Page 43 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

w An increase of the temperature of the pumped liquid.
w An increase in the velocity or flow of the fluid.
w Separation and reduction of the flow due to a change in the
viscosity of the liquid.
w Undesirable flow conditions caused by obstructions or sharp elbows
in the suction piping.

w The pump is inadequate for the system.
The focus should be on resolving cavitation problems by increasing the
external pressure on the fluid or decreasing its vapor pressure. The
external pressure could be increased by:
w Increasing the pressure at the pump suction.
w Reducing the energy losses (friction) at the entrance to the pump.
w Using a larger pump.

The vapor pressure of the fluid is decreased by:
w Lowering the temperature of the fluid.
w Changing to a fluid with a lower vapor pressure.
At times, simply removing aspirated air venting the pump will have the
same effect.

Cavitation: A practical discussion

Consider the following
I need a pump to raise cold water at 10 gallons per minute. There is an
open well with water 40 fi below ground level.
w Do I need a PD Pump?
w Do I need a Centrifugal Pump?
w Should the pump be small, medium, or large?

The reply
No pump in the world can lift cold water 40 ft from an open well in a
suction lift condition because the water would evaporate before it
comes into the pump. The reason lies in the basic head formula:
si x 2.31
H=
SP. gr.