Figure 12-19.Enlarged tube.
Figure 12-20.Heat blister on a fire row tube.
HEAT BLISTERS differ from tube en-
largements in that they affect only one side of the
tube, usually the side toward the fires. Blisters
appear as egg-shaped lumps on the fireside. They
indicate that the tube has been heated to the
softening point and has blown out under boiler
pressure. Heat blisters always indicate the
presence of waterside deposits. If the deposit is
brittle, as scale or baked sludge, blistering breaks
the deposit and allows the boiler water to quench
the hot metal before the tube bursts. Heat blisters
are most commonly found on the fire row
generating tubes; they are rarely found on
superheater tubes or economizer tubes. A typical
heat blister is shown in figure 12-20.
SAGGING is the term applied to tubes that
appear to have dropped downward toward the
furnace under their own weight. This type of
deformation results from semiplastic flow of the
tube metal, caused by extremely mild overheating.
A momentary condition of low water is probably
the most common cause of sagging. If the boiler
has been cooled slowly, and if the distortion is not
so severe as to interfere with the designed flow of
combustion gases, sagged tubes may still be
continued in service.
WARPING is similar to sagging except that
the distortion is haphazard rather than in one
direction. Warping usually occurs as a result of
sudden cooling of the tubes after they have been
overheated. Cooling a boiler too rapidly after a low-
water casualty is a typical cause of warped tubes.
MELTING can occur as a result of a serious
low-water casualty. If the tube temperature
becomes high enough, the tube metal actually
melts and runs down into the furnace. A cluster of
fused tubes that resulted from melting is shown in
figure 12-21. Melting of aluminum economizer
parts can cause tremendous damage to a boiler.
The molten aluminum from overheated economizer
parts reacts so violently with the iron oxide coating
on the steel tubes below that the heat of the
chemical reaction may melt the steel tubes even
though the furnace temperature is not high enough
to melt them.