Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/388231
www.machinerylubrication.com | September - October 2014 | 47 after a general upward trend in temperature beginning almost five days before the failure. The temperature spike resulted in a sudden increase in vibration and elevated bearing temperatures on both ends of the fan. The lubricating oil level in the system reservoir had also dropped to very low levels. Figure 9 shows the lubrication cooling pump used to move oil through a radiator attached to the fan inlet. A seal on the pump had failed, and oil was leaking out during operation. The location of the pump caused the leaking oil to be sucked into the fan. There- fore, routine visual inspections by operating personnel did not detect a pool forming. This allowed the oil reservoir level to drop at a steady rate with no visual cues of a problem. The failure occurred during an operating period with record cold temperatures. The unit needed to be available for full-load operation and had to run regardless of the suspected bearing condition. In order to accomplish this, the reservoir was refilled with new oil. Lube oil filtration carts were placed directly on both bearing drains and monitored continuously. To determine if the bearing condition was stable or deterio- rating, lube oil contamination parameters were monitored and tracked regularly, as shown in Figure 12. This allowed the unit to operate until after the severe weather had passed. Once the unit was taken down for repair and the top half of the bearing removed, the damage was evident, as seen in Figure 11. The Babbitt material worked as designed, melting in the absence of lubrication to protect the shaft. After oil was restored to the bearing, the Babbitt cooled and reformed, enabling the machine to continue to operate until an outage could be scheduled. The bear- ings were replaced during a brief scheduled outage, and the machine returned to normal operation. References Leader, M. "Understanding Journal Bearings" Wikipedia Contributors. "Babbitt (metal)." Wikipedia, The Free Encyclopedia. http://en.wikipedia.org/wiki/Babbitt_(metal) ML FIGuRE 8. A large centrifugal fan FIGuRE 9. A lube oil pump and radiator (note the dark area where oil and dirt collected during operation) FIGuRE 10. A leaking pump seal FIGuRE 11. Previously melted Babbitt reformed after restoring lubrication Failed Pump Seal 0 20 40 60 80 100 120 140 160 180 200 Delta T Ambient Ambient Vibration Oil ISO Temperature FIGuRE 12. Lube oil contamination trend plot for a fan bearing