How a solenoid valve works
A coil wound around a tube creates a magnetic field when energised. A steel plunger inside the tube is pulled into the field against a spring, and the plunger carries or releases the seal that closes the valve orifice. Cut the power and the spring returns it.
That is the whole mechanism in a direct acting valve, and it is why they are limited: the coil has to generate enough force to lift the seal against the full line pressure acting on the orifice. Above a few millimetres of orifice at any real pressure, the coil would be enormous.
Direct acting against pilot operated
A pilot operated valve gets round the force limit by using the line pressure itself. The main seal is a diaphragm or piston with a small bleed hole. The solenoid only opens a tiny pilot orifice above the diaphragm; that lets the pressure on top drain away, and the line pressure underneath lifts the diaphragm.
The catch is that it needs pressure to work. Below the minimum differential, usually 0.3 to 0.5 bar, there is not enough force under the diaphragm to lift it and the valve stays shut even with the coil energised. Gravity-fed and low-pressure systems need a direct acting or an assisted-lift valve for that reason.
| Direct acting | Pilot operated | |
|---|---|---|
| Minimum pressure | Zero | Typically 0.3 to 0.5 bar differential |
| Maximum size | Small, to about 1/2 in | Any, common to 2 in and beyond |
| Response | Fast | Slightly slower |
| Coil power | Rises steeply with size | Small regardless of size |
| Suits | Low pressure, vacuum, small flows | Mains water, compressed air, larger flows |
Normally open, normally closed, and port counts
Normally closed is shut with the coil off and opens when energised. It is the default and the safe choice for most applications, because a power failure closes the valve. Normally open is the reverse, held open by the spring and closed by the coil, used where flow must continue on power loss.
A 2-way valve has an inlet and an outlet and simply opens or shuts. A 3-way valve has a third port and switches flow between two paths, or vents a line when it closes, which is what a single-acting air cylinder needs. 4-way and 5-way valves drive double-acting cylinders by pressurising one side while exhausting the other.
Choosing by what flows through it
Water valves need seals that suit the temperature, NBR for cold water and EPDM for hot, and a body material that will not corrode: brass for most, stainless for aggressive or potable-water service where NSF certification is required.
Air valves are usually pilot operated for anything over a few millimetres, and the seal material matters less. Lubricated air needs NBR; dry oil-free air suits most compounds.
Oil and fuel need Viton or PTFE seals and a body rated for the pressure. Steam needs PTFE seals and a stainless or bronze body rated for the temperature. Never use a water valve on oil; the seals swell and the valve sticks.
