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

28 2. THE MARINE BOILER PLANT
plant input. When the designed controller has integral action, special precautions
must be taken.
• Discontinuous input flows. Both the burner and the feed water system are characterised
by having discontinuities in the delivered output. Usually continuous
operation of the feed water valve is reliable down to an opening degree of 10%;
from here to fully closed the valve position is unreliable. To overcome this problem
the valve might be operated in on/off mode for low openings. As the burner
turndown ratio 2 cannot be infinite the burner has to be operated in on/off mode
for low steam consumptions.
• High disturbance bandwidth. The steam flow disturbance instantly affects the
steam pressure and as mentioned in Section 2.3 also the water level, creating
the shrink-and-swell phenomenon. The marine boiler control system receives
no prior information about the steam load changes which makes this disturbance
difficult to handle and requires frequent updates in the monitoring of the steam
pressure to be able to respond fast to the disturbance for good control performance.
• Shrink-and-swell. When present in the response from control input to control
variable it limits the achievable performance using single input single output
(SISO) strategies as it introduces a non-minimum phase zero. However, this
zero from feed water to water level is not pronounced in the one-pass smoke
tube boiler. The shrink-and-swell caused by engine load changes was previously
described and has a large affect on the water level.
• Measurement noise. Here we refer to the water level measurement. As mentioned
in Section 2.3 the water surface is rather chaotic causing some measurement techniques
to deliver a signal subject to noise. This makes it difficult to obtain a high
bandwidth of the closed loop system.
As can be seen, the factors contributing to complication of the boiler control fall into
two groups; One group is concerned with the supply systems and the actuators, and the
other group is concerned with disturbances and noise. The reason why plant nonlinearities
are not mentioned is that in the boiler family addressed in this project these are
not pronounced in the frequency band around the desired crossover frequency [Solberg
et al., 2007b].
The largest challenge is to improve the current water level control in such a way that the
boiler geometry can be minimised (by reducing the necessary distance between LWL
alarm and HWL alarm – see Section 2.3) without compromising steam quality and
steam output. Improved level control further makes it possible to increase efficiency
of existing boilers by moving the level setpoint closer to the HWL alarm level. It is
2 Turndown ratio is defined as the ratio between the maximum and minimum possible fuel flow which
results in an acceptable burner performance during boiler operation.