Solenoid valves are devices that use a solenoid to control valve activation. They are considered electromechanical control devices used to control liquid or gas flow. An electrical current runs through a coil to control the valve by moving a plunger. When the solenoid receives an electrical signal (energized), it channels the air supply directly to the plunger. When the electrical signal is removed (de-energized) the valve returns to its normal condition.

Find Solenoid Valves by Specification or See Our Directory of Suppliers

Solenoid valve working. Video Credit: Marine Mechanic / CC BY-SA 4.0

Solenoid valves are used to open and close a system. The media enters through the inlet port and flows through the orifice before it continues to the outlet port. This port is closed and opened by the plunger.

• Direct-acting: Solenoid valves can be considered direct-acting, which means that the plunger directly opens and closes the orifice inside the valve.

• Pilot-operated: Solenoid valves can also be considered pilot-operated, which means the plunger opens and closes a pilot orifice. The inlet pressure opens and closes the valve seal. This type of valve is also called a servo-type valve.  

More information can be found at Engineering360's How to Select Linear Solenoids and How to Select Rotary Solenoids pages. 

Applications

Solenoid valves are used in a wide variety of industries. They are used in machinery, devices, and equipment such as refrigerators and automatic faucets. Solenoid valves are commonly used in central heating systems to control the thermostat to regulate the flow of heated water to the heating element. They are also used in automatic irrigation sprinkler systems, air control, fluid control, and in pharmacology experiments.

For more information, read Engineering360's Valve Applications page.

Types of Solenoid Valves

Solenoid valves generally have two ports: an inlet and an outlet port. There are several types of solenoid valves that include three or more ports.

Three-way solenoids are used to operate single-acting actuators, such as diaphragm actuators. They are designed to only send air to one chamber of an actuator. Three way solenoids are used to interrupt or override an instrument signal for double-acting actuators with a pneumatic positioner.

Four-way solenoids provide a positive two directional action. They can be used instead of positioners to provide on-off operation of double-acting valves. When the solenoid is de-energized, it sends the full air supply to one side of the actuator and exhausts the other side to the atmosphere.

The effective size of a solenoid valve can be increased by servo or pilot operation. The term servo-operated applies to solenoid valves to indicate the main valve is fluid-powered and actuated by a small valve in a servo or pilot circuit.

Two and three way, normally open and normally closed solenoids.

Image Credit: Rostratransmission.wordpress.com 

Valve State

Powered states include normally open and normally closed.

Normally closed -- Normally closed solenoid valves use a spring to press the plunger tip against the opening of the orifice. This keeps the media from entering the orifice until the plunger is lifted by the electromagnetic field created by the coil.

Normally open -- Normally open valves close when the coil is energized, allowing the media to flow through the system. This valve state is best used for water, air, and noncorrosive liquids.  

Find Solenoid Valves by Specification or See Our Directory of Suppliers

Solenoid Valve Components

Solenoid valves come in a variety of sizes and materials in order to integrate within many fluid management systems. The body of the valve should be made of a material that is compatible with the system media to prevent premature failure of the valve, or contamination of the media. The most important components to consider when selecting a solenoid valve are the seal, coil, and the ports of the valve.

Solenoid valve components. Image Credit: Solenoid-Valve-Info.com

Seals

In general, seals are the most limiting factor when selecting a solenoid valve. When selecting the material for the seal it is important to consider the chemical properties of the media, and temperature and pressure of the media and system. The most common seal material choices are nitrile rubber (NBR), ethylene propylene diene monomer rubber (EPDM), Viton^®, and PTFE.

Solenoid Valve Coils

The solenoid valve coil converts electrical energy into linear motion. It is usually made of copper or aluminum wire wound around a hollow form. A ferromagnetic core, called the valve plunger, is placed inside the coil. As electric current flows through the coil, the lines of magnetic flux turn the plunger into an electromagnet, creating a magnetic field. The valve body orifice opens as the magnetic field pulls the plunger further up into the coil.  Polarity of the coil does not matter because most coils with lead wires use the same color wire for both terminals and the positive terminal can be connected to either wire without affecting the operation of the valve. There are two types of coils that can be used in a solenoid valve.

Solenoid coil. Image Credit: rostransmission

• Tape wrap coil- Tape wrap coils are composed of conductor wire wrapped around a spool or bobbin. The conductor wire, also known as magnet wire, has a thin insulation layer around it and the completed winding is protected by an additional layer of insulation tape. Tape wrap coils are best used in applications which have mild environments. They can be used for smaller production runs, but they have a lower tolerance for moisture.

• Encapsulated coils- Encapsulated coils, also called molded coils, are also composed of conductor wire wrapped around a spool or bobbin. The coil is then encapsulated in a suitable resin instead of a tape. Encapsulated coils can be used in environments with higher humidity levels and are made with stronger wires to protect against pull out.

Find Solenoid Valves by Specification or See Our Directory of Suppliers

Solenoid valve coils are available in different voltages and can be used with DC or AC electricity. It is often possible to change coils out in order to use a different voltage for the valve.

How solenoid valves work. Video Credit: Confessfletch / CC BY-SA 4.0

Solenoid Valve Ports  

Solenoid valves generally have two ports. The ports can come with either inside or outside threaded ports. In a tandem center solenoid valve, the pressure and tank ports are connected while the service ports are blanked. This allows system unloading while still providing isolation of the service lines. In a float center solenoid valve, the supply pressure port is closed. All others ports are interconnected. This allows the supply to be shut off while enabling the load to move or free wheel with flow available to other services.

Resources

Solenoid Valve Information

Nesbitt, Brian. Valves Manual International: Handbook of Valves and Actuators. Oxford: Elsevier, 2007. Print. 

Hundy, G. F., A. R. Trott, T. Welch, and A. R. Trott. Refrigeration and Air-conditioning. Amsterdam: Butterworth-Heinemann/Elsevier, 2008. Print.

Image Credit: Clark | Gems