Over a period of five years, the condensers of two sister 500 MW Units, in the Midwest, were retubed using a SEA-CURE® S44660 material. S44660 is a high alloy, super ferritic stainless steel. The S44660 material replaced the original Admiralty and copper/nickel condenser tubing. Several factors contributed to the decision to use S44660 as the condenser tube material. The critical factor in the decision was a pending lower copper discharge limit in the plant’s National Pollutant Discharge Elimination System (NPDES). The plant was unable to meet the lower copper limits with copper condenser tube material.
In service since the early 1970’s, Unit A and B condensers were subjected to conditions, including silting and operational upsets, which negatively affected condenser performance. A third 500 MW sister Unit, Unit C, operational in the mid 80’s, experienced similar conditions. All three units are based load coal units and use cooling towers to cool the circulating water. The cooling tower make-up comes from the Arkansas River, which also receives the effluent from the plant.
During the summer of 2011, shortly after the Unit B condenser retube, both A & B condensers showed degradation in performance. Through a series test and trials, the causes of degradation were identified and corrected. The cause was a combination of the tube material, lower river levels, higher than normal temperatures, macro fouling, and air in leakage.