3-2 Air Valves


3-2 air valves are used when the device to which the compressed air is being supplied is also one from which the compressed air must exit after the work is performed.

For example, when inflating a lift bag to move a large mass, compressed air is fed into it.

When the lift is complete, in order to deflate and remove the air bag, the air must be exhausted from it. The valve selected both to inflate and allow the bag to deflate is normally a 3-2 air valve.

What The Numbers Mean

The first number in the 3-2 type air valve indicates the number of ports the valve will have. The second number indicates the number of positions that the air control mechanism inside this valve can have.

2 Positions

Most compressed air valves have two positions, but there are some that have three. I will cover three position valves on another page.

Valve Ports

Port 1 or port A on the 3/2 air valve will be the supply port to which the compressed air supply from the air compressor will be plumbed. This port is normally a female NPT or female metric threaded hole into which an appropriately sized air fitting will be threaded.

Of course, the fitting thread size must match the thread of the port in the valve. The other side of the fitting must correspond to the air line size that will be connected to that fitting.

Port 2 or port B on a typical 3-2 air valve is the power or cylinder port. It's typically a female port into which the appropriate fitting size and style is threaded. You will plumb from that port to the single acting actuator.

Port 3, possibly marked 'E' or even number '5', is the exhaust port. On some 3-2 air valves this will be a female threaded port, and in others, simply an opening through which compressed air can escape from the valve when it's shifted to the off position.

Be a bit careful as to the number of ports, as some 3/2 air valves only appear to have two ports with the third port, the exhaust port, actually exiting through the top of the solenoid. Normally this type of valve cannot have the exhaust restricted in any manner as doing so will negatively affect the operation of that type of air valve.
If port 3 is in the body of the valve, an exhaust flow control can be installed there and used to meter the outflow of air, and in so doing, adjust the speed of the actuator in one direction.

Air Piloted Valves

If the valve is air-piloted, meaning that it's compressed air that operates this valve, then it will have an air port on one or both ends of the valve, as well as the three working ports. Usually these ports will be smaller than the working ports unless the working ports are 10-32 or 3 or 4 MM in size. Then the control air ports might be the same size. Look for these to be marked with two digit numbers, perhaps as 12 or 14.

NC Valves

Typically 3/2 compressed air valves will be NC, or normally closed.

This means that when the valve is not actuated it is in its resting state, and it is closed. Compressed air cannot pass through it. In this resting state there will be an open air path through the valve from the power or working port to the exhaust port.

NO Valves

There will be some cases where having air pass through the valve to the application when it is not actuated is necessary for that air circuit. That being the case, a 3/2 NO (normally open) valve would be selected. Normally open means that when the valve is at rest compressed air passes through it from the supply to the cylinder port, and the exhaust port is blocked.

Valve Actuators

All 3/2 valves have actuators that will operate or 'shift' the air valve when required. An external button, a toggle, a lever or perhaps a solenoid actuator would be visible actuators.

Inside the 3/2 valve there will likely be an internal actuator - a spring - which will shift the valve to the off position when the external valve actuator is not being used. If the 3-2 air valve will shift itself back off when the actuator is not being used, then it's known as a 'spring return' type valve.

For more information about 3/2 valves please click 3/2 valves, page two.