Page 339 - Introduction to Marine Engineering

Instrumentation and control 313

This signal is fed to the controller where it is compared with the desired
value. The two-term controller will provide an output signal which is fed
to both control valves. Each valve is operated by a different range of
pressure with a 'dead band' between the ranges so that only one valve is
ever open at a time. The arrangement is shown in Figure 15.37. Thus if
the auxiliary range pressure is high the dump valve opens to release
steam. If the pressure is low the make-up valve opens to admit steam.
This split range control principle can be applied to a number of valves
if the controller output range is split appropriately.

Steam temperature control of high pressure superheated steam is
necessary to avoid damage to the metals used in a steam turbine.
One method of control is shown in Figure 15.38. Steam from the
primary superheater may be directed to a boiler drum attemperator
where its temperature will be reduced. This steam will then be further
heated in the secondary superheater. The steam temperature leaving
the secondary superheater is measured and transmitted to a three-term

Steam temperature
transmitter
Figure 15.38 Steam temperature control