Hydro units powered by Francis turbines often experience operational zones with significant hydrodynamical and mechanical instabilities. These zones are referred to as the Rough Load Zones (RLZ). Typically, the instabilities occur at 30-50% loads, but under certain conditions they can also be present up to 70% of nominal load.
In the RLZ vortices form in the draft tube below the turbine due to the pressure pulsations forming below the turbine runner. This can lead to large rotor vibrational response, related to operation in the RLZ in the turbine region and in the thrust (axial) bearing region as well. The direct consequences of RLZ operation are often large axial loads which can be measured on the axial (thrust) bearing bracket and generally increased vibration levels in axial and radial directions.
One example of very prominent dynamical instabilities is on a 120 MW, 120 RPM unit where the RLZ extends from ~30 to ~80 MW (25 to 67%). The vibrational response in the axial direction is so large that the rotor vibrational displacements exceeded 2 mm peak-to-peak (vRMS = 1.3 mm/s). This poses a direct threat to the thrust bearing structure and the supporting concrete foundations. Therefore, cracks were found in the axial bearing foundations on this plant. An experimental identification procedure was performed, and suggestions provided to reduce the vibrational response in order to reduce stress levels.
Some additional units (15 to 50 MW rated power) were analyzed during the RLZ operation to compare the vibrational behavior. The main focus was on the analysis of dynamic instabilities and potentially dangerous consequences of RLZ operation. The goal was to present criteria for allowable (temporary) operation in the RLZ and to determine the relationship between the vortex and the machine vibrations.
Key words: Rough Load Zone (RLZ), vortex, draft tube, pressure pulsation, dynamic loads, axial loads