Solenoid Valves

A solenoid valve is an electro-mechanical valve that is used to control the flow of liquid or gas. The solenoid starts by converting an electrical signal into a mechanical movement. The signal is then sent to a coil and the movement then occurs inside of the valve. Solenoid valves are usually be described as pilot operated or direct operated/acting.

Solenoid valves are the most frequently used control elements in fluidics. A Solenoid valve is an electromechanically operated valve. The valve is controlled by an electric current through a solenoid: in the case of a two-port valve the flow is switched on or off; in the case of a three-port valve, the outflow is switched between the two outlet ports. Multiple solenoid valves can be placed together on a manifold. SME FZE Solenoid valves are amongst the most used components in gas and liquid circuits.

Special Metals and Equipments FZE Solenoid Valves used in oil and gas circuits, fluid power pneumatic and hydraulic systems, to control cylinders, fluid power motors or larger industrial valves. Automatic irrigation sprinkler systems also use solenoid valves with an automatic controller.


Solenoid valves can be categorized into different groups of operation.

  • Direct Operated Solenoid Valves
  • In-Direct Operated Solenoid Valves
  • Semi-Direct Operated Solenoid Valves
  • 3/2 Way Solenoid Valves


  1. Direct Operated Solenoid Valves

Direct Operated Solenoid Valves have the most simple working principle. The medium flows through a small orifice which can be closed off by a plunger with a rubber gasket on the bottom. A small spring holds the plunger down to close the valve. The plunger is made of a ferromagnetic material. An electric coil is positioned around the plunger. As soon as the coil is electrical energized, a magnetic field is created which pulls the plunger up towards the centre of the coil. This opens the orifice so that the medium can flow through. This is called a Normally Closed (NC) valve. A Normally Open (NO) valve works the opposite way: it has a different construction so that the orifice is open when the solenoid is not powered. When the solenoid is actuated, the orifice will be closed. The maximum operating pressure and the flow rate are directly related to the orifice diameter and the magnetic force of the solenoid valve. This principle is therefore used for relatively small flow rates. Direct operated solenoid valves require no minimum operating pressure or pressure difference, so they can be used from 0 bar up to the maximum allowable pressure. The displayed solenoid valve is a direct operated, normally closed 2/2 way valve.


2.In-Direct Operated Solenoid Valves

In-Direct Operated Solenoid Valves (also called servo operated, or pilot operated) use the differential pressure of the medium over the valve ports to open and close. Usually these valves need a minimum pressure differential of around 0.5 bar. The inlet and outlet are separated by a rubber membrane, also called diaphragm. The membrane has a small hole so that the medium can flow to the upper compartment. The pressure and supporting spring above the membrane will ensure that the valve remains closed. The chamber above the membrane is connected by a small channel to the low pressure port. This connection is blocked in the closed position by a solenoid. The diameter of this “pilot” orifice is larger than the diameter of the hole in the membrane. When the solenoid is energized, the pilot orifice is opened, which causes the pressure above the membrane to drop. Because of the pressure difference on both sides of the membrane, the membrane will be lifted and the medium can flow from inlet port to outlet port. The extra pressure chamber above the membrane acts like an amplifier, so with a small solenoid still a large flow rate can be controlled. Indirect solenoid valves can be used only for one flow direction. Indirect operated solenoid valves are used in applications with a sufficient pressure differential and a high desired flow rate, such as for example irrigation systems, showers or car wash systems. Indirect valves are also known as servo controlled valves.


3.Semi-Direct Operated Solenoid Valves

Semi-Direct Operated Solenoid Valves combine the properties of direct and indirect valves. This allows them to work from zero bar, but still they can handle a high flow rate. They look somewhat like indirect valves and also feature a movable membrane with a small orifice and pressure chambers on both sides. The difference is that the solenoid plunger is directly connected to the membrane. When the plunger is lifted, it directly lifts the membrane to open the valve. At the same time, a second orifice is opened by the plunger that has a slightly larger diameter than the first orifice in the membrane. This causes the pressure in the chamber above the membrane to drop. As a result, the membrane is lifted not only by the plunger, but also by the pressure difference. This combination results in a valve that operates from zero bar, and can control relatively large flow rates. Often, semi-direct operated valves have more powerful coils than indirect operated valves. Semi-direct operated valves are sometimes called assisted-lift solenoid valves.


4.3/2 Way Solenoid Valves

3/2 Way Solenoid Valves has three ports and two switching states. In each switching state, two of the three ports are connected. By activating the solenoid, the valve switches state and a different connection between the valve ports is established. The drawing below shows a direct operated 3/2 way valve. In the de-energized state, the medium can flow between from the port on the right side to the top port. In the energized state, the medium can flow from the left port to the right port. This is a called a normally closed 3/2-way valve.


Solenoid Valve Specificaton

Power Input : 0 to 30 Vdc

Maximum Powered Required : 400 mA

Type Of Operation : normally closed (NC) when deenergized

Connections : 1/4″, 1/8″ and 3/8″

Dimensions : 3.45″ x 3.25″ x 1″

Materials In Fluid Contact : Brass, Stainless steel (316 and 416), Aluminum.

Maximum Presuure : 1000 psig (68.9 bars)

Maximum Differential Pressure : 50 psid (345 kPa)

Leak Integrity : 1 x 10-9 sml/sec Helium; individually tested

Maximum Temperature : 174°F (79°C) inside, 130°F (54°C) outside surface at 24 Vdc