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.

Interlocks in a boiler fuel train are safety controls that ensure the burner only operates under safe conditions. They link fuel delivery to the essential preconditions of proper air supply and an established flame. If air is not adequate or no flame is detected, the interlocks prevent fuel from being supplied or shut the fuel valves off if a flameout or fault occurs. This prevents dangerous situations like fuel flowing without enough air or without ignition, which can lead to incomplete combustion, fuel buildup, or an explosion. In short, interlocks enforce a safe sequence so the boiler can only run when everything necessary for safe combustion is present.

Specific gravity describes the ratio of the weight of a liquid to the weight of an equal volume of water at the same temperature. Because weight is mass times gravity, and gravity is the same for both samples, this ratio is effectively the same as the ratio of their densities. In other words, specific gravity is the density of the liquid divided by the density of water at that temperature, a dimensionless number that shows how heavy or buoyant the liquid is relative to water. We use water as the reference point precisely because its density changes with temperature, so the comparison must be made at the same temperature. This concept differs fromDensity, which is an intrinsic property of a substance, as well as from viscosity and refractive index, which describe flow resistance and light-bending behavior, respectively.

The packing gland area is traditionally called the stuffing box. In a rotary pump, the shaft passes through a housing that holds packing material; the gland compresses this packing to form a seal and prevent leakage around the rotating shaft. This makes stuffing box the standard term for that area. The other phrasing is either just descriptive or not the established name used in pump terminology.