Fluid film bearings are mainly monitored with proximity probes. It is often stated that “you can’t detect early defects in fluid film bearings with normal vibration techniques (Velocity, Acceleration or a bearing condition unit)”. But in fact you can detect the effects of a fluid film bearing deteriorating with a normal accelerometer.
Under abnormal circumstances metal to metal contact might occur, leading to occasional high-frequency noise that can be detected with normal vibration equipment. The following case study is a great example of this and also using lubrication analysis as part of a maintenance program.
This case history covers a Production facility Extraction Fan which has been monitored as part of a site wide Condition Based Monitoring program. The drive of this program is to integrate condition monitoring techniques and to drive the maintenance program.
This fan unit had a motor that is a direct drive to a fan shaft, the fan shaft has a white metal fluid film bearing. The fan is a standard overhung centrifugal of about 2.5 meters diameter.
We flagged this reliability risk very early in the program for having a considerable 1 Order impact present in the PeakVue™ data (see Figure 1).
Figure 1 shows the initial Vibration data collected on the fan bearing with the 1 Order event present.
Vibration Trend Analysis:
Figure 2 is the PeakVue™ Trend of the bearing as it was nursed through until maintenance could be conducted.
This showed the initial level, reduction in levels after an oil flush, then a period of monitoring until another oil change and a bearing inspection.
As the initial fan data had a dominant 1 order event and was being monitored on a monthly basis, we needed to determine whether the event was consistent or deteriorating, and what possible causes were.
Oil samples were then taken (see image 1), on visual inspection the oil was in a very poor contaminated condition. The lab report (see image 2) stated a serious concern with particulate matter contamination and high levels of Sodium, Iron, and Tin.
The indications of Tin suggested probable sleeve (Babbitt) wear of the bearing.
Image 1 is the Fan DE bearing oil sample:
Image 2 shows the initial oil sample report and diagnosis with emphasis on the high Tin levels:
This fan was a critical component of the facilities production process, however with a plant upgrade planned for the very near future, the decision was made to closely monitor the deterioration of the assembly rather than to rectify this potentially expensive piece of equipment.
The temporary measure of an oil flush and change was conducted immediately, with a visual inspection planned for the next shutdown.
During the shutdown the bearing housing was split and the bearing shells separated. The damage to the bearing was very extensive with ‘scalloping’ of the Babbitt material evident in the direction of rotation forming a build-up at the end of the lower shell. The cause of this is most likely the failure of the lubricants ability to sustain an adequate oil wedge between the shaft and the bearing.
Image 3 shows the Fan DE white metal bearing upon inspection:
Image 4 shows the Lower half shell of the white metal bearing with a piece of ‘free floating’ Babbitt that was found in the sump:
In conclusion, with the company’s utilisation of all the available Condition Monitoring technologies and tools, they were able to monitor and be consistently and accurately informed of the state of deterioration of the bearing. This allowed them to implement a rolling program of temporary measures to stave off what was essentially an unserviceable critical machine until the Factory upgrade was conducted.
A reliable plant is a safe plant
…..an environmentally sound plant
….. a profitable plant
……a cost-effective plant
7th August 2018: Additional data as requested by JUAN CARLOS URQUIOLA
Below is the Velocity data from September 2017 and June 2018, there is a difference in the number of an amplitude of the 1 order as associated harmonics.
Below is the Acceleration high frequency spectrum, this shows two mounds of activity that has side bands of 1 order and 2 orders.