Understanding Solenoid Valves: How They Work, Types, and Industrial Applications

Solenoid valves are one of the most widely used components in automated fluid and gas control systems. Found in everything from domestic washing machines to large-scale industrial processing plants, solenoid valves provide fast, reliable, and remotely controllable flow switching. At Valves UK, we stock an extensive range of solenoid valves from ASCO — one of the world's most trusted manufacturers — and in this guide we'll walk you through everything you need to know.

What Is a Solenoid Valve?

A solenoid valve is an electromechanically operated valve that uses an electromagnetic solenoid coil to control the opening and closing of the valve. When an electrical current passes through the coil, it generates a magnetic field that moves a metal plunger or piston, either opening or closing the valve orifice. When the current is removed, a spring or the fluid pressure itself returns the plunger to its original position. This mechanism allows solenoid valves to be operated remotely via an electrical signal — making them essential components in automated process control, safety systems, and machine automation.

Direct Acting vs. Pilot Operated Solenoid Valves

In a direct acting valve, the solenoid coil directly lifts or closes the valve disc or plunger to control flow. These valves can open against zero pressure differential because the solenoid provides all the force needed. They are ideal for low-flow, low-pressure applications or where the valve must operate under zero differential pressure conditions.

Pilot operated valves use the fluid pressure to assist opening and closing. A small pilot orifice is opened by the solenoid, and the resulting pressure differential across the main disc lifts or closes it. This means the valve requires a minimum pressure differential to operate — typically around 0.5 bar — but in return they can control much larger flow rates with a smaller, less expensive solenoid coil.

Normally Closed vs. Normally Open

Solenoid valves come in two fundamental configurations. Normally Closed (NC): the valve is closed when de-energised and opens when power is applied. This is the most common configuration and is used where the default state should be no flow — for example, in safety shut-off applications. Normally Open (NO): the valve is open when de-energised and closes when power is applied. Used where flow must continue unless an active control signal is present. Choosing the correct configuration is critical for system safety — in the event of a power failure, you need to know whether the valve will open or close.

2/2, 3/2, and 5/2 Solenoid Valves

The port configuration of a solenoid valve is described by two numbers. The first number indicates the number of ports (connections) and the second indicates the number of positions (states). 2/2 Valves have two ports and two positions (open or closed) — the simplest type, used for basic on/off control of a fluid line. 3/2 Valves have three ports and two positions, directing flow between two different outlets — common in pneumatic pilot applications to control single-acting actuators. 5/2 Valves have five ports and two positions, used in pneumatic circuits to control double-acting actuators.

Common Media and Compatibility

Solenoid valves are available for use with a wide range of media, but it is critical to select the correct valve for your application. For water and neutral liquids, brass or stainless steel bodies with NBR or EPDM seals are appropriate. For air and inert gases, brass bodies with NBR seals are standard. For steam, stainless steel bodies with PTFE or metal seats rated for high temperatures are required. For aggressive chemicals, stainless steel or PTFE-lined bodies with PTFE or FFKM seals are needed. At Valves UK, our ASCO solenoid valve range includes products approved under WRAS (Water Regulations Advisory Scheme) for use with potable water — an important certification for UK water applications.

Voltage and Coil Options

Solenoid coils are available in a range of voltages including 12V DC, 24V DC (common in industrial control panels and machinery) and 110V AC, 230V AC (standard mains supply). Coil duty cycle is also important — continuous duty coils can remain energised indefinitely, while intermittent duty coils are designed for short bursts of operation. Always confirm the correct coil voltage and duty rating for your application.

Industrial Applications

Solenoid valves are used across virtually every industry: water treatment (controlling dosing pumps, filter backwash systems, and distribution networks); HVAC (zone control, chiller systems, and refrigerant management); food and beverage (hygienic process lines and CIP systems); chemical processing (automated dosing and mixing systems); oil and gas (emergency shut-off, pilot systems, and separator control); and pneumatics (controlling the air supply to pneumatic actuators and cylinders).

Conclusion

Solenoid valves are deceptively simple devices that play a critical role in industrial automation and fluid control. Selecting the right type — considering media, pressure, temperature, port configuration, and coil voltage — is essential to ensure reliable, long-term performance. At Valves UK, our team of flow control specialists can help you identify the perfect solenoid valve from our extensive ASCO stock. Contact us today or browse our full solenoid valve range online.