Civil Engineering Reference
In-Depth Information
TABLE 29-1. Application of Affinity Laws in a Case of Constant Impeller Diameter and
a Decrease in Impeller Rotation Rate
Q
(gpm)
H
1
(ft)
BHP
1
(hp)
E
1
(%)
Q
2
(gpm)
H
2
(ft)
BHP
2
(hp)
E
2
(%)
0
100
7.5
—
0
43
2.1
—
200
102
9.9
52
130
44
2.8
52
400
100
13.5
75
260
43
3.8
75
600
92
17.0
82
400
40
4.8
82
800
75
18.3
83
530
32
5.1
83
1,000
52
18.8
70
660
22
5.3
70
(From Culp, Gordon, and Williams, Robert,
Handbook of Public Water Systems.
Copyright
1986 by John
Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.)
Note:
In Fig. 29-8,
N
1
1,750 rpm and
N
2
1,150 rpm. Therefore, in this case,
N
1
/
N
2
0.66; (
N
1
/
N
2
)
2
0.43; and (
N
1
/
N
2
)
3
0.28.
An example of the application of the affinity laws showing a reduction in rpm is
given in Table 29-1 and in Figure 29-8. As the rpm is reduced, the flow and head
are correspondingly reduced. If the head / flow / efficiency curve is known for one pump
speed, then the affinity laws can be used to determine the curve for another pump
speed.
Centrifugal pumps produce variable flows as total pumping pressure changes, as
shown in Figure 29-8. Therefore, with centrifugal pumps it is incomplete to indicate
a particular pump capacity unless the pumping head is also described. The pump curve
shown in Figure 29-8 also illustrates that the efficiency of pumping is variable for
different head or pressure conditions. This particular pump curve indicates that the
power requirement increases with increasing flow capacity. Other centrifugal pumps,
however, may require greater power at lower pumping capacities because of the char-
acteristics of the pump.
NET POSITIVE SUCTION HEAD
Each liquid has distinctive vapor pressure, which varies with temperature. For example,
water has a vapor pressure of 0.6 ft (0.18 m) at 60
F (16
C). In pump and piping
systems, it is typical for the lowest pressure to occur at the pump impeller inlet. Should
the pressure at this point be below the vapor pressure of the liquid, the water will boil
and cavities will form. The cavities will collapse rapidly as they move into higher-
pressure regions in the pump. The collapse of these cavities, termed
cavitation,
is
accompanied by noise and vibration. Often the collapse of the cavities occurs against
the impeller or pump casing and causes erosion of the pump itself.
In order to design a pumping system in which the water will flow to the pump and
operate without cavitation, it is necessary to make certain that there will be a positive
suction pressure at the pump inlet. Also, as mentioned previously, the specific speed
of the pump must be limited to avoid cavitation. The available net positive suction
head (NPSH) at the pump inlet is defined as follows:
144
NPSH
(
P
P
)
h
(U.S. customary units)
a
vp
s
W
(29-3)
102
(
P
P
)
h
(metric units)
W
a
vp
s
