Optimisation of Marine Boilers using Model-based Multivariable ...

26 2. THE MARINE BOILER PLANT
of polluting gasses leaving the stacks. Keeping the oxygen level low also has the benefit
of improving combustion performance and hence increasing boiler plant efficiency.
2.5.1 Consumer
Of course the requirements for the boiler output (steam flow and pressure) depend on the
type of application which the steam is to be used for. The most stringent requirements
occur when the steam is to be used to drive a turbine. In this case the turbine efficiency is
very dependent on the steam temperature and further wet steam can damage the turbine
blades. However, the requirements are less stringent in the applications in which the
one-pass smoke tube boiler is used.
The consumer requirements fall into three categories:
• Plant stability: The capability of the plant to supply steam at a predefined temperature/pressure
to the consumer when in operation. This is mostly a matter of
correct sizing of the actuators. The consumer also plays an important role when
it comes to plant stability. The reason is that the steam flow disturbance is completely
controlled by the consumer as mentioned in Section 2.4. This means that
he can by choice of the disturbance profile cause tripping of the boiler if this profile
does not comply with the plant design. To avoid tripping the boiler is again a
matter of proper water level control. Basically this means that the control system
has to expect the worst from the consumer which is frequent, large steps in the
steam flow.
• Output variance: When purchasing a new boiler the consumer normally only
specifies a certain steam flow capacity at a certain pressure. For this reason it is
difficult to set up requirements for the control system regarding the output variance.
However, the following can be observed: the reason why the consumer
wants a constant pressure is that this implies a constant temperature (i.e. saturation
temperature), and all heat consuming applications and pipe systems are
designed for saturated steam pressure. Especially heat exchangers require constant
pressure as industrial processes often require a constant temperature.
• Steam quality: Water drops in the steam should be avoided in general. The reason
is that water from the drum can carry salts and oil into the steam supply line
damaging the pipe system. Further, water drops at high velocity can hit the pipe
walls in pipe bends causing wear on the pipeline. Keeping high steam quality is
a matter of appropriate water level control. The higher the water level, the higher
the risk of carrying water drops into the steam supply line. However, a new
mechanical installation above the steam space in the boiler drum has reduced this
dependency. In the boilers treated in this project the matter of keeping water
drops from entering the steam outlet can be formulated as a hard water level
constraint. If this constraint is exceeded water will get into the steam supply