The THIN-LIPPED RUPTURE, shown in
figure 12-16, is a fairly common tube deformity.
The rupture resembles a burst bubble; the open
lips are uniformly tapered to sharp, knifelike
edges, with no evidence of cracking or irregular
tearing of the metal. True thin-lipped ruptures
occur in economizer tubes, in generating tubes,
and, to a much lesser extent, in superheater tubes.
Ruptures of this type indicate that the flow of
steam or water was not adequate to absorb the
heat to which the tube was exposed; consequently,
the tube metal softened and flowed and then burst.
Thin-lipped ruptures may be caused by a sudden
drop in water level or by tube stoppage from plugs,
tools, and so forth, that were accidentally left in
resemble the thin-lipped ruptures except that the
edges are thick and ragged rather than tapered
and knifelike. Thick-lipped ruptures that occur in
mild steel generating tubes indicate milder and
more prolonged overheating than the overheating
that leads to thin-lipped ruptures. Abnormal firing
rates, momentary low water, flame impingement,
gas laning, and many other causes can produce
mild but prolonged overheating that can
eventually lead to thick-lipped ruptures. A typical
thick-lipped rupture in a generating tube is shown
in figure 12-17.
PERFORATION is the term used to de-scribe
any opening in a tube (other than a crack) that is
NOT associated with tube enlargement. The most
common kind of perforation
Figure 12-16.Thin-lipped rupture in a generating tube
Figure 12-17.Thick-lipped rupture in a generating
Figure 12-18.Thermal crack in a superheater tube.
is probably the pinhole leak. In many cases, the
first evidence of tube failure is a pinhole leak.
CREEP CRACKS, result from prolonged, mild
overheating or repeated short-time over-heating.
Cracks of this type are found most often in alloy
superheater tubes, but they can occur in mild steel
tubes as well. The tube is not usually enlarged
when a thermal crack exists; the cracked wall has
normal thickness, and the break has a dark
crystalline appearance. A typical example is shown
in figure 12-18.
TUBE ENLARGEMENT of the type shown in
figure 12-19 is relatively common in super-heater
tubes but rare in generating tubes. This uniform
enlargement of a portion of the tube is caused by
milder overheating than that which produces
cracks or ruptures. If an enlarged tube is continued
in service, it will almost certainly crack or break.