Condenser Tube Leak and Sodium Excursion with Caustic Gouging History
A 300 MW subcritical unit has a surface condenser with 304 stainless steel tubes and a divided waterbox. Over 10 days, boiler water cation conductivity rises from 0.08 to 0.65 µS/cm, sodium from 2 to 85 µg/L, and condenser hotwell sodium from 3 to 220 µg/L. The unit is on 100% condensate polishing, but the polisher effluent sodium is 4 µg/L. The condenser vacuum is normal; the circulating water pump strainers are clean. The plant uses river water with chloride 45 mg/L and cycles cooling water at 3.5 cycles. The boiler is phosphate-treated, and the drum has a history of caustic gouging. Operations wants to continue at full load until the weekend because the unit is the only source for a hospital complex. The chemistry limit for sodium in the boiler water is 10 µg/L, and the cation conductivity action level is 0.5 µS/cm.
1. What is the most likely source of the sodium and cation conductivity, and why is the condensate polisher not keeping the boiler within limits?
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The rising hotwell sodium (220 µg/L) with normal vacuum and clean strainers points to a condenser tube leak or a tube-to-tubesheet joint leak, not to makeup or a resin leak. A 304 stainless tube can pass river water at a small leak that is hard to see because the vacuum pulls air and water into the hotwell. The polisher is removing most of the sodium-effluent is 4 µg/L-but 4 µg/L feedwater at boiler cycles of concentration around 20 is enough to produce roughly 80 µg/L in the drum, which matches the 85 µg/L boiler sodium. The polisher is therefore working, but it is not a barrier for a continuing leak; it is being asked to remove a continuous sodium load and will eventually exhaust, and its effluent sodium will rise. The cation conductivity rise is consistent with chloride intrusion. The normal vacuum is not reassuring: a small tube leak can be masked by the condenser’s air-removal system.
2. What immediate actions would you take to protect the boiler and turbine, and how would you locate and isolate the leak?
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I would reduce load and start the condenser leak-detection procedure: isolate one half of the condenser at a time, watch hotwell sodium and cation conductivity for a step change, and use the divided waterbox to identify the affected half. If the leak is confirmed, keep the affected half out of service and continue at reduced load on one half, subject to condenser vacuum and circulating-water flow limits. Chemistry actions: increase blowdown, verify polisher performance and bypass not open, monitor sodium and chloride at the drum, and check for caustic gouging risk. The boiler is phosphate-treated and has a history of caustic gouging; sodium and chloride ingress can concentrate under deposits and cause corrosion. I would not continue at full load until the weekend. The hospital load is a real constraint, but the plant’s own contingency should be a reduced-load run or a temporary feed from the reserve, not a known condenser leak into the feedwater.
3. What return-to-service criteria would you set after the leak is repaired, and what would you do if the leak is in an inaccessible tube section?
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Before return, I would require: the leaking tube or joint identified and repaired by the approved plugging or retubing procedure; a hydrostatic or vacuum test of the repaired half; hotwell sodium and cation conductivity at or below the action levels with the polisher in service; a documented chemistry hold point before raising load; and a check that the repair does not leave a crevice that can initiate a new leak. If the leak is in an inaccessible section, plugging both ends is acceptable only if the tube is isolated and the plug is qualified for the service; if the tube cannot be isolated, the affected half stays out of service. The boiler should not be returned to full load until the drum sodium is below the 10 µg/L limit and the cation conductivity is below 0.5 µS/cm. I would also review the condenser tube material and the cooling-water chloride level; 304 stainless in 45 mg/L chloride at 3.5 cycles is a corrosion risk, and repeated leaks may justify retubing with a more resistant alloy.