Feedwater Systems and Water Treatment
Two goals: no scale, no corrosion
Feedwater treatment prevents deposits and prevents corrosion. External treatment (softeners, reverse osmosis, demineralizers, deaerators) removes or reduces impurities; internal treatment (chemicals fed to the boiler) conditions what remains. A softener exchanges calcium and magnesium for sodium - it lowers hardness but does not reduce TDS, alkalinity, or silica. That distinction is one of the most common exam traps.
Key tests: hardness, pH, conductivity/TDS, alkalinity, dissolved oxygen, sulfite residual, phosphate, silica, and iron/copper. Blowdown controls dissolved solids; surface blowdown skims from the water surface, bottom blowdown removes sludge. Too little blowdown causes carryover, foaming, and scale; too much wastes fuel, water, and chemicals.
Chemistry basics: oxygen scavengers such as sodium sulfite with a cobalt catalyst at lower pressures, and hydrazine, carbohydride, or similar alternatives at higher pressures where sulfite breaks down. Scale is controlled with phosphate/polymer programs matched to pressure, and pH with amines or caustic. Priming and carryover come from high TDS, high alkalinity, foaming organics, or sudden load swings.
Rule of thumb
If a sample is soft but conductivity keeps climbing, the answer is more blowdown, not more softener.