Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non-destructive Evaluation




Machine Design - Case Study No. 50: ASM Tech Paper Excerpt - Establishing Credibility in the Finite Element Model

Refined Model of Rotary Dryer Feed End TireIt was important that the FE model be a valid representation of the stresses occurring within the rotary dryer. The model would be valuable in evaluating whether or not the dryer could be modified in the field to remedy the chronic failures, or whether replacement was justified. If replacement were required, then the model would prove valuable in assessing the capabilities of potential manufacturers by specifically querying them on design weaknesses found in the model. The lead author felt that in order for the model to be valid the dryer needed to have the lifters represented in the model. The industry normally omits the lifters to simplify the model and they accept the assumption that the lifters contribute very little in terms of structural interaction with the shell. As was to be seen from the model. This was not the case. However seemingly small in magnitude, the lifters did in fact carry a structural load (Figure 1: left). They acted like structural angle iron supports.

Moderately Refined Finite Element Model of Rotary DryerThe size and complexity of the dryer, the unknown mechanical and thermal loadings, and the numerous cracks whose existence, location and direction had to be predicted by the model should have made it obvious that the task was impossible. However, the lead author's optimism apparently was contagious. Most frequently, a large structure has only one or a few areas of interest and the FE model mesh is coarse in all but these areas of interest. However, the dryer was cracking in numerous locations so that the large structure had to be modeled in some detail in its entirety (Figure 2: left). The second author's approach was to use a moderately refined full model, identify the most appropriate locations for high detail, and then insert those sections of high detail into the full model. The finite element analysis consisted of a family of 34 different FE models (e.g. Figure 1: above left), many of which had numerous submodels. The largest single model contained over 50,000 elements and required over 250,000 equations to be solved simultaneously.

A steady state material balance, and visual observations of product flow through the unit during the typical start-up, allowed a transient material balance to be estimated. The product weight distribution along the axis and the total product weight were derived from this transient material balance. The distribution of product weight around the circumference at each axial location was estimated from the visual observations during start-up. With this data, 1,450 vertical forces of appropriate magnitude and location were applied to the FE models to simulate product loads.

VALIDATION Elevation View of Shell underneath Feed-End Tire

If any location was well documented with failures, it was the feed end tire. The tire had been a continual maintenance problem since initial installation in 1987. Axial cracking occurred underneath the tire in the thicker reinforcing ring. The first finite element model results obtained in 1989 addressed this cracking, but the global model was never configured to obtain other results. The lead author gave a general description of the problems experienced on the dryer to the engineers at Applied Reliability who were to do the finite element analysis. They were not provided information regarding observed cracking. If the model were correct, it would have to show that there was a problem on the downstream side of the feed end tire. The model of the shell underneath the tire did in fact show that there was a highly stressed region located along the circumferential weld opposite the feed end side of the tire (Figure 3:left, circled). This weld had failed 120 degrees around the circumference. In addition, it showed that the lifters were actually loaded in the spill region (Figure 1: top left). The stress pattern on the lifters was such that an overload would tend to cause these lifters to bow in a manner similar to what had been observed in the field (Figure 4: lower right). The model was valid.Internal View of Rotary dryer Shell and Lifter Buckle

To be continued........

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