What are Solenoid Valves?

Solenoid valves (also known as electromagnetic valves) are valves controlled by an electric current. They consist of two main parts - the valve body and the solenoid (coil). The solenoid is made up of wound copper wire that encircles a core tube with a movable closing piston. The coil's task is to create a magnetic field using the passing electric current, which then moves the piston and either opens or closes the valve. Solenoid valves thus use electric current to convert into linear motion.

The use of solenoid valves is very diverse. They are suitable for applications for both liquid and gaseous media. They serve to close, open, dose, distribute, or mix in distribution systems. Classic uses include heating systems, irrigation, car washes, dishwashers and washing machines, cooling and air conditioning systems, medicine, dentistry, industrial cleaning equipment, and water tanks.

Solenoid valves can be found in the common two-way variant, or in more complex three-way and multi-way constructions used for switching flows and mixing. The valve bodies are most commonly made from brass, stainless steel, aluminum, or even plastic. Before selecting, it is always good to check whether the material corresponds to the intended use and is compatible with the given medium.

Valve Construction

A solenoid valve consists of a valve body on which the coil is mounted. The inlet and outlet are equipped with connections to allow the valve to be connected to piping. Inside the valve body, you will find a shading ring, spring, piston, and seal or diaphragm.

Open or Closed?

The two main categories of solenoid valves are NO and NC - normally open and normally closed. When an electric current acts on the coil, a magnetic field is created, the strength of which depends on the current, the number of wire windings, and the material of the movable core, also known as the piston. The magnetic field moves this piston and thus closes or opens the valve. Without current, the valve can either be closed or open.

In a normally closed valve, when the current is turned on, the piston is pulled upwards by the magnetic field. This opens the valve and allows the medium to flow. The magnetic field lifts the piston against the spring, which pushes it back down. When the current is interrupted, the magnetic field disappears, and the spring pushes the piston back to its original position. The valve is thus closed. This is the more commonly used variant for safety reasons in case of a power failure.

A normally open valve allows the medium to flow without the current being turned on. The piston is thus always raised, and the medium can flow through the valve. However, if the current is turned on, the magnetic field pushes the piston down and closes the valve. This variant is mainly used in applications where it is more energy-efficient to have the valve open for a long time.

Direct or Indirect Valve Control?

Solenoid valves also differ in the way they are controlled. They can either be directly controlled, where the coil directly opens the valve's passage, or indirectly controlled, meaning they are controlled by the pressure difference between the inlet and outlet. They are always installed in piping systems so that the arrow on their body points in the direction of flow.

Directly Controlled Valves

Directly controlled valves operate on a very simple principle. As an example, we will consider a normally closed valve. When the power is interrupted, the spring pushes the piston, closing the valve's passage and seating it on the seal. When the current is turned on, the coil pulls the piston up and opens the space. For a normally open valve, it is exactly the opposite.

These valves are used more for small flows where there is not much pressure. They do not require any pressure or pressure difference for operation and can be used even at zero pressure in the distribution systems.

Indirectly Controlled Valves

Indirectly controlled valves, sometimes also called pilot valves, operate on the principle of differential pressure between the inlet and outlet. In this case, the valve's passage is closed by a diaphragm that separates the inlet and outlet openings. There is a small balancing hole in the diaphragm through which the medium can flow into the chamber above the diaphragm from the inlet. This balances the pressure acting on the diaphragm, keeping it in the closing position. Again, as an example, we will take a normally closed valve. If the valve is not under voltage, the diaphragm is pushed down by the spring and the pressure that forms in the chamber above the diaphragm. The diaphragm thus closes the valve, and the medium cannot flow through. The chamber above the diaphragm is connected to the outlet opening by a small channel (pilot hole), which is closed by the piston. If the coil is under current, it lifts the piston up, allowing the medium to flow through the pilot hole into the outlet space. Since the pilot hole is larger than the balancing hole, the pressure above the diaphragm drops, while the pressure in the inlet opening remains the same and now therefore greater. It lifts the diaphragm, allowing the medium to flow into the outlet opening. The differences in pressure, or the value of the differential pressure, range from 0.3 bar to 1 bar. For a normally open valve, the operating principle is exactly the opposite.

These valves are suitable for larger flows.

Two-Way or Three-Way Valve?

Two-way valves are among the most basic solenoid valves. There is an arrow marked on their body that shows the direction of the medium's flow. For the valve to function correctly, this direction should be followed during installation. Two-way valves are used to open or close the flow. However, sometimes more complex applications are needed, such as mixing. For this purpose, three-way and multi-way valves are used.

Three-way valves have a total of three ports for connection to the distribution system. They can switch between two circuits or mix two circuits together. Some valves can handle both of these functions, depending on what is needed more at the time. The valve is always connected to only two ports at a time, through which the medium flows at that moment.

Valve Distribution Function

The medium flows through the inlet opening. The coil decides which outlet the medium will flow through. Without current, the medium will flow through the upper outlet. If the coil is under current, the piston will be lifted up, closing the path to the upper outlet but opening the path to the lower outlet. The medium will thus be redirected to the second outlet.

Valve Mixing Function

In this case, the valve has two inlets and one outlet. The coil decides from which outlet the medium will flow at any given moment. Without current, the medium will flow from the upper inlet to the outlet. Under current, the coil lifts the piston up, preventing flow from the upper inlet and opening access to the medium from the lower inlet.

Universal Valve Function

The valve operates in both directions. Either as a distribution or as a mixing valve. However, only two ports are in operation at any time.

Tork Solenoid Valve directly controlled open without voltage 230 VAC power supply water, gas, air brass body More information

Coil Power Supply

Coils are powered by either direct current or alternating current. Direct current coils have more windings than alternating current coils. They are less susceptible to dirt, and the lifting force in both the initial and lifted positions remains the same. However, the energy consumed and the magnetic force in direct current coils depend on the temperature. In alternating current coils, the dependence is less, but they are more susceptible to dirt, which can cause the coil to buzz. They are characterized by faster switching speed. If the piston is blocked, the coil may overheat. At the same voltage, the resistance in an alternating current coil is less than in a direct current coil.

The voltage range for coils is wide. For direct current, the voltage ranges between 12-48 V, and for alternating current, the voltage ranges between 110-230 V. Solenoid valves are usually sold with the coil, but it is also possible to find valves without a coil. Coils are fully replaceable, and for each valve, you can find a replacement coil if the existing one stops working. Replacement is quick and easy.

Danfoss Solenoid Valve indirectly controlled closed without voltage 230 VAC power supply water, gas, steam, oil built-in filter snap-on coil More information

What are the Differences Between Seal Materials?

The material of the seal and diaphragm should always be compatible with the given medium. For different media, a different type of seal is always more suitable.

NBR - or nitrile rubber is synthetic. It is characterized by high resistance to wear. It is an elastomer with high tensile strength. The risk of deformation is very low. It is suitable for water-based and glycol-based liquids, oils, and gaseous media such as air. The temperature should range from -10 to +90/100 °C.

EPDM - ethylene-propylene rubber. It is characterized by high resistance to aging, ozone, heat, and ultraviolet rays. The risk of deformation is very low. The temperature range is wider than NBR, from -30 ° to +140 °C. EPDM is most commonly used for water-based and glycol-based liquids and for steam applications.

ViTON - for applications with a temperature range of -15 to 220 °C. It is a fluororubber material resistant to a large number of chemicals, such as mineral oils, ozone, fuels, organic solvents, and more. It is characterized by excellent insulating abilities and resistance to vacuum. It is not very flexible and can easily get scratched.

FKM - another fluororubber material with similar properties to ViTON. The temperature range is from 0 to 100 °C. It is used in applications with water and glycols, for oils and gaseous media such as air.

PTFE - a very durable polymer with a temperature range of -10 to +150 °C. It is used for steam applications. It is characterized by very high resistance to aging and chemicals, high tensile strength, and low friction.