The Different Types of Solenoid Valves and How They Work

Solenoid valves are automatic flow control devices popular across different industrial applications. These valves feature a compact design, offer durable fluid service, require low power to operate and are quick-acting. How do these valves achieve unique performance characteristics? The answer lies in their designs and the electromagnetic action of the valve components.

Solenoid valve examples: 2-way, 3-way and 4-way styles. Photo credit: Emerson Automation Solutions, Fluid Control & Pneumatics

Solenoid valves comprise a valve body (containing orifices) and a solenoid. The solenoid consists of an inductive coil surrounding a ferromagnetic core, or plunger. Energizing the coil by passing electric signals creates a magnetic field through it. The magnetic field attracts the plunger and causes linear motion, moving the plunger within the valve body. The movement causes the sealing or opening of the various valve orifices. The magnetic force produced by energizing the solenoid is sufficient to overcome both inertia and the mechanical force of an opposing spring, thus, moving the plunger to a secondary position relative to orifice locations.

Solenoid valves are versatile and have diverse circuit designs; they can open or close the flow of fluids and control flow rates, as well as distribute or mix service fluids in a piping system. Typical application areas include automated process controls, on/off fluid service, hydraulic and pneumatic loops, water supply, fuel supply, air conditioning systems and wastewater treatment.

TYPES OF SOLENOID VALVES

There are different types of solenoid valves based on the mode of actuation and number of flow paths. These distinct features determine the suitability of a particular solenoid valve for an industrial application. When evaluating or selecting solenoid valves for different pipe systems, engineers and system designers weigh additional features like valve response time, coil protection, solenoid material, and valve and seal materials. A description of the different types of solenoid valves follows:

DIRECT-ACTING SOLENOID VALVES

The operation of these valves is solely dependent on the electromagnetic field. They do not require external pressure or a supplementary actuator to operate. Direct-acting solenoid valves can open or close even when there is no pressure difference across valves. The plunger will move in response to the electromagnetic field created by energizing the solenoid.

Stainless-steel and Numan-mount style solenoids.
Photo credit: Emerson Automation Solutions, Fluid Control & Pneumatics

A popular direct-acting solenoid valve is the 2-way valve that can be selected in the normally open or normally closed configuration. In a normally open solenoid configuration, a spring supplies the force to hold the seal away from the seat of the orifice, keeping the flow path open as long as the coils are de-energized. Applying electrical power creates an electromagnetic force that pushes the seal towards the seat and stops flow in the desired direction. The inverse is true for a normally closed circuit. Energizing the solenoid coil lifts the seal from the orifice seat, opening the flow path and permitting fluid flow.

Direct-acting valves have a compact design since the actuation mechanism is within the valve body. The flow rates and allowable pressure limits for fluids passing through direct-acting solenoid valves depend on the sizes of the orifices and the magnetic force that the solenoid supplies. They are suitable for applications having relatively low flow rates.

There is a unique type of direct-acting solenoid valve that can be selected in the normally open or normally closed configuration. These are called “bistable” or “latching solenoids”.

PILOT-OPERATED SOLENOIDS

The operation of these valves requires some assistance from the pressure of the service fluid. They are common in fluid applications with a high-pressure operating range and high flow rates. The solenoids for these valves provide sufficient forces to control the opening and the closing of pilot orifices. When the pilot orifice opens or closes, it permits or expels fluid in or out of the top cavity and holds the valve diaphragms in appropriate positions.

In pilot-operated solenoids, the service media moves through the pilot orifice of the solenoid and applies some pressure on the top side of the diaphragm, holding it in a closed position. The valves contain mechanical springs that keep the diaphragms against the main valve orifice to block flow paths. No fluid flows past the valve. Energizing the solenoid causes the expulsion of the service fluids out of the top cavity and releases them to the main pressure line. It creates a pressure difference between the lower and upper sides of the diaphragm. The differential pressure overcomes the force of the diaphragm and the springs to open the main valve.

The orifice of the pilot valve is usually smaller than that of the main valve. Such orientations enable small-sized pilot valves to operate large valves using small amounts of energy. By comparison, these valves operate slower than their direct-acting counterparts. They are suitable for high-pressure hydraulic systems, steam facilities and high flow rate industrial processes.

THREE-WAY SOLENOID VALVES

These solenoid valves provide additional flow control beyond routine on/off fluid service in a piping system. They are appropriate for mixing fluids drawn from two different sources or diverting fluids from a common inlet to two destinations. The orientation of these valves can have one inlet port and two outlets or two inlet ports and a common outlet. These valves can offer reliable service for either mixing or diverting fluids depending on their configuration.

When used for diverting the flow of fluids, the 3-way solenoid valve will feature a single inlet port and two outlets. In its initial setup, fluid will flow from the inlet orifice and move out through one outlet while the other remains in a closed position. Energizing the solenoid coils diverts the flow from the initial open orifice to the secondary path that was an initially closed outlet. In mixing applications, one of the inlets is initially open, and the other is in a closed position. The open inlet transfers the fluid through the valve and ejects it through the common outlet. After energizing the solenoid coil, the plunger moves to a position that blocks the initially open orifice, while the secondary orifice opens to permit the flow of fluids.

NAMUR MOUNTED SOLENOID VALVES

Most solenoid valves can be predictably fixed to actuators to facilitate automated valve operations in different applications. Many applications require standard solenoid valves having specific mounting patterns and installation styles. They are referred to as NAMUR mount valves.

Specific solenoid mounting designs fit pneumatic actuators specifically designed to accommodate this standardized mounting pattern. The valves can provide 3-way, 4-way or even 5-way flow paths. Energizing the solenoids of these valves causes plungers to move into different positions and open or close the orifices accordingly. NAMUR mount valves selected for pneumatic applications should meet the flow conditions of the pipeline and appropriate response time to guarantee the precision of pneumatic applications.

CONCLUSION

Solenoid valves can improve the reliability of diverse flow applications by providing an inexpensive way to remotely actuate valves with a size and design that fits any fluid flow operation. When selecting these valves, ensure they meet the flow conditions of the pipeline and valve body materials, and that the seals are compatible with the service media.

About the Author

Gilbert Welsford Jr. is the founder of ValveMan.com and a third-generation valve entrepreneur. He has learned valves since a young age and has brought his entrepreneurial ingenuity to the family business in 2011 by creating the ValveMan online valve store.