Machinery Lubrication May June 2011 - Welcome to ECN Digital ...

VIEWPOINT
HEAD
Low flow
cavitation
High
temperature
rise
Structured Failure Analysis
Strategies Solve Pump
Problems
BY HEINZ P. BLOCH, P.E.
Repeat pump failures are clear indications that the root
causes of problems were not found. Or, in case the problem
cause is known, someone must have decided to do nothing about
it. Either way, pursuing a structured failure analysis approach is
necessary to solve problems. Guessing or “going by feel,” will
never do. Fortunately, there are powerful methods we can use to
uncover the root causes of repeat problems and these are highlighted
in this article.
We define structured analysis as a repeatable approach which
can be learned and employed by more than one person. 1
Once an accurate analysis is documented, remedial steps can
be defined and implemented. Logic, common sense and a
process of elimination are applied in this process. Suppose it
could be established that a pump at location “A” suffers more
failures than an identical pump at location “B.” One would
determine what’s different on the pump in “A” and compare that
to “B.” All differences are found in component deviations (or
deviations from best practices) in one or more of the following
seven cause categories:
Lower impeller life
Discharge
recirculation
Lower bearing
& seal life
Reliability Curve
Suction
recirculation
Best Efficiency Point
Lower bearing
& low seal life
Cavitation
80% 110% % FLOW
Figure 1. Staying near the center of this
“Reliability Curve” is a wise course of action
• Faulty Design
• Material Defects
• Fabrication and/or Processing (Machining) Errors
• Assembly or Installation Defects
• Off-Design or Unintended Service Conditions
• Maintenance Deficiencies, including Neglect/Procedures
• Improper Operation
Searching for additional cause categories will not add value
because anything uncovered will, at best, be a subset of these seven.
However, if one systematically concentrates on eliminating five or
six of the seven categories in succession, one will arrive at the category
where a deviation exists. That will make it possible to
concentrate on understanding what led to the deviation. 2
The pump person must pay close attention to the under-appreciated,
generally non-glamorous “basics,” and do so before opting
for the generally costly, sometimes unnecessary, and often unprofitable,
high-tech solution. Pumps obey the laws of physics and
there is always a cause-and-effect relationship. It follows that even
seemingly elusive and generally costly repeat problems can very
often be eliminated without spending much money.
An integrated, comprehensive approach to failure analysis starts
by either describing the deviation, or by isolating the problem.
Next, such an approach encourages, or even mandates, careful
observation and definition of failure modes. The approach should
employ pre-existing or developed-as-you-go checklists and troubleshooting
tables. Many specific checklists have been provided by
pump manufacturers and helpful generic varieties can also be
found in a very large body of recent literature. 1-3
The “FRETT” Approach to Eradicating
Repeat Failures of Pumps
From observation and examination of a failed pump part one
identifies failure agent(s), realizing that there are only these four
possibilities: 3
• Force
• Reactive Environment
24 May - June 2011 www.machinerylubrication.com Machinery Lubrication