Cavitation-damage materials resistant

Saetre [102] has reported cavitation erosion in GRP pipe bends guiding sea water at around 10ms and at pressures of less than 1 bar. Hammond et aL [103] examined the cavitation damage resistance of FRP before and after sea water immersion using a modified ASTM G32 method. A stationary specimen was placed beneath a Ti-6A1-4V tip oscillated at 20kFIz with an amplitude of 25 pm. The materials studied were ... 

Selection of materials. Materials, like 18-8 steels and titanium, are resistant to cavitation damage. 

Using materials resistant to cavitation damage (Table 4.17) [116]. 

Areas of struetural metals affected by cavitation can be surfaced with welding wire or strip, overlay welding, or eoating with dense high-tensUe materials that resist cavitation damage (e.g. chromium stainless steel 18-8). 

The theoretical maximum suction lift at sea level for water (14.7 psi) (2.31 fi/psi) = 34 ft. However, due to flow resistance, this value is never attainable. For safety, 15 feet is considered the practical limit, although some pumps will lift somewhat higher columns of water. WTen sealing a vacuum condition above a pump, or the pump pumps from a vessel, a seal allowance to atmosphere is almost always taken as 34 feet of water. High suction lift causes a reduction in pump capacity, noisy operation due to release of air and vapor bubbles, vibration and erosion, or pitting (cavitation) of the impeller and some parts of the casing. (The extent of the damage depends on the materials of construction.)... 

In practice it is expensive, and therefore uneconomic, to produce a pump which operates completely free from cavitation. As a result it is usual for commercial pumps to operate in the NPSH range between inception and a point where erosion damage is unacceptable. The extent of this range may be increased by using impellers made from the more resistant materials shown in Fig. 8.77. The subject of cavitation in pumps has been dealt with extensively by Pearsall and Grist... 

As with other types of erosion, the superposition of a corrosion process also has to be taken into account where damage by cavitation occurs. Removal of material by corrosion after destruction of protective covering layers often represents the more intensive attack, and the corrosion resistance of the material is then the dominant property. 

A study of E-glass/PVC foam sandwich panels [101] ranked this kind of material above stainless steel in respect of cavitation resistance. On the other hand some investigations have reported damage, as the following examples illustrate. 

Subsurface metal shows evidence of mechanical deformation. As a general rule, cast alloys are likely to suffer more damage than wrought versions of the same alloy. Ductile materials, such as wrought austenitic stainless steels, have the best resistance to cavitation. 

Ranking for liquid erosion resistaiKe in a given situation is made difficult by the complications of definiiig both the fluid conditions that result in damage and the metal prcqpoties that influence erosion resistance. This is true for laboratory tests and for fidd evaluations. Even as late as 1960, attempts to rank materials for cavitation resistance produced ordy a qualitative comparison, because results fiom different sources varied widely in cavitation conditions and in amount of damage for the same material. 

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