Applications—Cavitation

The construction of a cryogenic valve with extended bonnet. (Courtesy of Fisher Controls.) 

Because no known material can remain indefinitely undamaged by severe cavitation, the only sure solution is to eliminate it. The greatest damage is caused by a dense pure liquid with high surface tension (e.g., water or mercury). Methods to eliminate cavitation include the reduction 

Post-Oil Energy Technology After the Age of Fossil Fuels 

Cavitation occurs when the pressure rises downstream of the vena contracta. When it reaches the vapor pressure of the process fluid, the vapor bubbles implode and release powerful microjets that will damage any metallic surface in the area. 

When operating conditions cannot be changed, one can select a valve with a treacherous flow path, either of the multiple-port or multiple-flow-path variety. Valves that are most likely to cavitate are the high-recovery valves (ball, butterfly, gate) having low FL and Fc coefficients (Table 2.67). Therefore, if cavitation is anticipated, low recovery valves with high Fc and FL coeffi- 

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The majority of industrial slurry-transport installations tend to be free of cavitation and related problems with pump suction performance, but in many other applications cavitation is important and must be considered in system design. In disposal pipelines operated by the dredging industry and also in pipeline applications such as those used by the phosphate extraction industry (for example in Florida, U.S.A.), the first (upstream) pump is susceptible to highly sub-atmospheric suction pressures (see Fig. 1). In these cases, cavitation is a common occurrence, and may be significant in determining the costs of operation. 

U.S. Environmental Protection Agency, CAW-OX Cavitation Oxidation Process, Magnum Water Technology, Inc., Application Analysis Eeport, EPA/540/SR-93/520,1994. 

Generation Spontaneous generation of gas bubbles within a homogeneous liquid is theoreticaUy impossible (Bikerman, Foams Theoiy and Industrial Applications, Reinhold, New York, 1953, p. 10). The appearance of a bubble requires a gas nucleus as avoid in the liquid. The nucleus may be in the form of a small bubble or of a solid carrying adsorbed gas, examples of the latter being dust particles, boiling chips, and a solid wall. A void can result from cavitation, mechan-ic ly or acoustically induced. Blander and Katz [AlChE J., 21, 833 (1975)] have thoroughly reviewed bubble nucleation in liquids. 

Resilient materials such as rubber and some plastics may be useful in certain applications, especially under conditions of low cavitation intensities. However, such materials are subject to disbondment at the metal and elastomer interface at high cavitation intensities, even if the exposure is brief. 

Actually, everything we said about bearings, mechanical seals, piping, TDH, system curves and mating the pump curve to the system curve, the affinity laws, cavitation, horsepower and efficiency arc as applicable to PD pumps as centrifugal pumps. 

Lightarowicz, A., Cavilating Jet Apparatus for Cavitation Erosion Testing, Erosion Prevention and Useful Applications, ASTM STP 664, p. 530-549 (1979)... 

The forms of corrosion which can be controlled by cathodic protection include all forms of general corrosion, pitting corrosion, graphitic corrosion, crevice corrosion, stress-corrosion cracking, corrosion fatigue, cavitation corrosion, bacterial corrosion, etc. This section deals exclusively with the practical application of cathodic protection principally using the impressed-current method. The application of cathodic protection using sacrificial anodes is dealt with in Section 10.2. 

Applications Plasma spraying is used to apply coatings for protection against wear and corrosion, to prevent erosion or cavitations, and to provide electrical insulation or conductivity. It can also be employed to produce bearing surfaces, abrasive properties or resistance to wetting by molten metals. The coatings can also be applied to facilitate the joining of different materials. 

In considering these tests it should be remembered that the phenomenon of cavitation-erosion is often accompained by corrosion effects and that a synergistic effect may operate between the mechanically and chemically induced forms of attack. In fact the term cavitation-erosion-corrosion may often be more applicable in describing the requirements of a test procedure. The subject has recently been discussed by Wood etal. °. 

Miscellaneous There are many interesting applications that arise from time to time that are outside the main stream of industry described above. Examples include desalination plant reactor cooling water circuits automobile body corrosion in situ) marine (vessels, piling, harbour installations) aircraft (in situ) packaging and cavitation monitoring. 

Cavitation erosion With increasing ship speeds, the development of high-speed hydraulic equipment, and the variety of modem fluid-flow applications to which metal materials are being subjected, the problem of cavitation erosion becomes ever more important. Erosion may occur in either internal-flow systems, such as piping, pumps, and turbines, or in external ones like ships propellers (36). 

Ultrasound can thus be used to enhance kinetics, flow, and mass and heat transfer. The overall results are that organic synthetic reactions show increased rate (sometimes even from hours to minutes, up to 25 times faster), and/or increased yield (tens of percentages, sometimes even starting from 0% yield in nonsonicated conditions). In multiphase systems, gas-liquid and solid-liquid mass transfer has been observed to increase by 5- and 20-fold, respectively [35]. Membrane fluxes have been enhanced by up to a factor of 8 [56]. Despite these results, use of acoustics, and ultrasound in particular, in chemical industry is mainly limited to the fields of cleaning and decontamination [55]. One of the main barriers to industrial application of sonochemical processes is control and scale-up of ultrasound concepts into operable processes. Therefore, a better understanding is required of the relation between a cavitation coUapse and chemical reactivity, as weU as a better understanding and reproducibility of the influence of various design and operational parameters on the cavitation process. Also, rehable mathematical models and scale-up procedures need to be developed [35, 54, 55]. 

Hydrodynamic cavitation reactors have been investigated for more than a decade now in the UDCT Department of Bombay University [63-66]. When applied to some industrially relevant reactions, the hydrodynamically created cavitation appeared to dehver on average an order of magnitude higher cavitation yields than the acoustic cavitation. In addition, the processing volumes could be up to about 100 times larger than in the conventional sonochemical reactors. So far, there is no information about the industrial applications of the hydrodynamic cavitation reactors, although some concepts have already been patented [67]. 

For general aspects on sonochemistry the reader is referred to references [174,180], and for cavitation to references [175,186]. Cordemans [187] has briefly reviewed the use of (ultra)sound in the chemical industry. Typical applications include thermally induced polymer cross-linking, dispersion of Ti02 pigments in paints, and stabilisation of emulsions. High power ultrasonic waves allow rapid in situ copolymerisation and compatibilisation of immiscible polymer melt blends. Roberts [170] has reviewed high-intensity ultrasonics, cavitation and relevant parameters (frequency, intensity,... 

Wear is the removal of surface material by one of three mechanisms erosion, abrasion, or cavitation. Erosion is the removal of a polymer s surface by abrasive materials carried in a fluid medium. We see this type of wear in plastic pipes used to transport waterborne slurries of minerals in mining operations and in vacuum transfer pipes used to convey powders in a stream of air. Abrasion is the result of two surfaces sliding against each other. We commonly observe abrasion of polymers in the fabrics of our clothes and upholstery. Cavitative wear is caused by voids in a liquid medium collapsing against a surface. It is essentially an impact process. Cavitation is a relatively uncommon cause of wear in polymers. Pump impellers are one of the few applications where polymers must resist this type of wear. 

Mettin R (2005) Bubble structures in acoustic cavitation. In Doinikov AA (ed) Bubble and particle dynamics in acoustic fields modem trends and applications, pp. 1-36. Research Signpost, Trivandrum... 

Apart from the classification based on the mode of generation of cavities, cavitation can also be classified as transient cavitation and stable cavitation [3]. The classification is based on the maximum radius reached (resonant size), life time of cavity (which decides the extent of collapse) in the bulk of liquid and the pattern of cavity collapse. Generation of transient or stable cavitation usually depends on the set of operating parameters and constitution of the liquid medium. Depending on the specific application under question, it is very important to select particular set of operating conditions such that maximum effects are obtained with minimum possible energy consumption. 

The reactor design in terms of ratio of the diameter of the immersion transducer to reactor diameter, liquid height, position of the transducers and characteristics of the cell plays a important role in deciding the cavitational activity distribution and hence the efficacy of sonochemical reactors for the specific application. Based on a critical analysis of the existing literature, following important design related information can be recommended ... 

With an increase in the diameter of immersion transducer relative to the reactor diameter, the cavitational activity increases till an optimum ratio, usually dependent on the application [51,52], The ratio mainly affects the level of turbulent... 

The important liquid phase physicochemical properties which affect the cavitation phenomena and hence the extent of cavitational effects for the given application include vapor pressure, viscosity and surface tension. 

Intensification can be achieved using this approach of combination of cavitation and advanced oxidation process such as use of hydrogen peroxide, ozone and photocatalytic oxidation, only for chemical synthesis applications where free radical attack is the governing mechanism. For reactions governed by pyrolysis type mechanism, use of process intensifying parameters which result in overall increase in the cavitational intensity such as solid particles, sparging of gases etc. is recommended. 

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