Solenoid valves, whether direct acting or pilot operated, are used to automatically control the flow of either liquids or gases, removing the need for manual operation. They function using an electromagnetic solenoid coil that opens and closes a valve, depending on whether it’s ‘normally’ closed (in which case the valve opens via the electromagnetic force), or ‘normally open’ (in which case it closes).
But how exactly do they work?
The actual method differs depending on what type of valve it is, but the general idea is that the coil, plunger, and sleeve assembly of the valve alternately allow and prevent the flow of whatever it is coming through. When the valve is normally closed, the plunger is held against the opening of the valve by a plunger return spring. Then when the coil is energised the plunger lifts via the magnetic field produced, allowing the flow. When it’s a normally open valve, the opposite happens – the plunger is lifted until the coil is energised, when it closes and seals the opening.
The main parts of a solenoid valve are as follows:
- Valve body – which the solenoid valve is connected to.
- Inlet port(s) – where the fluid or gas enters the valve.
- Outlet port(s) – where the fluid or gas leaves the valve.
- Coil (solenoid) – the body of the coil, inside the valve.
- Coil windings – a number of turns of enamelled wire looped around the material.
- Lead wires – connected to an electrical supply, these wires transmit the current.
Direct acting solenoid valves
The seal in a direct acting solenoid valve is attached directly to coil, which when not energised is in the closed position, only opening when the valve is energised. Two-way direct acting solenoid valves have one inlet port and one outlet port, and when closed (de-energised), the fluid pressure means the spring keeps the seal shut to stop the flow. When energised, however, the seal is pulled into the coil – opening the valve. Three-way direct acting solenoid valves work in a similar way, but have two valve seats and three ports – with one seal always open and one seal always closed, reversing when the coil is energised.
Pilot operated solenoid valves
While direct acting solenoid valves, as discussed, are attached to the opening and closing armature, in pilot operated solenoid valves it’s the fluid pressure which opens and closes the valves. Pilot operated valves are generally chosen for usage in situations where there’s going to be a higher pressure and/or higher temperatures, due to the higher flow rates they allow. To work, the fluid enters a little chamber situated right above the diaphragm, via the inlet port, which constricts against the diaphragm (in the case of a normally closed valve) and sustains the closing seal. When the coil is energised, the spring pressure pulls the diaphragm up, and the fluid from the chamber is forced back through the inlet port. Pilot operated solenoid valves are available as two-way valves or multi-way valves, similarly to the direct acting solenoid valves.