Page 79 - Instrumentation and Measurement

FIGURE 5.19 4/3 shuttle valve.
EXAMPLE 5.2
State what happens for the pneumatic circuit shown in Figure 6.20 when the push button is pressed
and then released. The right-hand box shows the initial position with the pressure source, i.e. the
circle with the dot in the middle, connected to a closed port and the output from the right-hand end
of the cylinder connected to the exhaust port, i.e. the open triangle. When the push button is pressed
the connections between the ports become those indicated in the left-hand box. The pressure source
is then connected to the output port and hence to the right-hand end of the cylinder and forces the
piston back against its spring and so from left to right. When the push button is released, the
connections between the ports become those in the right-hand box and the right-hand end of the
cylinder is exhausted. The piston then moves back from left to right.

FIGURE 6.20 Example.

5.4 Flow Control Valves

In many control systems, the rate of flow of a fluid along a pipe is controlled by a valve which uses
pneumatic action to move a valve stem and hence a plug or plugs into the flow path, so altering the
size of the gap through which the fluid can flow (Figure 5.21). The term single seated is used where
just one plug is involved and double seated where there are two. A single-seated valve has the
advantage compared with the double-seated valve of being able to close more tightly but the
disadvantages that the force on the plug is greater from the fluid and so a larger area diaphragm may
be needed.

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