POWER February 2011 - 18

Internal wear and operation at low flow
also cause pump cavitation. The impeller
regions subjected to a given pressure
will vary with progressive wear. Prevailing
pressures multiplied by effective impeller
areas will generate a thrust force. An
increase in this thrust force can greatly
contribute to bearing failures because
bearing life varies inversely and exponentially
with load. Moreover, on BFW pumps
with sleeve bearings and associated shaftdriven
worm gear lube supply pumps, excessive
thrust often leads to worm wheel
damage and loss of lubrication.
Pump operators and reliability professionals
need to pause and consider each of
these risk factors independently and in total.
Pump owners must accept that, as several
risk factors accumulate, there will be a
time when one seemingly small deviation
could result in catastrophic pump failure
and perhaps cause an outage of the entire
plant. Therefore, it should be a priority to
examine all probable causes and factors
that have recently combined or contributed
to costly repeat failures of BFW pumps.
Simply put, operating in the low-flow
range forces BFW pumps to run in the flat
portion of the H-Q curve. By the time pressure
sensors signal a small pressure difference
in the flat portion of the curve
and control action is initiated, large differences
in throughput will have occurred.
Flow control and load sharing are difficult
in the flat portion of a pump performance
curve, and long-term satisfactory pump
operation is simply not possible in the
forbidden low-flow range.
Rebuilding and Upgrading Are
Urgently Needed
The best possible remedial action is to
rebuild and upgrade all the plant's BFW
pumps, for example, and to then use controls-automated
or manual-that ensure
pump operation in the steep region, or
close to BEP. The next step is to couple the
upgrades with good operator training that
reinforces the BFW pump system design rationale
and proper operating procedures.
Rebuilding BFW pumps affords an opportunity
to upgrade older BFW pumps
to current designs and modern materials,
such as high-performance polymer-based
wear rings and throat bushings. Rebuilding
all pumps to original equipment manufacturer
(OEM) specifications should be
entrusted to either the OEM or a reputable
pump rebuilder.
Should you elect to rebuild your key
process pumps, consider adopting the following
criteria when preparing your purchase
order specifications.
18
For example, many process pumps are
equipped with slinger rings that supply
oil to the bearings. To perform well, these
oil rings (slinger rings) must be concentric
within 0.002 inches (0.05 mm) and have
a bore finish reasonably close to 32 RMS.
Oil rings will not survive long if the shaft
system is not truly horizontal or if the
depth of immersion in the oil is either too
much or too little. Although an ISO Grade
32 lubricant is required on pumps equipped
with sleeve bearings, mineral oils with this
viscosity grade are seldom satisfactory for
BFW pumps with rolling element bearings.
In theory, rolling element bearings will
benefit from ISO Grade 68 mineral oils. This
thicker ISO grade is required for rolling element
bearings, and major bearing suppliers
recommend ISO Grade 68 mineral oils up to
176F. However, using the thicker ISO Grade
68 mineral oil will slow down the oil slinger
rings and will, furthermore, risk depriving
sleeve bearings of the oil wedge on which
the shaft must ride.
Special issues and potential problems
thus arise if, in any pump, both sleeve type
and rolling element bearings share the same
bearing housing. In essence, viscosity is of
greatest importance, and each bearing type
fails if there is prolonged metal-to-metal
contact. To prevent this contact, the oil
film has to be thicker than the asperities
(the surface roughness) in the bearing and
journal surfaces. The oil film must also be
thicker than an occasional dirt particle traveling
in the oil. Most importantly, the oil
needs to be properly applied and must form
a suitable film on the surfaces where such
a film is most needed. To make a long story
short, a lubricant with all required performance
attributes, including film strength
and film thickness, must, at the same time,
allow slinger rings to function properly.
Only well-proven synthetic ISO Grade
32 lubricants will satisfy all of these requirements.
A preferred supplier usually
formulates such lubricants from a PAO/Diester
synthetic base oil to which an ionic
bonding agent has been added. The oil
will have to be viscosity Grade 32 and the
particular synthetic ISO Grade 32 formulation
must give users the protection and
film thickness/film strength properties of
ISO Grade 68 mineral oils. Though these
properties may not be needed elsewhere
in your facility, their use is appropriate in
BFW pumps that have recently and unexpectedly
proven repair-intensive.
While troubleshooting lubrication issues
on existing pumps, sludge in pump bearing
housings is often found. Water acts as a catalyst
that promotes sludge formation. Sludge
is thus often the result of water and atmowww.powermag.com
POWER
| February 2011
spheric dirt, in addition to oil ring (slinger
ring) debris. Exposure to airborne particulates
is unavoidable in some environments,
and water intrusion is possible in other environments.
Another possible source of water
is from cracked water jackets. There is some
irony in that situation, because with rolling
element bearings, cooling water may not be
needed in the first place.
The troubleshooter should also take
care not to mix two " virtually identical "
oils from different suppliers. There is ample
evidence that some oil suppliers skimp
on the amount and/or quality of additives
to reduce cost. If you are buying lubricants
based on lowest cost, then you are
encouraging suppliers to provide a " commodity
product, " and commodity lubrication
products may not be able to provide
the reliability desired.
During a recent plant visit, an oil slinger
ring that had been removed from a BFW pump
after the latest pump failure was measured.
Instead of staying within the allowable eccentricity
of 0.002 inches, this ring was about
0.06-inch eccentric-30 times the allowable
value. The ring was abraded on one side; it
also showed very serious discoloration, like
that shown in Figure 2. If you use slinger
rings, make your selection based on quality,
not price. Good ones will have gone through
an annealing step before finish-machining.
Again, be certain to subject slinger rings to
rigorous specifications and quality control.
On pumps furnished with rolling element
bearings, avoid slinger rings by insisting
on bearing housings configured
3. Three bearing housing protector
seals. Pay close attention when specifying
pump O-rings. Some configurations risk Oring
degradation (top left) due to contact with
sharp grooves. Others use a V-contoured ring
(top right) that tends to increase frictional drag.
Look for optimized designs (bottom) when
choosing an O-ring. Source: Heinz P. Bloch
http://www.powermag.com
POWER February 2011

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