Hale MBP Booster - Manual (Page 38)

37 Silencer Series Booster Pump, p/n: 029-0020-83-0
Operation
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Process of Cavitation
Cavitation occurs when a centrifugal pump attempts to discharge more
water than it is receiving. When cavitation occurs, bubbles are created
under the vacuum, formed near the eye of the impeller.
Cavitation is often referred to as “the pump running away from the fluid sup-
ply.” This means the operator is trying to pump more water out of the pump
than is going into the pump.
The formation of bubbles in the low pressure regions of the impeller cause
the impeller to “slip” in the water, since the impeller is designed to move liq-
uid not the air in the bubbles. (See Figure 3-5: “Sample, Cavitation
Regions,” on page 36.)
When increased discharge flow exceeds the intake, bubbles form in the low-
pressure region at the eye of the impeller. The pressure of the water in the
pump drops as it flows from the suction flange through the suction nozzle
and into the impeller.
As flow from the pump increases, the vacuum at the impeller increases. As
the vacuum increases, the water near the impeller eye begins to boil and
vaporizes.
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 dam-
ages the impeller and pump housing. Usually there are thousands of tiny
vapor pockets (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.
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 cavitation.