Condenser Tube Leak With the Condensate Polisher Out of Service
A 1,200 psig, 850,000 lb/h HRSG supplies steam to a turbine with a condenser using 90/10 CuNi tubes. The water treatment is all-volatile treatment with ammonia and hydrazine. For two weeks, boiler cation conductivity has risen from 0.12 to 0.45 µS/cm, steam cation conductivity from 0.08 to 0.30 µS/cm, and steam sodium from 1 to 4 ppb. Feedwater pH is 9.1, dissolved oxygen 5 ppb, iron 8 ppb, and copper 3 ppb. The condensate polisher is out of service for resin replacement. The condenser hotwell sodium is 6 ppb, and the cooling-water fluorescence dye test is inconclusive. The plant proposes to increase boiler blowdown from 1% to 3%, add more ammonia to raise pH to 9.6, and continue running until the polisher is back in service in five days.
1. Identify the most likely source of the conductivity and sodium excursion and explain why the proposed response is inadequate.
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The most likely source is a condenser tube leak. The simultaneous rise in boiler cation conductivity, steam cation conductivity, steam sodium, and hotwell sodium points to cooling water entering the condensate. Cation conductivity responds to chloride and sulfate from the cooling water, while sodium is a direct tracer. Increasing blowdown removes more boiler water but does not stop the source; it only raises makeup demand and can worsen the ingress if the leak remains. Raising pH to 9.6 with more ammonia does not remove sodium or chloride and increases the transport of copper corrosion products from the CuNi tubes. At 1,200 psig, steam cation conductivity of 0.30 µS/cm and sodium of 4 ppb already exceed typical targets, so five more days of operation risks superheater and turbine deposits and under-deposit corrosion.
2. What immediate steps would you take to confirm and isolate the leak, and what operating limits would force a shutdown?
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Sample the hotwell, condensate pump discharge, and each condenser half for sodium and chloride, since these are more specific than conductivity. Isolate one half of the condenser at a time and watch sodium and cation conductivity; a drop identifies the leaking half. If the leak is confirmed, plug or repair the tubes and return the half to service only after the chemistry response is stable. Check makeup water and attemperator spray as alternate contamination sources before assuming the condenser. Set clear shutdown limits: if steam sodium exceeds about 10 ppb, steam cation conductivity exceeds about 0.5 µS/cm, or the leak cannot be isolated, shut down rather than continue to the polisher repair date.
3. Why is the out-of-service polisher important, and how would you manage chemistry until repair?
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The condensate polisher is the ion-exchange barrier that removes ionic impurities from a condenser leak before they reach the boiler and superheater. With it out of service, the leak contaminants pass directly into the cycle. Until repair, reduce condenser load, isolate the leaking section, keep blowdown as high as chemistry limits allow, and monitor sodium, chloride, and cation conductivity at least hourly. Hold feedwater pH in the lower part of the all-volatile treatment range, about 9.1 to 9.3, to limit copper transport. Do not raise pH to 9.6 or add phosphate to mask the problem; those actions do not remove sodium and chloride and can increase copper corrosion or hide the true contaminant level.