High Axial vibration.

We have steam turbine generating set 12 MW, 3 Ph, 11kV, 0.8 pf(lag), 3000 rpm. Generator is coupled to steam turbine through gear box. Turbine rated speed is 7029 rpm which is coupled to 3000 rpm generator through gear box. The set has been installed and commissioned in 2008 and since then it is operating without any major problem.

Recently from last one year turbine was not able to load 12 MW due to gland sealing leakage on the turbine side and it was running at 9.0 MW. However there was no issue on the generator / electrical side. Last November’2015 we have taken up turbine rectification work. After turbine box up there was a mis-match in the turbine-gear box alignment. Accordingly we did the re-alignment between turbine and gear box. Subsequently it was necessary to do re-alignment between gear box and generator also with reference to turbine side.

We have taken trail run immediately after above work. TG set was running normal at rated speed and no-load. Shaft vibration and housing vibration were found normal.

Vibration in mm/s

TG Running Status Pedestal Bearing Horizontal Vertical Axial
Rated speed 3000 rpm Gen DE 0.50 0.35 0.22
Gen NDE 1.27 0.38 0.66

We noticed rotor earth fault problem during voltage build us. Rotating diodes found defective and same have been replaced. Subsequently we could get rated voltage on the generator terminals.

Rotor earth fault problem solved. TG trail running taken again. Generator reached rated speed and vibrations were normal. Generator voltage build up started and noticed bearing axial vibration increasing.

Vibration in mm/s

TG Running Status Pedestal Bearing Horizontal Vertical Axial
Rated speed 3000 rpm Gen DE 0.50 0.35 0.22
Gen NDE 1.27 0.38 0.66
3000 rpm & 6 kV Gen DE 1.32 0.24 2.65
Gen NDE 1.54 0.41 3.12
3000 rpm & 9 kV Gen DE 1.56 0.41 3.23
Gen NDE 1.74 0.61 5.56
3000 rpm & 11 kV Gen DE 1.65 0.41 4.56
Gen NDE 1.67 0.61 9.14

We tried on-line balancing (in-situ) on generator rotor. But there was no improvement in the vibration.

With all the above we feel that there may be an electrical influence on the pedestal axial vibration.