Direct-acting solenoids lift the seal with the coil alone and work from zero pressure difference, while pilot-operated types use the coil to open a small pilot that lets line pressure move a larger diaphragm or piston, so they need a minimum differential to shift. They come normally closed or normally open, two-way or three-way. A common job is venting the air off a valve actuator on a shutdown trip.
Coil on the trip solenoid is cooked, the valve won't hold air so the actuator keeps going to fail position.
A pilot-operated solenoid that misbehaves on a low-pressure line is often working exactly as designed: it needs a few psi of drop across it to shift.
Also heard as
- solenoid
- SOV
Related terms
Actuator
The powered device that opens, closes or positions a valve using air, hydraulic oil or an electric motor.
Fail-closed
Describes a valve that springs shut when it loses air, power or its control signal.
Spring-return
Describes a single-acting actuator that uses air to move one way and a spring to move back and fail safe.
Control valve
A power-operated valve that modulates flow on a controller signal to hold pressure, level, flow or temperature at setpoint.