure of the air. They do not require any line pressure to work, and can operate in vacuum conditions.

With direct acting, N/C valves, the solenoid rod is attached to a spool and kept in place by spring. When the solenoid is charged, the magnetic field causes the solenoid rod to lift, moving the spool and allowing air to pass through to the other side. In a N/O valve, the opposite happens – the spring keeps the spool in the open position.

Direct acting solenoid valves are of limited use and are only seen in about 10% of applications. This is because flow can be limited, and they consume a large amount of electrical power.

Internally piloted

Unlike direct acting solenoids, internally piloted valves work with system pressure to aid control, rather than against it. This makes them capable of controlling air flow using less power than is exerted by pressure in the line.

In internally piloted valves, the solenoid gates a smaller passage between the line and a cavity behind the spool. When this is opened, pressure in the line pushes the spool across, opening the valve. As the solenoid is controlling much smaller openings, it requires much less power to move compared to a directing acting solenoid valve.

Externally piloted

Externally piloted solenoid valves work in a similar way to internally piloted valves, but use air from an external source to assist valve movement, rather than pressure within the valve. This must come from upstream of the valve, but can also be provided from a separate circuit. This external air source is fed into an extra port on the valve. Externally piloted valves are typically used in low pressure, vacuum or alternative porting scenarios, where there is low, negative or no pressure in the valve itself to facilitate movement.

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