Centrifugal
pumps can be described as a rotating spade for fluids.
Centrifugal pumps operate by using kinetic energy to
move the fluids and utilize an impeller and circular
pump casing. With every rotation of a centrifugal pump,
an amount of fluid is driven out. The volume of the
donut in the centrifugal pump represents how many gallons
per minute that the pump is able to deliver.
The
head, or pressure, that the centrifugal pump will develop
can be determined by the amount of velocity that the
fluid has as it leaves the pump. The fluid is thrown
off the vane tips. There is no relative motion in the
center of the shaft but the fluids will move out to
replace the fluids thrown off the vane tips. When this
happens a low-pressure area is created at the shaft’s
center, which is also called the liquid inlet or pump
suction. Atmospheric pressure, or any external pressure
on the liquid supply will force more liquid into the
pump suction.
The
flow rate of the pump will determine how big the case
will need to be to handle the amount of water desired,
which is indicated by the inlet and outlet pipe sizes.
Two
or more impellers can be incorporated into the same
housing if it becomes impractical to make an impeller
large enough to get the desired head. This can be common
in well pumps where the pump must be located inside
the hole because it can be difficult to get a pump with
a diameter larger than the hole to go in without using
a hole stretcher. Hole stretchers are large and dangerous,
not to mention illegal in most states.
If
you lack head in your system however, the velocity of
the water will increase at it approaches the discharge
nozzle. This will cause pressure to decrease and will
generate a force that will displace the impeller at
a sixty-degree towards 240 degrees from the cutwater.
Likewise, if there is too much head on the system, some
water will re-circulate back through the cutwater causing
the velocity of the liquid to increase. This makes the
pressure fall and will force the impeller sixty degrees
from the cutwater.
