Page 41 - Know and Understand Centrifugal Pumps

Cavitation

In trod uct ion
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It is important to clarifv that the pump does not cavitate, although
people in the industry tend to say that the pump is cavitating. It is more
correct to say that the pump is in cavitation or the pump is suffering
cavitation. In reality it is the system that cavitates the pump, because
the system controls the pump.
Inadequate NPSHa establishes favorable conditions for cavitation in the
pump. If the pressure in the eye of the impeller falls below the vapor
pressure of the fluid, then cavitation can begin.

Vapor pressure
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Definition
The vapor pressure of a liquid is the absolute pressure at which the
liquid vaporizes or converts into a gas at a specific temperature.
Normally, the units are expressed in pounds per square inch absolute
(psia). The vapor pressure of a liquid increases with its temperature. For
this reason the temperature should be specified for a declared vapor
pressure.
At sea level, water normally boils at 212°F. If the pressure should
increase above 14.7 psia, as in a boiler or pressure vessel, then the
boiling point of the water also increases. If the pressure decreases, then
the water’s boiling point also decreases. For example in the Andes
Mountains at 15,000 ft (4,600 meters) above sea level, normal
atmospheric pressure is about 8.3 psia instead of 14.7 psia; water would
boil at 184°F.