Environmental Engineering Reference
In-Depth Information
component ( H t ), the pump total dynamic head is defined by solving for H p in
equation (5).
(V 2
V 1 )
(P 2
P 1 )
H p
=
+
+
Z 2
Z 1 +
H L
(13)
2 g
γ
For a pump supplying water between two reservoirs or tanks, the pump head
required to produce a given discharge can be expressed as
H p
CQ 2
=
Z
+
H L or
H p
=
Z
+
(14)
Z 1 )isreferredtoasthe
static lift . Figure 9.5 shows a system curve for a pipe having a static lift of 65m
and moderate friction losses. When the reservoir elevations are variable, a family
of system curves will exist, describing the varying Z values and corresponding
flow rates.
Figure 9.5 shows three pump curves, which represent a single pump with three
different impeller sizes. The intersection of the system and pump curves identifies
the flow rate and the head ( H p ) generated by the pump for each impeller size.
Figure 9.5 also includes pump efficiency, cavitation (net positive suction head
required, or NPSHr), and brake horsepower (bhp) information. For the system
represented in Figure 9.5, impellers A and B would be a good choice based on
efficiency. According to Figure 9.5, pump B will produce a head and flow rate of
93m and 480 l/s for this system. NPSHr represents the minimum upstream total
head, relative to absolute vapor pressure (datum) at the suction side of the pump
required to avoid cavitation levels that will decrease the pump performance. bhp
is the power added by the pump to the flowing water.
C is defined in equation (12), and Z (i.e., Z 2
Figure 9.5 Example system and pump curves
 
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