Page 73 - Instrumentation and Measurement

5.3 Directional Control Valves

Directional control valves are widely used in control systems as elements for switching on or off
hydraulic or pneumatic pressures which can then, via some actuator, control the movement of some
item. A directional control valve on the receipt of some external signal, which might be a mechanical,
electrical or pressure signal, changes the direction of, or stops, or starts the flow of fluid in some part
of a pneumatic/hydraulic circuit.
The basic symbol for a control valve is a square. With a directional control valve, two or more squares
are used with each square representing the positions to which the valve can be switched. Thus, Figure
5.8A represents a valve with two switching positions, Figure 5.8B a valve with three switching
positions. Lines in the boxes are used to show the flow paths with arrows indicating the direction of
flow (Figure 5.9A) and shut-off positions indicated by terminated lines (Figure 5.9B). The pipe
connections, i.e. the inlet and outlet ports of the valve, are indicated by lines drawn on the outside of
the box and are drawn for just the ‘rest/initial/neutral position’, i.e. when the valve is not actuated
(Figure 5.9C). You can imagine each of the position boxes to be moved by the action of some actuator
so that it connects up with the pipe positions to give the different connections between the ports.

FIGURE 5.8 (A) Two position and (B) three position valves.

FIGURE 5.9 (A) Flow path, (B) shut-off and (C) input connections.
Directional control valves are described by the number of ports and the number of positions. Thus, a
2/2 valve has 2 ports and 2 positions, a 3/2 valve 3 ports and 2 positions, a 4/2 valve 4 ports and 2
positions, a 5/3 valve 5 ports and 3 positions. Figure 5.10 shows some commonly used examples and
their switching options and Figure 5.11 the means by which valves can be switched between
positions.

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