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Nowadays, most of the seafaring ships use high pressure and high temperature water tube boilers. This water tube boilers use less amount of water to produce large quantities of steam. Because of this reason, the internal parts of the boiler are subjected to continuous high temperature and pressure. Moreover, the reactions between the steam and water inside the boiler are highly complicated and requires close attention and monitoring.
The most important aspect to ensure efficient running of the whole system is to maintain adequate amount of water level inside the boiler drum. The water tube boilers are also known for sudden variations in the boiler pressure, which can lead to various detrimental and dangerous conditions in and around the boiler. Thus an extremely careful control of the boiler water system is necessary. The boiler water level control system depends of a number of elements.
The main aim of these controlling elements is to prevent the occurrence of two main effects that take place inside the boiler drum. The two effects are Swell and Shrinkage which can lead to a variation in the peak and trough of the water level gauge reading
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The level of the boiler water inside the drum is determined with the help of a water level gauge glass mounted on top of the boiler. It is a known and accepted fact that when the boiler is operating, the water level in the water gauge glass for a particular amount of water in the boiler drum, will be higher than that shown when the boiler is shut down. This happens because as the water reaches the boiling temperature steam bubbles that produces in the water increases the overall volume and pressure inside the boiler drum. This results in a higher level of the boiler gauge glass. As the pressure inside the boiler drum increases, some quantity of water will flash off due to the high temperature and would result in the formation of steam bubbles. These steam bubbles would further cause the drum level to rise.
However, if there arises a sudden demand for steam, the pressure inside the drum will drastically reduce. This would result in a condition of more steam than water inside the boiler drum It is to note that even if the volume of water reduces in the drum, the greater mass of steam will increase the level of water inside the drum. Moreover as the mass of the water further reduces, even more amount of steam is produced due to high temperature, which increases the level of water inside the boiler drum even further. This effect of water level rise is known as "swell".
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Now, as the load on the boiler gradually reduces, the pressure inside the boiler drum will increase, which eventually would lead to reduction in bubble formation. With the reduction in the steam bubbles, the level of water inside the boiler drum would come back to normal. With the introduction of cold water inside the boiler, the temperature would further reduce and would result in a sudden drop in the boiler drum water level. This effect of drop in the water level inside the boiler drum is known as "shrinkage".
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Malek, M. (2004) Power Boiler Design, Inspection, and Repair: As per ASME Boiler and Pressure. New York: McGraw-Hill Professional Engineering