2nd Class Power Engineering (2B2) Practice Exam 2025 – Complete Study Resource

The correct answer is that the two-element feedwater control system measures both drum water level and steam flow to adjust the feedwater flow accordingly. This method uses these two variables to maintain the water level in the steam drum within a desired range, which is crucial for efficient boiler operation and to prevent issues such as water carryover or dry firing.

In a two-element system, the water level is a vital input for measuring the amount of water in the drum, while the steam flow provides a measure of the steam being produced. By correlating these measurements, the system can make timely adjustments to the feedwater flow, ensuring that there is enough water to meet the steam generation demands, thus maintaining operational safety and efficiency.

Using only the drum water level is insufficient for accurately controlling feedwater flow because it does not account for the variability in steam demand. In comparison to one-element systems that focus solely on drum level or other types of systems which may incorporate more variables, the two-element approach strikes a balance between simplicity and effectiveness by integrating the steam flow aspect into its control strategy.