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 the boiler.

Serious THICK-LIPPED RUPTURES 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 describe 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 tube.

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.

THERMAL CRACKS, sometimes called 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.