HALE AP, Technical Manual

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Figure 11. Low Pressure Regions
B. The pressure of the water in the pump drops as it flows from the suction
flange through the suction nozzle and into the impeller.
C. As flow from the pump increases, the vacuum at the impeller increases. As
the vacuum increases, the boiling point of water in that vacuum decreases
until it reaches a point near the impeller eye where it boils and vaporizes.
D. Once the vapor pockets, or bubbles, enter the impeller, the process begins to
reverse itself. As the vapor reaches the discharge side of the pump, it is sub-
jected to a high positive pressure and condenses back to a liquid.
E. The sudden change from vapor to liquid generates a shock effect that dam-
ages the impeller and pump housing. Usually there are thousands of tiny va-
por pockets (bubbles) rather than a few large ones. It is the collapsing (or im-
plosion) of these bubbles that causes the characteristic sound of cavitation
that has been described as rocks tumbling in the pump.
3.5.2 Warning Signs Of Cavitation
As a pump approaches cavitation, certain warning signs appear on the gauges that monitor the
suction and discharge pressures. These signs are discussed in the following subparagraphs.
3.5.2.1
Discharge Pressure
In a properly functioning pump, an increase in RPM will increase the discharge pressure and vol-
ume. An increase in engine RPM that does not cause an increase in the pump discharge pres-
sure, is the most reliable indication that a pump is approaching cavitation.
3.5.2.2
Vacuum Compound Gauge
The operator should not depend entirely on the vacuum (compound) gauge to indicate when a
pump is nearing cavitation: The vacuum gauge is usually tapped into the intake chamber several
inches away from the leading edge of the impeller eye where the greatest amount of vacuum oc-
curs. The vacuum gauge does not take into account ambient temperature nor atmospheric pres-
sure and is not accurate near zero on the vacuum scale.
3.5.3
How To Prevent Cavitation
Monitoring current operating conditions, knowing the capabilities of the equipment, and regular
inspection are the best protection against cavitation.