Feedwater Systems and Water Treatment
Core concepts
Feedwater systems deliver treated water at the right pressure, temperature, and purity. The deaerator heats water to saturation, strips dissolved oxygen and CO2, and stores hot feedwater; chemical oxygen scavengers (sodium sulfite, or hydrazine where permitted) remove the remainder. External treatment - softener, demineralizer, reverse osmosis - removes hardness before it reaches the boiler. Internal treatment uses phosphate and polymer programs to keep residual hardness as mobile sludge, plus amines to protect condensate return lines.
Control targets include boiler water pH near 10.5 to 11.5, low hardness, controlled alkalinity, and a dissolved solids limit set by the manufacturer and monitored by conductivity. Blowdown is the tool: bottom blowdown removes sludge and sediment from low points; surface (continuous) blowdown from the water surface controls dissolved solids.
Failure modes to know: scale (insulating deposits that overheat tubes), carryover, priming and foaming (wet steam from high solids, oil, or sudden load swings), oxygen pitting, and caustic embrittlement.
Traps
The favorite trap asks which blowdown controls dissolved solids (surface, not bottom). Another mixes mechanical deaeration with chemical scavenging.
Worked example
If conductivity climbs above limit at steady load, open the continuous blowdown valve slightly and re-sample. Do not use the bottom blowdown to trim solids, and never blow down a boiler that is low on water.