Figure 12-21.Melted cluster of tubes.
Figure 12-22.Lamination of a tube wall (fabrication detect).
MECHANICAL FATIGUE CRACKS occa-
sionally occur in boiler tubes from such purely
mechanical processes as flexing. Cracks of this type
can usually be identified by a clean, bright break
through a major portion of the metal thickness.
These cracks begin on the outside circumference of
TUBE WALL LAMINATION is shown in
figure 12-22. This lamination or layering occurs
during the fabrication of the tube. It is the most
common material defect found in boiler tubes.
FOLDED or UPSET TUBES are a result of
defective fabrication. A folded tube is shown in
figure 12-23. This defect resembles a heat blister in
appearance, but the folded tube shows no wall
thinning and has a depression on the side of the
tube opposite the bulge.
STRETCHED or NECKED TUBES are also a
result of defective fabrication. A stretched or
necked tube is shown in figure 12-24.
FIRESIDE TUBE DEPOSITS can produce
many of the scars and deformities just described.
Basically, tube deposits cause tube failure because
they lead to localized overheating of the tube