Water Treatment, Chemistry, and Corrosion Control
Domain focus
Questions test pretreatment, internal treatment, blowdown, deaeration, pH control, oxygen scavenging, and corrosion monitoring. The goal is to prevent scale, corrosion, and carryover without hiding problems.
Core rules
- Scale insulates and causes overheating; corrosion thins metal and creates localized failure.
- External treatment removes hardness; internal treatment controls residual hardness, alkalinity, and oxygen.
- Deaeration removes dissolved oxygen; chemical scavengers handle residual oxygen.
- Blowdown controls dissolved solids and silica; cycles of concentration are limited by the most restrictive impurity.
- pH, conductivity, phosphate, silica, and iron are routine tests; trends matter more than single readings.
Worked example
Make-up silica is 20 mg/L and the boiler limit is 150 mg/L. Allowable cycles = 150/20 = 7.5. Blowdown fraction = 1/7.5 = 0.133, or 13.3% of feedwater. The reasoning: at steady state, silica in equals silica out; blowdown must reject the fraction that would otherwise concentrate. If silica rises, increase blowdown or improve make-up quality.
Traps
Using pH as a substitute for alkalinity; overfeeding oxygen scavenger; ignoring hideout and carryover; and calibrating conductivity against the wrong temperature compensation. Also do not confuse total hardness with dissolved solids.