Water-Lifting Devices – Pumps - Practices of Irrigation and On-farm Water Management - page 449

necessary during re-priming. Automatic priming of a pump is achieved by the use
of a recirculation chamber which recycles water through the impeller until the pump
is primed, or by the use of a small positive displacement pump which supplies water
to the impeller.
12.4.7 Cavitation
Pump cavitation is defined as the formation of cavities on the back surface of an
impeller and the resulting loss of contact between the impeller and the water being
pumped. These cavities are zones of partial vacuum which fill with water vapor as
the surrounding water vaporizes due to the reduced pressure in the cavities. The cav-
ities are displaced with the flowing water along the pump impeller surfaces toward
the outer circumference of the impellers. As they move toward the circumference,
the pressure in the surrounding water increases, and the cavities collapse against the
impellers with considerable force. The force created by the collapse of the cavities
often causes erosion and rapid wear of the pump impellers as well as a characteristic
noise during pump operation.
The process of cavitation is caused by the reduction in pressure behind the
impellers to the point that the water vaporizes. Thus, it can be caused by any com-
bination of factors which allow pressure to drop to that point, including inadequate
submergence or excessive suction lift, so that little pressure is available to move
water into the pump, high impeller speeds which cause extremely low pressures to
be generated behind the impellers, restricted pump intake lines which prevent water
from moving readily into the pump, and high water temperatures which decrease
the pressure at which water vaporizes.
Cavitation can occur in all types of pumps and can create a serious problem.
In some cases of mild cavitation, the only problem may be a slight drop in effi-
ciency. On the other hand, severe cavitation may be quite destructive to the pump
and result in pitting of impeller vanes. Since any pump can be made to cavitate,
care should be taken in selecting the pump for a given system and planning its
installation.
Pump manufacturers specify the Net Positive Suction Head required (NPSHr)
for the operation of a pump without cavitation. Pump cavitation can be avoided by
assuring that the net positive suction head available (NPSHa) is always greater than
that required (NPSHr) by the pump.
12.5 Description of Different Types of Centrifugal Pumps
12.5.1 Turbine Pump
A turbine pump is basically a centrifugal pump mounted underwater and attached
by a shaft to a motor mounted above the water (Fig. 12.1 ) . The shaft usually extends
down the center of a large pipe. The water is pumped up this pipe. Turbine pumps are