Boiler Water Chemistry, Treatment and Blowdown
Core concepts
Chemistry protects the boiler from scale, corrosion, caustic embrittlement, carryover and foaming. Learn hardness, alkalinity, pH, dissolved oxygen, total dissolved solids (TDS), silica, chlorides, conductivity and suspended solids.
Treatment methods include base exchange softening, demineralisation, reverse osmosis, dealkalisation, oxygen scavenging (sulphite, hydrazine substitutes), phosphate treatment, amine treatment and filming amines. Understand internal treatment versus external treatment.
Blowdown controls dissolved solids. Bottom blowdown removes sludge; surface (continuous) blowdown controls TDS and is taken from the water surface where concentration is highest. Blowdown rate depends on feedwater TDS and the maximum permitted boiler water TDS.
Traps
- Believing blowdown should be continuous and heavy; it wastes energy and should be minimised.
- Confusing pH too low (acidic corrosion) with pH too high (caustic attack).
- Forgetting that oxygen pitting occurs mainly in feed systems and at low load.
- Assuming softened water is pure water; it still contains dissolved solids.
Worked rule of thumb
Blowdown % = (feedwater TDS / (maximum boiler water TDS − feedwater TDS)) × 100. With feedwater TDS 200 mg/L and limit 2 000 mg/L, blowdown ≈ 200/1 800 × 100 ≈ 11 %.