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The Practical Pumping Handbook ................................................................................. ii ....... i:ii iiiiiii

When an anticipated pump performance is identified on the
Characteristic Pump Performance Curve at (for example) 500 gpm at
60 feet total dynamic head, it should be noted that the 500 gpm will
only be achieved when the differential pressure across the pump is at 60
feet of head. If the pressure in the system changes at any time, causing a
change in the total dynamic head across the pump, the flow rate will
also change accordingly. As the total dynamic head increases, the flow
will decrease, and as the total dynamic head decreases, the flow will
increase. Further details on this topic are discussed in Chapter 3.
2.2.3.1 The best efficiency point (BEP)

It has already been stated that the Best Efficiency Point (BEP) is the
most stable condition at which the pump can operate. Therefore, in
order to achieve the highest degree of reliability possible for the pump,
it should operate as close as possible to the BEP. If the operating flow
moves away from the B EP, that reliability decreases. Consequently most
pump users will attempt to operate their pumps within a range of 70%
to 120% of the BEP. Depending on the hydraulic design of the pump
and the service involved, it may be necessary to operate even closer to
the BEP.
While the pump can still operate outside this range, it does so at the
expense of the reliability of the seals and bearings, as a number of other
conditions start to be a factor which will detrimentally impact these
items. Other parts of the pump, such as the impeller, volute and shaft
will also be subjected to the adverse operating conditions which can
contribute increased erosion and fatigue impact to these items.

2.2.3.2 Pump run-out
As every centrifugal pump does not operate reliably beyond a certain
flow rate, the published performance curve for that pump is dis-
continued at that point. This is referred to as the Run-Out condition.
Operation of the pump beyond that point (and often, even approaching
that point) will cause damage to the pump and will also frequently
overload the motor driver.

The damage caused at high flow rates will frequently be a result of
cavitation as the increase in flow rate through a pump requires a much
higher Net Positive Suction Head. Further details on this matter will be
discussed in Chapter 4.

2.2.3.3 Minimum flow point
On every pump curve, a number of 'minimum' flow points can be
identified, depending on the operating requirements and equipment
reliability standards of the individual end user.

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