Solenoid Valve Types – A Comprehensive Guide

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.

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.

Bi-stable solenoid valve

A bi-stable or latching solenoid valve can be switched by a momentary power supply. When de-energized, the valve stays in the position it switched to. Therefore, it is not normally open or closed, as it stays in the current position when no power is applied. They accomplish this by using permanent magnets rather than a spring. This gives the benefit of reduced power consumption.

2-way solenoid valve

2-way solenoid valves have two ports, an inlet, and an outlet, and are used to allow or block flow. The flow direction through the valve is critical to ensure proper operation. Typically, an arrow indicates the flow direction on the body of the valve.

3-way solenoid valve

A 3-way solenoid valve usually features three ports, each serving a distinct purpose: one for the inlet, one for the outlet, and one depending on the valve’s configuration and application (exhaust, return, another inlet, or another outlet). Here are the common designations for these ports:

• P (pressure) port or inlet port:This is where the pressurized fluid or gas enters the valve.
• A (actuator) port or outlet port:This is the working port connected to the device or actuator that the valve is intended to control, such as a single-acting pneumatic cylinder or a process line.
• E (exhaust) port or R (return) port:When the valve is switched to the exhaust position, this port vents the fluid or gas from the actuator or device back to the atmosphere or a reservoir. Depending on the valve’s configuration and application, this port might also serve as an alternative inlet or outlet in some systems.

The specific function of the ports depends on the state of the solenoid valve (energized or de-energized) and the design (normally closed or normally open).

• Normally closed (NC):When the valve is de-energized, the P port is closed, and no flow is allowed from the inlet to the outlet. The A port is typically connected to the E or R port, allowing the actuator to exhaust. When energized, the valve opens the flow from the P port to the A port and closes off the E or R port.
• Normally open (NO):When the valve is de-energized, the P port opens to the A port, allowing flow from the inlet to the outlet. The E or R port is closed. When energized, the valve switches to close the flow from P to A and opens the connection between A and E or R, allowing the actuator to exhaust.

Housing material

The solenoid valve’s housing and body material must be compatible with the media. Common options are brass, stainless steel, PVC, aluminum, and cast iron.