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High Vibration Caused By Hydraulic Unbalance On A 144 MW Francis Turbine Unit
A portable vibration device was used to perform measurements on a 333 rpm, 144 MW, Francis turbine hydro unit. The unit was recently refurbished and the main purpose of these measurements was to identify the root cause of large radial shaft displacements near the turbine guide bearing. It was identified that the vibration levels increased with load and at 120 MW were considered too high. This practically limited the maximum load that this unit could be operated in safely. The vibration levels at maximum load exceeded the bearing clearance which is unacceptable, even for a short period of time.
Using appropriate diagnostic techniques, it was shown that the cause for the unacceptable vibrational state was the hydraulic unbalance of the turbine rotor. The vibrations at the turbine bearing increased continuously with the flow rate because the turbine rotor was asymmetric. Water intake on the turbine produced radial forces on the blades that were not balanced out. The radial force rotated with the rotor causing a vibrational response at 1x rotating frequency which increased with load (flow rate). Additionally, magnetic unbalance of the generator was also present. This was identified with changes in the vibrational behaviour observed at the generator bearings when the excitation was on and the load increased to change the magnetic state in the generator air gap.
In addition, measurements were performed by slowly opening the wicket gate and increasing the rotational speed from nominal up to ~490 rpm. Shaft displacement at the upper and lower guide bearing indicated that there existed behaviour similar to a critical speed effect (high 1x amplitude and phase shift) at ~350 rpm which is very close to the nominal speed. Since the critical speed should be much higher, the only realistic explanation is that
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