Feedwater, Water Treatment, and Blowdown
What this domain tests
This domain covers what is in the water, what it does to steel, how chemicals and blowdown control it, and how often you test. Expect questions on scale versus sludge, oxygen pitting, carryover, cycles of concentration, and the correct blowdown procedure.
Impurities and their damage
- Hardness (calcium and magnesium) forms scale on hot surfaces; scale is an insulator and overheats tubes.
- Suspended solids form sludge, which is removed by bottom blowdown. Dissolved solids do not settle; they concentrate until carryover begins.
- Dissolved oxygen causes pitting, especially in feed lines and the economizer; mechanical deaeration plus an oxygen scavenger such as sodium sulfite handles it.
- Low pH promotes corrosion, high pH promotes caustic attack; pH is normally held mildly alkaline and controlled with amines or caustic.
Blowdown practice
- Continuous (surface) blowdown skims the surface where dissolved solids concentrate; bottom blowdown removes sludge from the mud drum or low points.
- Bottom blowdown is done with the burner at low fire, opening the valve fully for a short time, then closing and confirming it seats. Never leave a blowdown valve partly open - it will wire-draw and leak.
- Cycles of concentration estimate how many times the solids have concentrated: % blowdown = feedwater TDS ÷ (maximum boiler water TDS − feedwater TDS) × 100.
Worked example
Feedwater carries 150 ppm dissolved solids and the boiler water limit is 3,000 ppm. % blowdown = 150 ÷ (3,000 − 150) × 100 = 150 ÷ 2,850 × 100 = 5.3%. Meaning: for every 100 lb of feedwater, about 5.3 lb must be blown down to hold the limit. Reasoning: if the limit doubles, the blowdown percentage roughly halves - but never below the minimum needed for sludge removal.
Traps that cost marks
- Thinking bottom blowdown removes dissolved solids; it removes sludge. Dissolved solids leave with continuous blowdown.
- Blowing down when the water level is already low, or with the burner at high fire.
- Over-treating with sulfite or phosphate, which creates its own deposits.
- Skipping daily testing because the boiler "looks fine" - carryover and scale are invisible until damage is done.