TGM believes that forced outages can be avoided with proper maintenance and periodic assessments performed during a short outage. Unfortunately, we see all too many examples of too few inspections and too little maintenance.
Here’s an example from one of our recent projects. The picture below is a gearbox bearing on a line shaft gearbox that had not been inspected for several years. One bearing is on one side of the bull gear and there is another just like it on the other side. As you can see, the bearing is fully wiped – it is a wonder it is still functioning. If the bearing had failed, the entire gear set would have collapsed, necessitating a compete replacement. The gear set is expensive but the real loss would be the substantial downtime for the plant as a new gear set is manufactured.
An investigation of the cause revealed very dirty oil and water in the oil but the root cause was alignment issues. We found improper spacing between the drive shaft couplings which put stress on this combination thrust and radial bearing. The increased heat from this stress led to oil degradation, made worse by the water contamination. Consequently, poor lubrication caused the bearing to wipe. Alignment issues can also be detected through vibration changes or abnormal wear patterns on the complete set of bearings. These “running assessments” are crucial to predicting problems before they cause serious damage.
It is relatively inexpensive to inspect the bearings every two years and damaged bearings can usually be replaced during the outage window. However, an incident such as overheating, abnormal vibration, or water ingress is evidence of potentially serious problems and must be addressed immediately. Any extra effort required to keep the oil clean and relatively water-free will also signal the need for an early inspection/overhaul. (See also our past Turbine Tips for maintaining the Lube Oil system and also note that re-Babbitted bearings should be UT inspected for proper bonding of the Babbitt to the shell.)