Boiler Feedwater Treatment and Control: Scale, Corrosion, and Blowdown
What this domain tests
Feedwater treatment questions test your understanding of external and internal treatment, the chemistry that drives scale and corrosion, and the control of dissolved and suspended solids through blowdown. Expect items on softeners, deaerators, oxygen scavengers, phosphate and amine programs, conductivity-based blowdown, and the interpretation of test results.
Core rules and concepts
- External treatment removes impurities before the boiler: softening (ion exchange replacing calcium and magnesium with sodium), demineralisation, and deaeration.
- Internal treatment conditions what remains: phosphate for scale control, polymers for dispersion, amines for condensate pH, and oxygen scavengers such as sodium sulfite or hydrazine.
- Deaeration removes dissolved oxygen and carbon dioxide by heating water to saturation and venting the gases. Oxygen pitting is the most common cause of boiler tube failure.
- Scale is mainly calcium and magnesium salts; it insulates the tube and causes overheating and failure. Sludge is suspended matter removed by bottom blowdown.
- Blowdown control: continuous (surface) blowdown controls dissolved solids using conductivity as the indicator; bottom blowdown removes sludge. Cycles of concentration = boiler water TDS / feedwater TDS.
Worked example
Feedwater chloride is 40 mg/L and the boiler water limit is 400 mg/L. What are the permissible cycles of concentration, and what blowdown percentage is required if steam output is 10 000 kg/h?
Reasoning: cycles = 400/40 = 10. Blowdown fraction = 1/cycles = 1/10 = 10% of feedwater. Feedwater = steam + blowdown, so blowdown = steam x (1/(cycles - 1)) = 10 000/9 = about 1111 kg/h. If chloride climbs to 60 mg/L in the feed, cycles fall to 6.7 and blowdown must increase; the conductivity controller would open further to hold the setpoint.
Traps that cost marks
- Using TDS or conductivity for cycles without confirming the same test method on both feed and boiler samples.
- Confusing softening (removes hardness only) with demineralisation (removes all ions).
- Forgetting that sulfite is not used where steam is used for food processing or where high pressure causes breakdown to corrosive gases.
- Treating a rising boiler water pH as always good; high pH with caustic concentrates can cause caustic embrittlement or gouging.