I assume here you do not have intercooling.

It seems to me that you should have replaced the complete set of stages/bearings not only the 3rd stage. Because if the flow condition is offset (due to gas dirtiness / deposit or/and wear and tear etc.) the 3rd stage will be operating the most off design due to flow "stacking" effect across the impeller stages (especially if you do not have IGV's or some other process control method to correct/mitigate that effect). So now that you have set back the 3rd stage to its original design parameters by replacing your3rd stage set [bearing + rotor] you are still standing at same place as far as wear & tear, performance degradation, and even unbalance, of 1st and 2nd stage are concerned.

Going even further in the speculation, the behavior of high vibration trip could also possibly mean that the unbalance that occurred at first place has part of its root cause mostly in 1st / 2nd stage systems.

With such poor info and description - this is again just speculation.

Hi
thanks rotw
actually machine tripped because of 3rd stage vibration and we just replace impeller and bearing of it.
after dismantle the machine we observed 3rd stage impeller in bad shape because of excessive wear. but 1rd and 2rd stage had no problem.
we knew most of failure causes refer to 3rd (high speed) stage experimentally but we have no idea for after maintenance trips.
its even condition monitoring couldn't help us because of unusual trend.

For many of these machines, the pinion and impeller are an assembly that are matched and balanced as an assembly. If you replaced the impeller, but reused the shaft and pinion gear, you may have a poor balance. If your vibration is collected from non-contacting proximity probes (Bently Nevada), you could have high run-out in the probe areas. We experienced a similar problem recently. We found that the vendor had failed to burnish the probe areas and we had high electro-mechanical run-out.

Johnny Pellin

thanks Mr. Pellin
probes area was a good point.
its bearings are 5 shoes tiiting pad and its not show 1x and 90 degree phase (or 30 less or more).
as i knew shaft unbalance observed on previous condition. may it different with different bearing type? tilting pad, 3 lobe, lemon type and etc.