Page 336 - Introduction to Marine Engineering

310 Instrumentation and control

Air
supply To diaphragm
chamber on
valve
Orifice

Nozzle Valve
Flapper stem
Bellows
o-u—Spring

I Signal
j from
controller
Figure 15.35 Valve positioner

Actuator operation
The control signal to a correcting unit may be pneumatic, electric or
hydraulic. The actuating power may also be any one of these three and
not necessarily the same as the control medium.
Electrical control signals are usually of small voltage or current values
which are unable to effect actuator movement. Pneumatic or hydraulic
power would then be used for actuator operation.
A separate pneumatic power supply may be used even when the
control signal is pneumatic, as described in the previous section.
Hydraulic actuator power is used where large or out of balance forces
occur or when the correcting unit is of large dimensions itself. Hydraulic
control with separate hydraulic actuation is a feature of some types of
steering gear, as mentioned in Chapter 12.

Control systems
Boiler water level
A modern high-pressure, high-temperature watertube boiler holds a
small quantity of water and produces large quantities of steam. Very
careful control of the drum water level is therefore necessary. The
reactions of steam and water in the drum are complicated and require
a control system based on a number of measured elements.