Inspired solution
The stability margin of the replacement rotor-bearing system was analyzed throughout the entire range of loading conditions. Particular emphasis was placed at 25% load, where the rotor bearing system has the lowest loading. Bearing metal temperature data were also collected during the turbine tests and revealed additional clues to the root cause of turbine vibration (Figure 12). The metal temperature “mirrors” the changes in calculated bearing pressure changes induced by the governor valve sequencing. This is clearly confirmed by a comparison of the trends in Figures 10 and 12.
The subsynchronous vibration experienced by this plant for many years before the upgrade project had been sporadic and insignificant enough to have no appreciable impact on total measured vibration. After all, if a variable isn’t measured, trend analysis isn’t possible. This “dormant” unstable condition was theorized to be the cause of the sudden increase in the subsynchronous component due to an unknown and unexpected excitation. In this particular case, investigators determined that the rubbing condition (illustrated in Figure 6) was the excitation source causing the sudden increase in subsynchronous, or oil whip instability, vibration.
But what caused the original rubbing, especially given that the turbine was initially started and loaded without any evidence of rubbing? Evidence of the rubbing was clearly seen in a photo of the labyrinth seals taken after excessive vibration was observed during the November 20 coast-down (Figure 13).
The solution to both observed problems was to increase the stability margin of the rotor-bearing system by modifying the bearing geometry. This one modification increased the stability margin of the HP rotor-bearing system under rubbing conditions and nozzle valve bearing-loading conditions.
The original HP bearings were of the “partial center slot” type with a 0.96-inch slot in the lower half. The HP bearings were modified by increasing the slot width by 0.61 inch—to 1.5 inch—in order to increase the HP rotor bearing system’s stability margin.
Modification of the bearings—including their removal, preparation of the drawing with the modified geometry, machining of the lower half, and reinstallation—was performed in less than one week. The unit was restarted on December 2 and demonstrated a clear reduction in subsynchronous vibration, which enabled the unit to return to commercial operation.
—Craig C. Jennings (craig.jennings@exeloncorp.com) is a senior rotating equipment engineer for Exelon Power.
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