Hydrocarbon Processing - April 2022 - 24

Maintenance and Reliability
ertheless, accept our premise that we
need focus and must concentrate on realistic
events and solutions.
To begin, we observe that work execution
requires emphasis on any two of the
three attributes shown in FIG. 1. If a manager
demands quality work done cheaply
and quickly, we must tactfully train that
manager to let us know which two will
be accepted, since achieving all three
will be impossible. Frankly, it is in everybody's
best interest that our work output
be good. That leaves it to the manager to
accept that good is your unyielding standard,
but he/she may instruct you to either
be fast (and expensive) or cheap (and
slow, by prevailing standards).
In this context, a useful focus is readily
obtained by accepting that all machinery
failures, be they turbo-compressors or
machines making cotton-tipped ointment
applicators or doorknobs, will exTake
any two!
GOOD
CHEAP
FIG. 1. Good managers know that competent
staffers will give their employer any two of
these three. One of these three attributes
will be unachievable.
FAST
perience distress attributable to only one
of seven possible categories:1
1. Design deficiencies
2. Material defects
3. Processing and manufacturing
deficiencies
4. Assembly errors
5. Off-design or unintended service
conditions
6. Maintenance deficiencies (neglect,
procedures)
7. Improper operation.
Of equal importance is recognizing
that the basic agents of machinery component
and parts failure mechanisms are
always force, a reactive environment, time
and temperature (FRETT). Each of these
four can be further subdivided into steady,
transient, cyclic or durations labeled very
short, average length, long and so forth.
The gasket case. An expert failure investigator
will take pleasure in reaffirming
the above by stating that regardless of
whether the component is made of plastic,
steel, rosewood or porcelain, when it
breaks the basic agents of failure mechanisms
are found in FRETT, and FRETT
alone. Once this fact is accepted, we have
a starting point for investigating our first
example, a leaking gasket. Looking at
the data, it will not be difficult to determine
which of these four basic agents can
be crossed off our list. If only 6 wk had
elapsed since the gasket was inserted between
a pump discharge nozzle and its
adjacent pipe flange, it would be safe to
delete " time " from the acronym FRETT.
Suppose the remaining five gaskets in
the various flanges associated with the
downstream discharge piping had been in
service for close to 4 yr and were of the
same material composition as the leaking
gasket. In that case, we might cross
off both " reactive environment " and an
abnormal operating " temperature. " That
would then leave us with " force. "
Clamping forces are a function of bolting
torques and bolting procedures, bolt
sizes, fabrication methods, hardness and
bolt materials. All of these must harmonize
with well-established guidelines-
failures will result when we deviate from,
or disregard, established procedures. If
we allow several deviations to exist at the
same time, failures become a certainty.
Data relating to bolt size, material, gasket
deformation, flange condition, method
and sequence of bolt tightening will
lay bare the cause of gasket leakage. Of
course, the human factor remains. If the
two workers working on that flange were
denied necessary tools or time, we can
state that the case of the leaking gasket
may have its roots in a measure of somebody's
flawed thinking.
The case of the overheated bearing.
Our second example involves a pump
bearing that showed signs of temperaturerelated
failure. We must resist the temptation
to say that the " bearing failed, therefore
the bearing is at fault. " Instead, we
might carefully look at failure frequency
and failure record. If the pump manufacturer
has used 600,000 identical bearings
in 300,000 pumps of the same model designation,
we have no reason to re-engineer
the pump for a larger bearing.
As we then look at the seven possible
FIG. 2. Effect of preload on bearing life.1
24 APRIL 2022 | HydrocarbonProcessing.com
FIG. 3. The oil ring on the right ran " downhill "
and contacted a stationary part. Abraded
bronze particles contaminated the lubricant
and caused short bearing life.
failure cause categories, we should cross
off numbers 1, 2, 4, 5, 6 and 7. We look
more closely at (3), " processing and manufacturing
deficiencies. " If the shaft was to
be 75-mm diameter with a plus-tolerance
of 0.02 mm-0.03 mm but we carefully
measure and record 75.04 mm, and the
bearing bore was to be 74.99 mm with
a minus tolerance of 0.01 mm-0.02 mm
but is measuring 74.96 mm, there will be
an interference fit of 0.08 mm (0.003 in.).
According to the trend curve in FIG. 2,
such an interference fit will cause bearing
operation deep in the high-preload range
http://www.HydrocarbonProcessing.com
Hydrocarbon Processing - April 2022

Table of Contents for the Digital Edition of Hydrocarbon Processing - April 2022
