Feedwater Treatment and Water Chemistry
What this domain tests
Feedwater Treatment and Water Chemistry tests why untreated water damages a boiler and how external and internal treatment prevent scale, corrosion, and carryover. Tests, dosages, and blowdown control are all fair game.
Core rules
- Impurities: hardness (calcium and magnesium), alkalinity, dissolved oxygen and carbon dioxide, silica, iron, suspended solids, and total dissolved solids.
- Damage: scale insulates tubes and causes overheating; oxygen pits metal; carbon dioxide lowers condensate pH; high TDS and alkalinity cause foaming and carryover.
- External treatment: sodium zeolite softening exchanges hardness for sodium, demineralizers and reverse osmosis remove dissolved solids, and the deaerator heats water to saturation and strips oxygen and carbon dioxide.
- Internal treatment: an oxygen scavenger such as sodium sulfite, phosphate for residual hardness control, amine or neutralizing chemicals for condensate pH, caustic or coordinated phosphate for boiler pH, and polymer dispersants.
- Control: blowdown rate is set from conductivity or TDS, backed by tests for hardness, alkalinity, pH, sulfite, phosphate, silica, and conductivity.
Worked example
Makeup water contains 250 ppm TDS and the boiler water limit is 2500 ppm. Cycles of concentration = 2500 / 250 = 10. Blowdown as a fraction of feedwater = 1 / COC = 1/10 = 10%. At 20,000 lb/h of feedwater, blowdown is 0.10 x 20,000 = 2000 lb/h and steam flow is the remaining 18,000 lb/h. Raising the limit or lowering makeup TDS would reduce the blowdown and its heat loss.
Traps that cost marks
- Confusing an oxygen scavenger with a scale preventive; sulfite does not soften water.
- Assuming a softener removes silica or dissolved solids; it only exchanges hardness.
- Sampling boiler water without cooling it, so the reading flashes off and misleads.
- Blowing down on schedule alone instead of on conductivity results.