Solenoid Valve - How They Work

Figure 1: 2/2-way solenoid valve

A solenoid valve is an electrically controlled valve. The valve features a solenoid, which is an electric coil with a movable ferromagnetic core (plunger) in its center. In the rest position, the plunger closes off a small orifice. An electric current through the coil creates a magnetic field. The magnetic field exerts an upwards force on the plunger opening the orifice. This is the basic principle that is used to open and close solenoid valves.

Quick solenoid valve notes:

• Only used for clean liquids and gases.
• Indirect operated valves require a pressure differential to function.
• Are used to close, open, dose, distribute, or mix the media with 2 or more inlets/outlets.
• Fast acting.
• Options for manual overrides, ATEX, gas approval, media separation and more.
• Can get hot as it requires energy to switch and stay in that position (depending on type).
• Common in heating systems, compressed air, vacuum, irrigation, car washes, etc.

Table of Contents

• How does a solenoid valve work?
• Circuit functions of solenoid valves
• Solenoid valve types
• Solenoid valve working principles
• Selection criteria
• Solenoid valve applications
• FAQ

How does a solenoid valve work?

Figure 2: Components of a solenoid valve; coil (A); armature (B); shading ring (C); spring (D); plunger (E); seal (F); valve body (G)

A solenoid valve consists of two main components: a solenoid and a valve body (G). Figure 2 shows the components. A solenoid has an electromagnetically inductive coil (A) around an iron core at the center called the plunger (E). At rest, it can be normally open (NO) or normally closed (NC). In the de-energized state, a normally open valve is open and a normally closed valve is closed. When current flows through the solenoid, the coil is energized and creates a magnetic field. This creates a magnetic attraction with the plunger, moving it and overcoming the spring (D) force. If the valve is normally closed, the plunger is lifted so that the seal (F) opens the orifice and allows the flow of the media through the valve. If the valve is normally open, the plunger moves downward so that the seal (F) blocks the orifice and stops the flow of the media through the valve. The shading ring (C) prevents vibration and humming in AC coils.

Solenoid valves are used in a wide range of applications, with high or low pressures and small or large flow rates. These solenoid valves use different operating principles that are optimal for the application. The three most important ones are explained in this article: direct acting, indirect acting, and semi-direct acting operation.

Circuit functions of solenoid valves

Solenoid valves are used to close, open, dose, distribute or mix the flow of gas or liquid in a pipe. The specific purpose of a solenoid valve is expressed by its circuit function. An overview of 2-way and 3-way solenoid valves is below. For an in-depth understanding of symbols and understanding circuit function diagrams, view our valve symbol page.

2-way solenoid valve

A 2-way solenoid valve has two ports, an inlet and an outlet. Flow direction is critical to ensure proper operation, so there is typically an arrow indicating the flow direction. A 2-way valve is used to open or close the orifice. Figure 3 shows an example of a 2-way solenoid valve.

Figure 3: 2-way solenoid valve

3-way solenoid valve

A 3-way valve has three connection ports. Typically, it has 2 states (positions) it can be in. So, it switches between two different circuits. A 3-way valve is used to open, close, distribute, or mix media. Figure 4 shows an example of a 3-way solenoid valve.

Figure 4: 3-way solenoid valve

Solenoid valve types

Normally closed solenoid valve

Figure 5: Operating principle of normally closed solenoid valve: de-energized (left) & energized (right)

For a normally closed solenoid valve, the valve is closed when de-energized and the media cannot flow 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 the flow through the valve. Figure 5 shows the operating principle of a normally closed solenoid valve in the de-energized and energized states.

Normally open solenoid valve

Figure 6: Operating principle of normally open solenoid valve: de-energized (left) & energized (right)

For a normally open solenoid valve, the valve is open when de-energized and the media can 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 is then seating in the orifice and closing it, which prevents media from flowing through the valve. Figure 6 shows the operating principle of a normally open solenoid valve in the de-energized and energized states. A normally open solenoid valve is ideal for applications that require the valve to be open for long periods of time as this is then more energy efficient.

Bi-stable solenoid valve

A bi-stable or latching solenoid valve can be switched by a momentary power supply. It will then stay in that position with no power. Therefore, it is not normally open or normally closed as it stays in the current position when no power is applied. They accomplish this by using permanent magnets, rather than a spring.

Solenoid valve working principles

Direct acting

Figure 7: Direct acting solenoid valve working principle and components: coil (A); armature (B); shading ring (C); spring (D); plunger (E); seal (F); valve body (G)

Direct acting (direct operated) solenoid valves have a simple working principle, which can be seen in Figure 7 along with the components. For a normally closed valve, with no power the plunger (E) blocks the orifice with the valve seal (F). A spring (D) is forcing this closure. When power is applied to the coil (A), it creates an electromagnetic field, attracting the plunger up, overcoming the spring force. This opens the orifice and allows the media to flow through. A normally open valve has the same components, but works in the opposite way.

The maximum operating pressure and flow rate are directly related to the orifice diameter and the magnetic force of the solenoid valve. Therefore, a direct acting solenoid valve is typically used for relatively small flow rates. Direct operated solenoid valves require no minimum operating pressure or pressure difference, so they can be used from 0 bar up to the maximum allowable pressure.

Indirect acting (servo or pilot operated)

Figure 8: Indirect acting solenoid valve working principle

Indirect acting solenoid valves (also called servo operated or pilot operated) use the pressure differential of the medium over the valve inlet and outlet ports to open and close the valve. Therefore, they typically require a minimum pressure differential of around 0.5 bar. The working principle of an indirect acting solenoid valve can be seen in Figure 8.

The inlet and outlet ports are separated by a rubber membrane, also called diaphragm. The membrane has a small hole so that the medium can flow to the upper compartment from the inlet. For a normally closed indirect acting solenoid valve, the inlet pressure (above the membrane) and supporting spring above the membrane will ensure that the valve remains closed. The chamber above the membrane is connected by a small channel to the low-pressure port. This connection is blocked in the closed position by the plunger and valve seal. The diameter of this "pilot" orifice is larger than the diameter of the hole in the membrane. When the solenoid is energized, the pilot orifice is opened, which causes the pressure above the membrane to drop. Because of the pressure difference on both sides of the membrane, the membrane will be lifted and the medium can flow from inlet port to outlet port. A normally open valve has the same components, but works in the opposite way.

The extra pressure chamber above the membrane acts like an amplifier, so a small solenoid can still control a large flow rate. Indirect solenoid valves are only used for media flow in one direction. Indirect operated solenoid valves are used in applications with a sufficient pressure differential and a high desired flow rate.

Semi-direct acting

Figure 9: Semi-direct acting solenoid valve working principle

Semi-direct acting solenoid valves combine the properties of direct and indirect valves. This allows them to work from zero bar, but they can still handle a high flow rate. They look similar to indirect valves and also feature a movable membrane with a small orifice and pressure chambers on both sides. The difference is that the solenoid plunger is directly connected to the membrane. The working principle of a semi-direct acting solenoid valve can be seen in Figure 9.

When the plunger is lifted, it directly lifts the membrane to open the valve. At the same time, a second orifice is opened by the plunger that has a slightly larger diameter than the first orifice in the membrane. This causes the pressure in the chamber above the membrane to drop. As a result, the membrane is lifted not only by the plunger, but also by the pressure difference.

This combination results in a valve that operates from zero bar and can control relatively large flow rates. Often, semi-direct operated valves have more powerful coils than indirect operated valves. Semi-direct operated valves are also called assisted-lift solenoid valves.

3-way direct acting

A 3-way solenoid valve has three ports, so depending if you want a mixing (two inlets and one outlet) or diverting (one inlet and two outlets) affects operation. Certain valves can also work in both directions, which is called a universal circuit function. However, only two ports are connected in each state. Figure 10 shows an example of a 3-way direct acting solenoid valve.

Figure 10: 3-way direct acting solenoid valve working principle

Only two ports are connected at one time. In Figure 10, the plunger has an orifice on the top and bottom of it with two valve seats. At any given time, one is open and one is closed to route the media in the desired flow direction. Below are examples of circuit functions for a normally closed valve (opposite for a normally open).

• Diverting 3-way solenoid valve: Figure 10 would have one inlet (bottom left) and two outlets (top and bottom right). No power has the plunger blocking the bottom orifice, meaning the media goes from the inlet to the top outlet. When power is applied, the plunger is forced up closing the top outlet. This routes the media from the inlet to the bottom right outlet.
• Mixing 3-way solenoid valve: Figure 10 would have two inlets (top and bottom right) and one outlet (bottom left). No power has the plunger blocking the bottom orifice, meaning the media goes from the top inlet to the outlet. When power is applied, the plunger is forced up closing the top outlet. This routes the media from the bottom right inlet to the outlet.
• Universal 3-way solenoid valve: These valves act similar to a diverting 3-way solenoid valve. Looking at Figure 10, media can flow in either direction but still only two ports are connected at any given time.

Solenoid valve approvals

Depending on the application, certain approvals for the valve may be needed. Having a valve with a certain approval ensures it meets the demands of the application. Common approvals are:

• UL/UR: Underwriters Laboratories inspects and certifies products with respect to their safety
• Drinking water: Ensures it is suitable for drinking water purpose. For more information, refer to our drinking water application page or view our online selection of water solenoid valves. Common drinking water approvals are:
  • Kiwa: Drinking water approval for the Dutch market.
  • NSF: Drinking water approval for North America.
  • WRAS: Compliance with Water Supply Regulations in UK or Scottish Byelaws for material safety and mechanical testing.
  • KTW: Approval for plastics and non-metallic materials for use with potable water in Germany.
  • ACS (Attestation De Conformite Sanitaire): Drinking water approval for France.
  • Watermark: Certification for plumbing and drainage products for sale in Australia and New Zealand.
  • FDA: US Food and Drug Administration
• ATEX version: ATEX certification is provided for explosion protection. For more information, refer to ATEX Directives for valves and fittings.
• CE certification: CE certification signifies conformity with high safety, health and environmental protection requirements for all products in the European Economic Area. For more information, refer to our CE certification page.
• Gas version: Valves for gas applications have DVGW approval for use in gas burning appliances as automatic shut-off valve. For more information, refer to our Gas Approval Regulation page or view our online selection of gas solenoid valves.
• IP rating: The IP rating of a valve explains its protection to dust and water. For more information, refer to our IP rating page.

Special solenoid valve features

• Electric power reduction: A short current pulse is fed to open or close the valve and the electric power is reduced far enough to hold it in position. This helps in conserving energy.
• Latching: Latching or pulse coil version provides a solution for applications with low frequency switching. The valve is energized by a short electric pulse to move the plunger. A permanent magnet is then used to keep the plunger in that position with no additional spring or magnetic field. This lowers power consumption and heat development in the valve.
• High Pressure: High pressure versions are designed for pressure requirements up to 250 bar.
• Manual override: Optional manual override feature provides better safety and convenience during commissioning, testing, maintenance, and in case of a power failure. In some versions, the valve cannot electrically actuate when the manual control is locked.
• Media separation: Media separation design allows isolation of the media from the valve’s working parts, making it a good solution for aggressive or slightly contaminated media.
• Vacuum: Valves that do not require a minimum pressure differential are suitable for rough vacuums. Universal direct acting or semi-direct acting solenoid valves are well suited for these applications. For more stringent leakage rate requirements special vacuum versions are available.
• Adjustable response time: The time it takes the valve to open or close can be adjusted, typically by rotating screws on the valve’s body. This feature can help prevent a water hammer
• Position feedback: The switching status of a solenoid valve can be indicated with an electrical or optical position feedback as a binary or NAMUR signal. NAMUR is a sensor output that indicates the on or off state of the valve.
• Low noise: Valves have a damped design to reduce the noise during the closing of the valve.

Selection criteria

It is essential to understand your application before selecting a solenoid valve. Some important selection criteria are as follows:

• Type of solenoid valve: Determine whether your application requires a 2-way or 3-way solenoid valve.
• Housing material: Determine valve housing material based on the chemical properties and temperature of the media, but also the environment the valve is in. Brass is generally used for neutral media. Stainless steel has good chemical, temperature and pressure resistance. PVC and polyamide are commonly used as they are cost efficient. However, they are also used in high end applications with aggressive chemicals. Take into account that the mechanical parts, such as the stainless steel plunger and spring are in contact with the medium and must be compatible as well. Special medium separated valves are available where these parts are separated from the fluid by a membrane. Refer to Select the right housing material for your solenoid valve for more information.
• Seal material: Seal material should be selected based on the chemical properties and temperature of the media. NBR, EPDM, FKM (Viton) and PTFE (Teflon) are common options. Refer Select the right seal material for your solenoid valve for quick reference on chemical resistance of seal materials.
• Voltage: Solenoid valves are available in AC and DC versions, with each there are small pros and cons. Refer Choosing an AC or DC coil for a solenoid valve for more information.
• Valve function: Depending upon the operating time period, you can choose a normally open or normally closed valve. The majority of solenoid valves are normally closed. If the valve opening time is longer than closing time, a normally open valve is preferred and vice versa. Bi-stable, or latching, is also an option.
• Pressure: The valve must be able to withstand the maximum pressure required for your application. It is equally important to note the minimum pressure as high-pressure differential can cause the valve to fail.
• Operation type: Determine if your application requires a direct, indirect or semi-direct operated solenoid valve.
• Temperature: Ensure that the valve materials can withstand the minimum and maximum temperature requirement of your application. Temperature consideration is also essential to determine valve capacity as it affects the viscosity and flow of the fluid.
• Response time: The response time of a valve is the time needed for a valve to go from the open to closed position or vice versa. Small direct acting solenoid valves react much quicker than semi-direct or indirect acting valves.
• Approvals: Ensure that the valve is appropriately certified depending upon the application.
• Degree of protection: Ensure the valve has the appropriate IP rating for protection from dust, liquid, moisture and contact.

Solenoid valve applications

Figure 11: Water tanks use solenoid valves to control the inflow or outflow of water

Common domestic and industrial solenoid valve applications include:

• Refrigeration systems use solenoid valves to reverse the flow of refrigerants. This helps in cooling during summer and heating during winter.
• Irrigation systems use solenoid valves with automatic control.
• Dishwashers and washing machines use solenoid valves to control the flow of water.
• Air conditioning systems use solenoid valves to control air pressure.
• Solenoid valves are used in automatic locking systems for door locks.
• Medical and dental equipment use solenoid valve to control the flow, direction and pressure of the fluid.
• Water tanks use solenoid valves to control the inflow or outflow of water, often in combination with a float switch.
• Car washes to control the water and soap flow.
• Industrial cleaning equipment

FAQ

What is a solenoid valve used for?

A solenoid valve is used to open, close, mix, or divert media in an application. They are used in a wide variety of applications from dish washers, cars, and irrigation.

How can you tell if your solenoid is bad?

If the solenoid valve fails to open or close, is partially open, makes a humming noise or has a burned-out coil, you need to troubleshoot the valve solenoids. For more information, refer to the troubleshooting guide.

How do I choose a solenoid valve?

When choosing a solenoid valve, it is important to know your media. Depending upon the media and flow requirement, choose the material, orifice size, temperature, pressure, voltage, response time and certification required for your application. Refer to Solenoid valve selection guide for more information.

What is a solenoid?

A solenoid is an electric coil that is wrapped around a ferromagnetic substance (such as iron) that acts as an electromagnet when the current is passed through it.

How does a solenoid work?

When the electric current is passed through the coil, an electromagnetic field is created. This electromagnetic field causes the plunger to move up or down. This mechanism is used by solenoid valves to open or close the valve.

Additional solenoid valve type articles

This article only discussed 2-way and 3-way solenoid valves. Proportional and pneumatic solenoid valves are discussed in separate articles here:

• Proportional solenoid valves
• Introduction to pneumatic solenoid valves
• 3/2-way pneumatic solenoid valves
• 5/2 and 4/2-way pneumatic solenoid valves

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