EEI POSS Practice Test 5

Keeping the Drum Level in the Band

¹In a drum-type boiler, water and steam meet in a large horizontal vessel called the steam drum, mounted at the top of the boiler. Water flows down from the drum through unheated pipes, picks up heat in the furnace tubes, and returns to the drum as a mixture of water and steam bubbles. The steam separates in the drum and leaves through the top, while the water circulates again. Drum level is one of the most closely watched readings in the plant, and the operator's job is to keep it within a narrow band near the middle of the gauge glass.

²A level that is too low is dangerous because the furnace tubes depend on a steady flow of water to stay cool. If the drum runs low, some tubes may be starved of water while the fire is still heating them, and their metal can overheat and fail within minutes. A level that is too high is dangerous for a different reason. Steam separators in the drum need open space to work, and when the water rises into them, droplets of water are carried out with the steam. This carryover can deposit solids in the superheater and can send slugs of water toward the turbine.

³Drum level does not always move the way an operator might expect. When the turbine suddenly calls for more steam, drum pressure drops for a moment. The lower pressure lets more of the water flash into bubbles, and the bubbles swell the mixture so the level rises, even though water is leaving the boiler faster than before. This is called swell. A sudden drop in steam demand does the opposite: pressure rises, bubbles collapse, and the level falls. This is called shrink.

⁴Because of shrink and swell, a controller that looked only at drum level would cut feedwater just when the boiler needs more. Most large boilers therefore use three-element control, which compares steam flow with feedwater flow and uses drum level only to trim the result.

⁵Operators do not rely on the controller alone. Before trusting a remote level indication, they compare it with the gauge glass at the drum. If the readings disagree, the operator treats the level as uncertain and investigates before making any large change in feedwater flow.