Fluid Flow
CENTRIFUGAL PUMPS
Cavitation can be a very serious problem for centrifugal pumps. Some pumps can be designed
to operate with limited amounts of cavitation. Most centrifugal pumps cannot withstand
cavitation for significant periods of time; they are damaged by erosion of the impeller, vibration,
or some other cavitation-induced problem.
Net Positive Suction Head
It is possible to ensure that cavitation is avoided during pump operation by monitoring the net
positive suction head of the pump.
Net positive suction head (NPSH) for a pump is the
difference between the suction pressure and the saturation pressure of the fluid being pumped.
NPSH is used to measure how close a fluid is to saturated conditions. Equation 3-19 can be used
to calculate the net positive suction head available for a pump. The units of NPSH are feet of
water.
NPSH = Psuction - Psaturation
(3-19)
where:
Psuction
=
suction pressure of the pump
Psaturation
=
saturation pressure for the fluid
By maintaining the available NPSH at a level greater than the NPSH required by the pump
manufacturer, cavitation can be avoided.
Pump Laws
Centrifugal pumps generally obey what are known as the pump laws. These laws state that the
flow rate or capacity is directly proportional to the pump speed; the discharge head is directly
proportional to the square of the pump speed; and the power required by the pump motor is
directly proportional to the cube of the pump speed. These laws are summarized in the following
equations.
(3-20)
V n
(3-21)
Hp n2
(3-22)
p n3
Rev. 0
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