PROBLEM STATEMENT:
This machine was experiencing Elevated Vibration response at 1x RPM. The dominant response was in the Vertical Direction on the outboard bearing support.
PRELIMINARY ASSESSMENT:
- Unbalance
- Coupling Alignment Error (Vertical Offset)
- Bearing Assembly Alignment Error
- Natural Frequency excited by Pump 1x RPM response.
- Amplified Piping Response
FINAL CONCLUSIONS:
1. Natural Frequency excited by Pump Residual 1x RPM response.
RECOMMENDATIONS / OPTIONS:
- Change Rotor Speed to Avoid Resonance
- Add Mass: Reduce Natural Frequency
- Remove Mass: Increase Natural Frequency
- Add Stiffness: Increase Natural Frequency
- Remove Stiffness: Reduce Natural Frequency
- Add Damping: Reduce Resonance Amplification
OVERALL RESPONSE:
Plumbing Line Modifications
Overall response showed “step” changes in response in the Vertical Vibration levels at the pump outboard bearing housing. The step changes were associated with maintenance outage unit restarts.
Foundation Modifications
Notations indicate significant structural modifications to this unit. There appears to be a shift in amplitude that is associated with each recent change.
SPECTRUM RESPONSE:
Synchronous (1x RPM)
The Spectrum response shown indicates a dominant 1x rotor speed (5,100 RPM / 85 Hz) vertical vibration level at the pump outboard bearing location. Also shown is the Blade Passing Frequency (BPF) at 5x RPM.
Harmonic (BPF)
TIME WAVEFORM RESPONSE:
The Time Waveform shows a dominant synchronous response with a slight higher frequency distortion due to the 5x RPM blade passing frequency.