The solenoid valve is a practical way to control liquids and gases electronically. Throughout industries from automotive to food service, hydraulic and pneumatic devices play a key role in modern mechanics. For a better understanding of their function and terminology, we have outlined some basic concepts.
- Open or Closed
- Poppet or Spool
- Direct or Pilot
- 2, 3, or 4 Way Valve Functions
By understanding these four characteristics, you will gain a foundational understanding of solenoid flow control, useful in any field or application requiring electronic flow control.
1. Normally Open or Normally Closed
Solenoids are normally described in the unpowered state when describing their function. By activating the solenoid, the inverse state will be created. By activating the solenoid, the inverse state will be created.
2. Poppet or Spool
Poppet: A poppet valve uses a cover that is held in place by a spring or by the pressure of the medium flowing through it. By means of a pilot (described below), the solenoid adjusts the position of the poppet by opening or closing the passage. For poppet valves to accommodate the pressures acting on the poppet, the solenoid actuation force must be higher than that used for spool valves.
Spool: Spool-type valves have seals along their bores. Depending on the position of the spool, the alignment of the seals changes to allow or stop the flow. As spool-type valves have a smaller surface area, they can be actuated using less powerful solenoids since they are less sensitive to flow pressure. They also have a limited flow rate as a drawback.
3. Direct or Pilot
Direct: A direct-acting solenoid works by directly altering the position of the poppet or spool to open or close a valve. While direct-acting valves are fast and have simpler design principles, they are less capable in high-flow applications.
Pilot: A pilot-operated solenoid valve has a poppet or diaphragm that is held closed by a spring and the differential pressure between the inlet and outlet ports. On either side of the poppet, a smaller secondary channel links the inlet and outlet ports. As the solenoid controls flow through this secondary channel, a small amount of medium flows to the outlet port, which begins to equalize the pressure acting on the poppet until it opens to allow for full flow. It is slower to operate a pilot valve than a direct-acting valve, but it allows for a smaller solenoid to control a higher flow rate.
Also, read Basic Valves and their Types
4. Functions of 2, 3, or 4-way valves
2 Way: The flow rate is controlled by a valve between the inlet and outlet ports.
3 Way: In most three-way valves, one port serves as an inlet and two as outlets. A solenoid is used to direct flow from an inlet port to an outlet port. For mixing functions, three-way valves can also use two ports as inlets and one port as an outlet.
4 Way: Four-way valves have four or five ports. The valve has 1 inlet port and 2 outlet ports like a 3-way valve, but whatever outlet port is off becomes a secondary inlet port that routes the flow to the 1 or 2 exhaust ports. There are some 4-way valves that also have 3-way positioning. The solenoid is also capable of a third position, which blocks all fluid from traveling through the valve to maintain the loop’s pressure, rather than just choosing between two paths of travel (2-way positioning).
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