The problem of increased vibrations was noticed when two new Francis turbine driven hydro-units (5 MW, 750 rpm) were commissioned.
Both unit’s behavior vibro-dynamical response was identical with a passage through critical speed at exactly 600 rpm (10 Hz). Vibrational velocities measured on the upper generator bearing exceeded 20 mm/s (RMS value) during the critical speed operation and were at acceptable limits 2-3 mm/s during normal operation. However, even during normal operation, the vibration levels intermittently increased and decreased without obvious cause.
Several unsuccessful measurement campaigns, with different approaches, were tried before the measurements presented in this article were conducted.
The full identification was conducted in a couple of steps with simultaneous multi-channel configuration which was proven to be of great importance to successfully diagnose the problem. The main conclusion was that there were significant irregularities in spiral and draft tube foundations, which allowed the unit to vibrate at 10 Hz resonant frequency.
The solution was proposed with to improve the unit foundations. It involved high-pressure concrete injection and addition of dozens of anchoring elements on spiral and diffusor in order to reinforce the bond.
After these actions – the resonant frequency was increased into the ~16 Hz range and both units are able to operate adequately in all available operating regimes and achievable rotational speeds. The critical speed is between the nominal and highest achievable rotational speed following 5 MW load rejection and is also below the theoretical overspeed.