Page 342 - Introduction to Marine Engineering

Instrumentation and control

Desired value

Seawater Pump
Temperature control
transmitter valve
t Sea water""""'^
Sea water outlet
JL inlet
Slave
controller
Main Cooler
engine

Figure 15.40 Cooling water temperature control

The control system shown in Figure 15.40 uses a combination of
cascade and split range control. Cascade control is where the output
from a master controller is used to automatically adjust the desired value
of a slave controller. The master controller obtains an outlet
temperature reading from the engine which is compared with a desired
value. Any deviation acts to adjust the desired value of the slave
controller. The slave controller also receives a signal from the water inlet
temperature sensor which it compares with its latest desired value. Any
deviation results in a signal to two control valves arranged for split range
control. If the cooling water temperature is high, the sea water valve is
opened to admit more cooling water to the cooler. If the cooling water
temperature is low, then the sea water valve will be closed in. If the sea
water valve is fully closed, then the steam inlet valve to the water heater
will be opened to heat the water. Both master and slave controllers will
be identical instruments and will be two-term (P+I) in action.
Another method of temperature control involves the use of only a
single measuring element (Figure 15.41). A three-way valve is provided
in the cooling water line to enable bypassing of the cooler. The cooler is
provided with a full flow of sea water which is not controlled by the
system. A temperature sensing element on the water outlet provides a
signal to a two-term controller (P+I). The controller is provided with a
desired value and any deviation between it and the signal will result in an
output to the three-way control valve. If the measured temperature is
low, more water will be bypassed and its temperature will therefore
increase. If the measured temperature is high, then less water will be
bypassed, more will be cooled and the temperature will fall A simple