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When considering Figure 4.1, it is evident that the problem results
from the pressure of the liquid dropping below its vapor pressure in the
eye of the impeller. This is what creates the vapor bubbles in that area.
Consequently, cavitation can usually be avoided or stopped, simply by
increasing the pressure of the liquid before it enters the suction nozzle
of the pump. This will ensure that the pressure in the eye area does not
fall below the vapor pressure, and therefore no vapor bubbles will be
created and no cavitation will exist.
Much of the critical pressure drop that is created as the liquid moves
into the eye of the impeller can be attributed simply to the loss of
energy of a liquid moving from a static environment (the pump
suction) to a dynamic environment in the rotating impeller. However,
other design factors may occasionally play a part, such as the entrance
angles of the impeller vanes as they relate to the velocity of the liquid.

4.4 Net positive suction head

The Pressure Energy needed to avoid the formation of vapor bubbles in
the eye of the impeller in the cavitation process, is referred to as the Net
Positive Suction Head (NPSH). The design criteria of each impeller
require the supply of a minimum level of NPSH for its optimum
performance, and are identified as the Net Positive Suction Head
Required. It is strictly a function of the pump design and its rotational
speed.
The pressure energy required by the pump is made available from the
system in which the pump operates. In this form it is identified as the
NPSH Available and is solely a function of the system design.
Consequently, to avoid Cavitation damage, the NPSH Available must
be greater than the NPSH Required.

Figure 4.2. NPSH balance diagram

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