Environmental Engineering Reference
In-Depth Information
is that when two pumps are online one can be shut down during low
demand. This allows the remaining pump to operate close to its opti-
mum efficiency.
Series
operation is achieved by having one pump dis-
charge into the suction of the next. This arrangement is used primarily
to increase the discharge head, although a small increase in capacity
also results.
3.3 CenTrifugal PuMPs
The
centrifugal pump
(and its modifications) is the most widely
used type of pumping equipment in water/wastewater operations. This
type of pump is capable of moving high volumes of water/wastewater
(or other liquids) in a relatively efficient manner. The centrifugal pump
is very dependable, has relatively low maintenance requirements, and
can be constructed out of a wide variety of construction materials. It
is considered one of the most dependable systems available for water
transfer.
3.3.1 description
The centrifugal pump consists of a rotating element (
impeller
)
sealed in a casing (
volute
). The rotating element is connected to a drive
unit (
motor/engine
) that supplies the energy to spin the rotating ele-
ment. As the impeller spins inside the volute
casing, an area of low pressure is created in the
center of the impeller. This low pressure allows
the atmospheric pressure on the liquid in the
supply tank to force the liquid up to the impel-
ler. Because the pump will not operate if no low-
pressure zone is created at the center of the impeller, it is important that
the casing be sealed to prevent air from entering the casing.
Key Point:
A centrifugal pump is a
pumping mechanism whose rapidly
spinning impeller imparts a high veloc-
ity to the water that enters, then converts
that velocity to pressure upon exit.
To ensure that the casing is airtight, the pump employs some type
of seal (
mechanical
or
conventional packing
) assembly at the point
where the shaft enters the casing. This seal also includes lubrication,
provided by water, grease, or oil, to prevent excessive wear.
From a hydraulic standpoint, note the energy changes that occur
in the moving water. As water enters the casing, the spinning action
of the impeller imparts (transfers) energy to the water. This energy is
transferred to the water in the form of increased speed or velocity. The
liquid is thrown outward by the impeller into the volute casing, where
the design of the casing allows the velocity of the liquid to be reduced
which, in turn, converts the velocity energy (
velocity head
) to pressure
energy (
pressure head
). The process by which this change occurs is
described later. The liquid then travels out of the pump through the
pump discharge. The major components of the centrifugal pump are
shown in Figure 3.7.
