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

24 2. THE MARINE BOILER PLANT
steam bubbles in the water which raises the surface. See Figure 2.3. Occasionally this
cone seems to collapse creating a wave on the surface. In summary, one must keep in
mind that when speaking of controlling the water level it is the number returned from a
water level measurement that is controlled and not the actual chaotic water surface.
Figure 2.3: Water surface in one-pass smoke tube boiler. Note that the surface is rising towards the flue gas
pipes.
2.4 Consumer and Load Characteristics
The steam consumers onboard ships are mostly represented by ruthless on/off operation
of valves. Even on tank ships the controllers for the cargo pumps, driven by steam
turbines, react very fast compared to the marine boiler dynamics. However, generally
there is a mixture of load situations listed below:
• Ramps: turbine units changing load will generate a ramp in the steam flow disturbance.
• Steps: on/off switching of e.g. heating sections for cargo or fuel.
Regarding the step disturbances they are usually in the range from 10-30%, however,
there is no onboard boiler equipment which prevents larger steps from occurring. However,
at this point, by informing the consumer, AI tries to limit the load changes to about
10 %
min to avoid tripping1 the boiler.
The steam load is interpreted as a disturbance in the boiler control system. The reason
is that the consumer has full authority of the main steam line. This means that in theory,
if sufficient firing is sustained, the consumer can empty the boiler for steam/water if the
consumption is high enough.
The type of disturbance and the frequency with which the disturbance occurs differ
from one ship type to another. However, also within the same ship type the disturbance
1 Tripping here refers to the forced burner shutdown caused by activating the water level alarms.