We were engaged to perform vibration measurement and analysis of a 260 MW Francis turbine and generator and recommended a solution to mitigate an excessive vibration problem.
The subjected unit was put into commercial operation in early 2022 and was reported to have high shaft vibration (> 800 µmpp) at the turbine guide bearing (TGB) during partial load operation of 150-170 MW since late 2022. The OEM-supplied monitoring and diagnosis system revealed the dominant frequency at 1 Hz (0.3X turbine speed). Hence, the draft tube instability due to operating the unit in the rough load zone (RLZ) was suspected.
Machine description
The machine train is a vertical Francis turbine rigidly coupled with the 32-pole generator with a nameplate output of 260 MW and an operating speed of 187.5 rpm. The turbine and generator rotors are radially supported by an upper guide bearing (UGB), lower combined bearing (LCB), and turbine guide bearing (TGB) from top to bottom, respectively. The rotor is supported axially by a thrust bearing at LCB. All radial and thrust bearings are tiling pad hydrodynamics bearings.
The unit is well instrumented with X-Y proximity probes at UGB, LCB, and TGB for relative shaft vibration. In addition, velocity sensors are attached to each bearing housing to measure casing vibration and axial vibration at LCB. An inductive proximity switch keyphaser is installed just above the X probe at TGB.
Vibration data discussion
The on-site vibration measurement showed the maximum shaft vibration amplitude of almost 900 µmpp occurred during 150 MW operation at TGB and was drastically reduced as the MW was raised to 170 MW and above as shown in Figure 1. The full waterfall spectrum at TGB in Figure 2 reveals the purely forwarded 0.94 Hz (0.3X turbine speed) dominant frequency caused by draft tube instability due to unavoidable partial load operation. Figure 3 indicates an excessive shaft centerline movement at TGB during startup at TGB. In addition, an elliptical synchronous (1X) orbit, which is considered abnormal for a vertical machine with a tilting pad journal bearing (TPJB), was observed. Hence, the possible root cause of the vibration problem could be over-clearance at the TGB.
Figure 1: Amplitude trend of direct shaft vibration
Figure 2: Full waterfall plot of TGB
Figure 3: TGB’s shaft orbit at 150 MW and shaft centerline during startup and normal operation
Remedy action and result
The TGB was inspected by OEM. The pads’ clearances in the up/downstream direction were found over specification and adjusted accordingly. The comparison of amplitude trends, shaft centerline during startup and normal operation, and shaft orbit at 150 MW, where the maximum vibration amplitude due to operation in RLZ occurred, are presented in Figure 4 to Figure 6, respectively. It can be seen that the vibration amplitudes and shaft centerline movement at TGB were significantly reduced with the synchronous (1X) orbit becoming more circular.
Figure 4: Comparison of amplitude trends before and after TGB gap adjustment
Figure 5: Comparison of shaft centerline during startup and normal operation
Figure 6: Comparison of TGB’s shaft orbit at 150 MW
Tags: #hydroturbine, #drafttube instability, #rough load zone, #vortex rope, #Francis turbine, #machinosis
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