Solenoid Valves

A solenoid valve is an electrically operated switching valve placed between the positioner (or I/P) and the actuator. Its job is binary and safety-critical: when de-energised, it either blocks the air path or dumps it — and that single choice defines how the valve fails when the electrical signal is lost.

What the porting means

• 3/2 normally energised (NE) — the classic trip valve arrangement: energised, supply passes to the actuator; de-energised, the actuator vents. The valve fails to its spring position when the loop signal is lost.
• 3/2 normally de-energised — used where the valve must stroke on loss of power, e.g. vent-to-fail-open service.
• 5/2 for double-acting actuators — switches pressurised air between the two piston sides; the de-energised porting decides which side vents and which connects to supply or to a fail-action volume tank.

Selection notes

• Flow capacity (Kv / Cv) sets venting speed — an undersized solenoid turns a fast-trip valve slow. For large actuators, pair with a quick-exhaust or check the dump path separately.
• Failure mode of the solenoid itself — a stuck solenoid defeats the whole trip path. Critical service often uses two solenoids in a partial-stroke-tested arrangement (per IEC 61511 practice).
• Coil rating and certification — voltage, duty cycle (ED%), Ex certification for the area class.
• Manual override — a local override helps maintenance but must be secured to defeat accidental operation.

Where it sits in the loop

Typical chain: instrument air → filter-regulator → solenoid → positioner → actuator (or solenoid directly on a single-acting actuator for on-off duty). On double-acting cylinders, the solenoid may switch both sides — see piston cylinders.