Solenoid Valve: Working Principle and Type Selection

The solenoid valve is an automatic basic element used to control the direction of the fluid, belonging to the actuator;

It is usually used for mechanical control and industrial valves to control the direction of the medium, so as to control the valve switch.

Working principle of solenoid valve

The solenoid valve has a closed cavity, and there are through holes at different positions.

Each hole leads to different oil pipes.

There is a valve in the middle of the cavity, and two electromagnets are on both sides.

The valve body will be attracted to which side when the magnet coil is energized.

By controlling the movement of the valve body, different oil drain holes will be blocked or leaked.

The oil inlet hole is normally open, and the hydraulic oil will enter different oil drain pipes, and then push the piston of the oil cylinder through the oil pressure. 

The piston drives the piston rod, which drives the mechanical device.

In this way, the mechanical movement is controlled by controlling the current of the electromagnet.

Classification of solenoid valve

1. Solenoid valves are divided into three categories in principle:

1) Direct acting solenoid valve:

Principle: When energized, the solenoid coil generates electromagnetic force to lift the closing piece from the valve seat and open the valve;

When the power is cut off, the electromagnetic force disappears, the spring presses the closing element on the valve seat, and the valve closes.

Features: It can work normally under vacuum, negative pressure and zero pressure, but the diameter generally does not exceed 25mm.

2) Distributed direct acting solenoid valve:

Principle: It is a combination of direct action and pilot type. When there is no pressure difference between the inlet and outlet, the electromagnetic force will directly lift the small pilot valve and the main valve closure piece upward in turn and open the valve after power on.

When the inlet and outlet reach the starting differential pressure, the electromagnetic force will pilot the small valve, and the pressure in the lower chamber of the main valve will rise and the pressure in the upper chamber will drop, so that the main valve can be pushed upward by using the differential pressure;

When the power is cut off, the pilot valve uses spring force or medium pressure to push the closing piece to move downward to close the valve.

Features: It can also operate under zero pressure difference, vacuum and high pressure, but the power is large, so it must be installed horizontally.

3) Pilot operated solenoid valve:

Principle: When the power is on, the electromagnetic force opens the pilot hole, the pressure in the upper chamber drops rapidly, forming a high low pressure difference around the closing piece, the fluid pressure pushes the closing piece upward, and the valve opens;

When the power is cut off, the pilot hole is closed by the spring force, and the inlet pressure quickly forms a low to high pressure difference around the valve closing piece through the bypass hole chamber.

The fluid pressure pushes the valve closing piece downward to close the valve.

Features: The upper limit of the fluid pressure range is high, which can be installed arbitrarily (to be customized) but must meet the fluid pressure differential conditions.

2. The solenoid valve is divided into six subcategories according to the difference in valve structure and material and principle:

Direct acting diaphragm structure, step by step heavy plate structure, pilot diaphragm structure, direct acting piston structure, step by step direct acting piston structure, pilot piston structure.

Precautions for selection of solenoid valve

1. Applicability

The fluid in the pipeline must be consistent with the medium calibrated in the selected solenoid valve series model.

The fluid temperature must be less than the calibration temperature of the selected solenoid valve.

The allowable liquid viscosity of the solenoid valve is generally below 20CST, and it shall be noted if it is greater than 20CST.

When the working differential pressure and the maximum differential pressure of the pipeline are less than 0.04MPa, such as ZS, 2W, ZQDF, ZCM series and other direct acting and multi-step direct acting types shall be selected;

When the minimum working pressure difference is greater than 0.04MPa, pilot operated (differential pressure) solenoid valve can be selected;

The maximum working differential pressure shall be less than the maximum calibration pressure of the solenoid valve;

Generally, solenoid valves work in a one-way way, so pay attention to whether there is a back pressure difference. If there is a check valve, install it.

When the fluid cleanliness is not high, a filter should be installed in front of the solenoid valve. Generally, the solenoid valve requires a better cleanliness of the medium.

Pay attention to the flow diameter and nozzle diameter;

Generally, the solenoid valve is only controlled by two positions of switch;

If conditions permit, please install bypass pipe to facilitate maintenance;

In case of water hammer, the opening and closing time adjustment of the solenoid valve shall be customized.

Pay attention to the influence of ambient temperature on the solenoid valve.

The power supply current and power consumption shall be selected according to the output capacity.

The power supply voltage is generally allowed to be about ± 10%.

It must be noted that the VA value is high during AC starting.

2. Reliability

The solenoid valve is divided into two types: normally closed and normally open;

Normally closed type is selected, which is powered on and powered off;

However, when the opening time is long and the closing time is short, the normally open type should be selected.

For life test, factories generally belong to type test items. To be exact, there is no professional standard for solenoid valves in China, so we should be careful when selecting solenoid valve manufacturers.

When the action time is very short and the frequency is high, the direct action type is generally selected, and the fast series is selected for large caliber.

3. Safety

Generally, the solenoid valve is not waterproof. If conditions do not permit, please select the waterproof type, which can be customized by the factory.

The maximum calibrated nominal pressure of the solenoid valve must exceed the maximum pressure in the pipeline, otherwise, the service life will be shortened or other accidents will occur.

All stainless steel type shall be used for corrosive liquid, and plastic king (SLF) solenoid valve shall be used for strong corrosive liquid.

Corresponding explosion-proof products must be selected for explosive environment.

4. Economy

There are many solenoid valves that can be used universally, but the most economical products should be selected on the basis of meeting the above three points.

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