Environmental Engineering FOURTH EDITION

Water Supply 13 1

Pump Characteristic Curves

The total head developed by a pump, the power required to drive it, and the resulting
efficiency vary with the capacity. The interrelations of head, power, efficiency, and
capacity are shown in Fig. 6-14 and are calledpump characteristic curves:

0 The head capacity curwe H-Q shows the relationship between capacity and total

head. Pumps are often classified according to the shape of the head capacity curve.

The ejjiciency curve rpQ shows the relationship between capacity and efficiency.

The power curve P-Q shows the relationship between capacity and power input.

System Head Curves

Head loss in a pumping system increases with increasing flow through the system, and
can be shown graphically as a system head curve like that in Fig. 6-15. The system
head loss for any flow rate is the sum of friction head loss and the total static head in
the system. Static head is present whether the pump is operating or not, and is plotted
as the lower portion of the system head curve.
Friction losses, including minor losses, are determined as outlined in the previ-
ous section of this chapter. The system piping and fittings may be converted to one
equivalent pipe, and head losses for several flow rates may be determined readily from
the Hazen-Williams nomograph. Alternatively, the head loss through all pipes and
fittings may be computed for a single flow rate, and losses for other flow rates may be
determined from the relationship

0.54

-=[?I Qi.

Q2

7

160 80 8

180 90

C

70 .g 0

60 E

50 '

140

U

c - 100

RI

9 80 40

2 60

8 120

c

c

30 4-

(u

40 20

20 10

0 0

0 2 4 6 8 10 12 14 16 18 20 22

(6.15)

Capacity in 100 G.P.M.

Figure 6-14. Pump characteristic curves.