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Shell fatigue


In considering the fatigue life of our Kilns it has been suggested that both the circumferential and longitudinal welds on the kiln shell are the weakest points and are expected to fail after a pre determined time. From my information [theOEM] has not supported this. Do you have examples or witnessed any kilns where these welds have failed as a result of reaching their design fatigue life and not due to other contributing factors such as refractory failure, mechanical damage or other catastrophic failures?

That circumferential welds are the weakest points on the kiln in terms of bending strength is a fact demonstrable per the stress analysis performed during each kiln's design stage used to determine shell plate thickness and the points at which thicknesses change. The idea that these points are expected to fail after a pre determined time is not something we've heard stated before. We do talk about this bending stress undergoing 100% reversal with each turn of the shell and fatigue taking its toll in the long run. For that reason the OEM's have drawn from experience and limit the shell bending stress to 1,500psi in the hot end and 2000psi elsewhere (typical values; some manufacturers vary from this but not significantly). Given that mild steel has yield strength of about 36,000psi this gives a factor of safety ranging from 24:1 to 18:1. That is their way of managing long term fatigue; i.e. high safety factors. Many kilns never fail or even come close to failing over the very long haul. We think it is safe to opine that most likely it takes more than simple fatigue; things like manufacturing/installation defects, unusual loading conditions, changing to higher speeds, higher than expected shell temperatures, bad alignment and excessive shell flexing etc. etc. to cause failures. There are plenty of examples to draw from here as well. Given the many failures we do see it is difficult if not impossible to say which of these many factors including fatigue, was the chief culprit. But obviously we see more problems with older kilns rather than new ones so we must say the older the kiln the higher the influence of fatigue contributing to failure. If failure is to happen then it is natural to say that the weakest point on the shell will be the likely place for it to happen. This in no ways means it is expected or will happen at a predetermined time.



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