Question 148Steam Plants - Assistant Engineer

The correct answer is B) feed water flow as a sensed variable. The key difference between a two-element and a three-element feed water regulator control system is that the three-element system incorporates the feed water flow as an additional sensed variable, along with the steam flow and drum water level. This allows the three-element system to more precisely control the feed water flow to match the steam demand, resulting in better overall boiler water level control. The other answer choices are incorrect because they do not represent the additional sensed variable that distinguishes the three-element system. A two-element system does not measure feed water flow, while the other options (steam flow, fuel oil flow) are not the defining characteristic of the three-element system.