Hale RSD - Manual (Page 121)

121 Appendix F: Cavitation
Hale Products, Inc., Nov.2005, Rev-A
C av it atio n
❑
Appendix F: Cavitation
(See Figure F-1: “Sample, Cavitation Regions.”)
Cavitation can occur while pumping from draft, in relay, or from a hydrant
(although it is more likely from draft conditions). The operator must be aware of
the warning signs and immediately correct the situation.
Cavitation can damage the impeller and
other sensitive components, impair pump
performance, and reduce flow capacity.
The damage done during any one period of
cavitation is not great, but the effects are
cumulative. Implosions occurring during
cavitation break away or erode tiny pieces
of metal from the internal parts and the
pump casing. When enough metal has
been chipped away, the impeller becomes
unbalanced causing a strain and vibration
on bearings, bushings and shafts.
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 vac-
uum, formed near the eye of the impeller. Cavitation is often referred to as “the
pump running away from the fluid supply.” 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 liquid not
the air in the bubbles. (See Figure F-1: “Sample, Cavitation Regions.”)
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
vacuum increases, 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 sub-
jected to a high positive pressure and condenses back to a liquid.
Figure F-1: Sample, Cavitation
Regions