Page 313 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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8.2 Pump Characteristics 273

120
Tank full, source
110
at low level
100

90

80 Pumps
2 and 3
70 Pumps
Total head, ft 60 Pump 1 and 3

3
Pumps
Tank empty,
50
source at Pump 1 and 2
2
high level
40 Pump Range in
1 system head
30
Minimum demand Normal demand Maximum demand
20

10

0
0 10 20 30 40 50 60
Rate of pumping, MGD
Figure 8.5 Pump Selection in Example 8.4. Conversion factors : 1 ft 0.3048m;
3
1 MGD 3.785 MLD 0.0438 m /s 43.8 L/s.

closed. As the head falls past the point of maximum efficiency, normal discharge, or
rated capacity of the pump (point 1 in Fig. 8.4), the power continues to rise. If a cen-
trifugal pump is operated against too low a head, a motor selected to operate the pump
in the head range around maximum efficiency may be overloaded. Pump delivery can
be regulated (a) by a valve on the discharge line, (b) by varying the pump speed me-
chanically or electrically, or (c) by throwing two or more pumps in and out of service
to best advantage.
What happens when more than one pumping unit is placed in service is shown in
Fig. 8.4 with the help of a curve for the system head. Obviously, pumping units can oper-
ate only at the point of intersection of their own head curves with the system head curve. In
practice, the system head varies over a considerable range at a given discharge (Fig. 8.5).
For example, where a distributing reservoir is part of a system and both the reservoir and
the source of water fluctuate in elevation, (a) a lower curve identifies head requirements
when the reservoir is empty and the water surface of the source is high, and (b) an upper
curve establishes the system head for a full reservoir and a low water level at the source.
The location and magnitude of drafts also influence system heads. Nighttime pressure dis-
tributions may be very different from those during the day. How the characteristic curves
for twin-unit operation are developed is indicated in Fig. 8.4. Note that the two identical
pumping units have not been selected with an eye to highest efficiency of operation in