Local reduction of the static pressure p to the vapor pressure pv of the liquid causes vaporization of the liquid and cavitation. Internal pressure drops are due to a) impeller inlet velocity head and inlet passage loss and b) blade loading and loss within the impeller. In order to prevent a substantial decrease of impeller pressure rise, the sum of these pressure drops should not exceed the difference between Pin andpv, the head equivalent of which is called "net positive suction head" or NPSH:

Insufficient NPSH leads to cavitation and loss of pump pressure rise. That is because the impeller can become filled with vapor, in which case the density p of the fluid within the impeller is then reduced by orders of magnitude. This in turn, as can be seen in Eqs. 18-22, results in essentially zero pump pressure rise; that is, total loss of pump performance.

Eq. 24 substituted into Eq. 22 yields the local static pressure above vapor pressure in terms of the NPSH:

FIGURE 5 Pump stage internal pressure development. Total pressure rise AP = pgAH.

This, together with the foregoing pressure drops, which occur in the inlet region of the pump, is illustrated in Figure 5. The figure contains three plots of p along the representative streamline from 1 to 2, m being distance along this line in the meridional plane. These plots are for the suction side or trailing face of an impeller blade, the pressure side or driving face, and the average or mid-passage position. The middle or average pressure plot is readily described by Eq. 25 in terms of the local average W-distribution. The local blade-to-blade static pressure difference pp — ps arises from the torque exerted on a strip of fluid between the blades and approximated here via blade-to-blade average velocity components in Newton's Second Law for Moments of Forces:

dm dm

- = 2p pVm d( UVu) Pp Ps nb a dm n where, for ease of illustration, the blade-to-blade polar angle difference AS is taken equal to 2p/nb, the actual value of AS being slightly less than this due to the thickness of the blades. Thus, for example, too small a number of blades nb results in a larger value ofpp — ps and a lower minimum static pressure in the inlet region of the impeller.

The density reduction in a cavitating impeller is difficult to predict analytically; therefore, empirical relationships for acceptable levels of NPSH have been developed and will be presented further on, as guidelines for design and performance prediction are developed.

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