In late 1988 and early 1989, two of the four high pressure feedwater heaters at each of the three generating units at the Tennessee Valley Authority's (TVA) Thomas H. Allen Fossil Plant were replaced. In the Fall of 1997, after 8 1/2 years of service, a significant number of tubes failed suddenly in the two Allen unit 3 replacement heaters. Within a 14 month period, one of the unit 3 replacement heaters had 97 tube failures and the other had 171 failures. The mode of these tube failures was determined to be stress corrosion cracking (SCC) initiated at a tube roll transition located within the tubesheet. In order to prevent continued progression of tubing damage and to restore failed tubes to service, TVA employed CTI Industries to install stainless steel inserts in the inlet and outlet ends of each tube in the affected heaters.
This paper discusses the failures, investigative techniques, probable root causes, and corrective measures evaluated and implemented to extend the service life of high pressure feedwater heaters at TVA's Allen Fossil Plant.
TVA is a federally owned and operated electric power company serving the Tennessee Valley. TVA's total net electric power generating capability is approximately 28,000 megawatts from a combination of fossil-fired, nuclear, hydroelectric, combustion turbine, and pumped storage plants.