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What's a 3/2 compressed air valve?

Recently, I published an article on this site titled: What's a 2/2 compressed air valve? Here, in the next installment in this series, is information on a slightly more complex air valve; the 3/2 style.

The first number in the 3/2 air valve, the three, 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 3/2 valves will have numbers or letters etched, cast or painted near each of their three "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.

The third port in a 3/2 air valve is an "exhaust" port and if numbered, it could be a 3 or a 5. If the port designations in a 3/2 valve are letters, then port 'A' would be the supply port and port 'B' the working port, with the third port normally being an 'E'.

As in the 2/2 valve there may be one or two additional ports in the ends of the 3/2 valve to allow an air signal line or lines to be connected. If this is the case, this 3/2 valve will either be single, or double air piloted.

The 2 in a 3/2 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 and air will either flow to the application upon actuation, or it will be prevented from flowing.

Most 3/2 compressed air valves will be NC, or normally closed. When the valve is not actuated, it's normal state is closed, and compressed air cannot pass through it.

If your application calls for air to flow through the valve when it's not actuated, that the circuit needs air to be flowing through this valve when it is at rest, then a NO or normally open configured valve would be selected.

All 3/2 valves have actuators that will operate or 'shift' the air valve. An external button, or toggle, or perhaps a solenoid actuator would be the visible actuator. Inside, there will likely be an internal actuator – a spring - which will shift the valve to the off position when the external actuator is not being used.

If the external actuator is 'detented', then when the valve is operated, it will stay in it's last selected position until an operator changes it. Detented means it will stay where it's put! This is useful when an operator needs to actuate the valve, and then manually perform another operation while the air valve feeds air to the application.

Unlike it's less complex 2/2 valve cousin, the 3/2 valve is used when a compressed air supply is needed to an application or device that uses compressed air to power it, yet in itself has no integral air pathway to atmosphere. Therefore, when the device has performed it's function, and it's time to 'deflate' it or to let the compressed air back out, the third port in the 3/2 valve comes into play.

When the compressed air supply through the valve is shut off internally, a pathway back through the valve to atmosphere will be opened, to allow the compressed air to escape. The air supply is shut, so the compressed air flowing to the valve cannot flow through it, and the compressed air that was formerly in the device or application can now bleed back down the air line through the valve to exhaust.

So, what type of devices are these?

Usually they are single acting type actuators. One comes to mind immediately; "Air springs".

Both Firestone and Goodyear (among others) manufacture "air springs". These are devices that look like tires, but rather than have an opening in the middle of the doughnut where the rim goes, they are closed on both sides with steel plates. In one side there will be an air port to which an air line from a 3/2 valve can be connected. These "air springs" are mounted on their sides, picture a tire lying flat after you've taken it off your car, and can generate huge actuation forces. Force equals pressure times area, and the “piston” size inside an air spring can be huge. The application of air springs mirrors that of typical air cylinders, yet offer large capacity at a fraction of the cost of an air cylinder of a necessary size to generate the same force as the air spring.

Another application for 3/2 valves is single acting air cylinders. Whether they are spring extend or spring retract, an air supply is required to operate the SA cylinder. A 3/2 valve is designed to do just that.

A couple of more points; the 3/2 valve can have the exhaust port plugged, and voila, you have a 2/2 valve.

If the cost of the valve is the same, you can use a 3/2 air valve anywhere you might use a 2/2 valve. Since 2/2 valves always have to have the “working port” ultimately plumbed to atmosphere, that there is an exhaust port in a 3/2 valve offers no obstacle to it’s use.

If you have a double acting air cylinder, and you don't have a 4/2 or 5/2 (more on these valves next article) available, you can use two 3/2 valves to operate any cylinder that requires two supply lines in order for it to extend and retract.

At my e-book entitled All About Air Valves - Volume One is now available. If you are interested in more information about air valves, do visit the site and download a copy. This first e-book is an introduction to air valves, and focuses on the 2/2 iteration. Future volumes will focus on 3/2 valves, and then the 4/2 & 5/2 configurations.

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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