Environmental Engineering Reference
In-Depth Information
Centrifugal pumps may be single stage with a single impeller, or
they may be multiple stage with several impellers through which the
fluid flows in series. Each impeller in the series increases the pressure
of the fluid at the pump discharge. Pumps may have 30 or more stages
in extreme cases. In centrifugal pumps, a correlation of pump capac-
ity, head, and speed at optimum efficiency is used to classify the pump
impellers with respect to their specific geometry. This correlation is
called specific speed and is an important parameter for analyzing pump
performance (Garay, 1990).
The volute of the pump is designed to convert velocity energy to
pressure energy. As a given volume of water moves from one cross-sec-
tional area to another with the volute casing, the velocity or speed of the
water changes proportionately. The volute casing has a cross-sectional
area that is extremely small at the point in the case that is farthest
from the discharge (see Figure 3.8). This area increases continuously to
the discharge. As this area increases, the velocity of the water passing
through it decreases as it moves around the volute casing to the dis-
charge point.
As the velocity of the water decreases, the velocity head decreases
and the energy is converted to pressure head. There is a direct rela-
tionship between the velocity of the water and the pressure it exerts;
therefore, as the velocity of the water decreases, the excess energy is
converted to additional pressure (pressure head). This pressure head
supplies the energy to move the water through the discharge piping.
3.3.4 Pump Characteristics
The centrifugal pump operates on the principle of an energy trans-
fer and, therefore, has certain definite characteristics that make it
unique. The type and size of the impeller limit the amount of energy
that can be transferred to the water, the characteristics of the mate-
rial being pumped, and the total head of the system through which the
water is moving. For any one centrifugal pump, a definite relationship
exists between these factors along with head (capacity), efficiency, and
brake horsepower.
3.3.4.1 Head (Capacity)
As might be expected, the capacity of a centrifugal pump is directly
related to the total head of the system. If the total head on the system
is increased, the volume of the discharge will be reduced proportion-
ately. As the head of the system increases, the capacity of the pump will
decrease proportionately until the discharge stops. The head at which
the discharge no longer occurs is known as the cut-off head . As pointed
out earlier, the total head includes a certain amount of energy to over-
come the friction of the system. This friction head can be greatly affected
by the size and configuration of the piping and the condition of the valv-
ing of the system. If the control valves on the system are closed par-
tially, the friction head can increase dramatically. When this happens,
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