Pumps can last for decades when operated correctly.
by Jim Elsey
January 24, 2018

Suction specific speed:
As suction specific speed (NSS) decreases, the requirement for minimum flow can also be relaxed. The NSS for the example impeller is 7223 (based on U.S. Customary Units [USCU]) which is well below any normal concerns that designers express. Normally pumps with NSS approaching 11,000 are a concern for applications at low minimum flows due to suction recirculation issues. Refer to published technical papers by Simon Bradshaw and Jerry L. Hallam for more information on this subject. Due to the lower suction specific speed for the example pump, the minimum flow concern can be relaxed.

vane overlapFigure 2. Example with no vane overlap (above) and with vane overlap (below)

Impeller vane overlap:
The example pump has a five-vane semi-open impeller. See Figure 2 for an example of vane overlap. On a design scale, the vane overlap on this impeller is high and thus mitigates potential issues with recirculation. Impellers with lower vane overlap will facilitate discharge pressure to recirculate flow back to the suction due to less interference from the vanes. If your pump has little to no vane overlap, add that mitigating component to your reliability equation.

In this case, the vane overlap mitigates issues with operation to the left of BEP. Note that many services, such as slurry and trash applications, require impellers be designed with little to no overlap.

Impeller inertia:
Because of the small size and weight of the example impeller, it will contribute lower radial forces on the rotor dynamics. It follows intuitively that an impeller weighing 100 pounds will have more effect than one that weighs five pounds. While I cannot offer an actual reliability formula, in this case the impeller weight would be a direct relation.

Overall impeller-to-eye diameter ratio, or D1 to D2:
The ratio of the overall diameter of the maximum impeller size compared to the diameter of the inlet eye may sometimes be a concern. Note this ratio is directly related to NSS (refer to NSS technical papers).

The example impeller is of little concern. The overall maximum diameter (D2 dimension) is 6.0625 inches and the eye (D1 dimension) is 3.14 inches.

In summary, the larger the suction eye is in relation to the overall diameter, the more likely there will be an issue with recirculation at lower flows.

Specific speed:
Specific speed (NS) is a mathematical expression for the overall geometry of the impeller. In this particular instance, there is a five-bladed impeller and we are looking for a curve shape that has a continuous rise to shutoff as the curve moves right to left from the BEP design flow point back to shut-off (zero flow).

By virtue of the curve shape and the anticipated intersection of the example system curve to the Pump Curve, this shape indicates that the pump will not hunt or oscillate at the low flows approaching minimum flow and shutoff.

NPSH margin, fluid temperature and vapor pressure:
Another guideline is the net positive suction head (NPSH) margin (NPSH available as compared to NPSH required). The higher the margin, the more stable the pump will be at low flows. This is partially due to the fluid’s initial temperatures and subsequent temperature increases (delta T) that can occur when the fluid is recirculating at the suction eye and nearby impeller blade edges. As the fluid temperature increases, the corresponding vapor pressure will also increase to the point of cavitation bubble formation and subsequent collapse causing vibration/shock and erosion.

The lower the fluid temperature and the higher the NPSH margin, the more stable the pump will be at the lower flows and approaching minimum flow.

Shaft stiffness/deflection ratio:
While not an option in this example due to the fluid’s acidic properties, the use of a solid shaft would reduce the shaft deflection ratio (L3/D4) from 143 to 64. Shaft deflection ratio should be one of the major components in the equation for pump reliability at low flows. From the mathematical formula aspect, keep in mind the reverse (indirect) relationship. The lower the number, the better.

Design to counter deflection due to unbalanced radial forces at low flows:
All fully compliant ANSI pump manufacturers must design the shaft and bearing system for less than 0.002 inches of shaft deflection at the seal faces when operating at minimum flow. The design for the example pump stays well below that requirement (actual deflection of 0.0004 inches as compared to design maximum 0.002 inches).

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