Figure 12-23.Tube fold (fabrication defect).
Figure 12-24.Stretched or necked tube (fabrication defect).
metal. The accurate identification of tube deposits is
often a necessary part of determining the cause of
FIRESIDE TUBE DEPOSITS include soot, slag,
corrosion products, and high-temperature oxide.
SOOT is a broad term used to cover all of the
ash products (other than slag) that result from
combustion. These ash products include carbon,
sand, salts such as sodium sulfate, and other
materials. Soot deposits are usually powdery or ashy
on the tube surfaces near the top of the boiler; but
they tend to be packed solid on drums, headers, and
the lower ends of the tubes.
SLAG is not a powdery or packed ashlike soot;
rather, it is a saltlike material that is fused to the
tube surfaces. Slag is objectionable on boiler tubes
because it retards the transfer of heat to the tube
metal and because it may cause gas channeling, with
consequent local overheating of tube metal that is
not covered by the slag. Most slags on boiler tubes
are soluble enough to be controlled by periodic
washing of firesides. The main way to prevent slag is
to avoid burning fuel oil that is contaminated with
CORROSION DEPOSITS seldom form major
fireside deposits. Occasionally, however, bulky
deposits of ferrous sulfate may form as the result of
the combination of soot and large amounts of water.
These deposits have been known to travel away from
their original location and adhere to remote rows of
generating tubes. The deposits can usually be
removed by water washing and mechanical cleaning.
The source of the water leakage should be found and
corrected. Also, the location of the original deposit
should be found, and the area should be carefully
inspected for signs of corrosion.
HIGH-TEMPERATURE OXIDE is the term
applied to heavy fireside layers of mixed iron oxides
formed by overheating of the tube metal. Low water
is a frequent cause of high-temperature oxide on the
tube firesides. The high-temperature oxide has a