The solenoid valve is a basic automatic component used to control the direction of fluid flow and is considered part of the actuator.
It is widely used in mechanical control systems and industrial valves to regulate the fluid direction, thereby controlling the valve switch.
Solenoid valve working principle
The solenoid valve has a closed cavity with multiple through holes located in different positions. Each of these holes leads to a separate oil tube. In the center of the cavity there is a valve, with two electromagnets positioned on each side. When the magnetic coil is energized, the valve body will be pulled to either side. This movement of the valve body regulates the oil flow by blocking or opening different oil drain holes.
The oil inlet hole is normally open, allowing hydraulic oil to flow into the various oil drain tubes and exert pressure on the oil cylinder piston. This, in turn, drives the piston rod and the connected mechanical device. In essence, mechanical movement is controlled by regulating the current in the electromagnet.
Solenoid valve classification
1. Solenoid valves are divided into three categories in principle:
1) Direct acting solenoid valve:
Principle: The solenoid coil generates electromagnetic force when energized, which lifts the closing part of the valve seat and opens the valve. When the power is cut off, the electromagnetic force disappears, causing the spring to press the closing piece back onto the valve seat, thus closing the valve.
Features: It is capable of operating under vacuum, negative pressure and zero pressure conditions, although the diameter is generally limited to 25 mm or less.
2) Direct acting distributed solenoid valve:
Principle: The valve operates through a combination of direct action and pilot control. When there is no pressure difference between the inlet and outlet, the electromagnetic force directly lifts the small pilot valve and the closing part of the main valve, causing the valve to open after the power is turned on.
When an initial differential pressure is reached between the inlet and outlet, electromagnetic force pilots the small valve, causing the pressure in the lower chamber of the main valve to increase and the pressure in the upper chamber to decrease. This creates a differential pressure that pushes the main valve upward.
When the power is turned off, the pilot valve uses spring force or medium pressure to push the shutoff piece down, closing the valve.
Features: The valve can work under zero pressure difference, vacuum and high pressure conditions. However, it requires a large power input and must be installed horizontally.
3) Pilot operated solenoid valve:
Principle: When power is applied, electromagnetic force opens the pilot orifice, causing a rapid drop in pressure in the upper chamber. This creates a large pressure difference around the closure piece, and the fluid pressure pushes the closure piece upward, opening the valve.
When the power is cut off, the pilot hole is closed by spring force. The inlet pressure quickly creates a low to high pressure difference around the valve closure piece through the bypass port chamber, causing fluid pressure to push the valve closure piece downward, closing the valve.
Features: The fluid pressure range has a high upper limit and can be installed in any location (customizable) as long as it meets the fluid pressure differential requirements.
2. Solenoid valve is divided into six subcategories according to the difference in valve structure and material and principle:
Direct Acting Diaphragm Structure, Heavy Plate Structure Step by Step, Pilot Diaphragm Structure, Direct Acting Piston Structure, Direct Acting Piston Structure Step by Step, Pilot Piston Structure.
Precautions for selecting solenoid valve
1. Applicability
The fluid in the piping must be compatible with the medium specified in the selected model of the solenoid valve series.
The fluid temperature must be lower than the calibrated temperature of the selected solenoid valve.
The allowable liquid viscosity of the solenoid valve is generally less than 20 CST and should be observed if it exceeds 20 CST.
When the working differential pressure and the maximum differential pressure of the pipeline are less than 0.04 MPa, direct-acting and multi-stage direct-acting types such as ZS, 2W, ZQDF, ZCM series should be selected.
If the minimum working pressure difference is greater than 0.04 MPa, a pilot-operated solenoid valve (differential pressure) can be selected.
The maximum working differential pressure must be lower than the maximum calibrated pressure of the solenoid valve.
Solenoid valves typically operate in a unidirectional manner, so it is important to consider the presence of backpressure differences. If necessary, a check valve must be installed.
If the fluid is not clean enough, a filter must be installed before the solenoid valve. The solenoid valve requires a higher level of average cleaning.
Consider the flow diameter and nozzle diameter.
The solenoid valve is usually controlled by only two switch positions.
If possible, a branch pipe should be installed to facilitate maintenance.
In case of water hammer, the opening and closing time of the solenoid valve must be adjusted as necessary.
Take into account the impact of ambient temperature on the solenoid valve.
The current and power consumption of the power supply must be chosen based on the output capacity.
The power supply voltage can normally deviate by ±10%.
It is important to note that the VA value is high during AC startup.
2. Reliability
Solenoid valve comes in two types: normally closed and normally open.
When selecting a solenoid valve, it is typically recommended to choose the normally closed type, which is on and off.
However, if the opening time is long and the closing time is short, it is better to choose the normally open type.
Factory life tests are a common type of test item. However, it is important to note that there is no official standard for solenoid valves in China, so care must be taken when choosing a manufacturer.
For cases where the action time is very short and the frequency is high, the direct action type is generally preferred. For larger calibers, the rapid series is the best choice.
3. Security
Generally, solenoid valves are not waterproof. If conditions do not permit a standard solenoid valve, it is recommended to select a waterproof type, which can be customized by the factory.
It is important to ensure that the maximum calibrated pressure rating of the solenoid valve exceeds the maximum pressure in the pipeline, as failure to do so may result in reduced service life or other accidents.
For corrosive liquids, it is recommended to use a stainless steel type solenoid valve, and for strongly corrosive liquids, a king plastic solenoid valve (SLF) is recommended.
In explosive environments, it is necessary to select corresponding explosion-proof products.
4. Economy
Many solenoid valves are suitable for universal use, but it is important to choose the most economical options that meet the three criteria mentioned above.
