The valves are operated by a pilot medium under pressure, usually compressed air but also oil, water or steam.
The valve is equipped with a pneumatic actuator supplied by a three-way solenoid valve. The pressure of the pilot medium enters the actuator cylinder and acts on the piston, which allows the seal to open or to close through the stem. The return of the seal into its rest position is usually achieved by a return spring that can be found in the pneumatic actuator.
In the double acting configuration there is no return spring and the pilot medium is used both for opening and for closing the valve.