Boiler Swell and Shrinkage
Concept – Boiler Swell and Shrinkage
Boiler Swell and Shrinkage as it relates to single element feed water control is a concept that may be difficult for new power engineers to understand. As the steam demand increases from a plant logic may tell you that the drum level in a boiler will decrease and consequently the feed water pump or valve will increase flow to the boiler. However what is actually going on is that the increase in steam demand causes a localized drop in steam drum pressure in the boiler which affects the boiling point and density of the water and steam. The boiler water side is a mixture of water and steam bubbles and the slight drop in drum pressure causes an immediate increase in the volume of steam bubbles in the water space and consequently the steam drum water level increases hence the concept of boiler swell. The result of this boiler swell is that at a time when more steam and more boiler water is needed the feed water pump or valve actually decreases flow to the boiler due to the increase in steam drum water level. This is the opposite of what is needed to maintain a steady steam drum water level. Conversely when the plant steam demand decreases this causes a localized increase in steam drum pressure. This increase in pressure causes many of the steam bubbles on the water side to collapse resulting in a decrease in steam drum water level which is termed boiler shrinkage. The decrease in steam drum water level will lead to an increase in flow from the feed water valve or pump which is the exact opposite of what is desired at a time of decreased plant steam demand. The conditions of swell and shrinkage combined with single element feed water control can lead to wide steam drum level fluctuations which is undesirable for steady steam plant operation.
Analogy to explain this concept is that opposites attract.
Logic may tell you that as there is an increase in steam demand then the water level in the steam drum should decrease as more steam is demanded then more water is also (as it converts to steam to meet this increased demand). Conversely as steam demand decreases and less steam is needed from the boiler then the boiler steam drum level should increase. However in fact the opposite is occurring with regards to steam drum level and in turn the feed water control.
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