Hale 2CBP - Manual (Page 17)

Operating Procedures Sec II- 5
Booster Pumps
More complete and detailed information can
be found in the relief valve manual.
THE PRESSURE INDICATOR ON THE
PANEL IS ONLY A ROUGH INDICATION
OF TPM SETTING. ALWAYS USE THE
ABOVE PROCEDURE TO PROPERLY SET
THE TPM RELIEF VALVE SYSTEM.
CAVITATION
Cavitation can occur while pumping from draft,
in relay, or from a hydrant. The operator must be
aware of the warning signs and correct the
situation, or serious damage to the pump and
impeller will occur.
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.
The way to eliminate cavitation is to increase the
flow to the pump, decrease the amount of water
being discharged from the pump, or reduce the
pressure in the pump by decreasing engine
speed.
Cavitation
Cavitation occurs when a centrifugal pump is
attempting to discharge more water than it is
receiving. It is often referred to as “the pump
running away from the supply."
Process of Cavitation
1. When increased discharge demand exceeds
the intake, bubbles form in the low-pressure
region (eye) of the impeller.
2. The pressure of the water in the pump drops
as it flows from the suction flange through
the suction nozzle and into the impeller.
3. 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.
4. 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 subjected to a high
positive pressure and condenses back to a
liquid.
5. The 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
(bubbles) rather than a few large ones. 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.
Figure 2-4 Low Pressure Regions