Page 334 - Introduction to Marine Engineering

308 Instrumentation and control

Correcting unit

The controller output signal is fed to the correcting unit which then
alters some variable in order to return the system to its desired value.
This correcting unit may be a valve, a motor, a damper or louvre for a
fan or an electric contactor. Most marine control applications will involve
the actuation or operation of valves in order to regulate liquid flow.

Pneumatic control valve
A typical pneumatic control valve is shown in Figure 15.34. It can be
considered as made up of two parts—the actuator and the valve. In the
arrangement shown a flexible diaphragm forms a pressure tight
chamber in the upper half of the actuator and the controller signal is fed
in. Movement of the diaphragm results in a movement of the valve
spindle and the valve. The diaphragm movement is opposed by a spring
and is usually arranged so that the variation of controller output
corresponds to full travel of the valve.
The valve body is arranged to fit into the particular pipeline and
houses the valve and seat assembly. Valve operation may be direct acting
where increasing pressure on the diaphragm closes the valve. A reverse
acting valve opens as pressure on the diaphragm increases. The
diaphragm movement is opposed by a spring which will close or open
the valve in the event of air supply failure depending upon the action of
the valve.
The valve disc or plug may be single or double seated and have any of
a variety of shapes. The various shapes and types are chosen according
to the type of control required and the relationship between valve lift
and liquid flow.
A non-adjustable gland arrangement is usual. Inverted V-ring
packing is used to minimise the friction against the moving spindle.
In order to achieve accurate valve disc positioning and overcome the
effects of friction and unbalanced forces a valve positioner may be used.
The operating principle is shown in Figure 15.35. The controller signal
acts on a bellows which will move the flapper in relation to the nozzle.
This movement will alter the air pressure on the diaphragm which is
supplied via an orifice from a constant pressure supply. The diaphragm
movement will move the valve spindle and also the flapper. An
equilibrium position will be set up when the valve disc is correctly
positioned. This arrangement enables the use of a separate power
source to actuate the valve.