Introduction to solenoid valve
The solenoid valve is an electromagnetic part of a control valve. A solenoid valve is an electromechanically operated valve used for controlling liquid or gas flow. Solenoid valves are the prime means of enabling/disabling air supply to pneumatic actuators and are a critical element in the pneumatic actuation circuits of single and double acting pneumatic actuators. Solenoid valves may de-energized or energized from the safety or control system responsible for operating the valve, depending on the project specifications, to drive the main isolation valve to the desired position.
Construction of solenoid valve
The most common type of solenoid valve are available in two port design with one inlet & outlet port. Special design may have three or more ports & also manifold-type design.
Different parts of a two port, normally-closed, direct-acting solenoid valve are:
- Valve body
- Inlet port
- Outlet port
- Coil windings
- Lead wires
A spring is housed in the coil which keeps the plunger in position. In normally closed position the spring presses the plunger tip against the orifice opening due to which there is no flow of the fluid form the inlet port to the outlet port. When the electric current is passed through the coil the spring is energized allowing the plunger to lift-up from the position and allowing the passage of fluid through the orifice. When the electrical current is removed, the valve returns to its de-energized state.
Besides the plunger-type actuator which is used most frequently, pivoted-armature actuators and rocker actuators are also used.
Working of solenoid valve
Solenoid valves are the prime means of enabling/disabling air supply to pneumatic actuators and are a critical element in the pneumatic actuation circuits of single and double acting pneumatic actuators. Solenoid valves may de-energized or energized from the safety or control system responsible for operating the valve, depending on the project specifications, to drive the main isolation valve to the desired position.
Unless otherwise stated, for shutdown valves and on-off valves with ‘fail safe’ action, the solenoid shall be de-energized to vent air out of the actuator and drive the valve to the safe (closed or open) position (spring force should push the actuator to the desired position).
For blowdown valves, the solenoid may be de-energized or energized by the safety system, depending on the project specifications. This aspect has to be critically worked out with the Process Dept., taking into account the adequacy of the blowdown valve tailpipe and flare header sizing, as well as when and how the blowdown valves are supposed to operate. For blowdown valves with solenoids supposed to energize to fail open the valve, the solenoid circuits should be line monitored.
Specification of solenoid valves
Unless otherwise specified, solenoid valves should be of the universal type, with Class H coil insulation, low power rated (consumption ≤ 6 W), with coil voltage of 24 V DC and certified flameproof as per IEC 60079-1 for the applicable hazardous area.
Checkpoints for solenoid valves
The Instrumentation engineer should check that the maximum instrument air pressure expected at the solenoid valve location should be sustainable by the solenoid valve body. Solenoid valves should be fitted with quench diodes suitable for continuous operation. Vent ports of the solenoid valves should be fitted with sintered Stainless Steel dust excluders. Flying leads shall not be used instead terminals shall be specified for solenoid.