Valves can have two or more ports and control the flow of the media between those ports. The circuit function of the valve describes the different switching states it has. For a systematic representation, symbols are used. This article explains the logic behind (solenoid) valve symbols.
Valves are appointed with two numbers, for example a 2/2-way valve. The first number indicates the number of connection ports. The second number is the number of switching states. A 2/2 way valve has two pipe connections (inlet and outlet) and two switching states (open and closed). The designation Normally Closed (NC) or Normally Open (NO) determines if the valve is closed or open in de-energized state. A 3/2 way valve has three ports and two switching states. In each switching state, a different port is closed off. More ports and switching states are possible.
For each state of the valve, a single square is drawn. A 2/2 valve has two states (open/close) and is therefore represented by two adjacent squares. In each square is shown how the medium can flow between the ports. This is done with arrows, that indicate which ports are connected and what is the flow direction. Closed ports are indicated by a 'T'. To indicate which square is active when the solenoid is electrically energized, a little actuator symbol is used on both sides. On the left a solenoid symbol is used to show that the left square is the energized state. On the right a spring symbol is used for the rest state.
The table below shows alternative controls, however they are less relevant for solenoid valves.
The major part of the solenoid valves are normally closed 2/2-way valves. This example shows a 'normally open' 2/2 way valve. The open and closed state are again displayed with two rectangular squares. It often happens that the actuator symbols (spring and coil) are left away, so it becomes unclear which state is the electrical energized state. Also pay attention that the left and right square are swapped by some manufacturers. This can lead to confusion, especially when the actuator symbols are left out.
3/2-way valves have two positions and three connection ports. These valves can be used for multiple applications, such as switching between two circuits, or actuating a hydraulic cylinder. The symbols below show different circuit functions of 3/2 way valves.