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

with the necessity of throttling back the pump discharge valve, and this
can result in a variety of hydraulic and mechanical problems in the
pump.
The ramifications of all such situations are generally imposed on the
maintenance department. Unfortunately, the training in that group has
traditionally been limited to the physical change-out of the parts when a
breakdown occurs. As the underlying cause of pump failure often
extends well beyond the failed item, these maintenance methods will
effectively reinstall the same old problem. This is particularly
concerning when we realize that over 80% of all pump failures tend to
manifest themselves at the mechanical seal or the bearings, which then
act in a manner similar to a fuse in an electrical system.
When a fuse in an electrical system fails, it doesn't mean there is
anything wrong with the fuse! In fact we understand that the problem
is almost always somewhere else in the system. In spite of this, when a
seal or bearing fails, we rarely look for the real problem. Instead, we
simply replace the offending part. While that will occasionally solve the
problem, the simple change-out of a seal or bearing rarely provides
long-lasting relief from the problem. Consequently, we have to review
the two different types of pump problems- those that are either
operational or reliability in nature- as well as some sources of these
problems, and some tools for identifying their underlying causes.

11.2 Operational problems

This is the type of problem where the pump simply doesn't produce the
hydraulic results for which it was intended. A typical example of this
occurs when the centrifugal pump isn't pumping enough liquid
through the system. While this is often blamed on an inadequate pump,
there are many other conditions that could be to blame, such as is
shown in Figure 11.1. The problem may be a blocked inlet line or air
entering the inlet line or even, in an appropriate case, a lack of priming.

11.2.1 Cavitation problem solving
Another example is when the pump is operated so far away from its
design point that it begins to vibrate as a result of a variety of hydraulic
conditions identified on Figure 11.2.
However, the biggest problem in all this is to identify which of three
hydraulic conditions we are faced with when the common symptoms of
noise and vibration are experienced.
This is accomplished by the throttling of the discharge valve which
reduces the flow through the pump and creates three possible scenarios.

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