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What is a Solenoid Valve?

Posted by ussolid on Nov 25th 2020

What is a Solenoid Valve?

Solenoid valves are magnetically operated valves. The are sometimes called electromechanical valves. The electrical part refers to the solenoid. The solenoid is a cylindrical coil of wire that, when energized, produces a magnetic field. This magnetic field is used to move the ferrous plunger, which directly or indirectly closes (or opens) the valve.

There are several different classifications of solenoid valves, based on a few different parameters. The first is based on the acting mechanism:

Direct Acting- Direct acting solenoid valves use the plunger to directly close the orifice (small opening). The are almost exclusively smaller valves, as larger direct acting valves would require a very large solenoid. These solenoid get pricey, so instead pilot operated solenoid valves are used.

Pilot Operated- Pilot operated valves use the pressure of the fluid to close and seal the orifice. This allows for a larger orifice without a larger solenoid.

The second type of classification is based on the resting state of the valve:

Normally Closed (N.C.)- Normally closed valves are closed when at rest (unenergized). This is the more common of the valves.

Normally Open (N.O.)- Normally open valves are open when at rest (unenergized).

Lastly, valves can also be classified on the directional flow of the material:

Unidirectional- Material must flow in the designated direction, usually indicated by an arrow

Bidirectional- Material flow can occur in either direction


Figure 1.1 shows a typical direct acting valve. It is made of two distinct parts. The metal body, which has the threaded female connections, is made of 2 brass pieces that form a seal with a gasket. The gasket material is the main factor in determining the possible usages for the valve. Gaskets made of NBR cannot be used with petrochemical products. Gaskets made of VITON are safe for any use. The second portion of the valve is the part responsible for opening and closing. The black enclosure houses the solenoid that, when energized, creates a magnetic field. The magnetic field acts on a ferrous plunger that will then open a small orifice. In direct acting valves, this is the main orifice. In pilot operated valves, the small orifice only releases pressure from the liquid on the gasket. When the pressure is released, the gasket is free to move, effectively unplugging the larger orifice. Pilot operated valves need pressure to operate effectively.

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As mentioned earlier the effective size of the solenoid valve can be increased by pilot or servo operation. Servo operated valves are unidirectional as the inlet and outlet pressures are essential for operation of the valve.

According to Figure 1.3 there are small holes in the servo diaphragm. Here is how they work.

When the valve is closed the fluid seeps through the diaphragm holes (Figures 1.3. and 1.4 )and inlet pressure acts onto the diaphragm holding the diaphragm in the closed position. This means that the higher the inlet pressure the higher is closing power. This is not the case with direct acting valves.


When the valve is in the open position (for N.C. valves, this means they are energized) only a small plunger is moving against the inlet pressure but NOT the entire diaphragm which seeps the fluid that used to pushing the diaphragm out through the small orifice (1.2. picture). That action reduces pressure acting on the diaphragm and finally the coil has the energy to open the entire orifice.

Lastly, solenoid valves are designated by their pipe threading. So a ¾” solenoid valve is, typically, ¾” of NPT threading. There are other standards used elsewhere in the world, but as NPT is standard in the United States, it is the assumed standard for valves for sale in the U.S.A. Our brand, U.S. Solid, used almost exclusively NPT threading, and will clearly denote when any other kind of thread is used.

Solenoid valves are an incredible piece of engineering. They are used in cars, irrigation systems, and dozens of other applications. There simplicity and cost effectiveness also makes them a great product for the DIY enthusiast. Coming up soon will be examples and uses of solenoid valves in the real world! Check back on our blog in the coming days.