In August of 2006, Detroit Edison contacted CTI Industries of Orange, CT about the possibility of restoring failed tubes at St. Clair Station’s, Unit 7, #5 IP Feedwater Heater. Stress Corrosion Cracking (SCC), throughout the desuperheating section of the 6+ year old YUBA heater, resulted not only in the loss of the #5 IP heater, but also forced the #6 HP heater to be taken out of service due to an isolation problem between the heat exchangers, and to also eliminate the damage caused by the hot drain from # 6 to 4 after a long isolation. With both IP and HP heaters in the Unit 7 train taken out of service for a long period of time, more than 100MW unit was derated from the heat loss penalty suffered. This then became a top priority to address.
Independent Eddy Current Testing indicated the SCC was confined to the desuperheating section of the heater, an area extending from the tubesheet to a depth of approximately 12 feet in. Testing revealed no damage indication at the inlet side of the FWH. With a new unit months away from delivery, sleeving all the outlet tube ends to a depth extending beyond the desuperheating section was the only possible alternative to restore the heat-rate.
Detroit Edison contacted CTI to discuss options. Detroit Edison elected to use Seamless-Alloy 800, to best combat the SCC. A decision was reached to size the inserts 13’ long, which would extend beyond the target area by one foot. The inserts would be expanded hydraulically, full-length, then mechanically expanded at the first inch (tubesheet) and last inch (discharge end), to completely shield the problem area.