tube ends between the headers and the seal plates,
water drum ends of generating tubes, and return
bends in economizer tubes. General fireside thin-
ning 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 burn-ing
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
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
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.