Page 337 - Introduction to Marine Engineering
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Instrumentation and control 311
When a boiler is operating the water level in the gauge glass reads
higher than when the boiler is shut down. This is because of the
presence of steam bubbles in the water, a situation which is accepted in
normal practice. If however there occurs a sudden increase in steam
demand from the boiler the pressure in the drum will fail. Some of the
water present in the drum at the higher pressure will now 'flash off and
become steam. These bubbles of steam will cause the drum level to rise.
The reduced mass of water in the drum will also result in more steam
being produced, which will further raise the water level. This effect is
known as "swell', A level control system which used only level as a
measuring element would close in the feed control valve—when it
should be opening it.
When the boiler load returns to normal the drum pressure will rise
and steam bubble formation will reduce, causing a fall in water level.
Incoming colder feed water will further reduce steam bubble formation
and what is known as 'shrinkage' of the drum level will occur.
Steam from boiler
Auto/manual
selector
Figure 15.56 Boiler water level control
