Pilot valves are often used in larger refrigeration plants with many consumers. A pilot valve is an add-on to the in-line valve. It enables a small force to operate the larger valve like in a hydraulic piston.
In the illustration, the pressure inside the temperature sensor and coil enables the main valve to operate. The control is mechanical.
Two pilot valves are also added to a large in-line valve in the illustration as well. One is an external thermo element for controlling an injection. This makes the valve work as an expansion valve. The other is a pressure control valve securing that the evaporating pressure does not fall under a certain temperature.
The pilot valve system enables an application to use standard in-line valves, adding on pilots for specific use or modification of functionality. This is done with ease at a small cost. Pilot valves come in standard sizes and can operate in in-line valves from 3/8" to 6/8".