A modern high pressure, high temperature water tube boiler holds a small quantity of water and produces large quantities of steam. Very careful control of the drum water level is therefore necessary. The reactions of steam and water in the drum are complicated and require a control system based on a number of measured elements.
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 higher increase in steam demand from the boiler the pressure in the drum will fall. 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 results in more steam being produced, which will further raise the water level. This effect is known as ‘swell’. A level control system which is used only level as a measuring element would close 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.
The problems associated with swell and shrinkage are removed by the use of a second measuring element, ‘steam flow’ . A third element ‘feed water flow’ is added to avoid problems that would occur if the feed water pressure were to vary.
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| Boiler Water Level control |
A three element boiler water level control system is shown in figure above. The measured variables or elements are ‘steam flow’, ‘drum flow’ and ‘feed water flow’. Since in a balanced situation steam flow must equal feed flow, these two signals are compared in a differential relay. The relay output is fed to a two term controller and comparator into which the measured drum level signal is also fed. Any deviation between the desired and actual drum level and any deviation between feed and steam flow will result in controller action to adjust the feed water control valve. The drum level will then be returned to its correct position.
A sudden increase in steam demand would result in a deviation signal from the differential relay and an output signal to open the feed water control valve. The swell effect would therefore not influence the correct operation of the control system. For a reduction in steam demand, an output signal to close the feed water control valve would result, thus avoiding shrinkage effects. Any change in feed water pressure would result in feed water control valve movement to correct the change before the drum level was affected.
References
“Introduction to Marine Engineering” by D.A. Taylor