Reliability Engineering SnapshotTM

Machine Design - Case No. 54: ASM tech paper excerpt: Finite Element Model of Feed-end Lifter Stress

It was important to verify that there was an interrelationship between the lifters and the shell. The best place to verify this was at the feed end lifters. It was at the intersection between two rows that fractographic and SEM examination revealed a bending fatigue failure. Would the FE model also show such a relationship? Did the lifters play an integral role in the failure? The high-resolution model revealed high shell stress at the intersections between the two rows (Fig. lower left). A typical fracture is shown in the lower right figure. If the lifters didn't have a space between them there would be no problem. It also showed that although the maximum principle tensile stress between the lifters was relatively small at the top of the dryer's rotation, the alternating principle stress was five times greater. In terms of fatigue, this high alternating stress and related strain, was severe.

PROVING THE FINITE ELEMENT THERMAL MODEL

It was crucial that the FE thermal model be as accurate as possible. Therefore, it was important to obtain accurate and representative field data. Fortunately, the rotary dryer's temperature service life was obtained by reviewing the process computer database at four hot locations. It was easy to see exactly what the most severe temperature swings looked like. From 70 individual product runs, a representative time-temperature profile was selected and utilized as input data for the FE thermal calculations.

A conventional heat transfer analysis was conducted on the shell wall at each row of the lifters (not including the lifters). This was done in order to obtain the convection coefficients that would be used in the two-dimensional FE thermal models of the walls with the lifters. The output of each 2-D model was the temperature profile of the shell and the lifter at that particular location along the axis of the dryer. Inside and outside shell temperatures, derived by conventional heat transfer methods on the wall, and the FE method, matched at the shell, remote from any lifter. This validation allowed the FE models to be used to determine the temperature of the lifters. The convection coefficients obtained and used in the FE models were effective convection coefficients that included the effect of product insulating the inside wall. This was because for any given axial location along the dryer, there was some inside surface around the circumference that was in direct contact with the hot gas and some that was completely covered with up to a few feet of product. This complication was readily resolved by recognizing that the rotating dryer allowed averaging the inside surface conditions and the inside convection coefficients derived in the thermal loading procedure.

Prior to running the model of the entire unit, a model was configured for an individual row. This row was in what was believed to be the field of influence for any thermal strains. The model showed a very low temperature difference at the lifter's weld interface (figure: left). Infrared images of the shell at the same location closely paralleled the thermal profiles of the 2-D FE thermal models. With this validation, the temperatures from the 2-D models were then extracted and applied to the 3-D models. To be continued.....