Feedwater, Steam, and Water Treatment
Water treatment questions are usually about consequences: what impurity causes which failure, and which test controls it.
External treatment
Softeners exchange calcium and magnesium for sodium and are regenerated with brine; demineralizers and reverse osmosis remove nearly everything; deaerators strip oxygen by heating water to saturation and venting, targeting very low residual oxygen (roughly 7 ppb or lower where specified).
Internal treatment
- Phosphate precipitates residual hardness as removable sludge.
- Sulfite scavenges leftover oxygen; roughly 8 parts sulfite per part dissolved oxygen, with a small residual maintained.
- Caustic or amine raises pH and protects condensate return lines from carbonic acid attack.
- Polymers/chelants disperse sludge and prevent scale adhesion.
Problems and causes
- Scale insulates tubes and causes overheating and blistering.
- Oxygen pitting appears as localized deep pits, usually in feedwater and economizer areas.
- Caustic embrittlement attacks stressed metal at riveted or rolled joints; controlled by pH and nitrate or coordinated phosphate programs.
- Foaming, priming and carryover send water, silica and solids into steam lines and superheaters; controlled by blowdown, antifoam and level control.
- Silica carryover forms deposits on turbine blades.
Traps
Blowdown controls dissolved solids, not hardness - a softener failure will not be fixed by more blowdown. And a rising boiler water conductivity with falling feedwater quality usually means a softener breakthrough, not over-blowdown.
Worked rule
Cycles of concentration = boiler water solids / feedwater solids. If feedwater chloride is 20 ppm and the boiler limit is 200 ppm, maximum cycles are 10; blowdown percentage needed is roughly 100 / 10, or 10 percent of feedwater flow.