Flexible shaft
The original steam turbine design included bearing vibration measurement in only one direction. The upgrade’s scope of work added additional, orthogonal vibration measuring capability on all bearings. This new instrumentation also allowed operators to gather shaft average centerline data, which was extremely helpful in diagnosing the new HP turbine’s vibration problems.
Figure 9 illustrates the average centerline of bearing 2 from cold start-up conditions (bottom of the plot) to the relatively hot shutdown conditions (last point to the right). The plot clearly indicates a shift of the rotor centerline toward the right side of the bearing during operation. Additionally, the rotor’s stationary position was displaced, relative to its pre-starting position, by almost 5 mils after stopping, in both vertical and horizontal directions.
The only explanation for this behavior is either actual movement of the shaft center to the right side of the bearing (in the opposite direction of the expected shaft locus) or relative movement of the sensor with respect to the bearing center.
Tracking sensor movement relative to the shaft. Bearing No. 2’s vibration sensors were installed in the pedestal cover at a relatively long distance from the rotor surface. Thermal expansionâcaused movement of the pedestal cover relative to the bearing resulted in the misleading indication that the rotor position at rest, before starting, and after shutting down had changed.
Investigators determined that the induced movement of bearing 2’s pedestal cover did result in the erroneous conclusion that the rotor position at rest was different at cold and hot conditions and that this was not part of the root cause of the vibration increase.
Factoring in the steam turbine valve sequence. Steam is admitted to the HP turbine through eight different nozzles located in the periphery of the HP turbine’s first stage. The nozzles are designed to gradually open during start-up to carefully control steam flow into the turbine’s governing stage. Investigators found that the order in which the eight nozzles are sequenced affects the bearing loading as the direction of the reaction force on the rotor changes (Figure 10).
Whenever nozzle valve 3 opens (at around 60 to 70 MW), a bearing load increase is observed. Bearing loading is clearly reduced after nozzle valve 4 opens at approximately 50% load (about 100 MW). The governor valve sequence related to 25% and 50% steam admission flow corresponds to the lightest load condition of the rotor, and it unloads bearings 1 and 2. A moment is also introduced that affects loading conditions on bearings 1 or 2, depending on the nozzle valve sequence.
Measuring the uneven movement of bearings 2 and 3 thrust pedestals during thermal expansion. The horizontal thermal expansion of the bearing pedestal between the HP and the IP-SFLP turbines was measured by installing dial gauges at the base of the pedestal base plate. Measurements revealed uneven movement of the pedestal referenced to the shaft centerline.
Measurements of the horizontal movement of the pedestal between HP and IP-SFLP turbines revealed uneven thermal growth displacement. This movement induced angular deviation between the rotor and bearings 2 and 3. This deviation caused movement of the rotor at bearing 2, away from the normal shaft centerline position on the right side of the bearing.
Pedestal movement readings at the generator and governor ends of the driveline were significantly different (Figure 11). The relative end-to-end change in pedestal position was a maximum of +6 mils and a minimum of â5 mils. This displacement caused the observed angular deviation of the bearing with respect to the rotor centerline. The lower chart in Figure 11 shows the loading condition of the unit during measurement of the pedestal’s horizontal movement.
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