POWER February 2011 - 16

Deferred Maintenance Increases
Pump Failures
If your facility has recently seen an upsurge in bearing failures on
boiler feedwater (BFW) pumps, you are not the only plant experiencing
these unnecessary and costly failures. The failure causes are often
elusive, which is why plants have so many unresolved repeat failures.
During an economic downturn, maintenance outlays and training
funds frequently are among the first to be curtailed. Maintenance
managers are then able to authorize only those repairs they deem
absolutely necessary to keep the plant running. In effect, these decisions
often encourage treating the pump failure symptoms and discourage
looking for the true root causes of failures. In time, the plant
will incur higher maintenance expenditures and an increase in forced
outages caused by deferred maintenance. The underlying causes of
pump failures must be determined and corrected if plants are to continue
reliable operation.
Why Bearings Go First
One plant component that is often the victim of deferred maintenance
is the boiler feed pump, especially those in the 200-kW
to 2,000-kW range. The effects of deferred maintenance often
appear as increased wear and tear on lubricated components. For
example, we currently see fewer BFW pump bearings reaching the
end of their design life.
Also, and in spite of using better lubricants and installing
bearing protector seals, pump bearings tend to be the parts that
fail first. Seeking to avoid bearing problems, many facilities have
placed greater emphasis on vibration-monitoring programs, allowing
operators to initiate pump shutdown a short time before
disaster strikes. Having shut down just in time, people then congratulate
themselves for accomplishing what management has
decreed; that is, repairs were deferred until absolutely necessary
and only the glaringly defective parts got replaced.
Deferring a full repair does not address the root cause of the failure,
and repeated failures put both equipment and human resources
at risk. Although the failed part is the weakest link in the component
chain and needs to be replaced, something else is pushing the weakest
link toward premature failure. We often forget that repeat failures
are the precursor to far more serious events. More failed pumps per
time period shorten the mean-time-between repairs (MTBR) metric.
This is most noteworthy because statistics, described in the Pump
User's Handbook: Life Extension, found that a serious fire occurs for
every 1,000 pump failures.
Underlying Causes of Failure
Some facilities that are worried about unexpected BFW and other
large pump outages decide to operate two or more pumps in parallel.
However, for reasons of gaining power efficiency, the performance
curves of many of these pumps were originally designed
to be relatively flat at flows approaching shutoff. The resulting
pressure rise from operating point to shutoff is then either insufficient
or nonexistent (Figure 1). As one or more pumps are
operating too close to shutoff, shafts deflect and bearings are
overloaded. The oil film that must separate bearing rolling elements
from stationary elements thins and heats up the bearings,
causing a vicious failure cycle. High load, a thin oil film, and
high metal temperature combine to result in premature bearing
failure. Depending on the bearing cage type, these failures range
16
from gradual and detectable to sudden, difficult-to-detect-inadvance,
and just plain catastrophic.
Running fewer pumps in parallel would result in each pump
operating closer to its best efficiency point (BEP). In contrast,
operating too many pumps often causes one or more to operate
in the prohibited low-flow range, especially if their respective
head-capacity (H-Q) curves (Figure 1) are not identical. Internal
recirculation and progressive wear increase the difficulty
of successfully operating pumps in parallel. Mechanical parts
distress and seemingly small deviations from the least-risk geometry
of best available designs now converge, and the BFW
pump will become involved in a string of seemingly random
failures. Impeller erosion and loss of internal clearances will
have occurred as well.
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POWER | February 2011
1. On the level. A typical " flat " pump performance or head-capacity
curve with undesirable low-flow characteristics is illustrated. The best
efficiency point (BEP) is found at 1,750 rpm. Source: Heinz P. Bloch
NPSH
BEP
800
750
700
650
600
550
500
Head vs flow
40
20
100
90
80
70
60
50
40
Horsepower
Shutoff
400
800
1,200
Gallons / min.
Note: NPSH = net positive suction head.
2. New slinger available. This photo shows a new slinger ring
(left) next to an abraded slinger ring (right). Courtesy: Heinz P. Bloch
1,600
1,750
Flow at BEP
2,000 2,400
Total head (ft)
NPSH in feet
Efficiency (%)
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POWER February 2011

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