Here is how to draw the 3/2 compressed air valve.
Until you get familiar with the process, I can almost guarantee that you will do a lot of erasing and changing of the lines, so I recommend that you do your drawing in pencil, and have an eraser handy too.
Start by drawing a square on your paper, a couple of inches square. You will find it easier if you use graph paper to help keep the lines straight.
After you've drawn your box, add three lines inside the square as shown in Fig. 21.
The black outer square box depicts the valve body. The three little lines inside the square box represent the internal air passages and the air ports of the 3-2 air valve.
You cannot tell from the outside how many positions a valve will have. The positions of the valve are selected by shifting the internal flow paths of a particular valve using some sort of a valve actuator.
Some circuit designers show the lines that are depicting the ports extending slightly through the wall of the valve when drawing their valve schematics. Some do not. I do not.
Fig. 22 shows the same box, but now we've connected up two of the ports, and modified the other one slightly, as shown in the graphic.
The black circle signifies the compressed air supply. It is flowing from the circle up to and into the 3/2 valve. This is a simple way to show that this valve is connected to an air supply, which is connected to the plant air mains, which are, in turn, connected to the compressor.
Compressed air cannot pass through the valve in Fig. 22 because the supply port inside the valve is closed. That is indicated by the small line teeing the top of the line coming from the air supply.
If this 3/2 valve was at rest, that is to say that there are no external valve actuators affecting the valve position, then it would be an NC (normally closed) air valve.
Note the line drawn from the top of the box to the bottom left, and that the line has an arrow head. This shows the open flow path of the air through this valve when it is in this position. The open flow path is from the top port, through the valve, and out the bottom port where the arrow head is located.
Please erase the small vertical lines that I asked you to draw in Fig. 21.
In Fig. 22 the port that is blocked or closed is the supply port. The port at the top of the box is the actuator or application port that is connected from that port by an air line to the end application ( a tool, actuator etc). The port that has the arrowhead pointing to it is now the exhaust port.
With the valve shifted into the position shown in Fig. 22, or if this is the at rest position for this valve, the supply is shut off and the actuator port is opened to exhaust which allows the compressed air to exhaust from whatever actuator is being used, to atmosphere.
Fig. 23 shows the same valve, but now in its shifted or alternate position. With the valve in this position, the supply air is moving through the valve to an application somewhere, and the other port, the exhaust port, is now blocked.The air path inside this air valve will stay as shown in Fig.23 until this valve is shifted back, and then the flow path will revert to the one shown in Fig. 22 again.
Fig. 24 takes the two drawings that we've made - Fig. 22 and Fig. 23 - and places them side by side in the same drawing, which is how the valve would be drawn on the air circuit schematic.
This depicts the flow paths of one 3/2 valve. The 3/2 valve has 2 positions, each position depicted by the drawing on one side or the other of the single valve schematic.
The way Fig. 24 depicts the valve, with the supply air coming into and through the valve, shows that when the valve is shifted to this position, then it is open, or passing.
If the positions are reversed, as shown in Fig. 25 just below, then the valve is closed, or non-passing.
Without the actuators being shown in the drawings, a viewer cannot tell where this valve should be when it is at rest. In that case the air supply symbol, the circle, is drawn on the right side of the valve schematic.
Sometimes when the actual air circuit is stopped, one or some of the air valves making up that machine circuit, may be deliberately left in an actuated position. The supply line may be shown on the left in these circumstances. The overall circuit schematic will show the details if that is the case.
You will need valve actuators to complete this 3/2 valve schematic. Here's how to draw them.