Page 342 - Centrifugal Pumps 2E

Chemical Pumps Metallic and Nonmetallic 307

Sump Pumps

Submersible or immersible sump pumps are a line of chemical pumps
that have been derived from the combination of parts of the horizontal
and vertical pumps (Figure 15-15). These pumps fit into wet sumps that
may be 3 ft to 20 ft deep.
These pumps employ the casing and impeller of a horizontal pump and
the support head, motors, and sometimes a casing cover of the vertical
pump. To prevent critical speed frequencies, the shaft is supported by
line bearings usually having a centerline to centerline distance of 5 ft for
1750 RPM and 3 ft for 3550 RPM. The column supporting the bearings
and the discharge pipe is made of compatible material for the liquid in the
sump. The bearings are either product lube, grease lube, or external lube
which has been centrifuged or filtered. Typical applications are shown in
Table 15-1.
The depth of the pump relative to the sump is called setting. Setting
goes from the bottom of the strainer to the bottom of the mounting plate.
The sump pump is usually required at the mounting plate; therefore,
when the impeller is selected, the frictional loss through the discharge
pipe as well as the static head above the minimum liquid level has to be
added to what is required at the mounting flange. These additional hy-
draulic losses may require a motor larger than would normally be used
for the same selection as a horizontal pump. These pumps also require a
minimum submergence to prevent vortexing or entrained air from enter-
ing into the suction (Figure 15-16).
There are no standards for the location in the sump of pumps of this
type. Many users employ the suggested applications shown in the Hy-
draulic Institute Standards for sump design. The mounting plates for
these pumps are usually plain carbon steel or carbon steel with an epoxy
coating on one side. Sometimes stainless steel is used.
Sometimes it is desirable to pump the liquid to a level below the suction
of the pump. A tailpipe is used to achieve this; however, the liquid level
has to be above the impeller centerline when the pump is started (Figure
15-17). The tailpipe allows the liquid level to be pumped down to as
much as 10 ft below the end of the flange of the suction pipe. The use of a
tailpipe reduces the cost of the initial pump. The disadvantage, however,
is that air can be pulled into the back of the casing thus reducing the over-
all performance of the pump.

Self Priming

The self-priming chemical pumps are also an offshoot of the horizontal
pumps (Figure 15-18). These are usually available in 316 or ductile iron.