Page 89 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

Figure 6-18
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Solid contamination will destroy the tolerance between the OD of the
eye and the bore of the pump housing.
This specific tolerance governs the efficiency of the pump.
The tolerance between the OD of the impeller eye and the internal
bore of the pump housing is set at the factory based on the temperature
of the application and thermal growth of the pump metallurgy. This
tolerance tends to open with time for a number of reasons. Among
them: erosion due to the passage of fluid, the lubricating nature of the
liquid, suspended solids and sediment will accelerate the wear,
cavitation damage, play in the bearings, bent shafts and unbalanced
rotary assemblies, and any hydraulic side loading on the shaft and
impeller assembly.

Wear bands
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Some pump companies will design replaceable wear bands for the OD
of the impeller eye and the bore of the pump housing. It’s said that the
pump loses 1.5% to 2% efficiency points for every one thousandths wear
in a wear band beyond the factory setting. Therefore, by changing wear
bands, the pump is returned to its original efficiency. Because of this,
the term wear band is a misnomer. A better term would be ‘efficiency
band’ (Figure 6-19).

The replaceable wear bands can also be made in a machine shop in a
pump maintenance function. It is important that the new wear band
material is made of a non-galling, and non-sparking material softer than
the pump housing metallurgy. Plastic, composite, fiberglass and carbon .
graphite wear band are perfectly good. Be sure the material is
compatible with the pump’s metallurgy and the pumped liquid. It’s not

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