Plant Hydraulics - Public Water Systems - page 920

Fig. 26-22. Combining flow—branch angle 90 —loss coefficient K 23 ( Source: Reference 7; cour-
tesy of the BHR Group Limited)

onset of cavitation depends on the pressure, size, opening, and manufacturing of valves
and pumps. For a more comprehensive discussion, see References 12 and 13.

Pump Cavitation
Pump cavitation occurs when the absolute pressure at the inlet of the pump drops
below the vapor pressure of the liquid being pumped. When the liquid or water enters
the eye of the impeller, the velocity increases and is accompanied by a drop in pressure.
If the pressure drops below the vapor pressure of the water, the water will vaporize
and the pumped liquid will consist of water and bubbles and / or vapor pockets. As the
water moves through the pump (impeller), it reaches an area of higher pressure and
the bubbles / vapor pockets collapse. If this collapse occurs on a solid boundary, which
in this case is the impeller, severe damage will occur and the pump will lose capacity.
Thoma's cavitation number is: 15,16

4/3
KN s

(26-22)

10

6