How a Solenoid Valve Works. Pilot Operated Solenoid Valve. Indirect Acting solenoid valve.

A Solenoid Valve is an Electromechanical Valve that is commonly used to Control the Flow of Liquid or Gas.

They Act as control units which when Electrically Energized or De-Energized either shut-off or allow Fluid Flow.

Parts of a Solenoid Valve

A solenoid valve mainly consists of a Solenoid, a Movable Plunger with spring and Valve Body.

Solenoid Valve States

Powered states of the solenoid valve include
Normally Open, Normally Closed and Bistable.

Normally Closed solenoid valves:

These valves use a spring to press the Plunger tip against the opening of the Orifice.
This prevents the media that is either gas or liquid from entering the orifice until the plunger is lifted by the Electromagnetic field created by the Coil.

Normally Open Solenoid Valves:

In Normally open solid valve, the valve is opened when de-energized and the media can flow through it. When energized it creates an electromagnetic field that forces the plunger downwards, closing the orifice which prevents the media from flowing through the valve.

Bistable solenoid valve.

A Bistable or latching solenoid valve can be switched by a momentary power will then stay in that position with no power. Therefore, it is not normally open or normally closed as it stays in the current position when no power is applied. They accomplish this by using permanent magnets rather than a spring.

Types of Solid Valves based on Working Principle

There are various solenoid valve types but the main variants are either

1.Pilot Operated

2.Direct Acting Solenoid Valves.

Pilot Operated Solenoid Valves utilize the System line pressure to open and close the main orifice in the valve body.
While direct operated solenoid valve is directly actuated by the solenoid and does not use the system line pressure to open or close the main valve Orifice.

Working of Pilot Operated Solenoid Valves.

Pilot operated solenoid valves are also called servo operated or indirect acting solenoid valves. They use the differential pressure of the medium over the valve inlet and outlet ports to open and close the valve. So, they require a minimum differential pressure of around
0.5 bar.

The Inlet and Outlet ports are separated by a rubber membrane called diaphragm. The diaphragm has a small hole, so that the medium can flow to the upper compartment from the inlet.

For a normally closed in direct acting solenoid valve the inlet pressure that is the pressure above the diaphragm and the supporting spring above the diaphragm will ensure that the valve remains closed.

The Chamber above the membrane is connected by a small pilot hole to the low-pressure port. (FIG BELOW)

This Pilot hole is blocked in the Closed position by the plunger. (FIG BELOW)

The diameter of this pilot orifice is larger than the diameter of the hole in the diaphragm.
When the solenoid is energized it creates an electromagnetic field that forces the plunger upwards. (FIG BELOW)

Now the pilot orifice is opened which causes the pressure above the diaphragm to drop. (FIG BELOW)

Because of the pressure difference on both sides of the diaphragm, the diaphragm will be lifted and the medium can flow from inlet port to outlet port.

A normally open valve has the same components but works in the opposite way. The pilot operated solenoid valves are only used for media flow in one direction.
They are used in application with sufficient pressure differential and a high desired flow rate.

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