Excessive vibration problem was detected on commissioning of two new Francis turbine driven hydrogenators of 5MW and 750RPM.

Both units experienced the same problem, with critical speed below the nominal speed, precisely at 600RPM. Vibration velocities measured on generator bearing exceeded 20mm/s RMS during the critical speed operation. In nominal operation vibration velocities were at acceptable levels of 2-3mm/s RMS.

Before the measurements both OEM and end user unsuccessfully tried several different approaches to finding the root cause and solving the problem.

The thorough identification was finally conducted, involving multichannel simultaneous and continuous measurements that included several different measurement configurations. The conclusion was that significant irregularities in concrete foundations were present, which caused machine critical speed to be at 10Hz and machine was working in resonance.

Finally the solution was proposed that included re-firming the concrete foundations, which resulted in shifting the critical speed between 925-950RPM.

Both units were repaired and put in operation that allows full availability at all operating conditions.