Page 71 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

Cen trifug a I pumps
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Centrifugal pumps perform the same function as PD pumps, but they
do it differently. These pumps generate pressure by accelerating, and
then decelerating the movement of the fluid through the pump.
The flow, or gallons per minute, must be available to the pump’s
suction nozzle. This flow, or energy, in centrifugal pumps is called
NPSH or Net Positive Suction Head (discussed in Chapter 2). These
pumps, like their PD sisters, cannot generate flow. No pump in the
world can turn three gallons per minute at the suction nozzle, into four
gallons per minute out of the discharge nozzle. The fluid enters into
through the suction nozzle of the pump to the eye of the impeller. The
fluid is trapped between the veins or blades of the impeller. The
impeller is spinning at the velocity of the driver. As the fluid passes from
the eye, through the blades toward the outside diameter of the
impeller, the fluid undergoes a rapid and explosive increase in velocity.
Bernoulli’s Law states that as velocity goes up, the pressure goes down,
and indeed there is a low-pressure zone in the eye of the impeller. The
liquid that leaves the outer diameter of the impeller immediately slams
into the internal casing wall of the volute, where it comes to an abrupt
halt while it collects in the ever-expanding exit chamber of the volute.
By Bernoulli’s law, as velocity goes down, the pressure increases. The
velocity is now converted into head or pressure available at the
discharge nozzle. Because the impeller diameter and motor speed is
mostly constant, the centrifugal pump can be considered to be a
constant head or pressure device. The theoretical curve of the
centrifugal pump is seen in Figure 64.

In reality, these pumps lose some head (pressure) as energy is channeled

Flow

Figure 6-4

R 54