A recent call to troubleshoot a horizontally-split multistage pump reminded me of my good old days working for an OEM. When an OEM de-stages a standard design, an important engineering step is properly sizing the center bushing. Unfortunately, when requested to de-stage a multistage pump, some repair shops do only that. They take one (or several, depending on the target flow/head/power conditions) of the stages off, replace it with a straight-through bushing and leave all the remaining internal components intact.
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Consider a split-case four-stage pump, as shown in Figures 1 and 2.
In this example, this four-stage design will be de-staged to bring its flow from 2,450-gpm/840-psi differential to 2,170-gpm/690-psi differential.
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The initial four-stage design flow pass is depicted in Figure 3.
Note the high axial thrust acting on each impeller. However, due to back-to-back arrangement, the four impellers completely balance their respective thrusts.
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When Stage #1 is removed from the unit, no other modifications are made to the center bushing or any other components. The axial thrust increases tremendously (see Figure 4).
The axial load for this unit is absorbed by size 7410 ball bearings. Calculated L10 life is reduced from 600 years to 215 hours! No wonder the unit's bearings start to fail soon after the repaired (de-staged) unit is re-installed.
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The correct approach should have been to change the size of the center bushing to compensate for the unbalanced axial thrust created due to impeller removal. Calculations are shown in Figure 5.
While the calculations above are somewhat simplified (parabolic pressure distribution changes their values and would need to be calculated more precisely), they are intended here to show conceptually the approximate difference in magnitude of axial force acting on the bearings. A significant change in bearing life would result if a center bushing diameter is not adjusted to compensate for the unbalanced thrust due to de-staging.
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The lesson here is not to cut corners when doing repairs or engineered upgrades, but pay attention to important engineering principles. Neglecting these principles can cause problems. Engineered upgrades of multistage pumps should have as much, if not more, attention to technical details as the initial OEM design.
Pumps & Systems, September 2008
