Civil Engineering Reference
In-Depth Information
CHAPTER 29
Pumping Systems
INTRODUCTION
This chapter addresses a few basic applications for pumping systems commonly used
in the waterworks industry. Application of pumping systems requires an understanding
of both the pumping unit and how the pumping unit functions within the system. It is
necessary to determine how the inlet conditions will affect the performance of the
pump. Surge, or water hammer, is addressed in Chapter 26, ''Plant Hydraulics.''
Figure 29-1 shows pump classifications.
1
Each class finds application in various
industries. Positive-displacement pumps are used for low-volume applications where
precise delivery is required, such as chemical feed applications, and where high-
viscosity fluid pumping is required. Certain classes of rotary positive-displacement
pumps are invaluable in fluid power transmission. However, the centrifugal pump is
the workhorse of the waterworks industry.
In a centrifugal pump, the rotation of the impeller draws liquid from the suction
side into the pump and converts the rotational momentum into pressure energy by
means of the volute or diffusion vanes. Figure 29-2 shows the two types of casings
used in centrifugal pumps.
SPECIFIC SPEED
Impellers in centrifugal pumps are classified as radial flow, propeller or axial flow, and
mixed flow, as shown in Figure 29-3. Figure 29-3 also shows the relationship between
the impeller type and specific speed. Specific speed is the speed (in revolutions per
minute) at which a given impeller would operate if reduced proportionately in size so
as to deliver a capacity of 1 gpm (0.063 L / s) against a total head of 1 ft (0.31 m).
2
Specific speed is defined as:
rpm
Q
N
(U.S customary units)
s
H
3/4
To calculate
N
s
in terms of foot-pound units when the flow and head are expressed in
metric units:
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