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. 126: Thermal Stress of a Restrained System on a Kiln


High thermal transitions that force a restrained system to grow when it cannot freely grow will create very high thermal stresses. So much so that designing a restrained system can be impractical. This is one case where it is better to go with the flow and allow the system to grow freely.
Infrared Scan of Dryer Hot-end
Figure 1 Figure 2
Figure 1 shows the hot end of a rotary dryer. The unit does not have fire brick in it. It is made of an alloy material. The unit turns on what is called a "tire." Filler blocks are placed inbetween the shell and the inner diameter of the tire. The filler blocks were welded to the shell's reinforcing ring (see Figure 3 for detail). It was a continuous weld that went all the way around each of 32 filler blocks. Figure 2 shows the thermal infrared image of the dryer during operation. It is obvious from the thermal image that although the shell and reinforcing ring (see Figure 3 for detail) grew uniformly, the filler blocks did not grow uniformly with the reinforcing ring. There was about a 50° to 100° F. difference between the filler block and the reinforcing ring. This thermal difference caused the filler blocks to start cracking immediately after being put into service. Within a few years the cracking had completely freed the filler blocks from the reinforcing ring. The blocks were held in place by the remaining weld.
Figure 3 Figure 4
The thermal stress took care of itself by completely breaking the filler blocks free. This wasn't the best way to run, but Production people somehow don't understand the phrase "I wouldn't do that if I were you." Instead ten years later, yes, ten years later I found a crack in the reinforcing ring, Figure 3. I was able to say "YOU'RE SCREWED!" Needless to say, the unit stayed down until I could repair it. Chasing the crack, as shown in Figure 4, I found that the crack had started at the corner of the filler block and traveled underneath the tire and also into the shell. Investigation with dye-penetrant revealed that the crack had compounded and split at two separate locations. Trying to isolate the single crack from the compound crack did not go well. Figure 5 shows where the hole was drilled to isolate the crack on the right side which we were going to repair. The compound crack to the left of the hole would be left for later when we could take an extended outage. However, the crack to the right of the drill hole in Figure 5 had a compound crack of its own. When checked from the inside of the dryer, that same crack can be seen as branching to the left of the drill hole, Figure 6, and then extending underneath the filler block.
Figure 5 Figure 6
The crack on the outside of the dryer ran underneath the tire along the edge of the filler block. Figure 6 shows a different story. This repair is going to be a mess. I guess we need to rephrase that old saying "Pay me now or pay me later." In the industrial maintenance business the phrase should go something like this:

"Pay me now .... or pay me later ... with interest."

The repair will be showcased in an upcoming article of MACHINE DESIGN.


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