Keywords: Failure Analysis; Fatigue Analysis; Finite Element Analysis; Design Recommendations; Tumbler; Shaft; Metallurgical Investigations
We performed a failure analysis and made design recommendations on the shaft of a tumbler operating at a chemical plant. Its 6-inch diameter shaft had failed during normal operation – causing the tumbler to drop out of position.
Stress and fatigue analysis was performed to determine the cause of the shaft failure. A three-dimensional finite element model was created, including detailed representation of the drive and the support shafts, the tumbler front and back panels, and the tumbler’s main body. The tumbler components were simulated with enough modeling detail to accurately account for weight distribution effects on the shaft to tumbler weldment, where failure occurred. ANSYS was used to construct the model and perform the analyses.
The following loads were applied to the tumbler:
1. Tumbler empty weight and counter weight.
2. Process weight assuming the tumbler being full 80%.
3. Increase in the temperature slowly from room temperature to 210°F.
4. Internal pressure of 10 psi.
Stress analysis results indicate the highest tensile stress of 17,585 psi on the bottom of the driving shaft where it enters the tumbler front panel (Point A). The highest compression stress of 16,494 psi occurs when Point A rotates 180 degrees. This location is the most vulnerable to fatigue failure. Based on the photographs presented by the inspection company, the failed section was in the middle of the curved filet and closer to the weld connecting the shaft to the front plate. The redesign of this connection should be conducted in order to improve the reliability, safety, and durability of the tumbler.
Another high stressed area is at the circumferential seam between the cylindrical and lower conical portions of the tumbler shell. In addition, the stress is increased due to the prying action imposed by the shaft on the front plate. This area of the tumbler shell should be redesigned based on the results of metallurgical examinations of the welded patch and the extent and the nature of the damage behind it. Further metallurgical examinations of other cracked areas are recommended to determine the cause of these cracks.
We perform Failure Analysis on equipment for clients in industries including manufacturing, petrochemical and energy.