QUESTIONING THE JOLTER THEORY
Experience with three similar dryers within the corporation revealed that cracking always initiated at the end of the lifter and propagated into the shell (Figure 1: left). The inspection reports went on to indicate that there was poor welding technique in all of the failed welds. There was additional ultrasonic test information that indicated those welds without visible cracks did in fact contain cracks. Still, the early assumption was that the jolters were the cause for lifter cracking underneath the jolter area. However, during this failure analysis the actual service exposure of the lifters in the region of the jolters was calculated. Results of the calculation indicated that there was some 90 minutes between impacts on the same lifter, or 16 impacts per day. The jolters were blamed for lifter cracking 12 months after initial start-up, which corresponded to 5,840 impacts. This number was considered too low to initiate fatigue cracking (Ref 1). This suggested that vibration waveform analysis be utilized to determine more information regarding the details of the load caused by the hammer blows.
QUESTIONING THE THERMAL STRESS THEORY
|The initial investigators assumed that all lifters were cracking due to thermal stresses. However, no infrared imaging had been done to support this assumption. Subsequent inspection of the dryer's discharge-end during this analysis revealed that the lifters had creep yielded (Figure 2: left). Further investigation lead to questions. Normal design values for alternating thermal stress was on the order of 40,000 cycles for a 20-year design life (Ref 2). If the rotary dryer was operated in the worst possible manner having an inconceivable four thermal shocks per unit start-up, the dryer would have experienced only 3,300 cycles by 1999. The thermal stress failure model was initially proposed in 1989, only 12 months after lifter cracking was first detected. This corresponded to 350 thermal stress cycles; this too, was suspiciously low. If thermal stress was the cause for failure then the two hottest rows of lifters should have been cracked, and they were not cracked. Were the relief grooves effective in only this region? Was there a significant temperature difference between the lifters and the shell to induce a stress? Infrared thermography of the dryer shell (Figure 3: right) did not show a large thermal differential along the weld profile; the worst case was 400 C (104 F). The result placed the thermal stress model in question.
To be continued.....
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