Softener Hardness Slip and Condensate Corrosion
A 150 psig, 20,000 lb/hr steam boiler uses a sodium zeolite softener and returns about 60% condensate. The softener was regenerated two days ago, but the lab reports 2.5 ppm total hardness in the softener effluent, boiler water OH alkalinity 650 ppm, silica 140 ppm, and pH 10.9. Feedwater dissolved oxygen is 0.06 ppm, and condensate pH is 6.7 with elevated copper and iron. The boiler cycles of concentration are about 8, based on chloride. The operator proposes to increase blowdown, add more caustic to raise boiler pH to 11.5, and skip the oxygen scavenger because the deaerator is "working." The plant discharges to a sewer with a pH limit of 6.0-10.0 and a copper limit.
1. Interpret the analytical results. What are the two separate corrosion/chemistry problems?
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The first problem is softener hardness slip. A sodium zeolite softener should produce essentially zero hardness; 2.5 ppm total hardness after regeneration means the unit is exhausted early, channeling, bypassing, or has damaged resin. Hardness in the feedwater can scale the boiler and, with high alkalinity, contribute to foaming and carryover. The second problem is condensate corrosion. A condensate pH of 6.7 with elevated copper and iron is characteristic of carbonic acid attack from dissolved CO2, not a boiler-water pH problem. Oxygen at 0.06 ppm in the feedwater is also above a typical target for a 150 psig boiler, so oxygen pitting is a risk. The high boiler-water OH alkalinity and silica suggest the cycles are already high, and adding more caustic to reach pH 11.5 would increase the potential for caustic attack and carryover without addressing the condensate pH.
2. Evaluate the operator's three proposed actions: more blowdown, more caustic, and skipping the oxygen scavenger.
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More blowdown may be needed to control silica and alkalinity if cycles are too high, but it does not fix the softener hardness slip; the hardness will continue to enter the boiler and scale surfaces. Adding caustic to raise boiler water to pH 11.5 is not the right treatment for condensate pH 6.7. The condensate is acidic because of CO2, and it needs a volatile amine feed to the steam/condensate system, not more boiler caustic. Raising boiler pH also increases the risk of caustic embrittlement and carryover, especially with high alkalinity and hardness. Skipping the oxygen scavenger is wrong: 0.06 ppm dissolved oxygen is not zero, and a deaerator alone does not guarantee protection during shutdown, standby, or condensate return. The scavenger should be fed and controlled by testing, with the deaerator checked as well.
3. Recommend a treatment and monitoring plan that also respects the sewer discharge limits.
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Place the softener on a verified regeneration cycle and test hardness after each regeneration and at the softener outlet; investigate resin condition, brine flow, and bypass. If hardness cannot be held near zero, take the softener out of service and use an alternate water source. Test boiler water for conductivity, OH alkalinity, silica, and chloride, and adjust blowdown based on those results rather than a fixed increase; keep cycles within the treatment program's limits. Feed a volatile amine to raise condensate pH into the appropriate range and an oxygen scavenger to control feedwater oxygen, with testing at the deaerator outlet and condensate return. Monitor iron and copper in the condensate to confirm the corrosion rate is falling. Before increasing blowdown, check the sewer pH and copper limits; if blowdown pH exceeds 10.0, neutralize or segregate it as required. Document all changes and retest after stabilization.