FIRESIDE TUBE DEPOSITS can produce many of the scars and deformities just described. Basically, tube deposits cause tube failure because they lead to localized overheating of the tube

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 tube failure.

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 seawater.

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 rather layered appearance; it resembles corrosion products and is often wrongly called scale.

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