Hale Q Series Muscle - Manual (Page 194)

 Cavitation
194 MUSCLE (Midship) Pumps Installation, Operation, and Maintenance Manual
p/n: 029-0020-63-0
Once the vapor pockets (bubbles) enter the impeller, the process begins to reverse
itself. As the vapor reaches the discharge side of the pump, it is subjected to a high
positive pressure and condenses back to a liquid.
This sudden change from vapor to liquid generates a shock effect that damages the
impeller and pump housing. Usually there are thousands of tiny vapor pockets (or
bubbles).
It is the collapsing (or implosion) of these bubbles that causes the characteristic
sound of cavitation that has been described as rocks tumbling in the pump.
G.2 WARNING SIGNS OF CAVITATION (DISCHARGE AND
GAUGES)
Discharge Pressure
In a properly functioning pump, an increase in RPM increases the discharge pressure
and volume. An increase in engine RPM that does not cause an increase in the pump
discharge pressure is the most reliable indication that a pump is approaching cavita-
tion.
Vacuum Compound Gauge
Do not depend entirely on the vacuum (compound) gauge to indicate when a pump is
nearing cavitation.
The vacuum gauge is usually installed several inches away from the leading edge of
the impeller eye where the greatest amount of vacuum occurs. The vacuum gauge
does not take into account ambient temperature nor atmospheric pressure and is not
accurate near zero (0) on the vacuum scale.
G.3 ELIMINATING CAVITATION
To eliminate cavitation, the operator must be aware of the warning signs listed above.
Low barometer, high elevation, and elevated water temperature also contribute to cav-
itation.
The most common way to eliminate cavitation is to decrease the amount of water
being discharged by decreasing engine speed or closing discharge valves. However,
this also results in a reduction of flow.