Page 49 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

Internal re-circulation
This is a low flow condition where the discharge flow of the pump is
restricted and the product cannot leave the pump. The liquid is forced
to recirculate from high-pressure zones in the pump into low-pressure
zones across the impeller.
This type of cavitation originates from two sources. First, the liquid is
circulating inside the volute of the pump at the speed of the motor and
it rapidly overheats. Second, the liquid is forced to pass through tight
tolerances at very high speed. (These tight tolerances are across the
wear bands on enclosed impellers, and between the impeller’s leading
edges and the volute casing on opened impellers.) The heat and the
high velocity cause the liquid to vaporize.
With the pump disassembled in the shop, with open impellers, the
damage is seen on the leading edge of the impeller blades toward the
eye of the impeller, and on the blade tips toward the impeller’s OD.
With enclosed impellers, the damage reveals itself on the wear bands
between the impeller and the volute casing. See the illustration (Figure
3-2).
To correct this condition with an opened impeller, it’s necessary to
perform an impeller adjustment to correct the strict tolerance between
the blades and the volute. Some back pullout pumps are designed with
jack bolts on the power end of the bearing housing to easily perform
this adjustment without pump disassembly.
This condition cannot be corrected on pumps with an enclosed
impeller. You need to relax the restricted discharge flow on the pump.
The problem could be a clogged downstream filter, a closed discharge
valve, an over-pressurized header (back-pressurizing the pump), or a

CLOSED
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DISCHARGE

Figure 3-2
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