Page 154 - Know and Understand Centrifugal Pumps

Shaft Deflection

The seal chamber internal bore corresponding to the posterior of
the mechanical seal OD.

Interpreting the evidence
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Let's interpret the physical evidence that you might see at these close
tolerances and their source. To begin:
1. You might see gouge or wear marks all around the circumferences
of close tolerances on the rotary elements, and a corresponding
wear spot at approximately 60" from the cutwater on the stationary
elements.

This would be induced or caused by operation, if the plant
operators strangle valves to control production flow. Any other
discharge flow restriction (clogged filter, pipe obstruction, or
un-calibrated automatic valve) would produce the same
evidence. Talk with the plant engineer about this situation and
show him the evidence on the pump.
This could also be induced by design, if the pumps are
oversized, or by high velocity and friction head in discharge
piping of inferior diameter.
This could be induced or caused by maintenance in cases where
the mechanic installs a check valve in reverse, or uses inadequate
practices when rebuilding valves, cutting and placing flange ring
gaskets at pipe joints, or exchanging and replacing incorrect
valves. Our recommendation is to always use good maintenance
practices.

L

Of the three sources of problems, design, operation and maintenance, the mechanic is
really responsible for a small part. The truth is that the majority of pump problems I
begin with changes to design, and plant operations after the system was
commissioned.

If the condition should be occasional, the solution could be to
install a variable speed motor. If the condition is permanent, the
solution could be to reduce the impeller diameter, replace the
pump, or increase the diameter of the pipe. If normal operations
require living with the condition, then increase the diameter of the
pump shaft to improve the L/D factor.