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

22 2. THE MARINE BOILER PLANT
Figure 2.2: Three cross sections of the water tubes in the WHR boiler. z is distance from the entry to the
boiler.
2.3 The Water Level
It is important to keep both pressure and water level in the boilers close to their setpoints.
Especially the water level is difficult to control. In this section we will explain
why.
For safety reasons it is necessary to keep the water level above a certain level in order
to have sufficient cooling of the metal parts, and to ensure a high steam quality it is
also important to keep the water level below a certain level. The lower level is indicated
by a low water level (LWL) alarm and the high level is indicated by a high water
level (HWL) alarm. In general, a normal water level (NWL) is defined and used as
setpoint in the level controller. The difficulties in keeping the level around this setpoint
arise from what is known as the shrink-and-swell phenomenon. This phenomenon introduces
an initial inverse response on the water level seen from both the steam flow
and engine load disturbance but also from the feed water flow and burner load. This
inverse response refers to the process control term non-minimum phase and basically
means that the process variable considered responds to an input by first moving in the
opposite direction before it moves in the long term direction. Details are listed below:
• Engine load changes:
– Swell: Occurs under start of the engine and under positive load changes.
When the power delivered to the WHR boiler increases the mass fraction of
steam in the boiler increases. At low pressure, steam occupies much more
space than water, which leads to large amounts of water being pushed into
the oil-fired boiler, and the water level increases.
– Shrink: Occurs under engine shutdown and negative load changes. When
the power delivered to the WHR boiler decreases the mass fraction of steam
decreases. This allows for more water in the WHR boiler, which is pumped
from the oil-fired boiler in which the water level decreases.
• Steam load changes:
z