CENTRIFUGAL PUMP OPERATION
It may also be possible to stop cavitation by reducing the NPSHR for the pump. The NPSHR is
not a constant for a given pump under all conditions, but depends on certain factors. Typically,
the NPSHR of a pump increases significantly as flow rate through the pump increases.
Therefore, reducing the flow rate through a pump by throttling a discharge valve decreases
NPSHR. NPSHR is also dependent upon pump speed. The faster the impeller of a pump rotates,
the greater the NPSHR. Therefore, if the speed of a variable speed centrifugal pump is reduced,
the NPSHR of the pump decreases. However, since a pump's flow rate is most often dictated
by the needs of the system on which it is connected, only limited adjustments can be made
without starting additional parallel pumps, if available.
The net positive suction head required to prevent cavitation is determined through testing by the
pump manufacturer and depends upon factors including type of impeller inlet, impeller design,
pump flow rate, impeller rotational speed, and the type of liquid being pumped.
manufacturer typically supplies curves of NPSHR as a function of pump flow rate for a particular
liquid (usually water) in the vendor manual for the pump.
Centrifugal Pump Characteristic Curves
For a given centrifugal pump operating at a constant speed, the flow rate through the pump is
Figure 11 Centrifugal Pump Characteristic Curve
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
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. Additional information may
be found in the handbook on Thermodynamics, Heat Transfer, and Fluid Flow.