Boiler Water Treatment Fundamentals — Oxygen, Scale, Alkalinity, Condensate

Direct answer

A boiler programme has four independent jobs and failing any one of them destroys the asset: remove dissolved oxygen, prevent hardness scale, hold the correct alkalinity, and protect the condensate return. Deaeration is mechanical first — a working deaerator at 102–105 °C leaves roughly 7–20 ppb oxygen, and chemical scavenging is there to finish that, not to replace it. Sodium sulphite is the low-cost scavenger for boilers below about 60 bar but adds dissolved solids and therefore blowdown; DEHA and carbohydrazide add no solids, passivate metal surfaces and are the choice for high-pressure or food-adjacent systems where sulphite carryover is unwanted. Hardness that reaches the boiler forms calcium carbonate and calcium sulphate scale — 1 mm of scale can add 5–8 % to fuel consumption, and a hard-water excursion through a failed softener will do more damage in a week than a year of imperfect dosing.

The four jobs

Oxygen. Dose sulphite to a boiler water residual of typically 20–40 mg/L as Na₂SO₃ in low-pressure boilers; DEHA to 0.1–0.2 mg/L residual. Catalysed sulphite reacts in seconds; uncatalysed can take minutes and is unreliable at feedwater temperatures below 80 °C.

Scale. Softened feedwater is the baseline. Internal treatment with phosphate converts residual hardness to a non-adherent sludge, held at 20–40 mg/L PO₄ in the boiler water with a polymer dispersant to keep it mobile. Chelant programmes (EDTA/NTA) handle higher hardness but attack metal if overfed.

Alkalinity and pH. Boiler water pH is usually held at 10.5–11.5 with caustic or phosphate; below 10.5 corrosion accelerates, above 11.5 caustic embrittlement and foaming become the risk. Control total dissolved solids by blowdown — every plant that skips conductivity-based blowdown eventually carries over.

Condensate. Carbon dioxide from bicarbonate breakdown forms carbonic acid in the return line and thins the bottom of horizontal pipe runs. Neutralising amines (morpholine, cyclohexylamine, DEAE) raise condensate pH to 8.2–9.0; filming amines coat the surface instead. Blend selection depends on how far the steam travels — cyclohexylamine carries furthest, morpholine drops out early and protects near the boiler.

Blowdown and cycles

Cycles of concentration equal feedwater conductivity divided by boiler water conductivity. Running more cycles saves fuel and water but concentrates silica and alkalinity; silica above roughly 150 mg/L in the boiler water risks turbine and valve deposits. Continuous surface blowdown on conductivity control plus a short periodic bottom blowdown for sludge is the standard arrangement.

Common failures

Sulphite residual holding but oxygen pitting continuing means the sample point is downstream of the corrosion — usually the feed line before the injection point. Phosphate disappearing between tests means hardness ingress, not dosing error; check the softener regeneration first. Foaming and carryover almost always trace to high TDS, oil contamination from a process leak, or excess alkalinity.

What to send with your enquiry

Boiler pressure and type, steam output, feedwater and makeup analysis, percentage condensate return, deaerator presence and temperature, current chemicals and residuals, and any food-contact or steam-in-contact requirement (FDA 21 CFR 173.310 for boiler water additives) that restricts which amines you may use.