In this article, a case study of defective rolling element bearing detection will be discussed.
Figure 1 - Bearing Inner Race
The machine train is a 60 Hz,186 kW induction motor driven oil float screw compressor which comprises of 5-lobe male rotor and 4-lobe female rotor for air compression service in a coal fired power plant. The unit is supported by steel frame with associated piping connection inside the enclosure with sound/noise absorption. Vibration measurement and machine condition assessment is performed using portable vibration data collector once a month.
In December 2023, the abnormality in vibration characteristics indicate potential of defective bearing at the compressor drive end (DE) and non-drive end (NDE). Based on vibration data and our experience working on the same/similar machines, it was recommended to replace the bearing.
The highlighted information to take away from this article is to emphasize that although the overall velocity vibration amplitudes did not reach alarm limit according to the ISO 10816 recommended criteria, the rolling elements bearing could be severely damaged as shown in the below figure. Therefore, vibration characteristics at each stage of bearing failure should be carefully reviewed and analyzed before remedy action can be taken.
Figure 2 - Bearing Outer Race
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1. Velocity Amplitudes
Referring to following figures that show trend plot of velocity vibration at compressor DE, at normal condition, the vibration amplitude was about 2.5 mm/s RMS. The trend plots show that the amplitude had been increasing since the last three measurements. The amplitude increased about 10 to 50% from the averaged level.
According to the ISO 10816-3 standard, if the amplitudes are lower than 4.5 mm/s RMS, it is acceptable to continue operate. This vibration amplitude is within Zone A boundary for newly installed and commissioned machinery which can be considered suitable for unrestricted long-term operation.
Figure 3 - Velocity Amplitude Trends
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2. Velocity Spectrum
The spectrum plot of velocity vibration in all directions at compressor DE showed "non-synchronous" peaks at 2.x, 4.x, 7.x, 8.x, and 10.x orders. The source of 4.x order or 133 Hz is not confirmed, it is most likely lobe passing frequency (female rotor speed of 32.5Hz x 4), which has always existed since commissioning and unit installed. However, machine information needs to be confirmed.
Figure 4 - Velocity Spectrum Plots.
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3. Demodulated Acceleration Amplitude
Although the increased velocity amplitudes are within acceptable range, the demodulated acceleration amplitudes increased over 200% as shown in the following figures. This evidence implies abnormal impaction or metal-to-metal contact adjacent to the measurement points. So, the defective bearing problem was suspected. This has to be addressed quickly.
Figure 5 - Demodulated Acceleration Trends.
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4. Demodulated Spectrum
Following figures are spectrum plots from the demodulated acceleration signal in horizontal and axial direction at compressor DE. It shows broad brand frequency domination, implying to poor lubricating condition and the non-synchronous peaks at 1.x, 2.x, 7.x, 8.x, and 10.x. These components imply bearing frequency with high potential of defective bearing problem. As the velocity vibration amplitudes was increasing, significant damage of bearing surface can be expected. This was confirmed as shown in the bearing picture above.
Spectrum data were compared to the historical data recorded from previous compressor overhaul which also supports this hypothesis. The current and historical vibration spectrum plots are shown below.
Figure 6 - Demodulated Acceleration Spectrum Plots on 15 December 2023. The red marks are non-synchronous peaks, potential indication of defective bearing.
Figure 7 - Reference demodulated acceleration spectrum plot, recorded on 1 July 2023, before previous overhauling. Note that it is just five months before recurrent.
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5. Specific Recommendation
To monitor and detect the faulty rolling element bearing problems, following practices are recommended.
Vibration measurement interval should be about one month or less.
Not rely on velocity vibration data only (overall amplitude and spectrum).
Always measure demodulated acceleration amplitude and spectrum data. The setting is based on equipment manual or some reference data from real cases.
Observe "non-synchronous" peaks in the demodulated spectrum plots as the indication of defective bearing. This is very helpful when the bearing numbers or the geometry data are not available.
The broadband frequencies peaks are likely to increase.
Always keep reference data of the similar problem occurred on the same machine or sister units. This data is the most valuable in real data analysis.
Note that this technique might not be able to prevent emergency damage from installation error, miss operation and/or maintenance. And keep in mind that the problem could occur with the recently installed bearing.