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unable to move. This can be considered as the maximum Head the
pump can develop and, at this point, the pump will continue to run,
but will be unable to push the liquid any higher in the pipe. Under
these conditions, liquid is agitated in the pump casing, but there is no
flow passing through the pump, therefore the flow rate is Zero at this
Maximum Head.
If we cut holes in the discharge
pipe at progressively lower levels,
the Head is effectively reduced,
and the pump will develop an
increasing flow rate. By graphically
depicting these results as shown in
Figure 2.4, the characteristic pump
performance curve is drawn.
It should be noted that this curve
is not completed down to Zero
Head, as a centrifugal pump does
not operate reliably beyond a Figure 2.4: Effect of reducing head on
certain Capacity. Consequently, at capacity
that point, the curve is usually
discontinued.
This curve identifies the Capacity
which this pump can develop, and
the Total Head it can add to a
system and is, therefore, usually
referred to as the 'Head-Capacity'
curve. In addition, when depicted
as in Figure 2.5, it is frequently
referred to as the 'Single Line
Curve' as it displays the per-
formance of the pump when one Q
particular impeller diameter is
installed and the pump is run at a Figure 2.5- Single line pump performance
predetermined speed. curve

.2.1 Efficiency
The Head-Capacity capability requires a certain amount of power to be
supplied to the pump. The actual quantity of power required will be
dependent on how efficiently the pump operates.
The Efficiency represents the percentage of the total power used in the
direct development of the Capacity and the Total Head. In general
terms, Efficiency is the work produced by a machine divided by the
work supplied to that machine.

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