1. Overview
Focused on trip that took place on February 19, around 4:11 PM.
Includes data to help understand the nature of the trip.
Roughly 2 minutes prior to the trip, there is an increase in motor amps and gearbox vibration. This period will be referred to as the “event”.
Vibration data acquired from the Bently Nevada Series Monitoring System.
Motor Voltage/Current data provided by Utility.

2. Plot Sequence Introduction
The following plots indicate overall trend values for the following components:
Motor
Gearbox
Compressor
They are broken down by a broad time trend, then immediately followed by a trend during the ~ 2 minute “event”.
The radial probe plots contain 4 each (X-Y) from the drive and non-drive end.
Description of points are located at the top-right of each plot.

3. Motor Overall Vibration Trend
Onset of “event”

4. Higher Resolution Leading Up to Trip

5. Gearbox Low Speed Vibration Trend
Onset of “event”

6. Higher Resolution Leading Up to Trip

7. Low Speed Axial Thrust
Onset of “event”
Away from thrust brg.

8. Higher Resolution Leading Up to Trip

9. Low Speed RPM
Onset of “event”

10. Higher Resolution Leading Up to Trip

11. Gearbox High Speed Vibration Trend
Onset of “event”

12. Higher Resolution Leading Up to Trip

13. Compressor Vibration Trend
Onset of “event”

14. Higher Resolution Leading Up to Trip

15. High Speed Axial Thrust
Onset of “event”
Away from thrust brg.

16. Higher Resolution Leading Up to Trip

17. High Speed RPM
Onset of “event”

18. Higher Resolution Leading Up to Trip

19. Trend Plot Summary
All points react to some degree; however, the gearbox points indicate significantly more response.
The low speed axial thrust probe indicates significant movement away from the thrust bearing on the bull gear shaft is taking place during the “event”.
The speed of the compressor does decrease by approximately 17 RPM over the “event” period, just prior to trip.
The compressor axial thrust probe indicates some movement is present; although, very little.
Highest overall vibration on the high speed pinion shaft PIN DE X – 635 probe at 3.2 Mils.

20. Closer Look at the PIN DE X – 635 Data
Since this is the probe with the highest level data, more information is presented to better understand why.
Data includes:
Spectral
Time Waveform
Orbits
Shaft Centerlines

21. Spectral Data
5.5 Hz Peak Component ~ 0.69 Mils with additional low frequency noise
1x Operating Speed of the Pinion Shaft / Compressor ~ 0.5 Mils

22. Summary of Spectral Data
1x Operating Speed (250 Hz or CPM) levels are relatively low at 0.5 Mils.
Low level harmonics are present that indicate low level looseness on pinion shaft – some to be expected due to journal bearing design but more likely due to instability as seen during the “event”.
Low frequency noise was not expected and further research determined the source that will be covered in the upcoming slides.

23. Time Waveform Data Prelude
The following data was taken from the high speed pinion shaft probes.
Data taken during a time period when the unit was operating satisfactorily as well as the “event”.
The first time waveform is from only one probe to illustrate the cyclic motion from 1x operating speed.
The second plot is simply the same plot with the other three probes overlaid on top.
The third, fourth and fifth plots illustrates what is happening to these signals during the “event” period.

24. Time Waveform Data – Normal 1 Probe
Steady State Time Waveform Signal that’s Sinusoidal in Nature

25. Time Waveform Data – Normal 4 Probes
Steady State Time Waveform Signal that’s Still Sinusoidal in Nature (all 4 Pinion Shaft signals)

26. Time Waveform Data – “Event” Onset
Early stages of irregular shift in signal as “event” initiates

27. Time Waveform Data – “Event” 4 Probes
1x Still Consistent; however, the entire signal is very inconsistent. This creates the noise in the spectrum.

28. Time Waveform Data – “Event” 4 Probes
Overall Peak to Peak levels in excess of 3.2 Mils.

29. Pinion Time Waveform Summary
Prior to the “event” period, the time waveforms look precisely as they should.
When the “event” period is taking place, the inconsistent shifting of the time waveforms indicate violent movement of the shaft. This is also present on the bull gear shaft, just at slightly lower levels. However, is not present on the motor or compressor.
Again, generates lots of low frequency noise as seen in the spectrum.

30. Pinion Shaft Orbits During “Event”
Filtered 1x Orbits

31. Pinion Shaft Orbits During “Event”
Unfiltered 1x Orbits

32. Pinion Shaft Centerlines During “Event”
Ends here
Ends here
Starts here
Starts here

33. Compressor – DE Pinion Centerlines
Ends here
Starts here
Pinion DE
Compressor DE

34. Orbit and Shaft Centerline Thoughts
1x Filtered Orbits are what you would expect to see. The 1x signal is relatively consistent throughout the “event” period.
The Unfiltered Orbits show the extent of the violent shaft movement taking place during the “event” period.
The shaft centerlines indicate the pinion shaft is high and to the left on the DE and low and to the right on the ODE. This should be consulted with G’Box OEM to ensure where the design intended.
When Pinion shaft drifts during “event”, the compressor shaft does not move. Movement is taking place within the gearbox.

35. Potential Mechanisms
Why is the gearbox shaft so unstable? With the previous data, this leads to either a changing load on the compressor, mechanical damage within the gearbox or potential power quality problem on the motor.
As the gearbox comes back online and operates similarly after each start, a common occurrence is noted.

36. Process Data

37. Process Data Very little fluctuation in process
Only sampling at 1 sample per second
Additional trips seen with lower suction/discharge pressures and flow (ratios similar)
Trips are almost identical each and every time they occur
Units are down for roughly 8 hours before restart

38. Odd Occurrences Just Prior to Trip Cycle
~0.18 Second Impulses

39. That Again Lead Up to This

40. Incoming Power Feed Data
Minor but Amperage Fluctuation Present
Amperage Fluctuation and Overall Increase during “event”

41. Incoming Power Feed Data
Amperage Fluctuation and Increase during “event”

42. Incoming Power Feed Data
High Resolution Shows ~30 Amp Swing

43. Incoming Power Feed Data
That intensifies to ~125 Amp Swing

44. Comparative Amperage Modulation
HC Recycle Compressor
BFW pump
Separator pump

45. What Does This Mean
By taking the amperage and voltage from the provided data, at roughly a 0.18 second interval, an amperage swing is taking place that was recorded as high as to as low as
Rough calculations estimate this as a significant horsepower swing (maintaining a consistent power factor for rough figures). And it’s taking place at a rate of seconds.

46. What Does This Mean
Could this attribute to a jerk reaction within the motor at a rate proportional to the cyclic fluctuation of amperage?
Could a jerk, or rate of change in acceleration, that would be similar to a twitch or surge that would manifest itself through the motor be present? Physically, with the motor shaft “frozen” with a strobe light, the shaft would make sudden twitches by darting slightly ahead, then slightly behind the frozen position…?

47. What Does This Mean
This jerk could cause a momentary loading or unloading of the gearing within the gearbox (causing the shifting of the signals as seen) as well as cause the axial movement on the low speed shaft due to the angular designed teeth on the gears.
The irregular movement could also interfere with the oil wedge within the bearings and allow the shifting of the shaft centerlines and inconsistent unfiltered orbits as well.

48. More Data to Support
Emax (motor current signature analysis) data was acquired on several motors that are fed from the feeder that supplies power to the problem compressor and amperage fluctuations at the 0.18 second period are present with as much as 65 amp fluctuations.

49. Source of Fluctuation
A nearby reciprocating compressor is operating at 5.5 Hz (0.18 second period). There is roughly a 10 amp fluctuation that is expected due to the design of the reciprocating operation (push and pull).
Why would the amperage be so much higher down the line?
What could fluctuations at 0.18 seconds in the current do to the operation of an AC Induction motor?