FIRESIDE BURNING occurs when the rate of heat transfer through the tube wall is so reduced that the metal is overheated. Waterside deposits can cause fireside burning, but most serious fireside burning occurs when a tube becomes steam bound or dry. Figure 12-13 shows the coarse, brittle appearance of tube metal that has suffered fireside burning.

STEAM GOUGING occurs when steam jets out of a hole in an adjacent tube. Steam gouging can be identified by the extremely smooth surface of the cavity, together with the irregular shape of the

Figure 12-11.-General fireside thinning of a generating tube.

Figure 12-12.-General fireside thinning of a stainless steel superheater tube (results of fuel ash damage).

the cavity. As maybe seen in figure 12-14, a steam gouge looks as though the metal has been blasted away and the cavity polished.

TOOL MARKS, such as chisel cuts or hammer scars, can usually be identified without too much trouble. As shown in figure 12-15, tool marks do not resemble corrosion effects in any way.

TUBE DEFORMITIES AND FRACTURES comprise another category of boiler tube damage that covers abnormal bends, blisters, bulges, cracks, warps, sags, and other breaks or distortions. Like the cavities and scars previously discussed, tube deformities and fractures are fairly easy to distinguish by visual observation.

Figure 12-13.-Fireside burning.

Figure 12-14.-Fireside steam gauge.

Figure 12-15.-Fireside tool marks.

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 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 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.

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