Page 245 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

4. Identify the causes supporting the evidence. Example: What could
cause low pressure? The cause is the origin of the failure.
The causes of low pressure, for example, could be either hydraulic or
mechanical. In many cases of failure analysis, asking ‘Why?’ and ‘What?’
and answering those questions, until you can no longer ask ‘why’, will
almost always get you to the answer. If all evidence leads to a
mechanical reason for the failure, the problem is probably maintenance
induced. If the evidence leads to a hydraulic reason for the failure, the
problem is either operations or design induced. In cases where the
‘reason for failure’ was not determined, a more extensive analysis is
necessary. The additional analysis is recommended to take advantage of
the pump supplier experience in identifying the root cause.

A paper mill was using an ANSI en d suction process pump with clear water service.
The motor was desig ped properly. The pump axial t ,hrust bearing ran hot, failing after
three months of ope )ration. It was replaced with ai i identical bearing. This ran during
three months and al so failed. All pump component s were investigated and found that
they complied with the specificatioi ns. These facts eliminated the defects of materials
~
as a cause.
All the failed parts in the unit were inspected to assure that they were manufactured
according to specifications. This ste p eliminated defects from the factory.

The maintenance team, that is to sa ~y, the mechanics, were found competent and they
followed the correct maintenance p rocedures. This information eliminates the defects
by maintenance.
Although the pump was being run at 25% of the BEP (Best Efficiency Point), it was
designed for this type of service. This eliminated improper operation or running
outside of the Sweet Zone.
There remains only one cause to er :plore, the design of the pump. The manufacturer
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was contacted and their design grolrp stuoIcu tf1C >IWdLIUII. DV UUCIIIIIU UdtdllCC IIUIC~
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on the face of the impeller, they reduced the heat generated by 70°F. The
manufacturer also incorporated a flin! jer ring lubrication system, a bigger oil reservoir
to cool the oil and improved the oi I circulation. The plant has not experienced a
failure of the pump since !the redesigned pump was ir
ed pump was installed.

Why is this pump in the shop?

Did you ever notice that the building or area in the plant called the
‘maintenance shop’, is actually the Pump Hospital? The shop may have
twelve workbenches, but ten benches have a pump in some stage of
surgery. You go into the shop and ask someone ‘Why is this pump in

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