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Power Eng 2nd Class study materials
Power Eng 2nd Class study resources

Power Eng 2nd Class: Water Treatment, Corrosion, and Chemistry

Power Eng 2nd Class · Work through your exam, one topic at a time.

All guide sections

Water Treatment, Corrosion, and Chemistry

This domain links chemistry to tube failures. The exam tests whether you can choose the right treatment, interpret a water analysis, set blowdown, and explain a corrosion mechanism by its appearance and location. Expect one cycles-of-concentration or blowdown calculation.

Treatment stages

  • External: softening (sodium zeolite) removes hardness but not dissolved solids; demineralisation (cation and anion exchange) and reverse osmosis remove dissolved solids.
  • Internal: phosphate buffers and precipitates residual hardness; polymer conditions sludge; sulphite, DEHA or tannin scavenge residual oxygen; amine raises condensate pH.
  • The deaerator does the bulk of oxygen removal, typically to about 7 ppb. Chemical scavenging is a backstop, not a substitute for a working deaerator.
  • Target pH: boiler water roughly 10.5-11.5 for steel, condensate 8.5-9.5. Low pH causes general and pitting corrosion; excessive caustic in high-stress areas causes caustic gouging and embrittlement.

Monitoring

  • Conductivity is the routine proxy for dissolved solids; blowdown holds it at target. Silica limits tighten as pressure rises because silica carries over into turbines.
  • Carryover produces wet contaminated steam, superheater deposits and turbine erosion. Causes include high dissolved solids, high drum level, foaming and sudden load swings.
  • Scale insulates and overheats tubes; sludge and iron oxide cause under-deposit corrosion, which is often the more damaging of the two.

Worked example

Make-up water contains 20 mg/L chloride; boiler water shows 200 mg/L. Cycles of concentration = 200/20 = 10. Blowdown as a fraction of feedwater = 1/COC = 10%, so 50,000 kg/h of feedwater needs 5,000 kg/h of blowdown. If steam purity requires boiler water chloride at or below 100 mg/L (COC of 5), blowdown doubles to 10,000 kg/h and the heat and chemical loss rises - the trade-off against makeup cost and treatment capacity.

Traps

  • Confusing softening with demineralisation when asked which removes dissolved solids.
  • Overdosing sulphite; its breakdown products raise sulphate and conductivity and can contribute to corrosion.
  • Ignoring condensate quality, although most boiler iron arrives from the condensate system.
  • Judging deposit condition from a manhole view and missing under-deposit pitting at the burner line.