In a three-element drum level control, which variables are used to maintain stable drum level?

• A. Drum level, steam flow, and feedwater flow (and sometimes feedwater pressure).
• B. Drum level only.
• C. Steam flow and feedwater temperature only.
• D. Fuel flow and air flow.

Answer: (A)

In a three-element drum level control, the system keeps the water level stable by using three signals: the drum level itself (the variable you want to maintain), the steam flow (the load signal representing water leaving the drum), and the feedwater flow (the manipulated input that adjusts how much water is added). The controller blends the level error with a forward signal from steam flow so it can preemptively adjust feedwater flow when load changes occur. More steam means more water removal, so the control increases feedwater to counteract the drop in level and hold it steady. Sometimes feedwater pressure can be used as an additional predictor when a direct flow measurement isn’t available, but the core trio used to maintain stability is drum level, steam flow, and feedwater flow.