tube ends between the headers and the seal plates, water drum ends of generating tubes, and return bends in economizer tubes. General fireside thinning of a generating tube is shown in figure 12-11.

A rather unusual type of general fireside metal loss sometimes results from the combination of extremely high tube temperatures and the burning of fuel oil that contains vanadium compounds. The vanadium compounds carried in the flame can cause rapid oxidation of metal at high temperatures. This type of damage is unusual in water-cooled parts of the boiler, since critical temperatures are not usually attained. Figure 12-12 shows a stainless steel superheater tube that has suffered this type of general thinning as a result of fuel ash damage.

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.