fluid_mechanics

660 Chapter 12 ■ Turbomachines
500
8 in. dia
50%
55
60
63
65
Head, ft
400 7
300
6
200
100
0
NPSH R
5
0 40 80 120 160 200 240 280 320
0
Capacity, gal/min
15
65
63
60
50
55
40 bhp
30
25
20
15
10
NPSH R , ft
F I G U R E 12.12 Performance
curves for a two-stage centrifugal
pump operating at 3500 rpm. Data given for
three different impeller diameters.
of the flowrate and reaches a maximum value at some particular value of the flowrate, commonly referred
to as the normal or design flowrate or capacity for the pump. The points on the various curves
corresponding to the maximum efficiency are denoted as the best efficiency points 1BEP2. It is apparent
that when selecting a pump for a particular application, it is usually desirable to have the
pump operate near its maximum efficiency. Thus, performance curves of the type shown in Fig.
12.11 are very important to the engineer responsible for the selection of pumps for a particular flow
system. Matching the pump to a particular flow system is discussed in Section 12.4.4.
Pump performance characteristics are also presented in charts of the type shown in Fig. 12.12.
Since impellers with different diameters may be used in a given casing, performance characteristics
for several impeller diameters can be provided with corresponding lines of constant efficiency
and brake horsepower as illustrated in Fig. 12.12. Thus, the same information can be obtained from
this type of graph as from the curves shown in Fig. 12.11.
It is to be noted that an additional curve is given in Fig. 12.12, labeled NPSH R , which stands
for required net positive suction head. As discussed in the following section, the significance of
this curve is related to conditions on the suction side of the pump, which must also be carefully
considered when selecting and positioning a pump.
Cavitation, which
may occur when
pumping a liquid, is
usually avoided.
12.4.3 Net Positive Suction Head (NPSH)
On the suction side of a pump, low pressures are commonly encountered, with the concomitant
possibility of cavitation occurring within the pump. As discussed in Section 1.8, cavitation occurs
when the liquid pressure at a given location is reduced to the vapor pressure of the liquid. When
this occurs, vapor bubbles form 1the liquid starts to “boil”2; this phenomenon can cause a loss in
efficiency as well as structural damage to the pump. To characterize the potential for cavitation,
the difference between the total head on the suction side, near the pump impeller inlet,
p sg V s2 2g, and the liquid vapor pressure head, p vg, is used. The position reference for the elevation
head passes through the centerline of the pump impeller inlet. This difference is called the
net positive suction head 1NPSH2 so that
NPSH p s
g V s 2
2g p v
g
(12.24)
There are actually two values of NPSH of interest. The first is the required NPSH, denoted
NPSH R , that must be maintained, or exceeded, so that cavitation will not occur. Since pressures
lower than those in the suction pipe will develop in the impeller eye, it is usually necessary to determine
experimentally, for a given pump, the required NPSH R . This is the curve shown in Fig.
12.12. Pumps are tested to determine the value for NPSH R , as defined by Eq. 12.24, by either
directly detecting cavitation, or by observing a change in the head-flowrate curve 1Ref. 72. The second
value for NPSH of concern is the available NPSH, denoted NPSH A , which represents the head
that actually occurs for the particular flow system. This value can be determined experimentally,
or calculated if the system parameters are known. For example, a typical flow system is shown in