Water Treatment and Boiler Feedwater
What this domain tests
This domain covers the chemistry that protects boilers from scale, corrosion, and carryover. Questions test external and internal treatment, blowdown control, dissolved oxygen removal, and the limits on boiler water chemistry.
Core rules to have at your fingertips
- External treatment: softening (ion exchange removes Ca²⁺ and Mg²⁺), demineralisation (removes all ions), reverse osmosis, and deaeration.
- Internal treatment: phosphate (precipitates hardness as sludge), sulphite (scavenges oxygen), amine (raises condensate pH), and chelant (binds hardness).
- Scale: calcium and magnesium salts deposit on hot surfaces and reduce heat transfer - even a thin layer raises tube metal temperature and can cause failure.
- Corrosion: dissolved oxygen causes pitting; low pH causes acid attack; high pH causes caustic embrittlement. Control pH (typically 10.5–11.5 for boiler water) and remove oxygen.
- Blowdown: controls dissolved solids (TDS) and silica. Continuous blowdown from the drum, intermittent from the bottom. Blowdown rate = (feedwater TDS − make-up TDS) / (boiler water TDS − feedwater TDS) approximately.
- Carryover: caused by high TDS, high alkalinity, or foaming; controlled by drum internals and blowdown.
Worked example
Feedwater TDS = 200 mg/L, boiler water limit = 3 500 mg/L, steam production = 20 kg/s. Assume TDS in steam is negligible.
Blowdown fraction = feedwater TDS / (boiler water TDS − feedwater TDS) = 200 / (3 500 − 200) = 200/3 300 = 0.0606. Blowdown rate = 0.0606 × 20 = 1.21 kg/s.
If dissolved oxygen in feedwater is 0.05 mg/L and the limit is 0.007 mg/L, a deaerator plus sulphite scavenger is needed - mechanical deaeration alone typically leaves about 0.005–0.01 mg/L, so chemical scavenging polishes the remainder.
Traps that cost marks
- Confusing softening (removes hardness only) with demineralisation (removes all ions).
- Forgetting that sulphite adds dissolved solids - it must be controlled.
- Treating blowdown as a fixed rate rather than a function of TDS.
- Ignoring the effect of high pH on caustic embrittlement.
- Assuming a deaerator alone achieves zero oxygen.
Exam strategy
For chemistry questions, state the impurity, its effect, and the treatment. For blowdown, show the mass balance.