I85PREP
⌘ K
Power Eng 5th Class study materials
Power Eng 5th Class study resources

Power Eng 5th Class: Water Treatment and Boiler Feedwater

Power Eng 5th Class · Work through your exam, one topic at a time.

All guide sections

Water Treatment and Boiler Feedwater

What the exam tests

This domain tests feedwater quality, the damage caused by impurities, and the treatment methods used to prevent scale, corrosion, and carryover. It includes external softening, internal chemical treatment, deaeration, blowdown, and routine testing.

Impurities and their effects

  • Hardness (calcium and magnesium) forms scale on hot surfaces, reducing heat transfer and risking overheating. Scale is a thermal insulator and a major efficiency loss.
  • Dissolved oxygen causes pitting corrosion, especially in feedwater lines and economizers. Carbon dioxide lowers pH and causes acidic corrosion.
  • High alkalinity and high pH can cause caustic embrittlement or foaming. High TDS and silica contribute to carryover and steam contamination.
  • Carryover is liquid water leaving with steam; it can foul superheaters, turbines, and steam traps.

Treatment methods

  • External treatment: water softeners exchange hardness for sodium; demineralizers remove dissolved solids; reverse osmosis reduces TDS. Softening does not reduce TDS.
  • Internal treatment: phosphate controls scale, sulfite or hydrazine scavenges oxygen, amines neutralize carbon dioxide, and chelants control deposits in some systems.
  • Deaeration heats feedwater and strips oxygen and other gases. Mechanical deaeration plus chemical scavenging is standard.
  • Blowdown controls boiler water TDS. Bottom blowdown removes sludge; surface blowdown controls dissolved solids. Over-blowing wastes energy; under-blowing causes carryover.

Worked example

Feedwater TDS is 200 mg/L and the maximum allowable boiler water TDS is 2,000 mg/L. What percentage blowdown is needed?

Blowdown % = feedwater TDS / (maximum boiler water TDS − feedwater TDS) × 100 = 200 / (2,000 − 200) × 100 = 200 / 1,800 × 100 ≈ 11.1%. The reasoning is that solids enter with feedwater and leave only in blowdown; the blowdown rate must dilute incoming solids to the limit.

Traps that cost marks

  • Believing a softener removes dissolved solids; it only removes hardness.
  • Using sulfite as a scale inhibitor instead of an oxygen scavenger.
  • Ignoring pH and alkalinity while focusing only on hardness.
  • Blowing down too little to save energy, causing carryover.
  • Forgetting that deaeration is both mechanical and chemical.