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What are pneumatic solenoid valves?

Pneumatic solenoid or control valves are the “Lego blocks” of pneumatic control system. They are bought as functional blocks and integrated into the system. Every type of control valve has a symbol. Something like 5/2, 5/3, 3/2 and similar and they always have a schematic. Usually on them like you can see on the picture bellow. If these schematics look confusing do not worry there are rules on how to read them and they are pretty simple.

So the first thing is to cover the components of the schematics:

Pneumatic solenoid components

Here is an example of the schematics of a simple solenoid valve. This is a so called 3/2 control valve.

Schematic of a 3-2 control valve

On this schematics you can recognize different symbols:

1. Square– denotes a state of pneumatic control valve i.e. its position. Usually there are two and three states of a pneumatic control valve.
2. Actuator– denotes what force is changing the states (squares) or what actuates the valve: a solenoid (top one), a button, a spring (bottom one)….
3. Arrows– show the direction of the air in the square. For example the air flows from port 2 to port 3 in the schematics above
4. T – denotes that the port is closed. The air can not get into the control valve or escape it.
5. Numbers (1,2,3,4,5) – denote the physical connections on the control valve. The connections are ports to which you can attach pneumatic hoses.

Reading a pneumatic solenoid valve scheme

3/2 control valve

Schematic of a 3-2 control valve

Here we have 3 ports. These ports represent physical ports on the control valve body so they do not change. First we look at the right state. We can see that in this state air flows from port 2 to port 3 and the port 1 is closed with a T-symbol. We can see that by noticing what numbers are connected via arrows.

OK now let’s look at the left state of the same control valve. Imagine that the left state shifts over to the right. If we now look at the numbers lining up to the new state we can see that air flows from port 1 to port 2 and the port 3 is closed with a T.

So now you know what states a control valve has and how do these states direct air flow. But how are these states actuated? Here is where we look at the actuator symbols. On the left side we see a symbol for a solenoid and on the right side we see a symbol for a spring actuator. This means that the left state is actuated with a left actuator – solenoid and the right state is actuated with a spring. In different words in order to “engage” the left state of the valve you need to actuate the solenoid. This also means that the right state is the default one. Since if there is no power the spring will actuate the right state. This is a convention. The default state is always shown on the left side by this convention.

5/3 control valve

What about three state valves like:

5/3 control valve

Here the middle state is the default one and left and the right state are actuated with solenoids. Springs are actuating this default state. This is also a convention, that in a 3 state valve the middle one is the default one.

So if we analyze the 5/3 control valve schematic that explain it as:

The control valve has three states and 5 ports. Air does not flow inside a control valve in the default state the (All ports are blocked with a symbol T) If we actuate a solenoid valve on the left side we actuate the second state and now air flows from 1 to 4 and from 2 to 3, the port 5 is blocked. If we instead actuate the right solenoid then ports 4 to 5 are connected and ports 1 to 2 are connected. In this state port 3 is closed.

What about the numbers like 5/3? This just represent the number of ports, 5 in this case, and a number of states- 3 states in this case. That is why in the first example the symbol was 3/2 – 3 ports and 2 states.