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As one of the parts that is manufactured to the most exacting of
tolerances, the ball bearing does not deserve it's reputation in
the process pump market as being one of the two parts of a pump
that is constantly failing. Yet that is the reality. To alleviate
this situation, we need some understanding of what bearings do and
what we need to do in order to let that happen.
One of the challenges that face pump users quite frequently,
comes into play when more than one pump is required to operate at
the same time on the same system. With such multiple pump systems,
there are two arrangements where the operating characteristics of
the different pumps can be considered to provide a single combined
performance curve.
"One Size Fits All" most definitely does not apply in this area
of pumping. Even when we define a Slurry as a mixture of solid
particles in a liquid that is usually water, we are still
concealing a multitude of applications behind a simple sounding
phrase. The variety of solids that are handled in slurry form
covers an extraordinary wide range of products and waste
material.
To one degree or another, we've all done it! We've found
ourselves so far into the trees, we can't find the forest. In other
words, we get too close the problem to identify a logical solution.
On one occasion, I overheard a discussion on the merits of having a
new pump supplied with a stainless steel bearing housing. The logic
offered was that an acid was leaking from a valve above the
proposed location of the pump. The leak would land directly on the
pump bearing housing and cause considerable corrosion damage. This
discussion ranged back and forth for quite a while before someone
inquired, "Why don't we stop the leak!".
Any practical discussion of mechanical seal operation starts
with the acceptance that it takes more than just a good seal to
secure the leakage in a centrifugal pump. The seal must be properly
installed and protected in a strong pump that is properly designed
to accommodate a mechanical seal.
Once referred to as the "black box" inside a pump that no-one
really understands, the mechanical seal is that other part of the
process pump that is constantly failing. To help alleviate that
unworthy accolade we must understand the most important aspects of
the mechanical seal.
At the end of every batch, the pump was vibrating so badly, it
would shear the hold-down bolts and move off the baseplate. The
pump in question was used to empty the raw wort (essentially
unprocessed beer) from a large tank. The line to the pump suction
came from a bottom connection in the tank, through a 90 degree
elbow and a short section of line to the suction flange of the
pump.
There are three major problems associated with poor pump
piping.
1. There is a scarcity of accessible information available on the
topic.
2. No one pays any attention to it when installing a pump.
3. It can remain undetected and cause repetitive pump failures for
many years.
As a consequence of 1 and 2 above, most pumps are piped up
incorrectly. In fact when we look at the way many pumps have been
installed, it resembles a "plumbers nightmare."
Is this really cavitation I am dealing with, or is it air
entrainment or recirculation? The reason for that question is that
all three conditions have almost identical symptoms. Air
entrainment and recirculation will also cause the same
rumbling/rattling noise and high vibration as cavitation, as well
as the recognizable impeller pitting damage. The major difference
is that cavitation is an NPSH problem, while the other conditions
have nothing to do with the suction pressure or NPSH.
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