Figure 12-9. - General fireside circumferential grooving.

down the header and onto the tube ends, or as it drips directly onto the tubes. This kind damage is greater on the top of the tube than on the underside, but the groove may extend the entire circumference.

Fireside circumferential grooving may also occur on vertical generating tubes as a result of thin, damp deposits of soot on horizontal drums or headers. In fact, this kind of grooving can occur in any part of the boiler where leakage provides a sufficient supply of water. Large quantities of water trapped between the water drum and the boiler casing - as, for example, from a serious economizer leak - can produce general fireside grooving around the bottom of the rear generating tubes. An example of this general fireside circumferential grooving is shown in figure 12-9.

CRATERS are deep, irregular, straight-walled cavities in the tube metal. WATER TRACKS are closely related to craters; the tracks consist of wandering, straight-walled, canyon-like cavities in the tube metal. Both cratering and water track-ing occur almost exclusively at the header ends of water wall tubes and division wall tubes that are surrounded by refractory; they are caused by water becoming trapped between the tube metal and the surrounding refractory. Water washing of boiler firesides, without proper drying out, is a frequent cause of cratering and water tracking; of however, any leak higher in the boiler can also cause this type of damage. The size of the leak around and the angle of the tube upon which the water leaks determine, to a large extent, whether the resulting damage will be circumferential grooving, cratering, or water tracking. Both cratering and water tracking are shown in figure 12-10. GENERAL FIRESIDE THINNING consists of a uniform loss of metal over a relatively large area on the outside of the tube. Soot corrosion is by far the most common cause of general fireside thinning. The parts that are particularly subject to this kind of damage are superheater

Figure 12-10. - Fireside catering and water tracking.