We are having a turbine rotor which during High Speed balancing shows a critical speed range from 1500 RPM to 4000 RPM(actual critical speed 3200. I have read that critical band is +/-20%. But considering the above data it is about 50%.
We do not have earlier records as the rotor is more than 20 years old and gone under several repairs.
I just want to know is it possible that during time course the band has increased. If yes then what can be the reasons
Secondly can multiple repairs be a reason for this increase in band? If yes how to avoid it.