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

Minimum bearing design life:
For B73.1 ANSI pumps, the minimum bearing design life is L10 of 17,500 hours. The closer to minimum flow, the higher the radial thrust will be and it will shorten the bearing life. Perhaps a higher quality or different design bearing will offer better reliability. Ask your manufacturer for the L10 life of their bearings. Pumps with dual volutes, diffusors or with congruent impeller-to-casing centerlines will have fewer issues than pumps of single volute design.

Suction pressure:
Suction pressure has a large effect on axial bearing life in the case of HO1 configured pumps (end suction top discharge). The higher the suction pressure, up to the limits of the pump, the longer the axial bearings will last.

Suction energy, suction energy ratio:
Suction energy is another method for evaluation of the liquid momentum.

SE = (Deye) (N) (NSS) (SG) where D is the diameter of the impeller suction eye in inches. N is the pump speed in revolutions per minute (rpm). NSS is the Suction Specific Speed and SG is the Specific Gravity of the fluid. The particular SE in this case is 80.5 X 106 which is very low and of minor concern. Normally a SE of 160 X 106 would be the lower threshold for concern for a high suction energy pump.

It is also the corresponding factor to use in the ratio calculation. The corresponding ratio in this case is well below one (1) and so the negative effects of recirculation and cavitation are minimized.

For end suction pumps with a suction size of 6 inches or larger at 3,550 rpm or 10 inches on a 1,750 rpm machine there would be a higher concern for minimum flow boundaries.

Materials:
The damaging factors in minimum flow issues are the deleterious effects of cavitation damage on the impeller; material selection is one method for treating those symptoms. Discuss with the manufacturer or consultant regarding material choices and their resistance to cavitation damage.

Because the materials for the example pump were high on the resistance scale (Hastelloy), there was little concern. Inconel materials would be near the top of the scale while gray cast iron and aluminum would be near the bottom.

Pump speed:
Pump operating speed (N) is an important factor when evaluating allowable minimum flow borders. Based on 60 hertz supply, 1,750 rpm speeds and lower would be markedly better than 3,550 rpm or higher. Shaft deflection occurs twice during one revolution of the shaft so a pump operating at 3,600 rpm would deflect 7,200 times per minute. The deflections are accumulative in the shaft stress cycle analysis.

Factors outside the control of the manufacturer:
The basic system design, how the pump is selected and how it is operated and maintained has significantly more impact on the reliability and expected pump life than most all factors controlled by the manufacturer.

References
HI/ANSI/ASME B73.1 2012

ANSI/HI 9.6.3- 2012 Guideline for Allowable Operating Region (Rotodynamic Pumps)

The Pump Handbook (4th edition), Igor J. Karassik, Paul Cooper, Charles Heald et al

Centrifugal Pumps, Igor J. Karassik and Terry McGuire

Handbook for Pump Life Extension, Heinz Bloch and Allan R. Budris

The Effects of NPSH Margin, SE and Air on Pump Reliability. 1998 Texas A&M Pump Users Symposium, Allan R. Budris and Phillip Mayleben.

To read other articles in the 'Common Pumping Mistakes' column, go here.

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