Three units with identical design were inspected in the powerhouse. The units were vertical-shaft Francis- turbine-driven suspended-type with three guide bearings. The generator had laminated rotor rim and 26 field poles. The nameplate rating of generators: 1. Rated Output: 47,900 kVA, 2. Rated Voltage: 13,800 V, 3. Rated Speed: 277 rpm. During the generator inspection, major damages were found on multiple rotor spider support ledges in two units. In the third unit, only one linear indication was found on the support ledge inside corner. Galling was also observed around the support ledges which showed relative movements between rotor rim and rotor spider ledge. As soon as the damages were found, all three units were taken out of service for further investigation. The two units with major damages in support ledges remained out of service since there were serious risk of catastrophic failure. The unit with only a linear indication was inspected with phased array ultrasonic testing. The test results confirmed the existence of a crack and helped to determine the size of the crack. It also confirmed that no other cracks were present in other support ledges of this unit. The support ledge is the part of rotor spider arm that supports the laminated rotor rim. The primary purpose of the support ledge is to carry the rotor rim weight and all components attached to it, such as field poles, brake ring, rotating fans, etc. If all support ledges fail, the rotor rim may drop downwards which will consequently engage with stationary components, such as brakes and lower bracket, at rated speed. This could lead to a catastrophic damage to the unit and powerhouse. The instruction book from the original equipment manufacturer indicated that the two-piece wedge rim keys (the keys transferring the torque from rotor spider to rotor rim) were installed by 'driving', indicating that the rim was not shrunk. In this case, the rim will expand against rotor spider when going from stand still to rated operation. This design is sometimes referred to 'floating rim'. The rim radial expansion is due to centrifugal forces on rotor rim and field poles, electromagnetic forces between field poles and stator, and temperature increase in the rim itself. The sliding of rim on the surface of support ledge results in friction forces on support ledge in radial direction. When moving outwards, this can cause considerable tensile stress at the inside corner of support ledge. When the unit is shut down and the rim cools down, its diameter reduces and the rim slides back on support ledge toward rotor center, causing compressive stress on inside corner of support ledges. Depending on the amount of movement and number of start and stops, the friction between rotor rim and support ledge can cause galling which deteriorates the surface condition and increases the friction factor. With increased friction, the forces and stresses posed to support ledge also increases. Eventually the galling can damage the sliding surfaces to the extent that the support ledge starts to engage with the rim, in which case the large portion of expansion forces are transferred to support ledges. Axial vibrations of rotating parts pose cyclic load on the support ledge. A vibration measurement was performed on one unit and it revealed that in partial load the turbine had a rough zone that was causing excessive axial vibrations. The frequency of the vibrations was analyzed, and it corresponded to the number of thrust bearing shoes. An inspection of thrust runner, which was in two-sections, showed that the segment joints had developed a slight step that was beating the thrust shoes and causing vertical vibrations. The axial loads pose stress fluctuation in the inside corner of support ledges. Due to high number of load changes (once per revolution), this is considered as high cycle loading although with a relatively small stress on the inside corner of suppor
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