Valves, and more specifically - compressed air valves - are discussed on this and the linked pages.
And before we start discussing air valves, this page has information about Unloader Valves. This is a valve page that is sought quite often.
Compressed air leaking from an unloader valve may actually be a symptom of a leaking compressor tank check valve. This is another page that a lot of folks are looking for.
This is what that designation means.
The first -2- in an air valve is designated as a 2/2, refers to the number of ports that valve has. The second -2- refers to the number of positions that valve will have.
In the picture above, the second of the two air ports is hidden. It is directly opposite the port pictured.
I know that this is an NC valve, since I took the photo. You normally cannot tell by just looking at an air valve whether it is an NC or NO configured valve.
There are some circumstances where an air circuit designer might want air to be flowing through a specific valve when the circuit is turned off, and in those circumstances, might specify a 2-2 N.O. air valve.
Here is how to draw them 2/2 air valves if you are working on designing an air circuit.
As noted earlier, the first number in the valve designation is the number of working ports that the valve has, and the second number indicates the number of positions that the valve has.
A 3/2 air valve will have an in port, an out port (to the application) and an exhaust port, a total of three ports.
The second number in the a 3/2 valve designation is a two, so this valve will have two positions it can be shifted into.
At rest, the 3/2 air valve will either be open, with compressed air flowing through it and out the power or working port to the application, or it will be closed. When it is closed, compressed air will be stopped a the valve supply or in port, and the working port that let the air flow to the application when this valve was open, will now allow that same air to flow through the air valve to the exhaust port and out to atmosphere.
Here is lots more information on 3/2 air valves and their uses or, visit this page that shows you how to draw a 3/2 air valve.
The 4/2 valve will have four working ports, and it will have two positions into which it can be shifted when actuated.
The four ports will be:
The 5-2 air valve will have:
The 5/3 valve will have five ports...
The center positions are;
If you have experience with compressed air valves, you will be comfortable with the term manual actuator. This simply means that the valve is actuated by an operator (person) that pushes a button, moves a lever, toggles a switch, to make the air valve shift.
For complex air circuits that demand multiple and sequential operation of devices, the valve of choice will typically be solenoid operated.
This is my personal definition, and you may find that other people have different terms for the same thing. Make sure when you are talking to folks about valves that you understand their terms, will you?
For the novice, for the newcomer to the world of pneumatic fluid power, many of the terms used and products discussed will be confusing. I will clarify as much as possible, and continue to provide a huge amount of information on this site about those terms and products.