In a three-element feedwater regulating system, __________ are measured.

• A. pressure, temperature, and humidity
• B. fuel flow, air flow, and flame temperature
• C. boiler pressure, drum level, and feedwater temperature
• D. steam flow, feedwater flow, and water level

Answer: (D)

In a three-element feedwater regulating system, the three measured quantities are steam flow, water level in the boiler drum, and the actual feedwater flow. These three signals are fed into the control logic so the feedwater flow can be adjusted to match the boiler’s demand.

Steam flow serves as the primary load indicator—when more steam is demanded, the system must supply more water to prevent the drum level from dropping. The drum level (water level) tells you how much water is actually present in the boiler, so the controller can keep the water inventory within safe limits. The measured feedwater flow provides direct feedback on how much water is being added, allowing the system to correct any mismatch between demand (steam flow) and supply.

Together, these signals let the regulator anticipate changes in load and maintain drum level while ensuring the right amount of water is delivered. The other options mix signals that aren’t standard for this three-element scheme (for example, humidity or combustion signals, or boiler pressure and fuel temperature), so they don’t provide the complete, coordinated picture needed for three-element control.