Cavitation-damage

Cavitation damage is a fonn of deterioration associated with materials in rapidly moving liquid environments, due to collapse of cavities (or vapour bubbles) in the liquid at a solid-liquid interface, in the high-pressure regions of high flow. If the liquid in movement is corrosive towards the metal, the damage of the metal may be greatly increased (cavitation corrosion). 

It is often localized at areas where water changes direction. Cavitation (damage due to the formation and coUapse of bubbles in high velocity turbines, propellers, etc) is a form of erosion corrosion. Its appearance is similar to closely spaced pits, although the surface is usually rough. 

Indirect attack can also occur because of turbulence associated with flow over and around a deposit. Increased turbulence may initiate attack (see Chap. 11, Erosion-Corrosion and Chap. 12, Cavitation Damage ). 

Cavitation may be defined as the instantaneous formation and collapse of vapor bubbles in a liquid subject to rapid, intense localized pressure changes. Cavitation damage refers to the deterioration of a material resulting from its exposure to a cavitating fluid. 

Cavitation damage in the complete absence of corrosion has been demonstrated (e.g., roughening of polished glass in cavitating distilled water). However, in industrial situations it is highly probable that corrosion is a common contributing factor. 

Metal damage due to cavitation can be both rapid (Figs. 12.3A through D) and severe. However, cavitation damage can have a time dependency see Fig. 12.4. An incubation period may be observed... 

Figure 12.4 Schematic representation of typical variation of cavitation damage rate with exposure time. (Reprinted with permission of American Society for Metals from Metals Handbook, vol. 10, 8th ed.. Metals Park, Ohio, 1974, p. 162.)...

In general, cavitation damage can be anticipated wherever an unstable state of fluid flow exists or where substantial pressure changes are encountered. Susceptible locations include sharp discontinuities on metal surfaces, areas where flow direction is suddenly altered (Fig. 12.5), and regions where the cross-sectional areas of the flow passages are changed. 

Figure 12.5 Cavitation damage at bottom of tee at impact site of in-flowing water.

Figure 12.6 Cavitation damage on the discharge side of a regulating valve.

Figure 12.7 Cast iron pump impeller with severe cavitation damage.

Figure 12.8 Cavitation damage repeated on successive elements of a bronze impeller.

Several approaches are available to alleviate or eliminate cavitation damage problems ... 

Weld overlays of stainless steel or cobalt-based wear-resistant and hard-facing alloys such as Stellite may salvage damaged equipment. In addition, weld overlays incorporated into susceptible zones of new equipment may provide cost-effective resistance to cavitation damage. 

Of the various approaches to reducing cavitation damage, redesign of equipment is probably the most successful. The following have proven useful ... 

Figure 12.9 Typical vertical alignment of cavities resulting from cavitation damage in a diesel engine cylinder liner.

Figure 12.10 Jagged, cavernous pits typical of cavitation damage.

The cavitation damage in this spacer was due to vibrations from operation of the engine. The localized nature of the damage in this case is an illustration of a common feature of cavitation. Pits formed by initial cavitation damage become preferred sites for the development of subsequent cavitation bubble formation due to the jagged, irregular contours of the pit. This tends to localize and intensify the cavitation process, especially in later stages of pit development. 

Figure 12.11 Zones of cavitation damage segregated to both sides of a vertical opening.

Figure 12.13 illustrates severe damage suffered by a component of a cooling tower water pump. The jagged, undercut, spongelike metal loss characteristic of cavitation damage is apparent in Fig. 12.14. All damage occurred along the inner curvature of the specimen.

Severe cavitation damage on the suction side of the pump reveals insufficient water supply to the pump (insufficient net-positive suction head). Such a circumstance could be caused by partially clogged filters or screens upstream of the pump, or simply by insufficient feed of water to the pump. 

Figure 12.14 Typical appearance of severe cavitation damage in cast iron.

Examination of surface profiles in these pitted regions under a high-power microscope revealed jagged, undercut profiles free of deposits or corrosion products. This appearance is typical of cavitation damage. 

Figure 12.21 Cavitation damage on the internal surface of the condenser tube. Note longitudinal crack. The surfaces are covered with orange, air-formed iron oxides that formed subsequently to the removal of the condenser tube.

The pump has experienced graphitic corrosion. Figures 17.10, 17.12, and 17.14 illustrate typical appearances of graphitically corroded cast iron. In addition, cavitation damage (see Chap. 12) has produced severe metal loss in specific areas (see Fig. 17.13). The soft, friable corrosion products produced by graphitic corrosion are susceptible to cavitation damage at relatively low levels of cavitation intensity. 

Graphitic corrosion of the cast iron produced a soft, mechanically weak corrosion product that is highly susceptible to cavitation damage, even at relatively low cavitation intensities. The black coating on the impeller surface is visual evidence of graphitic corrosion. The spongelike surface contours are typical of cavitation damage (see Chap. 12). 

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