Rotor Welding: TVA Experience

Abstract

TVA has a fleet of 59 units with 187 large steam fossil turbine rotors, 68 fossil generators, and 27 auxiliary turbine rotors, in-service. The original equipment manufacturers (OEM) rotors have three designs some welded, others are shrunk on disk, but the majority of the rotors are mono-block. TVA has worked with OEM’s, third parties, and with-in TVA to perform weld repairs on rotors. Rotor Problems requiring welding: 1. Journal/gland damage due to lubrication failure and/or wear. 2. Attachment area damage due to cracks or distortion. 3. Attachment area due to changes in design. In the TVA fleet there are approximately 20 rotors that have been welded. Most of these have been successfully operated, but there have been 3 failures of welded rotors that will be discussed. Allen Fossil Plant Unit 3 Intermediate Pressure–Low Pressure Single Flow Turbine Rotor: In 1992 this rotor had a failure of the Curtis Stage. The decision was made to have the rotor welded and change the number of blades. Westinghouse (the OEM) performed the design and weld repair. In 2000 the unit experienced vibration, the unit was disassembled, and the weld repair had failed at the fusion line. An investigation of the failure revealed the weld had not been located at the designed location and the actual location of the weld was in a high stress area. Gallatin Fossil Plant Unit 4 High Pressure Turbine Rotor: TVA designed an upgrade to the Rateau Stage to change from axial entry to a tri-pin attachment. This required the welding of the disk. During welding, with a third party, cracks resulted that extended into the base material. This failed and additional base material was removed before welding. After 8 years of operation the rotor experienced vibration, the unit was inspected, and the weld was found with creep cracks in the heat affected zone (HAZ). An investigation of the failure revealed the expanded weld build-up relocated the HAZ into a high temperature, high stress region of the rotor. Widows Creek Fossil Plant Unit 7 Boiler Feed Pump Turbine Rotor: The rotor experienced stress corrosion cracking (SCC) of the fifth stage during operation in 2003. The rotor was weld repaired and returned to service in 2006. After 5 months of operation the rotor experienced vibration, was inspected, and the welded fifth stage and the sixth stage had experience SCC failures.