Internal recirculation

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 re circulate 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

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 arc 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

DISCHARGE

CLOSED

DISCHARGE

Figure 3-2

chock valve installed backwards, or operating the pump at or close to shut-off head. Maybe a variable speed motor could help under certain circumstances.

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