Hydrocarbon Processing - April 2022 - 25

Maintenance and Reliability
where high bearing temperatures and
short life are certain to exist. Excessive
preload will also exist if all tolerances
are in the center of their
permitted ranges, but the shaft
material is a type of stainless steel
that thermally expands up to 17%
greater than that of the tool steels
normally used in pump shafts.2
Hopefully, we also investigate
the condition of our oil rings
(FIG. 3), which, should be concentric
within 0.002 in. (0.05 mm).2
To stay within these recommended tolerances,
oil rings must be heat-stabilized.
So as not to run downhill and abrade as
in FIG. 3, oil rings cannot be allowed on
shafts that are out-of-horizontal. Both
shaft alignment and horizontality must be
near-perfect. If a lubricant is thicker than
allowed, the oil ring may simply not feed
enough oil into the bearings.
Whenever two or more deviations
In the event of equipment failures, process facilities
must find and remedy the root causes by pursuing
repeatable failure analysis and troubleshooting.
Guesses are rarely correct: they can lead to costly
and even dangerous fixes.
come across bearing housings with oil
rings designed to operate in ISO VG 32
lubricants but barely capable of operating
while immersed in an ISO VG 100 lubricant.
Repeat malfunctions were tolerated
and were soon accepted as the norm. The
reasons for inflated maintenance budgets
and random downtime events should have
been evident to experienced observers.
combine, failures are likely to become
more serious. We have seen two oil rings
[a maximum allowable eccentricity of
0.002 in. (0.05 mm)] with eccentriciThe
case of the missing drain holes.
For good measure, our third example again
involves an overheated pump bearing and
indications of black matter floating in the
reason for bearing distress. Access to a
cross-section view was available in this
instance and facilitated finding the likely
cause of overheating. It was noted that oil
could collect near the outboard sides of
the two bearings in FIG. 4. The pump manufacturer
forgot to drill a drain passage for
the oil trapped behind each bearing.
Since there were no drain holes, it
could be reasoned that trapped oil turning
to viscous tar or solid coke contributed to
the early bearing distress reported by one
of the manufacturer's Canadian customers.
When this was brought to the attenties
of 0.017 in. (0.43 mm) and 0.061 in.
(1.55 mm), respectively. We have also
lubricant. FIG. 4 helps us understand one
additional and often considered " elusive "
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Hydrocarbon Processing - April 2022

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