Solenoid valves are characterized by low power consumption, compact space requirements, speedy operation, and long cycle lives. They are actuated by the principle of electromagnetism.
How does a solenoid valve work?
In general, a solenoid valve consists of a coil, an orifice, or multiple orifices, and a linearly operating member that seals closed or leaves open.
In the solenoid’s de-energize state, the linearly operating member, commonly referred to as a slide, tube, or plunger, rests with respect to the orifice. By passing an electric signal through the coil of the solenoid valve, a magnetic field is created. When the ferromagnetic control tube, or plunger, is exposed to this field, it slides to its second position.
The magnetic field created pulls or forces the sliding member into this secondary position, commonly known as the energize position. This basic principle has led to a variety of solenoid designs and configurations.
Direct Acting Solenoids
Direct-acting solenoid valves use magnetically responsive, sliding plungers to open or close the main media flow path. This means that the valve is opened or close directly by the movement of the control tube element in response to the generate electromagnetic field.
The solenoid valve design eliminates the need for a separate electric or pneumatic actuator to stroke the valve open or closed. The result is a more compact design than most separately actuated alternatives. There are two configurations of a 2-way, direct-acting solenoid valve: normally open or normally closed.
A normally closed solenoid, with a spring to hold it in place, sits against the orifice, keeping the flow path closed when it is de-energize, or normalized. The magnetic field pulls on the magnetized sliding member, surpassing the spring force, and the seat lifts from the orifice to open the flow path. Once de-energize, the spring returns the valve to its normal closed state.
Unlike latching or switching solenoids, bistable solenoids do not use a spring to return the sliding member to its original position. When de-energize, this design uses permanent magnets instead of a spring to hold the ferromagnetic sliding member in place.
In its energize state, the magnetic field generated by the solenoid is stronger than the magnetic force generated by the permanent magnet holding the plunger in its initial position. The electromagnetic field lifts the sliding member from its initial position, pulling it into a secondary position. When de-energize, however, an additional permanent magnet holds the sliding member in this secondary position.
By running the current in the opposite direction, the coil can energize to return to its initial location, thereby reversing the polarity of the generated magnetic field, which repositions the sliding member from its second position. After being de-energize, the first permanent magnet once again holds the plunger in the initial position. Solenoid valves are technically switching between two stable, de-energize positions, hence the term switching or bi-stable valve.
Also, read Control Valve Sizing and Selection
Pilot Operated Solenoids
Direct-acting solenoids can prove limited in terms of line size, operating pressure, and flow capacity, but pilot-operated solenoid valves, also known as indirect-acting solenoids, can overcome these limitations by constraining the solenoid’s action to the control of a pilot orifice and using the service medium to assist in opening and closing the main valve.
In the same way as other internally or externally piloted valves, the main valve functions according to the disposition of the pilot valve. Typical pilot-operated solenoid designs use a diaphragm to seal the main flow path. There is a small orifice in the diaphragm that allows the service medium to accumulate both above and below this component. As a result of this orifice, pressure is equalize on both sides of the diaphragm.
In this pressure-balanced state, a light spring pushes against the diaphragm holding the main valve’s flow path closed. The solenoid valve, which serves as the pilot, 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 diaphragm’s orifice, which allows the service medium to equalize on both sides. Thus, when the pilot-solenoid opens, the service medium evacuates from the cavity above the diaphragm faster than it can reaccumulate through the pressure-equalizing orifice.
Due to this, the pressure exerted by the service medium below the diaphragm is greater than the pressure exerted by the service medium above the diaphragm. In the main valve, the service medium pushes up against the diaphragm and spring, thereby opening the primary flow path. Such valves can configure to operate normally closed or normally open.
The solenoid valve has two states, the energized position, and the de-energized position, but this does not necessarily mean that it can only act as a one-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 would have two inlets and one outlet in a mixing application. As soon as it is stroke into the secondary position, the valve will open the formally closed inlet, allowing media to flow to the common outlet and simultaneously shutting off the flow path from the formally open inlet.
The solenoid would have one common inlet and two outlets in a diverting application. In its initial position, this 3-way arrangement would allow media to flow from the common inlet to one of the two outlets while the secondary outlet would remain closed. When stroked to the secondary position, the flow path to the formerly open outlet closes, and the flow path to the formerly closed outlet opens, diverting flow here.
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