Condensate pH Excursion and Rising Iron in a Steam Plant
Your 150 psig steam plant returns about 65% of condensate. Over three weeks, lab results show condensate pH falling from 8.9 to 6.7, total iron rising from 0.2 to 1.5 mg/L, copper from 0.02 to 0.08 mg/L, and condensate conductivity from 6 to 18 µS/cm. Boiler water remains in control at pH 11.0, phosphate 6 mg/L, and 3,500 µS/cm. Feedwater hardness is 0.05 mg/L. Makeup is softened and represents 25% of feed; its alkalinity is 160 mg/L as CaCO3. Two process heat exchangers return condensate to a common header: one is cooled by tower water at 25 psig, the other heats a food-grade product. The chemical vendor proposes increasing the neutralizing amine feed rate. Production wants the issue resolved without a shutdown, and the plant discharges condensate to the sewer if it is off-spec.
1. Explain the likely corrosion mechanism and evaluate whether increasing neutralizing amine alone is an adequate response.
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The low pH, rising iron and copper, and rising conductivity indicate acid corrosion of the condensate system. The most likely source is carbonic acid formed when bicarbonate alkalinity in the softened makeup breaks down in the boiler to CO2, which returns with the steam and dissolves in the condensate. A process leak is the other strong possibility: tower water or product entering the condensate would also raise conductivity and could depress pH.
More neutralizing amine may raise the measured pH, but it will not remove dissolved CO2 or stop a leak. It can also mask the conductivity trend and raise cost. If a heat exchanger is leaking, amine treatment cannot protect the food product side and may hide a contamination path. The correct response is to prove the source first, then treat the cause: reduce alkalinity, stop in-leakage, and use a balanced amine program. Do not accept a treatment change as the sole fix before the leak question is answered.
2. Describe a leak-investigation plan that can be carried out without a full plant shutdown, including the measurements you would use.
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Sample each heat exchanger’s condensate outlet separately and compare pH, conductivity, hardness, chloride, sodium, and TOC with the common header. Isolate one exchanger at a time, if production allows, and watch the header conductivity and pH for a step change. Check the pressure relationship: if tower water is at 25 psig and the condensate header is at lower pressure, in-leakage is possible; if condensate pressure is higher, the leak direction would be outward. For the food-grade exchanger, test for product-specific markers and verify that the product side is at higher pressure than the condensate.
Use a portable conductivity meter and a cation conductivity column for fast trending. A rising sodium or chloride with low hardness suggests tower water; a rising TOC or product marker suggests product. Record which exchanger was isolated and the response time. If no leak is found, sample individual steam users and condensate branches to locate the source.
3. Assuming no process leak is found, what treatment and monitoring changes would you make?
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Reduce the source of CO2 by lowering makeup alkalinity: improve the softener, or move to demineralized or reverse-osmosis makeup for the 25% makeup stream. Use a combination of neutralizing amine to bring condensate pH into the 8.5–9.0 range and a filming amine to protect the return lines, with feed points distributed rather than at one location. Avoid overfeeding, because amine salts can raise cation conductivity.
Set targets and alarm limits: iron below 0.1 mg/L, copper below 0.02 mg/L, condensate pH 8.5–9.0, and conductivity back near the 6 µS/cm baseline. Trend daily until stable, then weekly. Review boiler alkalinity and cycles to ensure the CO2 load is not being concentrated. Document the treatment change and the reasons, including why amine alone was not accepted.