Cavitation coefficient

Fig. 17. Cavitation phenomenon in pumps showing cavitation bubble distribution and rate of weight loss as a function of cavitation coefficient at constant...

Table 2.67 listed, for the various valve designs, the liquid pressure recovery factors (F,J—which are related to the ratio between the valve pressure drop and the difference between the inlet and the vena contracta pressure—and the cavitation coefficients (Kc)—which are the ratios between the valve pressure drop at which cavitation starts and the difference between the inlet and the vapor pressure—of the application. The allowable maximum Ap before cavitation begins is Ap = Kc (pt - pv). As the FL and Kc values of the different valve designs drop, the probability of cavitation increases. 

While manufacturers will always provide the value of the valve conductance, C, they do not give the value of the conductance to the valve throat, C, so that it is not possible to calculate the friction coefficient, Cyj, directly from equation (7.8). However, equivalent information is normally made available for the special condition when the pressure in the valve throat is at the vapour pressure of the liquid being passed. This information is contained in the manufacturer-supplied cavitation coefficient, Kc, which is used in the following equation pair to define the pressure conditions at the onset of cavitation ... 

When choking occurs in the valve, the upstream and downstream pressures are related by a condition very similar to the equation pair (7.11) for cavitation, but now using equation (7.16) for the throat pressure, rather than (7.IS), and replacing the cavitation coefficient, Kc, by a new choking coefficient, K ... 

K is sometimes referred to as simply the pressure recovery coefficient . The two coefficients are identical for globe valves K = Kc, and often only K Is quoted. The choking coefficient is somewhat larger than the cavitation coefficient for a rotary valve, with the full-open value, Ku, taking a value of typically... 

Figure 8.3 Showing transients for valve travel, x, valve opening, /, and choking and cavitation coefficients Km and Kc.

Correlation of the friction coefficient at high-pressure ratios with the cavitation coefficient... 

Figure 9.2 Correlation between the squared friction coefficient and the cavitation coefficient.

Kc cavitation coefficient for a rotary valve at fully open Mu)... 

The cavitation coefficient (NPSHR/pump head) was about 0.08 and 0.12 for water Flow rate (m /s) respectively. 

The nature of the dissolved gas i.e. its polytropic constant (y), solubility in the cavitating medium etc. severely affects the cavitational activity due to its direct effect on the final collapse conditions. The magnitude of temperature reached at the collapse is affected by the amount of gas dissolved in the liquid medium. Final temperature reached by the adiabatically collapsing bubble (mainly depends on polytropic coefficient of the gas (y)) can be given by following mathematical relationship ... 

Typical manufacturer s values of Cv to be used with Eq. (10-29) require the variables to be expressed in the above units, with hv in ft. [For liquids, the value of 0.658 includes the value of the density of water, pw = 62.3 lbm/ft3, the ratio g/gc (which has a magnitude of 1), and 144 (in./ft)2]. For each valve design, tables for the values of the flow coefficients as a function of valve size and percent of valve opening are provided by the manufacturer (see Table 10-3, pages 318-319). In Table 10-3, Km applies to cavitating and flashing liquids and C applies to critical (choked) compressible flow, as discussed later. 

Cavite au Bioc de Trouzi ou tpreuve au Bloc de Plomb. French for Trauzl Block Test, also called Lead Block Expansion Test, but conducted somewhat differently from the procedure used in US or Germany. A brief description of Fr method, also called "Coefficient d Utilisation Pratique , is given in Vol 1, p IX of this Encyclopedia... 

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-... 

Thus, the presence of 03 as a background gas provides new sources for OH radical formation during sonolysis. The direct pyrolysis in the cavitation bubbles of volatile intermediates generated during ozonation also explains the improvement in the overall efficiency. In addition to these direct chemical effects, sonication also increases mass transfer coefficients of ozone from the bubbles to the solution, where the direct reaction of 03 with the substrates, or further radical formation, takes place. 

The cavitation contribution is determined following Pierotti s scaled particle theory, where AGcav is expressed as an expansion series in powers of the radius of the sphere which excludes the centers of the solute molecules from the solute, Rms (i.e., the sum of the solute and solvent radii Eq. 4-6). The expansion coefficients Kt (i = 0-3) are expressed in terms of properties of the solvent (the radius of the solvent molecule and the numeral density) and of the solution (temperature and pressure) ... 

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