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The rotary dryer was installed in 1987 as part of a new process. It was over 16 meters (52.5 ft.) in length and 2.1 meters (7 ft.) in diameter. It rotated horizontally on top of 4 rollers and operated 24 hours a day, 345 days a year. Approximately 4 million load cycles were imposed per year. The rotary dryer was a counterflow design with liquid coming in at the cold feed-end while hot air came in at the hot discharge end. Paddles that were affixed to the inside wall tumbled the product into the air (Figure 1: left). These paddles, known as "lifters," were distributed throughout the length of the dryer. The finished granular product exited the hot end of the unit. Within six months of initial installation and start-up, cracks were observed to have developed underneath the cold feed-end tire (Figure 2:lower left). In addition, numerous cracks initiated at the ends of nearly every lifter and traveled along the attachment weld (Figure 3: lower right). In addition, other critical components started failing. However, it was the problems with the cold feed-end tire and numerous lifter cracks that attracted the most attention during the early time frame. The original equipment manufacturer (OEM) was asked to review the actual loading conditions of the dryer. The review indicated that the dryer was overloaded. It appeared that the actual loading conditions were different from the design conditions given the OEM. Since it was a unique process, this could not have been foreseen by either the OEM team or the in-house process design team. |
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In 1989 an in-house team of engineers configured a finite element (FE) model for the reinforcing ring underneath the feed-end tire to solve the cracking problem of the ring. The impact stresses induced by the jolters (Figure 4: lower left) and the induced thermal stresses were reviewed separately. The jolters were a unique piece of equipment with hammers that hit the top of the shell to jar the product loose from the inside wall. Modifications recommended by the study were made to the reinforcing ring (i.e. install stiffener-rings) and to the lifters (i.e. cut relief grooves). No modifications were made to the jolters. These modifications were effective over the next few years. However, the lifter cracking problem gradually reappeared,  and the stiffener-rings started to crack. Immediately after this, a large 120 degree shell crack opened along the circumferential weld on the downstream side of the feed-end tire. This was where the failed stiffener ring was located. Also, pieces of the lifters were breaking off and jamming the equipment further downstream. Worse yet, what were suspected as being shell buckles appeared in two separate locations, one of them being next to the same tire. Repairs began to require an inordinate amount of maintenance manpower and this demand impacted other parts of the plant that also required maintenance resources. By 1997 it became clear that the problems with the dryer were increasing in frequency and becoming serious. Steps needed to be taken to address the failures. It was feared that nothing would be done until the level of maintenance became so high that a new duplicate dryer would be purchased. If the causes for the failures were not identified, there was good reason to believe that the same problems would start again in the new dryer. As it turned out, identification of the design flaw regarding the problems at the feed-end was only one part of the ensuing failure analysis. The failure analysis was soon recognized as a legitimate part of an iterative design procedure.
In early 1997 a decision was made to audit all visible cracking. A digital camera was used to document the cracking and these photographs were distributed to various plant managers via the computer network. However, closer inspection of the pictures would cast doubt on the presumed failure causes of the original design. |
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A DILEMMA WITH THE EXISTING FAILURE MODELS One of the dominant causes of failure was cracking of welds that attached the lifters to the shell. The established theory was that the lifters were failing due to high thermal stresses and jolter shock loads that were imposed upon the attachment welds. Grooves had been cut at the ends of the lifters to relieve both of these stresses, but the cracking continued. Early in the audit, it became evident that there was a distinct and repeatable pattern on the fracture surface of the lifters. The fracture path ran either along the top or bottom toe of the weld, but never through the throat (Figure 5: left). Additionally, the topography of the fracture surface showed chevron markings that indicated the direction of crack propagation. This pattern indicated that the cracks were starting from both sides of the lifter surface and traveling inwards along the weld metal/base metal interface. This type of failure was not expected if the cause was due to thermal stresses. |
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 A second observation on the majority of lifters was that the cracks initiating in the stress relief grooves progressed inwards at 45 degrees from the groove down to the weld (Figure 6: left). This observation did not seem to fit the existing failure theory either. The goal of the failure analysis was to find the common denominator that tied all of the failures together. Each proposed model would have to be tested against each failure. If the jolters were thecause for all of the lifter failures, why were similar failures appearing in areas where there were no jolters? If thermal stress was the cause for all of the lifter failures, then why were similar failures appearing in regions that were essentially cold, and more importantly, why were some of the cracks moving away from the welds and into the shell? Had something of importance been missed in the first failure analysis? Significantly, no samples of failed weldments were examined during the first analysis. It was suspected that somebody did not want to cut holes in the dryer shell.
To be continued............ |
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BACKGROUND