Introduction to Industrial Automation

5.7.1   Directional  Valves

Directional valves are devices that have the ability to control the direction of pressurized air in 

pneumatic installations. The control of the flow direction is mainly utilized for controlling the 

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  Introduction to Industrial Automation

behavior of the pneumatic cylinders. Each directional valve is characterized by the number of 

switching positions and by the number of pressurized airways that could be externally connected. 

Due to the manufacturing complexity of these valves, based on international standards and regu-

lations, the following simplified symbolic representations have been adopted:

 

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The switching positions of a valve are denoted by rectangles.

 

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The number of rectangles in a row denotes the number of switching positions that the valve 

has.

 

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The route of the flow for the pressurized air is denoted by directional lines within the 

rectangles.

 

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The isolation points (stops of air flow) are denoted by a tau (T).

 

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The external connections of the pressurized air lines are denoted by external lines to the 

rectangle that represent the rest state of the valve.

 

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The other operational positions of the valve are created from the translation of the block 

(left or right) until the point that the flow directions are aligned to the external connections.

 

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The exhausting of the pressurized air can be achieved either freely toward the environment 

or in a network of pipes for the collection of low pressure air.

Table 5.1 summarizes all the previous notations and illustrates some symbolic details. For bet-

ter comprehension of the symbols utilized for the valve representation, refer to Figure 5.9, where 

the real operation of the two most common valve symbols is indicated for the flow and non-flow of 

pressurized air as it is actually happening inside the valve, with respect to the valve’s symbol as well.

In Figure 5.10 the combined operation of a directional valve with three connection lines, two 

switching states, and a single-acting cylinder is presented. In one switching state (left) of the valve, 

the pressurized air is inserted from the line A and it moves the piston toward the right direction, 

while contracting the cylinder’s spring. In the second switching position (right) of the valve, line 

A is blocked, while the pressurized air in the cylinder is being released through the exhaust line 

B. Thus, in this position the piston is returning to its initial position, through the action of the 

returning spring.

In Figure 5.11, the operation of a valve with four connection lines and two switching states, 

combined with a double-acting cylinder is displayed. As has already become apparent, the valves 

can be coded directly from the number of connecting lines and, afterwards, the number of switch-

ing positions. For example, the valve shown in Figure 5.11 is coded as a four way, two-position 

valve. The normal switching position for a valve, or rest position, is the position where any actua-

tion mechanism of the valve is inactive.

In Table 5.2, the valve type, the initial position status, and the symbol for the most typical 

directional valves are presented. It is obvious that for a directional valve to operate in the various 

switching positions, a kind of actuation is needed, either from a human or from an automatic sys-

tem. This actuation can be manual, mechanical, pneumatic, or a combination of all these.