A solenoid valve is an electrically controlled valve that allows or prevents media flow through it. A solenoid valve works by having a plunger move up and down based on the magnetic field generated by the electrical solenoid. The plunger either opens or closes the orifice that the media flows through. Different designs, construction materials, and circuit functions allow solenoid valves to be used in various applications. Since they are electrically controlled, they can be controlled remotely and automatically. They are commonly found in water treatment, automotive, food processing, and other industrial applications.

How does a solenoid valve work?

A solenoid valve consists of two main components: a solenoid and a valve body. A solenoid has an electromagnetically inductive coil around an iron core at the center called the plunger. AC coils have a shading ring, which prevents vibration and humming.

At rest, the valve can be normally open or normally closed. In the de-energized state, a normally closed valve is closed. When current flows through the solenoid, the coil is energized, creating a magnetic field. This creates a magnetic attraction with the plunger, moving it and overcoming the spring force. The plunger lifts so the seal opens the orifice, allowing the media to flow through the valve. A normally open solenoid valve works oppositely. To learn more about individual components, read our solenoid valve parts article.

Solenoid valve types

Direct acting

A direct-acting solenoid valve uses the solenoid to open or close without differential pressure. These valves are often used to control the flow of gas or liquid in a system. Direct-acting solenoid valves have the fastest operation, are reliable, and have a compact design.

Indirect acting

Indirect-acting solenoid valves, whether servo-operated or pilot-operated, operate using pressure differences. They require a minimum pressure differential of about 0.5 bar. These valves include a diaphragm with a small hole that allows flow from the inlet to the outlet when the solenoid is energized and the pressure drops. This system amplifies the pressure, allowing a small solenoid to control a large flow rate. Indirect solenoid valves are used in applications with sufficient pressure differential and high desired flow rates, and they only allow media flow in one direction.

Semi-direct acting

Semi-direct acting solenoid valves blend the features of direct and indirect valves, enabling operation from zero bar while managing high flow rates. These valves resemble indirect valves with a movable membrane, a small orifice, and pressure chambers on both sides, but the solenoid plunger is directly connected to the membrane. When the plunger lifts, it directly opens the valve and a second orifice, causing the pressure to drop and the membrane to lift. This results in a valve operating from zero bar and managing significant flow rates. These semi-direct operated valves, also known as assisted-lift solenoid valves, often have more powerful coils than indirect-operated valves.

Normally open vs normally closed solenoid valves

Normally open solenoid valve

A normally open (NO) solenoid valve is open when de-energized, allowing the media to flow through it. When current is sent to the coil, it creates an electromagnetic field that forces the plunger downwards, overcoming the spring force. The seal sits in the orifice and closes it, preventing media from flowing through the valve.

A normally open solenoid valve is ideal for applications that require the valve to be open for long periods of time, as this is more energy efficient. They can also be used for safety reasons if the application requires the valve to be open with no power (e.g., to prevent overpressure).

Normally closed solenoid valve

A normally closed (NC) solenoid valve is closed when de-energized, which prevents the media from flowing through it. When current is sent to the coil, it creates an electromagnetic field that forces the plunger upwards, overcoming the spring force. This unseats the seal and opens the orifice, allowing the media to flow through the valve.

A normally closed solenoid valve is ideal for applications that require the valve to be closed for long periods of time, as this is more energy efficient. They can also be used for safety reasons if the application requires the valve to be closed without power (e.g., gas appliances).