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      • Published 17 Jan 2023
      • Last Modified 4 Sept 2023
    • 11 min

    A Comprehensive Guide to Solenoid Valves

    In this guide, we look at what solenoid valves are and how they work, plus explore the different types available.

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    Reviewed by Luqman Said Abdullah, Technical Support Engineer (April 2021)

    Throughout this comprehensive guide, we will look at how a solenoid valve works, get to know some of the various types of solenoid valves available, and explore what types of applications and environments they are most commonly used in.

    What is a Solenoid Valve?

    Solenoid Valve

    A solenoid valve is an electromechanically operated valve component used to control flow rates in fluid or air-powered mechanical systems. They are used in many applications for fluid control of water, air, oil or gas.

    Solenoid valves are incredibly useful for engineers and end-users due to their automatic operation. The requirement for an engineer to be physically present to operate a valve manually is removed with the use of solenoids as they can be installed and used anywhere that gas or fluid flow needs to be automatically controlled and monitored.

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    How Does a Solenoid Valve Work?

    Solenoid valves are essentially flow control units with the ability to restrict or enable flow. This depends on the type of valve in question and whether or not they are in a currently activated (energised) state at any given time. But how does a solenoid valve work in practice?

    In a standard spool or poppet-type valve, the solenoid valve connection is direct to a plunger, pivoted armature, or another similar device, which for most models will be spring-mounted in some configuration. This plunger is located inside a hollow tube, often called the solenoid core.

    When electrically energised, the wire coil wound around the outside of this core creates a magnetic field inside the tube. In turn, this influences the motion of the plunger. This then pushes or pulls the valve in a particular direction, opening or closing the flow system. The precise way in which the solenoid valve functions - either to open or close when automatically activated - will depend on the specific type and configuration of the valve and solenoid controller chosen, and thus its normal operating state.

    Normally Open and Normally Closed Solenoid Valves

    If a solenoid valve is normally open:

    • The valve will remain open whenever the solenoid is not receiving an input signal. The rate of flow through it will remain at whatever the maximum flow rate of the conduit run or plumbing pipeline would be under normal circumstances
    • This normally open type of solenoid will close the valve automatically (using the electromagnetic force generated in its coil) under an electrical input signal. In short, as the coil becomes energised in a normally open solenoid, the valve is pushed shut, and thus the rate of flow through it is automatically controlled
    • As soon as the electrical input signal ceases to be sent to the coil, it will return to its normal state and position. The flow will either be restricted or enabled accordingly

    The opposite of the above is true for a normally closed solenoid valve.

    Complex Solenoid Valves

    Some types and models of solenoid valve controllers employ additional components, including advanced circuit technologies, to offer more complex or responsive functions and features. For example, these might include the ability to manage proportional flow or pressure control, achieved through coil sensitivity to a broad range of input signal strengths.

    It’s worth noting that, in fluid dynamics terms, normally open (i.e. flow-enabled) valves typically imply the opposite meaning to normally open gates in electronic systems.

    Two-Way Solenoid Valve

    Two-Way Solenoid Valves

    Two-way or two-port solenoid valves are among the most common types available. In essence, they control one inlet and one outlet port. They work to open or close the valve whenever they receive an electrical activation signal.

    Two-way solenoid valves are sometimes called shut-off solenoids. They are available in both normally open and normally closed varieties, according to whether the application requires flow to be restricted or enabled under standard prevailing conditions.

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    Three-Way Solenoid Valve

    Three-Way Solenoid Valves

    Three-way or three-port solenoid valves have three input/output ports and two valve seals, one of which will always be open and one of which will always be closed in normal (inactive) mode.

    In response to an electrical input signal, the open/closed state of these two ports is reversed, allowing various solenoid valve operations to be controlled. Flow can thus be diverted between channels as required, in accordance with the way the port connections have been set up and connected within the larger pipeline system.

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    Direct Operated Solenoid Valve

    Direct Operated Solenoid Valves

    Within the solenoid valve types outlined above, there exists a wide range of more specific operating types and functions. One of these is a direct-operated solenoid valve, sometimes called direct-acting.

    With direct operated solenoid valves, the energised solenoid acts directly on the valve-sealing piston or armature. Therefore, the valve's ability to restrict or enable flow depends on the rated power - in other words, the physical strength - of the component.

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    Pilot Operated Solenoid Valve

    Pilot Operated Solenoid Valves

    With pilot-operated solenoid valves, the opening or closing of the valve is powered by the energy stored in the build-up of pressurised fluid or gas.

    Pilot-operated solenoids can be internally or externally piloted. Internally piloted solenoid valves are used for high pressure systems, usually found in larger pipeline setups where pressure differentials do the bulk of the work to open or close the valve. Externally piloted solenoids rely on a third-party component, such as a spring or armature, to push or pull the valve/diaphragm, working in conjunction with the typically lower-pressure differentials commonly encountered in smaller pipelines.

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    Solenoid Valve Types and Applications

    You can buy solenoid valves intended for use with many different liquids and gases. These valves include specialist configurations to cover industrial, production, and laboratory environments. The wide range of types available includes products intended for applications such as:

    • Hot and cold liquids
    • Compressed air
    • Steam
    • Mineral oil, natural oil and other oils (with and without additives)
    • Aqueous solutions
    • Aggressive and combustible gas

    Solenoid valves are also used in domestic and commercial appliances. Typical examples you might encounter regularly include:

    • Heating and water flow systems
    • Lawn sprinklers and irrigation networks
    • Car washes
    • Swimming pool drainage and top-up controls
    • Medical and dental equipment
    • Refrigeration and air conditioning
    • Paintball guns, pressure pumps and other compressed air tools
    • Calibration and test stands

    In terms of basic media compatibility and operating fundamentals, the options in the list below are all common variants. Many types of solenoid valves offer specific functionality in one or more of the following areas:

    • Water solenoid valve
    • Solenoid gas valve
    • Solenoid air valve
    • Pneumatic solenoid valve
    • Hydraulic solenoid
    • High-pressure solenoid valves
    • High-performance or fast-acting solenoid valve

    What Does a Solenoid Valve Look Like?

    Solenoid Valve Diagram

    A solenoid valve is often constructed in poppet or spool format as a basic model option. However, more specialised versions are available for tackling specific or demanding workloads, media, functions, and environments. Most solenoids are modularly constructed around a fundamental arrangement of a few key components, namely:

    • Valve body - This is the main functioning part of the solenoid valve, and includes the input-output ports, valve seats, and the orifice that controls the flow
    • Electrical solenoid coil - The electromagnet made from copper windings which influences the plunger motion when energised
    • Coil enclosure, housing or tube assembly - Typically a cylinder of non-ferrous metal, sealed at one end, which provides the channel within which the plunger moves in response to electromagnetic forces

    Other components you might find in more advanced systems include various arrangements of DIN terminals and connectors. This provides numerous options for subsequent output signals and onward controls to influence a range of behaviours further along in the system.

    What are Solenoid Valves Made of?

    The vast majority of modern solenoid valves are manufactured from robust and corrosion-resistant materials. Depending on the intended usage, some common solenoid valve materials might include:

    What to Consider When Purchasing a Solenoid Valve

    Many different types of solenoid valves are currently available. This is an inherent benefit of the component's great versatility - the wide range of solenoid types and functions means that it is usually possible to choose a valve that will precisely suit the role it is needed to perform and the application and environment where it will be installed.

    When choosing a solenoid valve to install in a particular pipeline or flow system, the key requirements to consider include:

    • Flow rate - A valve that is too large or overpowered for the required flow rate of a system won’t be cost-effective. Similarly, a valve that is too small or underpowered for the required flow rate will not provide an accurate or proper actuator function
    • Media compatibility - It is critical to always choose the appropriate valve for operation with water, air, steam, oil, fuel, or the particular media where it will be employed
    • Pressure - Operating pressure is among the greatest influences on the overall flow rate of a solenoid valve, so it must be balanced with the system requirements for optimal performance
    • Temperature
    • Process fitting or mounting type

    Manufacturer guidelines for each of the above criteria list the maximum safety rating in each case. As a result, you will need to ensure that your operating parameters fall below the specified ratings in all cases. You will also need to look into some of the solutions for solenoid valve adapters and mounts. Broadly speaking, these will often align with the type of pipeline where they will be installed, for example:

    • Compression fittings for gas or water pipes
    • Push-in or horsetail hose fittings
    • PVC fittings for PVC conduit
    • Various adapter types, such as sockets and reducer bushings

    For each type of solenoid valve, you will also need to consider the appropriate size and power requirements. This can be achieved by calculating a match between your precise usage needs or working conditions and the valve’s rated performance specifications in various areas. To this end, manufacturer guidelines should always be observed in each of the following areas. For liquid solenoid valves (where liquids are broadly considered incompressible), you will need to consider:

    • Rated valve flow capability
    • Flow rate - often expressed as volume per minute
    • Inlet and outlet pressure
    • Pressure drop across the valve
    • The specific gravity of the fluid in question

    For gas solenoid valves, where gases are considered compressible media, there are a couple of more complex considerations to consider. These might include gas temperatures at various stages of the pipeline, shifting flow rates expressed in cubic feet per hour, and resulting absolute pressures at both the inlet and outlet ports.

    Understanding BSP Thread Sizes

    Some solenoid valves are available in BSP (British Standard Pipe) thread sizes. It's imperative to choose the correct size of valve for your system or application. It is also essential to understand how BSP threads are measured.

    They can be confusing because the imperial (inches) you measure on a ruler do not match the stated BSP thread size. For instance, a 1/2 BSP fitting will measure 3/4 inch when measured with a ruler.

    A handy way to confirm the correct BSP thread size is to measure the thread diameter in millimetres (mm) and then convert it back again. You can see the details in the table below:

    BSP Thread Size

    Thread Diameter (mm)

    1/8 BSP


    1/4 BSP


    3/8 BSP


    1/2 BSP


    3/4 BSP


    1 BSP


    1 1/4 BSP


    1 1/2 BSP


    2 BSP


    3 BSP


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    SMC Solenoid Valve


    Explore the extensive range of solenoid valves available from leading brand SMC and shop online today with RS.

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    In conclusion, solenoid valves are essential devices used in a wide range of applications and environments. This guide has provided a comprehensive overview of how solenoid valves work and introduced the various types available on the market.

    From controlling fluid flow in industrial processes to regulating water supply and managing irrigation systems, solenoid valves play a vital role. Their versatility, reliability, and precise control make them a preferred choice in many industries. Understanding the fundamentals of solenoid valves and their applications empowers engineers and technicians to effectively utilise these devices. Whether it's automation, HVAC, medical equipment, or countless other industries, solenoid valves continue to contribute to efficient and controlled operations.

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