Directional Control and Flow Restrictor Valves

What valves do?


In general a distinction can be made between three types of valves:

Pressure valves are used to detect a certain pressure level and, for example, to limit the pressure
Check and flow restrictor valves to influence the air-flow attributes
Directional valves for the control of compressed air to the various devices

In the following sections we will first examine directional control valves. These effectively constitute pneumatic switches, which depending on the operating mode guide the pressure and volume flows via various distribution channels. Directional valves can be designed as individual valves or as valve aggregates.


Circuit Symbols for Directional Control Valves

In pneumatic systems directional control valves serve to operate the cylinders using compressed air. The symbols depicting depicting directional control valves in circuit diagrams are specified according to DIN ISO 1219-1. Here it is important to note that the symbols thus specified only characterise the functions of the valves but not the respective design types. The essential features of a directional control valve are

The number of terminals of the valve being used for control purposes(distribution channels)
The possible switch settings of the valve (e.g. allowing or preventing flow)

Together these two parameters determine the valve's designation. As such a 3/2 directional control valve has 3 channels and 2 switch settings.

In the circuit symbol each switch setting of the valve is depicted by a box. The various flow channels are depicted by lines, whereby an arrow specifies the respective flow direction. Closed or blocked channels are designated by bars inside the boxes. Finally inlet and outlet terminals are drawn onto the box in a way that representd the rest state of the valve.

Terminal designation is specified using numerals, as laid out in DIN ISO 5599-3. From time to time it is still possible to run across an outdated system using letters. The subsequent table provides an overview of the designations.

DIN ISO 5599-3

Type of channel

Labelling system


Compressed air channel



Working inlets/outlets



Ventilating inlets/outlets



A signal present here blocks the channel between terminals 1 and 2



A signal present here connects channels 1 and 2



A signal present here connects channels 1 and 4



Auxiliary control air


The following graphic shows an example of a directional control valve's circuit symbol.


The following information can be deduced about the valve depicted:

The symbol consists of two boxes, i.e. the valve has two switch settings.
Each of the two boxes has three inlets/outlets, i.e. the valve has three channels.
The inlets/outlets of the valve are depicted in the right-hand box; this represents the rest state of the valve.
In the rest state the compressed air inlet (1) is blocked, the working outlet (2) is connected to the ventilating outlet (3); the connected cylinder is thus vented.
In the working position (left box) the compressed air (1) is supplied to the working outlet (2), the valve is then open, while the ventilating outlet (3) is closed in this switch setting.

Due to the fact that this valve channel (1) is closed in the rest state, the precise valve designation in this case is "3/2-way valve, channel 1 normally closed".

For the sake of clarification here is a second example:


A comparison to the circuit symbol considered first shows that here only the two boxes have been "reversed", i.e. in this valve the rest position corresponds to the valve being open from (1) to (2), while in the operating setting (left-hand box) the valve is now closed. Accordingly, the precise valve designation here is "3/2-way valve, normally open from 1 to 2 in the rest position".

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