Feedwater Systems and Water Treatment
Feed the boiler clean and hot
Feedwater systems include the feed pump (centrifugal, reciprocating, injector), feedwater regulator, feedwater heaters, and the deaerator, which strips dissolved oxygen and preheats water, usually to within a few degrees of saturation. External treatment handles hardness - softeners exchange calcium and magnesium for sodium; dealkalizers and reverse osmosis reduce alkalinity and dissolved solids. Internal treatment adds chemicals: oxygen scavengers (sulfite for lower-pressure boilers, DEHA or hydrazine where sulfite is unsuitable), phosphate or polymer for scale control, caustic or amine for pH, and antifoam for carryover.
What you measure
Hardness (softener performance, zero is the target), conductivity or TDS (drives blowdown), pH (boiler water typically alkaline, around 10.5–11.5; condensate slightly alkaline), sulfite residual, phosphate residual, and dissolved oxygen. Scale is hard adherent deposit from hardness; sludge is soft settable solids removed by bottom blowdown; carryover is water leaving with steam, caused by high TDS, foaming, or high water level.
Traps
More chemical is not better - overfeeding phosphate or caustic causes deposition and embrittlement risk. Blowdown removes dissolved solids; it is not a cure for hardness leakage. Confusing surface blowdown (controls TDS) with bottom blowdown (removes sludge) is a classic question. Also, an oxygen scavenger cannot repair pitting already caused by poor deaeration.
Worked example
A boiler evaporates 20,000 lb/hr with feedwater chloride of 20 ppm and a boiler-water limit of 400 ppm. Required blowdown fraction = 20 ÷ (400 − 20) ≈ 5.3%, or about 1,050 lb/hr of blowdown. Doubling that blowdown for no reason roughly doubles the heat and chemical lost down the drain.