O’Donnell Consulting was asked to perform an evaluation of a hydroelectric turbine shaft. The shaft had been removed and sent to an outside shop for refurbishment and installation of two new sleeves (upper and lower). Initial machining in the area of the Turbine Guide Bearing (lower sleeve position) uncovered a “knurled” surface just below the outermost shaft material – possibly indicative of a spray or weld metal build-up type of repair.
Non-destructive testing (NDE) including Magnetic Particle (MT), Ultrasonic (UT) and Phased Array Ultrasonic (PA-UT) was performed on the shaft in the area of the planned lower sleeve installation. Further investigation determined that by removing an additional 0.7 – 0.8” of metal from the shaft, the “near surface” indications could be removed from the shaft. The deeper indications are believed to be non-metallic inclusions which probably resulted from the original fabrication (forging) process and, as such, have likely been unchanged in over fifty years of service.
After weighing the NDE evidence and determining the material of the shaft, we determined that the recommended course of action was to rebuild the shaft using a weld overlay method called Submerged Arc Welding (SAW). This is a mature, proven technology which has been employed frequently to repair both high speed rotors and low speed shafts. Part of this recommendation included further machining as described above to clean up the “near surface” inclusions prior to the weld overlay. In addition to supervising the SAW procedure, we performed a fracture mechanics analysis to determine the expected life of the shaft.