Water Treatment and Boiler Feedwater
Domain 6 - Water Treatment and Boiler Feedwater. This domain tests impurities, the damage they cause, external and internal treatment, blowdown control, and carryover.
Impurities and their damage
Hardness (calcium and magnesium) forms scale, which insulates tubes and leads to overheating and fuel waste. Dissolved oxygen causes pitting, especially in feed lines and economizers. High alkalinity and TDS promote foaming and carryover; silica can deposit on turbine blades; low pH corrodes condensate return lines. Feedwater treatment combines external softening or demineralization, deaeration, and internal chemical addition.
Internal treatment and blowdown
Oxygen scavengers such as sodium sulfite remove residual oxygen; phosphate and polymer programs keep hardness in suspension or prevent crystal growth; amines raise condensate pH. Blowdown controls dissolved solids: cycles of concentration = maximum allowable boiler water TDS ÷ feedwater TDS, and percent blowdown = 100 ÷ cycles. Continuous blowdown removes concentrated surface water; bottom blowdown removes sludge.
Worked example
Feedwater TDS is 200 ppm and the boiler water limit is 2,000 ppm. Cycles = 2,000 ÷ 200 = 10, so blowdown = 100 ÷ 10 = 10 percent of feedwater. At 50,000 lb/h feedwater, blowdown = 5,000 lb/h. If TDS rises above 2,000 ppm, increase blowdown or find the contamination source - do not simply add more chemical.
Carryover
Foaming and priming come from high TDS or alkalinity, oil contamination, a high water level, or a sudden load swing. Carryover carries solids into the steam header and can damage superheaters and downstream equipment.
Traps
Treating a softener as an oxygen scavenger; overfeeding sulfite; judging water quality by appearance; blowing down only when the sight glass looks dirty rather than by test result.