Carryover and Softener Failure at Peak Load
A 100 psig firetube boiler produces 20,000 lb/h of saturated steam. The makeup water is softened but the softener has been off-line for two days; raw water hardness is 180 mg/L as CaCO3. The boiler water chloride is 420 mg/L and the makeup chloride is 21 mg/L. The steam is carrying over badly enough to contaminate a food process, and the gauge glass shows a foamy, high water level. The boiler water analysis shows 4,600 µS/cm conductivity, 850 mg/L OH alkalinity, and 65 mg/L silica. The continuous blowdown is set at 3% and the operator has been using the bottom blowdown only once per shift. The feedwater temperature is 220°F, and the deaerator is operating at 5 psig. The plant wants to increase production to 22,000 lb/h for the next week.
1. Calculate cycles of concentration from chloride and compare with the blowdown rate. What does this indicate?
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Chloride cycles = boiler chloride divided by makeup chloride = 420 / 21 = 20. At 20 cycles, the 3% continuous blowdown is not controlling the chemistry adequately. With only blowdown, 3% would correspond to roughly 33 cycles, but the actual 20 cycles plus the high conductivity, alkalinity, and silica show the boiler water is concentrated and the softener failure is adding hardness.
The practical target should be set by the boiler manufacturer and water-treatment program, often around 8–10 cycles for this pressure, which would require about 10% blowdown. The 4,600 µS/cm conductivity and 850 mg/L OH alkalinity are consistent with foaming and carryover. Bottom blowdown once per shift does not substitute for continuous control.
2. Identify the causes of carryover and the immediate actions to protect the process and boiler.
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Immediate actions: reduce firing rate and steam demand; lower the boiler water level to the normal gauge-glass level, not the top; increase continuous blowdown and use controlled bottom blowdown to reduce dissolved solids; test makeup and boiler water for hardness, chloride, conductivity, alkalinity, silica, and iron. Regenerate or repair the softener and verify zero hardness before returning it to service. If carryover is contaminating food product, divert or stop steam to the process until steam purity is restored. Do not add antifoam as a substitute for blowdown; use only a treatment vendor's approved product.
The causes are high dissolved solids, high alkalinity, hardness from the failed softener, possible organic contamination, and a high water level. Check for oil or organic contamination, which can also cause foaming. Monitor steam and condensate conductivity.
3. What water-quality program and operating changes should be implemented before increasing production?
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Before increasing production, restore the softener and confirm hardness is essentially zero. Establish documented limits for conductivity, alkalinity, silica, and chloride based on the manufacturer and treatment vendor. Automate continuous blowdown with a conductivity controller and flow totalizer, and require bottom blowdown on a defined schedule. Improve feedwater quality: verify deaerator operation, oxygen scavenger, and pH control.
Investigate carryover with a steam-purity test and, if needed, inspect drum internals and separators. Increase production only after two or three stable sets of boiler water and steam tests at the higher load. Train operators to respond to softener failure by increasing blowdown, reducing load, and notifying supervision. Increasing output while carryover is active would worsen the food-contamination problem and could damage the boiler.