If it has an internal actuator (usually a spring) that actuator is in control of the valve when it is "at rest", and has shifted the internal mechanism into the valve's "at rest" position.

If the "at rest" position means the valve is closed (compressed air is not flowing from the in to the out port) then this valve is deemed as NC - normally closed.

On the other hand, when this valve's external actuator is not being operated, and the valve is at rest, if compressed air flows from the in port through the valve to the out port, then this valve is deemed NO - normally open.

For most applications the NC valve is selected, as most applications will want the compressed air shut off when the valve is at rest.

Actuated means that the valves external actuator is being operated by someone or something. This could be a push button, lever, toggle, whisker switch, or solenoid actuator.

Please not that in my dealings with compressed air clients, I differentiate between the actuator (the device that is used to actuate the valve) and the operator (who is a person that actuates the valve). Others may not use this same differentiation, so when you are talking to colleagues about compressed air, do make sure you understand their frame of reference.

Applications for the 2/2 air valve are varied. The 2/2 NC valve that folks most often come in contact with is the air gun or blow gun. This device contains (or is in itself) a 2/2 NC compressed air valve. When the external actuator (the trigger or button) is actuated, air will flow from the supply side out the nozzle. When the actuator is released, the internal actuator (the spring inside the gun) moves the internal seals back into place to stop the air flow.

You can see from the photo that the blow gun's working port is the gun barrel, and it has no stop on the end. The barrel is open to atmosphere. When the actuator is released, supply air stops flowing, and the compressed air that is in the barrel of the blowgun continues to vent to atmosphere until it reaches ambient atmospheric pressure. Aside from the blow gun referred to above, other applications for the 2/2 air valve include quality control, cleaning, cooling, drying...etc.; any application where the compressed air flows through the valve to atmosphere.

In a quality control application, a product may be moving down a conveyor system. When the product moves through a quality checkpoint (perhaps a weight scale or some sort of colour scanner) if that product doesn't meet the quality set points, a signal is sent to a PLC which in turn, sends a signal to an actuator on a 2/2 air valve. That valve shifts, and air flows through the valve to blow the defective part from the conveyor.

In another application for a 2/2 valve, parts may be exiting a manufacturing operation, and when they pass in front of an air knife of some sort, a signal is generated that actuates the 2/2 valve allowing it to blow air onto the part to dislodge unwanted material.

The actuator in the quality control or cleaning operations noted above is a solenoid.

The graphic just above shows a small 2/2 air valve with a solenoid operator and a DIN electrical connection.

There are devices on the market, when compressed air is flowed through them, that generate a cooling effect. They are used to provide simplistic cooling to control cabinets where the owner doesn't want or cannot have other forms of cabinet cooling. In this case, when the temperature inside the cabinet reaches a set point, a signal is generated which, in turn, opens the 2/2 air valve. Air flows to and through the cabinet cooler, and lowers the temperature in the cabinet to an acceptable level. Another signal is generated, and the air valve shifts closed until the high set point is again reached.

Other applications include air motors. As an example, there may be an air motor powering a fixture to turn tops onto bottles. When a bottle reaches the set spot, a signal is generated, the PLC fires the 2/2 valve, and the air motor starts to turn the cap. A regulator can be used to allow the air motor to stall out before it over torques the cap.

Compressed air driven pumps are another frequent user of 2/2 valves. Perhaps there's an immersible air pump in a tank. A float switch signals when the tank level reaches a set point, a signal is sent, and the pumps starts and runs until the level in the tank reaches a low set point.

Do you see the problem with the 2/2 air valve though?

If the working port is connected to something via an air line, and that air line doesn't ultimately lead to atmosphere, even when the valve is shifted to closed, compressed air will be trapped in the air line between the valve out port and the application.

This is why no 2/2 valve will be used to supply air to any device that is a closed vessel. Once the air valve is shut off, compressed air in the air line from the valve to the application, and compressed air in the application itself - both at full line pressure - cannot get back out.

2/2 air valves are available in a variety of port sizes - in some cases as small as 4 MM or 10-32. As the need for more air flow for the application is determined, larger and larger ported 2/2 valves are selected. The larger the working ports, normally the higher the flow through the valve.

For purposes of economy of manufacturing, larger air valves used for a 2/2 application may actually be 3/2 or even 4/2 styles, with unused ports being plugged to make these valves into a 2/2 configuration.

You see, it is often more economical to make larger bodied valves multi-ported and then to have the unused ports plugged for different applications than it is to purchase tooling to manufacture many different body styles. Different valve manufacturers have different ways of dealing with this issue.

Where applications exist, 2/2 valves can be stacked, they can be sub-based, or they can be plumbed individually for each need.

And I haven't forgotten. Here's where you learn to draw the schematic for a 2/2 air valve.





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