For a given centrifugal pump operating at a constant speed, the flow rate through the pump is dependent upon the differential pressure or head developed by the pump.
The lower the pump head, the higher the flow rate. A vendor manual for a specific pump usually contains a curve of pump flow rate versus pump head called a pump characteristic curve. After a pump is installed in a system, it is usually tested to ensure that the flow rate and head of the pump are within the required specifications. A typical centrifugal pump characteristic curve is shown in Figure 11.
There are several terms associated with the pump characteristic curve that must be defined.
Shutoff head is the maximum head that can be developed by a centrifugal pump operating at a
set speed.
Pump runout is the maximum flow that can be developed by a centrifugal pump
without damaging the pump. Centrifugal pumps must be designed and operated to be protected from the conditions of pump runout or operating at shutoff head.
*Positive Displacement Pump Characteristic Curves
Positive displacement pumps deliver a definite volume of liquid for each cycle of pump operation. Therefore, the only factor that effects flow rate in an ideal positive displacement pump is the speed at which it operates. The
flow resistance of the system in which the pump is operating will not effect the flow rate through the pump.
Figure 21 shows the characteristic curve for a positive
displacement pump.
The dashed line in Figure 21 shows actual positive
displacement pump performance. This line reflects the
fact that as the discharge pressure of the pump increases,
some amount of liquid will leak from the discharge of the
pump back to the pump suction, reducing the effective
flow rate of the pump. The rate at which liquid leaks
from the pump discharge to its suction is called slippage.
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