in cfbc boiler how to maintain steam drum level constant with 3 element control?
Three-element boiler drum level control is the most complicated form of process control. The reason being that, regardless of the firing method, a steam drum has a behavior called 'shrink', and 'swell', i.e., the level initially rises when the load increases due to the creation of steam bubbles, and viceversa. After a while, the 'normal' trend takes over. To overcome the unstable response from the drum level transmitter, three-element level control was designed. Basically, the controller, instead of responding initially to the level control signal, responds to the flow control one. I.o.w, when the steam load increases, it is the feedwater valve that responds first. By doing so, the level remains fairly constant, because the influx of cold water into the drum, offsets the 'swell' by generating 'shrink', and viceversa. Eventually, the level controller takes over and makes the smaller corrections necessary to maintain the level fairly constant. A more analytical explanation can be found in the book Power and Process Control, for the Plant Engineer and designer, of Michael J. G. Polony, McGraw-Hill Book Co., .
One common boiler level control system is to balance out flow of steam with inflow of water and then over ride that control when the level gets back to normal.
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