PlantFlow 3D › Guides › Solenoid valve vs control valve: on/off and modulating valves
Solenoid valve vs control valve: on/off and modulating valves
How a solenoid valve and a control valve differ, how each is wired to a PLC, what fail-open and fail-closed mean, and typical faults to look for.
On/off versus modulating
A solenoid valve is either open or closed. A coil, typically 24 V DC or 230 V AC, is switched by a PLC digital output or a relay. A control valve can sit at any position between 0 and 100 %: a positioner receives a 4–20 mA signal from a PLC analog output and moves the actuator to the matching opening.
Fail-safe position
Every valve must go to a safe position when power or signal is lost. A spring-return valve is described as fail-closed (FC) or fail-open (FO). A cooling-water valve is usually fail-open; a fuel valve fail-closed. In PlantFlow 3D the control valve FV-101 is fail-closed: below 3.6 mA it shuts.
Typical faults
Solenoid valves: burnt coil (commanded open but stays shut), stuck spool, wrong coil voltage. Control valves: lost signal, sticking stem, positioner out of calibration, air supply lost. A pump running against a shut valve is dead-headed: pressure rises and flow stops, which is why discharge pressure and flow alarms matter.
Wiring to the PLC
Solenoid coils connect to a digital output, with a flywheel diode on DC coils to absorb the switch-off spike. Positioners connect to an analog output as a two-wire 4–20 mA loop. Feedback switches or a position transmitter can return the actual valve position to the PLC.
Try it yourself
Open PLC & Process, run the pump in AUTO, set the FV-101 output to 50 % and then set the XV-101 fault to “Coil burnt” to see the pump dead-head.
Open the PlantFlow 3D simulator →Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.