Pneumatic solenoid valve: what it is and how it works
A solenoid valve uses electromagnetic force to operate. When an electric current goes through the solenoid coil, a magnetic field is generated determining the movement of a ferrous metal rod. This is the basic operating process of a solenoid valve.
Solenoid valves can be normally open or normally closed:
- Normally open (N / O), the valve remains open when the solenoid is not “energized”.
- Normally closed (N / C), the valve remains closed when the solenoid is not “energized”.
Why use a solenoid valve?
Solenoid valves eliminate the need for manual or pneumatic control of a pneumatic circuit and they only require an electrical input (and air pressure for for piloted valves) to operate, making them easy to program and install in a wide variety of applications.
What are the different types of solenoid valves?
As we shall see, the solenoid valves can be divided into the following categories: direct-drive or solenoid operated. Solenoid actuated valves can be further subdivided into internally or externally piloted valves and are sometimes referred to as servo-assisted solenoid valves.
In the case of direct-acting solenoid valves, the force generated by the solenoid shall be greater than the force exerted by the air pressure. They do not require any line pressure to operate and can operate under vacuum conditions.
With direct acting N/C valves, the solenoid rod is fixed to a coil and held in place by the spring. When the solenoid is charged, the magnetic field raises the solenoid rod, letting the air pass to the other side. The opposite happens in a N/O valve: the spring keeps the coil in the open position.
Direct-acting solenoid valves are of limited use and are only used in about 10% of applications. This is because the flow can be limited and consume a large amount of electricity
Unlike direct-acting solenoids, internally piloted valves operate with system pressure to facilitate control. This makes them able to control the air flow using less energy than is exerted by the pressure in the line.
In internally piloted valves, the solenoid controls a smaller passage between the line and a cavity behind the coil. When this is open, the pressure in the line pushes the coil by opening the valve. Since the solenoid controls much smaller openings, it requires much less power than a direct acting solenoid valve.
Externally piloted solenoid valves work in a similar way to internally piloted valves, but they use air from an external source to facilitate the movement of the valve, rather than the pressure inside the valve. This must come from upstream of the valve, but it can also be supplied by a separate circuit. This external air source is inserted into an additional port on the valve. Externally piloted valves are generally used in scenarios of low pressure, vacuum or alternative porting, where there is a low, negative or absent pressure in the valve itself to facilitate movement.
How is a solenoid valve controlled?
At the simplest level, solenoids can be controlled using a manual electric on/off power switch, which is sufficient in some applications. In most cases, however, more complex control is required using a control card. Control cards digitally set the valves to operate at timed intervals or they can be programmed to operate the valve when certain conditions are met, for example 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 line pressure? This will determine how much energy is required. It will also tell you whether adirect acting, internal or external pilot valve is required.
- How fast should the valve open or close? Pilot operated valves take longer to exchange than direct acting valves, but require less energy.
- Do you need an N/O or N/ C valve? The valve must be appropriate for its application. The most important consideration is potential effect of a power outage or valve failure: is it safer for the flow to stop or continue if this happens? If there are no safety considerations, you have to consider whether the line will be open or closed for most of the time. If a line is mostly in flow, a normally open valve will be needed. If the reverse is true, a normally closed valve will be required. Getting this wrong will lead to an increase in energy costs and the potential depletion of the solenoid.
- What is required flow rate, size of outputs and number of outputs? As with any valve, these factors entirely depend on valve function and on the system in which it is integrated.