Hardness Breakthrough, No Surface Blowdown, and a Deaerator That Is Only a Heater
A 30,000 lb/hr watertube boiler at 150 psig returns 60 % condensate; the other 40 % (12,000 lb/hr) is softened make-up. Feedwater total dissolved solids run about 250 ppm and the boiler water limit is 2,500 ppm. The softener has been on line 14 months and the daily hardness test has read 4 ppm as CaCO3 for five days. The maintenance supervisor calls it 'a small leak' and wants to rebed next quarter. There is no continuous blowdown; operators give a 20-second bottom blow twice per shift, and boiler water conductivity is climbing. The deaerator runs at 3 psig but its outlet is 215 °F, the vent is wide open, and sulfite residual has been 0 ppm for two weeks because 'the deaerator handles the oxygen.'
1. Quantify the hardness entering the boiler and decide whether it can wait until next quarter.
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Mass balance. 4 ppm × 12,000 lb/hr = 48,000 ppm·lb/hr = 0.048 lb/hr ≈ 1.15 lb/day, roughly 35 lb per month of hardness as CaCO3 entering the boiler.
Why that is not small. Whatever does not leave with blowdown deposits preferentially at the highest heat-flux surfaces - waterwall and burner-area tubes - as an insulating scale. Even a thin layer raises tube-metal temperature and fuel consumption, and the deposit grows fastest where the metal is hottest. Thirty-five pounds a month is more than enough to start a tube failure inside a quarter.
Decision: it cannot wait. Immediate actions:
- Switch to the standby softener or regenerate and re-test; confirm treated-water hardness is essentially nil before returning it to the boiler.
- If no standby exists, reduce firing rate, increase monitoring, and schedule a shutdown; do not accept hardness to the boiler.
- Add a dispersant/polymer only as a holding measure, and only with the water-treatment supplier's guidance - polymer does not replace a working softener.
- Start a daily log of hardness, conductivity, and blowdown so the trend, not a single reading, drives decisions.
2. Calculate the blowdown required at the stated solids limits and explain why bottom blow alone is the wrong tool. Include the heat cost.
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Cycles of concentration. COC = boiler water limit / feedwater TDS = 2,500 / 250 = 10.
Blowdown rate. With COC = 10, blowdown = steam / (COC − 1) = 30,000 / 9 ≈ 3,330 lb/hr, about 11 % of steam output.
Bottom blow is not a substitute. Bottom blowdown removes sludge and sediment near the mud drum outlet. It does almost nothing for dissolved solids, which are distributed throughout the water and concentrated at the steam-release surface. Continuous surface blowdown from the waterline is the correct control. Short, frequent bottom blows on a hot boiler also risk level upset and thermal shock at the tube sheet.
Heat cost. Saturated liquid at 150 psig is about 338 Btu/lb; feedwater at 220 °F is about 188 Btu/lb, so the blowdown carries off roughly 3,330 × 150 ≈ 0.5 MMBtu/hr, on the order of 1.3 % of fuel input for this boiler. That justifies a blowdown heat exchanger, and it also means the blowdown must go to a blowdown tank and cooler, not a floor drain. Continuous blowdown plus heat recovery is the package to propose.
3. Assess the deaerator and the zero sulfite residual. What is the corrosion consequence and what is the right sequence of correction?
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The DA is not deaerating. Saturation temperature at 3 psig is about 227 °F; the outlet is 215 °F, roughly 12 °F subcooled. A deaerator that cannot bring water to saturation cannot strip oxygen - it is acting as a feedwater heater. Look for an over-open vent wasting steam and chilling the trays, insufficient steam supply or a stuck PRV, damaged trays, or the unit running beyond its rated make-up flow.
Consequence. Undeaerated feedwater carries several ppm of dissolved oxygen into the feed line, economizer, and boiler, causing pitting - rapid, localized, and often through-wall in months. Pitting in an economizer or feed line is far more expensive than the DA repair.
Sulfite. The theoretical requirement is about 7.9 lb of sodium sulfite per lb of oxygen, with a typical catalyzed-sulfite residual held in the feedwater/boiler water range of roughly 20–40 ppm. A residual of 0 ppm means no scavenging at all.
Correct sequence:
- Fix the deaerator first - set the vent per the manufacturer, restore steam supply, verify 227 °F at the outlet and measure dissolved oxygen with a test kit.
- Restore sulfite feed and residual testing as backup, not as a substitute for deaeration.
- Watch TDS when feeding sulfite, since it adds dissolved solids and therefore blowdown.
- Inspect the feed line and economizer for pitting at the next opportunity; do not assume the system is clean because the boiler water tests look acceptable.