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The Practical Pumping Handbook iii .........................................................................................................

used in that pump. Maintenance may have tried many different models,
types and face material combinations, but the seal fails with the same
frequency every time. As it is logical to expect different seals to last
different periods of time between failures, it becomes evident fairly
quickly that this is a situation where the seal is simply acting as the
'fuse' in the system. Consequently, the underlying problem is obviously
elsewhere in either the pump or the system.
This is where sldll and experience come in to play. The condition
described above is one where the experienced troubleshooter would
immediately consider either the pump shaft or the piping arrangement,
depending on the pump model in question.

11.4.2.1 Hydraulic imbalance in a double suction pump

For example, a horizontal, double-suction pump may be fitted with a
90 ~ elbow mounted on the suction nozzle in such a way that the line
leading to the elbow is parallel with the axis of the pump shaft as shown
in Figure 11.4. When the liquid sweeps round the elbow it is
centrifuged out towards the long radius and feeds the eye on one side
of the impeller, effectively starving the opposing eye. This creates an
imbalance of the liquid in the pump casing that can cause an excessive
axial thrust to be imposed on the impeller.

Figure 11.4 D/S pump with suction elbow (Reproduced with permission of Goulds Pumps, ITT
Industries)

The normal outcome of such an arrangement is a consistently frequent
failure of the mechanical seal or (when packing is fitted) the bearing, at
the end of the shaft closest to the suction source. Such failure will
normally occur at approximately 6 month intervals regardless of the
type of seal or bearing that is installed.

11.4.2.2 Undersized shaft in an end suction pump
In a horizontal, end suction centrifugal pump, frequent and regular seal
failure with different seals indicates an undersized shaft that is being
subjected to excessive deflection.

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