What is Solenoid valve?

August 30, 2022


Nikita Bhagat

Nikita Bhagat, graduated in Electrical Engineering. Currently she is working at Lesics engineers Pvt.Ltd as a Product Developer. Nikita has done other projects such as Solenoid valve and Linear motor. Her area of interests are Machines and circuits. To know more about the author check this link

This hydraulic excavator has a very interesting solenoid valve at the heart of its operation(Fig:1a). The solenoid valve is so crucial that even minor wear and tear on its disc can cause the excavator to jam, leading to its failure. They are even used in sensor taps. The other major industrial applications of the solenoid valve are shown in (Fig:1b). In this article we are going to explore the impressive engineering behind solenoid valves starting from the simplest one.

Fig:1a - Hydraulic excavator has a solenoid valve
Fig:1b - Applications of 2/2 solenoid valve

Need of solenoid valve

Here is an interesting question for you. Some commonly used motorized valves in the industry are working perfectly, then why do we need solenoid valves? The answer is, the solenoid valves are insanely fast. A normal plate motorized valve takes 1 second for its operation, while the solenoid valve finishes its task in 0.03 seconds as shown in Fig:2.

Fig:2 - Motorized valve VS Solenoid valve

Design: 1 - Basic design of 2-by-2 Solenoid valve

Let’s see the construction of the simplest possible solenoid valve. I took An iron armature that is kept perpendicular (which is free to move) inside the pipe(Fig:3a). An electromagnet is placed around the armature and also attach a spring to the fixed plate at the top of the armature. The spring is compressed here and it always tries to keep the armature at the bottom as shown in Fig:3b below.

Fig:3a - Armature is perpendicular inside the pipe
Fig:3b - Spring is trying to keep armature at the bottom

When I energize this electromagnet with DC current, strong magnetic fields are generated around it. You can see in the Fig:4 the armature will experience a force causing it to move upwards and to align perfectly to the center of the coil. The valve is now open, and the water flows by(Fig:4).

Fig:4 - Valve is open and water flows by

Let me de-energized the coil, and see what will happen now, obviously the compressed spring have closed the valve immediately. With this design, we have achieved a simple 2-by-2 solenoid valve. It is called 2-by-2 because of its two ports - input and output and two operation states - ON and OFF.

Fig:5a - Solenoid valve in OFF state

Solenoid valve In OFF state

I noticed small issue with this design. In OFF state, if the fluid pressure increases, the armature can bend, which results in fluid leakage. Please have a look at Fig:6, it will help you to better understand this.

Fig:6-Armature is bent due to high fluid pressure

To solve this issue let’s make some design modifications. We can try to shift the coil and the armature to the top and instead add a solid thick barrier(Fig:7a) in the flow path. Now the valve is closed, and the fluid won’t be able to cross this region. Great! This design will has ensure that the armature does not bend due to water pressure.

Fig:7a - Valve Barrier - OFF state of valve

Solenoid valve In ON state

Lets energizes the coil, will you can see in Fig:7b the armature moved up and the valve is open now, allowing fluid to easily flow through the gap.

Fig:7b - 2/2 Solenoid valve with barrier- ON state

Now that the armature is safe from bending, but the high fluid pressure can still push the armature upwards which is resulting in fluid leakage(Fig:8).

Fig:8-In OFF state, the high fluid pressure pushes the armature upwards

Now we have seen, this design has a small issue. But good to hear that our smart engineers have solved this issue. Engineer came up with a perfect solution: just keep the same fluid pressure on both sides of the armature(Fig:9). Let’s see how this new design is achieved practically.

Fig: 9 -To prevent from the fluid leakage, keep same fluid pressure on both side of the Armature

Engineers used a flexible rubber diaphragm with two holes(Fig:10). Let’s fix this diaphragm with the valve body.

Fig: 10 -Diaphragm with the valve body

Coil de-energised (OFF)state

The tiny red hole on the diaphragm plays a major role here. You can see how the fluid reaches both the sides of the armature due to this small red hole. This means that, when the valve is closed, the fluid pressure acting on the armature gets canceled. This perfectly solves the previous problem. (Fig:11).

Fig: 11 -Fluid pressure over armature from chamber and valve body

Coil Energized (ON)state

Now the coil is energized, the armature is moving up, and the fluid inside the armature chamber escapes via the central hole. Here, the fluid in the armature region escapes and causes a sudden pressure drop. This pressure drops because the outflow via the central hole is higher than the inflow as illustrated in Fig below(12a). Because of this pressure difference, the diaphragm will bend as shown in fig(1b). Now the fluid can escape directly via the bottom of the diaphragm.

Fig:12a - High pressure outflow via the central hole is higher than the inflow
Fig:12b -Diaphragm has bent due to high fluid pressure

Now, our simple 2 by 2 solenoid valve design is almost over.

However, if you tear down a real 2 by 2 valve, you can see an additional component - an iron cap at the top of the cover. Why is it needed? Now, let’s see which design achieves that small distance first!

Fig:13- Solenoid valve with diagram and Iron cap

In the given FEA result, You can see that the solenoid valve with the extra iron cap reaches that distance in half the time of the other case. This reduces the response time of the solenoid valve. The reason for the fast movement is that the iron cap produces a strong magnetic pole. Now a quick and efficient design of 2/2 solenoid valve is ready.

Fig:14- FEA simulation representing the solenoid valve with iron cylinder is achieved at that distance very fast.

I hope you have a good understanding of 2-2 Solenoid valves. Stay tuned to our next part of this article, working of Excavator with 5/3 solenoid valves.

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