A solenoid is a type of valve that uses electromagnetism to actuate the subject device and is preferred for its low power consumption, compact space envelope, superior speed of operation, and long cycle life.
What Is The Operation Of A Solenoid Valve?
A solenoid valve is made up of a coil, an orifice or multiple orifices, and a linearly operating member that seals or leaves open the orifice.
In the de-energized state of the solenoid, the aforementioned linearly operating member, also known as a slide, tube, or plunger, rests in its initial position with respect to the orifice or orifices. When a solenoid valve is energized by passing an electric signal through the coil, a magnetic field is created. In response to this field, the ferromagnetic control tube, or plunger, slides to its secondary position with respect to the orifice or orifices.
To put it another way, the magnetic field created draws or forces the sliding member to this secondary posture, also known as the energized position. This fundamental premise has given rise to a plethora of solenoid designs and configurations.
Solenoids That Act Directly
A direct-acting solenoid valve is one that uses a magnetically responsive, sliding plunger to open or close the valve’s main media flow path. In other words, when the control tube element moves in response to the generated electromagnetic field, it directly opens or closes the valve.
This solenoid valve design incorporates valve actuation into the valve itself, eliminating the need for a separate electric or pneumatic actuator to open or close the unit. This enables a more compact design than is possible with most separately actuated alternatives. A 2-way, direct-acting solenoid valve can be configured in two ways: normally open or normally closed.
In the normally closed version of a solenoid, the seat on the sliding member, held in place by a spring, sits against the orifice, keeping the flow path closed when de-energized, or normal. When energised, the magnetic field’s pull on the magnetized sliding member overcomes the resistant spring force, and the seat lifts from the orifice, allowing the flow path to be opened. When de-energized again, the spring returns the valve to its normally closed position.
The opposite is true in a normally open orientation. In this case, the de-energized seat is held off the orifice by the spring force, keeping the flow path open. When the solenoid valve is energized, the magnetic field’s pull on the magnetized plunger overcomes the spring force, drawing the seat to the orifice and sealing the valve closed. When the valve is de-energized again, the spring force returns it to the normally open position.
Solenoids That Are Bistable
Bistable solenoids, also known as latching or switching solenoids, work on a similar principle, but they do not use a spring to return the sliding member to its original position. Instead of a spring, this design employs permanent magnets to hold the ferromagnetic sliding member in place when de-energized.
The pull of the magnetic field generated by the solenoid’s energized state is greater than the pull of the magnetic force of the permanent magnet holding the plunger in its initial position. When activated, the electromagnetic field’s pull lifts the sliding member from its initial position and pulls it to a secondary posture. When the sliding member is de-energized, an additional permanent magnet holds it in this secondary position.
To return the sliding member to its original position, energies the coil by running the current in the opposite direction, which will reverse the polarity of the generated magnetic field and thus pull the sliding member back from its secondary posture? When the first permanent magnet is de-energized, the plunger returns to its original position. The term switching or bi-stable solenoid valve refers to the action of technically switching between two stable, de-energized positions.
Solenoids with Pilot Control
While a traditional and bi-stable direct acting solenoid valve may be limited in terms of line size, operating pressure, and flow capacity, a pilot-operated solenoid valve, also known as an indirect-acting solenoid, can overcome these limitations to some extent by restricting the solenoid’s action to controlling a pilot orifice and using the service medium to assist in the opening and closing of the main valve’s flow path.
The main valve, like other internally or externally piloted valves, operates as a function of the pilot valve’s position. The main valve in a typical pilot-operated solenoid design uses a diaphragm to seal the main flow path. The service medium can accumulate above and below this component due to a small orifice in the diaphragm. The presence of this orifice allows pressure on both sides of the diaphragm to equalize.
A light spring provides enough resistance to push against the diaphragm holding the main valve’s flow path closed in this pressure-balanced state. The pilot solenoid valve controls a second orifice that connects the cavity on the top side of the diaphragm to the downstream flow path. Importantly, the pilot valve’s orifice is larger than the orifice that allows the service medium to equalize on both sides of the diaphragm. When this pilot-solenoid opens, the service medium evacuates from the cavity above the diaphragm faster than it can accumulate through the pressure-equalizing orifice.
This results in a differential pressure in which the pressure of the service medium beneath the diaphragm exceeds the pressure of the remaining service medium above the diaphragm. As a result, the service medium pushes against the diaphragm and spring, allowing the primary flow path in the main valve to open. Such valves can be configured to operate normally closed or normally open.
Though solenoid valves typically have two states, energized and de-energized, this does not always imply that a solenoid can only function as a two-way, on-off valve with one inlet and one outlet. Depending on the desired configuration, a 3-way solenoid valve will have either two inlets and one outlet or one inlet and two outlets.
The solenoid in a mixing application would have two inlets and one outlet. This 3-way solenoid’s initial position would close one of the two inlets while allowing media to flow from the remaining inlet to the common outlet. When the valve is stroked to the secondary position, it opens the formally closed inlet, allowing media to flow to the common outlet while simultaneously closing the flow path from the formally open inlet.
The solenoid in a diverting application would have one common inlet and two outlets. In its initial configuration, this three-way arrangement would allow media to flow from the common inlet to one of the two outlets while keeping the secondary outlet closed. When the flow path to the formerly open outlet is stroked to the secondary position, the flow path to the formerly closed outlet closes and the flow path to the formerly open outlet opens, diverting flow to this secondary path.
Mount Solenoids NAMUR
Solenoid valves used to pilot pneumatic actuators must have multiple ports. While a traditional solenoid valve can be nipple-mounted to a pneumatic actuator, a NAMUR mount solenoid valve refers to a specific class of solenoid valves with a standardized mounting pattern designed for specific installation on rotary pneumatic actuators.
NAMUR mount solenoids are available in 3-way/2-position, 4-way/2-position, and 5-way/2-position configurations.
- This type of 3-way solenoid will have one inlet, one outlet, and one exhaust port.
- This type of 4-way solenoid will have one inlet, two outlets, and one exhaust port.
- This type of 5-way solenoid will have one inlet, two outlets, and two exhaust ports.
Each will have an energized and de-energized position that affects the air flow path through the solenoid. Other considerations include hazardous area classification, specific approvals, the need to control the speed of the valve’s operation in both directions, and the selection of a double-acting or spring return pneumatic actuator.