What is a Pneumatic Solenoid Valve?
A solenoid valve, also known as an electrically-operated valve, is a valve that uses electromagnetic force to operate. When an electrical current is passed through the solenoid coil, a magnetic field is generated which causes a ferrous metal rod to move. This is the basic process that opens the valve and it works either directly or indirectly on the air.
Solenoid valves can be normally open or normally closed:
Normally Open (N/O), the valve remains open when the solenoid is not charged.
Normally Closed (N/C), the valve remains closed when the solenoid is not charged.
Why use a solenoid valve?
Solenoid valves eliminate the need for manual or pneumatic control of a pneumatic circuit and only require an electrical input (and air pressure for piloted valves) to operate, this makes them easy to program and install in a wide variety of applications.
What are the different types of solenoid valve?
As we shall see, solenoid valves can be split into the following broad categories: direct acting or solenoid piloted. Solenoid piloted valves can be further divided into internally or externally piloted valves, and are sometimes referred to as servo-assisted solenoid valves.
Direct acting
In the case of direct acting solenoid valves, the force generated by the solenoid must be greater than the force exerted by the pressure of the air. They do not require any line pressure to work, and can operate in vacuum conditions.
With direct acting, N/C valves, the solenoid rod is attached to a spool and kept in place by spring. When the solenoid is charged, the magnetic field causes the solenoid rod to lift, moving the spool and allowing air to pass through to the other side. In a N/O valve, the opposite happens – the spring keeps the spool in the open position.
Direct acting solenoid valves are of limited use and are only seen in about 10% of applications. This is because flow can be limited, and they consume a large amount of electrical power.
Internally piloted
Unlike direct acting solenoids, internally piloted valves work with system pressure to aid control, rather than against it. This makes them capable of controlling air flow using less power than is exerted by pressure in the line.
In internally piloted valves, the solenoid gates a smaller passage between the line and a cavity behind the spool. When this is opened, pressure in the line pushes the spool across, opening the valve. As the solenoid is controlling much smaller openings, it requires much less power to move compared to a directing acting solenoid valve.
Externally piloted
Externally piloted solenoid valves work in a similar way to internally piloted valves, but use air from an external source to assist valve movement, rather than pressure within the valve. This must come from upstream of the valve, but can also be provided from a separate circuit. This external air source is fed into an extra port on the valve. Externally piloted valves are typically used in low pressure, vacuum or alternative porting scenarios, where there is low, negative or no pressure in the valve itself to facilitate movement.
How is a solenoid valve controlled?
At the simplest level, solenoids can be controlled using a manually operated electrical on/off switch, which is sufficient in some applications. Most of the time however, more complex control is required using a control board. Control boards digitally set valves to operate at timed intervals or can be programmed to operate the valve when certain conditions are met, for instance when it receives a signal from a pressure switch. Solenoid valves can be controlled by a computer, making them easier to integrate into Industry 4.0 systems.
How to select a solenoid valve
The type of solenoid required will depend on several factors.
What is the pressure of the line? This will dictate how much power is required. It will also tell you whether a direct acting, internally or externally piloted valve is needed.
How quickly does the valve need to open or close? Piloted valves take longer to switch than direct acting valves, but require less power.
Do you need a N/O or a N/C valve? The valve must be appropriate to the application. The single most important consideration is the potential effect of a power cut or valve failure – is it safer for flow to stop or continue if this happens? If there are no safety considerations, then consider whether the line will be open or closed most of the time. If a line will be mostly in flow, then a normally open valve will be needed. If the reverse is true, then a normally closed valve will be needed. Getting this wrong will lead to increased energy costs and potential burnout of the solenoid.
What is the required flow rate, port size and number of ports? As with any valve, these factors depend entirely the valve function and what system it is being integrated into.
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