Page 340 - Introduction to Marine Engineering

314 Instrumentation and control

controller which also acts as a comparator. Any deviation from the
desired value will result in a signal to a summing relay. The other signal
to the relay is from a steam flow measuring element. The relay output
signal provides control of the coupled attemperator inlet and
bypass valves. As a result the steam flow is proportioned between the
attemperator and the straight through line. This two-element control
system can adequately deal with changing conditions. If, for example,
the steam demand suddenly increased a fall in steam temperature might
occur. The steam flow element will however detect the load change and
adjust the amount of steam attemperated to maintain the correct steam
temperature.

Boiler combustion control

The essential requirement for a combustion control system is to correctly
proportion the quantities of air and fuel being burnt. This will ensure
complete combustion, a minimum of excess air and acceptable exhaust
gases. The control system must therefore measure the flow rates of fuel
oil and air in order to correctly regulate their proportions.
A combustion control system capable of accepting rapid load changes
is shown in Figure 15.39. Two control elements are used, 'steam flow'
and 'steam pressure'. The steam pressure signal is fed to a two-term
controller and is compared with the desired value. Any deviation results
in a signal to the summing relay.
The steam flow signal is also fed into the summing relay. The
summing relay which may add or subtract the input signals provides an
output which represents the fuel input requirements of the boiler. This
output becomes a variable desired value signal to the two-term
controllers in the fuel control and combustion air control loops. A high
or low signal selector is present to ensure that when a load change occurs
the combustion air flow is always in excess of the fuel requirements. This
prevents poor combustion and black smokey exhaust gases. If the master
signal is for an increase in steam flow, then when it is fed to the low
signal selector it is blocked since it is the higher input value. When the
master signal is input to the high signal selector it passes through as the
higher input. This master signal now acts as a variable desired value for
the combustion air sub-loop and brings about an increased air flow.
When the increased air flow is established its measured value is now the
higher input to the low signal selector. The master signal will now pass
through to bring about the increased fuel supply to the boiler via the
fuel supply sub-loop. The air supply for an increase in load is therefore
established before the increase in fuel supply occurs. The required air to
fuel ratio is set in the ratio relay in the air flow signal lines.