Reliability Engineering Snapshot TM

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

 

 

 

Machine Design - Case No. 37: Welding Holidays Cause Leakage Problem

Furnace Stack Sample - Outside View

In terms of welding, "holidays" can kill you in the long run.

This was part of a metallurgical study that concerned the deterioration of a stainless steel gas stack on a scrubber. This sample was sectioned in various places. Pictured on the left and right are the outside and inside of the duct (respectively). When the contractor installed this duct he did not connect the various stack sections with a full penetration weld. Instead, he used an external and an internal band to connect the duct sections together. This is no big deal. The big deal is the inattention to detail. The inside band should be leak tight. That certainly was the intention. However, the welder didn't think so. He was close, and that was good enough for him. Click on the white arrows on the right side picture to see the close ups of the seal welds in these two locations. The welder skipped a couple of spots (known as "holidays") throughout the course of welding this band from the inside. The problem with this little lack of attention is that it allowed corrosive liquid to enter between the band and the duct. This kind of protected environment is conducive to crevice corrosion and concentration cell corrosion. Repeated start ups and shutdowns concentrate the liquid between the band and the duct. Needless to say, the exterior seam welds were leaking because of this aggravated situation. The full penetration weld shown in the left side picture at the top (click on it for closer detail) was cross sectioned and found to be in excellent condition. Of course there was no way for any corrosive liquid to concentrate around it, or for that matter, stay in place long enough to attack it.

Everyone was nervous that the duct was failing and needed to be replaced. That was not to be. A simple understanding of the failure mechanism and metallurgical proof that the material was in good condition was enough to thwart the doomsdayers. The band continues to leak, but nobody is as nervous about it as they use to be. A proper repair would cost more than the inconvenience caused by the nuisance weld repairs.

Furnace Stack Sample - Inside View

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Close Up Of Fabrication Weld Lack Of Fusion Lack Of Fusion