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What's a 2/2 compressed air valve?

Here's information on the simplest type of air valve; the 2/2 style.

The first 2 in the 2/2 air valve refers to the number of "working" air ports that are found in the valve body. That is, the number of ports that supply air to the valve, and channel the compressed air to whatever it is that the valve is supposed to do.

Most 2/2 valves will have numbers or letters etched, cast or painted near each of their two "working" air ports. If there are numbers near the ports, the number 1 would be the supply port to bring the compressed air to that valve. Port number 2 would be the working port from which air would flow to accomplish whatever task that you wanted that valve to do.

If the port designations are letters, then port 'A' would be the supply port and port 'B' the working port.

If the 2/2 valve is to be "air operated", that is an external air signal is to be used to shift the 2/2 valve, there will be another port. That port may not have a designation or it might say '12'. No, that's not a twelve, but rather indicates that air will flow from port 1 to port 2 when an external air signal operates that valve.

The second 2 in a 2/2 air valve indicates the number of positions that the internal valve mechanism has. In this case, two. When this valve is operated or actuated, it will either open or close. At rest, that is when the external valve operator has not been activated, the internal valve mechanism will either stay open or closed.

Most 2/2 compressed air valves are classified as NC. NC stands for normally closed. This means that when the valve is not actuated, it's normal state is closed, and compressed air cannot pass through it.

There are some applications for 2/2 valves where the flow of air through the valve when it is not being operated is desirous. A NO or normally open valve would then be selected. When this type of 2/2 valve is at rest, compressed air will flow through it, and it is only when the valve is actuated that the flow of air will stop.

All 2/2 valves will have actuators that will operate or 'shift' the air valve.

A compressed air blow gun is a good example. On it there will be a push button or a trigger of some kind. When the button is depressed or the trigger is pulled, the compressed air will flow through the gun and out the nozzle to atmosphere. When the actuator is released, an internal spring (a secondary actuator) will shift the internal valve mechanism back, and the air will stop flowing. A compressed air blow gun contains a 2/2 NC air valve.

Some other 2/2 valve actuators are whisker switches, toggles, push buttons, palm buttons, roller cams, electric solenoids or compressed air.

2/2 valves can have detented or non-detented actuators. If the actuator is detented, this means that when the operator actuates the valve, the actuator will stay in the position selected until it is again moved by the operator. Toggle switches for air valves are often detented. You flick the toggle in one direction to actuate the valve, and it will stay actuated until you move the toggle back. The detented 2/2 valve may not have an internal spring, though with standardization of manufacture, even a detented valve may have a spring, since various kinds of actuators may be affixed to that same valve body.

Non-detented valves do contain an actuator spring, and the internal valve mechanism will "spring" back to the other position when the operator releases the primary actuator, similar to what happens when the button on a compressed air gun is released.

At, an e-book entitled All About Air Valves - volume one - will soon be available (end August 06). If you are interested in more information about air valves, do visit the site and download a copy.

All 2/2 valves must, when operated, allow air to ultimately flow to atmosphere. If a 2/2 valve is selected to provide air to a closed tank or air vessel, then when the 2/2 valve is shifted to a closed position, the air will be trapped in the downstream line. You wouldn't select a 2/2 valve to provide compressed air to an air actuator, for example.

The exception to this rule are inflatable bladders that contain their own integral 2/2 valves; a bicycle or vehicle tire being good examples. When you connect your 2/2 air supply valve to the tire valve, the tire valve - itself a small 2/2 valve - is actuated by the supply valve fitting, allowing compressed air to flow into the tire. When the supply valve is no longer actuated, as long as the fill fittings is attached to the tire valve, the line is pressurized. That's why, when you pull the fill fitting away from the tire, you hear that characteristic "pssssst" as the air that's been trapped between the two valves is vented. The tire valve will have shifted back to being closed by the air pressure inside the tire, thus preventing the compressed air in the tire from escaping back to atmosphere.

Other 2/2 compressed air valves commonly encountered are those push button valves on the handle of air tools, and the air-horns that are commonly heard at sporting events.

The air-horns were designed as distress sirens for small boat operators. The push button on the top of the gas canister is the 2/2 valve, and it's the pressure of the compressed gas trapped in the canister that forces the valve shut (NC - normally closed) when the button is released.

I guess it's better to hear an air horn at a football game than indicating some poor boater's distress out on the lake; unless it's blasting into your ear, of course!

And as always, if you have any questions, please send me a message from the contact screen at my web site.

About the Author: Bill Wade's experience in compressed air and other industries spans decades; from field sales positions through to the corporate presidential office. His sales agency represents a select group of industrial firms. Mr. Wade writes about all facets of compressed air at

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