Combined Cycle Fundamentals

Al Ezzel Power  Plant 

OPERATOR TRAINING MANUAL COMBINED CYCLE FUNDAMENTALS

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result in a false low level condition known as "shrink." In some situations, the tuning of the steam drum level control loop may not respond quick enough to maintain proper drum level during such abnormal conditions. In this event, the Operator may be required to take MANUAL control of the level control valves. During unit operation, a constant flow of makeup feedwater must be supplied to the steam drums to compensate for the amount of steam exiting the drum. In the simplest feedwater control scheme, referred to as single element control, water is added to maintain the drum level at or near the centerline. As the level of water in the steam drum decreases, it is sensed by a level controller which opens the feedwater control valve. As the drum level increases back to normal, the feedwater control valve is modulated toward close until the amount of water being added is equal to the amount of steam being used. If steam usage is reduced, the drum water level will tend to increase slightly, and the feedwater supply valve will again modulate toward close. Steam drum pressure is the result of drum level combined with the generation of steam in the evaporator tubes. During unit operation, steam is generated inside the evaporator tubes which contain water and steam bubbles. If drum pressure decreases due to a sudden demand in the steam header, the steam bubbles will ‘swell’ due to the decreased pressure in the drum. Likewise, when the quantity of steam being used by the loads on the steam header decreases, the drum pressure will momentarily increase and the steam bubbles will ‘shrink’. Because of the huge number of bubbles within the boiler water side, this change in bubble size results in the lifting/falling of the water level in the evaporator tubes and in the steam drum. Refer to Figure 5-2 below.

Figure 5-2 Steam Drum Level Shrink and Swell

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