Water Treatment and Chemistry
Water Treatment and Chemistry
This domain tests scale, corrosion, carryover, blowdown, softening, deaeration, oxygen scavenging, pH, alkalinity, conductivity, and internal treatment. The exam asks why a treatment is used and how it is controlled.
Core rules
- Hardness causes scale; softeners exchange calcium and magnesium for sodium.
- Dissolved oxygen causes pitting; deaeration plus an oxygen scavenger protects feedwater and boiler metal.
- pH and alkalinity affect corrosion and caustic embrittlement risk; maintain within treatment limits.
- Conductivity estimates dissolved solids and guides blowdown.
- Carryover can be caused by high solids, high alkalinity, foaming, or mechanical separation failure.
Worked example
Feedwater contains 200 ppm dissolved solids; the boiler water limit is 2,000 ppm. Cycles of concentration = 2,000/200 = 10. Blowdown percentage = 100/cycles = 10%. Reason: at 10 cycles, one part blowdown removes the solids concentrated from ten parts feedwater, so about 10% of feedwater is blown down.
Traps
Do not use blowdown to correct poor pretreatment alone; do not overfeed oxygen scavenger without testing; do not confuse conductivity with hardness; and do not ignore carryover because it can damage superheaters and turbines.