Feedwater Systems, Pumps, Injectors and Water Level Control
Core concepts
Feedwater must overcome boiler pressure, so pump selection matters. Learn reciprocating, centrifugal and positive displacement pumps, injectors (steam jet), feedwater regulators (on-off, modulating, proportional) and economisers. Understand NPSH, cavitation, suction head, recirculation and minimum flow protection for centrifugal pumps.
Water level control is central. A three-element control uses drum level, steam flow and feedwater flow; a two-element system uses drum level and steam flow; single element uses level only. Know why single-element control struggles with shrink and swell during sudden load changes.
Traps
- Assuming a rise in indicated level always means more water; swell can show a false high level on load increase.
- Forgetting that injectors need a steam supply and a cold water supply, and fail if either is too hot or too cold.
- Ignoring pump recirculation causing overheating at low flow.
- Confusing feedwater regulator failure with level gauge failure.
Worked rule of thumb
For a boiler rated 5 000 kg/h, feedwater flow should match steam flow plus blowdown (say 5 %). Required feed ≈ 5 000 × 1.05 = 5 250 kg/h, and pump capacity is normally selected with a margin of at least 10–20 % above this.