Cavitation steels

Titanium resists erosion—corrosion by fast-moving sand-laden water. In a high velocity, sand-laden seawater test (8.2 m/s) for a 60-d period, titanium performed more than 100 times better than 18 Cr—8 Ni stainless steel. Monel, or 70 Cu—30 Ni. Resistance to cavitation, ie, corrosion on surfaces exposed to high velocity Hquids, is better than by most other stmctural metals (34,35). 

Examination of the microstructure of the cavitated surface will commonly disclose evidence of deformation such as twins (Neumann hands) in carbon steel and general cold working in other metals (Case History 12.6). Damage from cavitation can be differentiated from attack by a strong mineral acid, which can produce a similar surface appearance, by observing the highly specific areas of attack characteristic of cavitation. Acid attack is typically general in its extent (Case History 12.6). 

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. 

Changing the pump metallurgy to a more corrosion- and cavitation-resistant material, such as stainless steel, is a potential solution to this type of problem. Note, however, that all other cast iron pump components that have sustained graphitic corrosion should be replaced to avoid the possibility of galvanic corrosion (see Chap. 16) between retained graphitically corroded cast iron components and new components. 

Specifying a cavitation-resistant material such as hardened carbon steel, hardened stainless steel, or carbon steel overlaid with a cavitation-resistant material... 

Plate anodes were used for corrosion protection in order to avoid damage due to erosion and cavitation. These consisted of enamelled steel bodies in which a metal oxide-coated titanium anode 1 dm in surface area was fitted. The enamel... 

Practice for making and using U-bend stress corrosion test specimens Recommended practice for laboratory immersion corrosion testing of metals Method for vibratory cavitation erosion test Practice for recording data from atmospheric corrosion tests of metallic-coated steel specimens... 

The behavior of materials, particularly steel, in cavitating fluids results in an erosion mechanism, including mechanical erosion and electrochemical corrosion. The straightforward way to fight cavitation is to use hardened materials, chromium, chrome-nickel compounds, or elastomeric plastics. Other cures are to reduce the vapor pressure with additives, reduce the turbulence, change the liquid s temperature, or add air to act as a cushion for the collapsing bubbles. 

A liquid with a viscosity of 5cP, density of 45 lbm/ft3, and vapor pressure of 20 psia is transported from a storage tank in which the pressure is 30 psia to an open tank 500 ft downstream, at a rate of 100 gpm. The liquid level in the storage tank is 30 ft above the pump, and the pipeline is 2 in. sch 40 commercial steel. If the transfer pump has a required NPSH of 15 ft, how far downstream from the storage tank can the pump be located without danger of cavitation ... 

Location of the Reaction Flask. We found that irradiation from the ultrasonic cleaner is most effective when the flask is positioned in the bath to achieve maximum turbulence of the reagents. This "sweet spot" is the point of maximum cavitation and assures optimum energy transfer to the reaction medium. In practice, this focal point of intensity may move after several hours, possibly because of distortion of the steel bottom caused by local heating of the transducer. 

Amatol is made up in various proportions of ammonium nitrate to trinitrotoluene, such as 50/50, 60/40, and 80/20. The granulated, dried, and sifted ammonium nitrate, warmed to about 90°, is added to melted trinitrotoluene at about 90°, and the warm mixture, if 50/50 or 60/40, is ladled into the shells which have been previously warmed somewhat in order that solidification may not be too rapid, or, if 80/20, is stemmed or extruded into the shells by means of a screw operating within a steel tube. Synthetic ammonium nitrate is preferred for the preparation of amatol. The pyridine which is generally present in gas liquor and tar liquor ammonia remains in the ammonium nitrate which is made from these liquors and causes frothing and the formation of bubbles in the warm amatol—with the consequent probability of cavitation in the charge. Thiocyanates which are often present in ammonia from the same sources likewise cause frothing, and phenols if present tend to promote exudation. 

The experiments were carried out in the set-up shown in Fig. 1. The cores that were used for the experiments were cylindrically shaped Berea sandstone samples. The length of the cores was 20 cm and the diameter 7.62 cm. The porosity was about 0.25. The initial permeability was 100-300 mD for all samples. During an experiment a core was placed in a rubber sleeve to keep it fixed during the experiment. It was then placed in a steel vessel in which downhole reservoir conditions of up to 150 bars and 100°C were simulated. An acoustic horn was placed at one end of the core (see Fig. 1). The high pressure in the vessel made it possible to avoid cavitation. For pressures lower than 100 bars the influence of cavitation becomes noticeable. The space between the vessel and sleeve, which was filled with water, was pressurized to 180 bars to make the rubber sleeve completely seal off. 

Cavitation corrosion occurs in pumps that have flow conditions that allow bubble formation on the surface of impellers. These bubbles, upon formation, break with enough force to rupture the protective film of the stainless steel. Plants can prevent this by designing a system that avoids bubble formation (i.e., provide sufficient Net Positive Suction Head - NPSH - for the pump), by polishing rotating parts to remove bubble formation sites and by using alloys with greater corrosion resistance and strength88. 

Toughness can influence the performance of materials under conditions of erosion-corrosion. The soft metals are often more susceptible to erosion corrosion because they are more susceptible to mechanical wear. The toughness is a good criterion for the resistance to the mechanical erosion or abrasion, but this is not necessarily a good criterion to predict the resistance to the erosion-corrosion. Stellite (Co-Cr-W-Fe-C alloy), which has better toughness than 18-8 stainless steel, showed better resistance to cavitation erosion on a water brake.25... 

There are several ways to harden alloys. A certain procedure to increase the resistance to the erosion corrosion is the hardening by solid solution. One adds an element to another to produce a solid solution that is resistant to the corrosion by hardening the metal. The thermal treatment is also a method to harden a metal or alloy, but it changes the microstructure and can induce a greater susceptibility to corrosion. Hardening by cold work is also an important procedure and it is the reason for using stainless steel to resist cavitation erosion. This material, initially hard, attains an even harder surface by cold work and becomes more resistant to attack and erosion. 

For transferring water, a positive displacement, screw type pump is necessary centrifugal pumps will introduce cavitation when they have either a restricted imput or output stream. Even with a screw type pump, it is advisable to insert a bubble trap between the pump and the tube. Such a trap consists of a steel cylinder, perhaps a foot high, in which the water is pumped into near the top and extracted from the bottom. Bubbles will rise to the top where the air they carry can be removed by a valve on the cylinder. In addition, perhaps due to the wall thickness of 1/2 , we have found it necessary to pressurize the tube to 100 psi to obtain accurate data. Bath clear (VWR Scientific, Inc.) or iodine can be added to the water to control bacterial growth. 

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