Page 81 - Instrumentation and Measurement

The movement of the stem, and hence the position of the plug or plugs in the fluid flow, results from
the use of a diaphragm moving against a spring and controlled by air pressure. The air pressure from
the controller exerts a force on one side of the diaphragm, the other side of the diaphragm being at
atmospheric pressure, which is opposed by the force due to the spring on the other side. When the air
pressure changes then the diaphragm moves until there is equilibrium between the forces resulting
from the pressure and those from the spring. Thus, the pressure signals from the controller result in
the movement of the stem of the valve. There are two alternative forms, direct and reverse action
forms (Figure 5.23) with the difference being the position of the spring. The valve body is joined to
the diaphragm element by the yoke.

FIGURE 5.23 (A) Direct action and (B) reverse action.
5.4.1 Fail-Safe Design
Fail-safe design means that the design of a plant has to take account of what will happen if the power
or air supply fails so that a safe shut down occurs. Thus, in the case of a fuel valve, the valve should
close if failure occurs, while for a cooling water valve the failure should leave the valve open. Figure
5.24 shows a direct acting valve which shuts down the fluid flow if the air supply to the diaphragm
fails.

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