Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

Roiling Element Bearings and Lubrications 545
because the system pressure is very low, oil mist is a reliable lubrication
method. Proper lubrication system operation can be interlocked with
pump operation or an alarm system, assuring adequate lubrication.
The savings due to lower preventive maintenance labor requirements,
equipment repair cost avoidance, and reductions in unscheduled production
outage events have been very significant and cannot possibly be
overlooked by a responsible manager or cost-conscious manufacturing
facility. Oil mist systems have become extremely reliable and can be used
not only to lubricate operating equipment, but to preserve stand-by, or
totally deactivated ("mothballed") equipment as well.
The majority of centrifugal pump installations using oil mist lubrication
have, as of 1991, applied open, once-through mist flow. However,
closed systems with virtually no losses to the environment do exist and
should be the preferred configuration in the future 14, 2],
Selecting Rolling Element Bearings for Reduced Failure Risk
Less than 10% of all ball bearings run long enough to succumb to normal
fatigue failure, according to the Harden Corporation [13]. Most
bearings fail at an early age because of static overload, wear, corrosion,
lubricant failure, contamination, or overheating. Skidding (the inability
of a rolling element to stay in rolling contact at all times), another frequent
cause of bearing problems, can be eliminated by ensuring the bearing
will always be loaded. With pairs of angular contact bearings, preloading
may be necessary.
Actual operations have shown that better bearing specification practices
will avert the majority of static overload problems. Other problems
caused by wear, corrosion, and lubricant failure, contamination and
overheating can be prevented by the proper selection, application, and
preservation of lubricants. Oil viscosity and moisture contamination are
primary concerns, and higher-viscosity lubricants are generally preferred
[3]. The detrimental effects of moisture contamination are described
in Table 20-3.
Unlike API pump bearings, which petrochemical companies often specify
for a B-10 life of 40,000 hours, ANSI pump bearings are selected on
the basis of an expected 24,000-hour life. Nominally, this means that
90% of the ANSI pump bearings should still be serviceable after approximately
three years of continuous operation. However, the failure statistics
quoted by MacKenzie [13] indicate that conventionally lubricated
ANSI pump bearings do not even approach this longevity. Lack of lubrication,
wrong lubricants, water and dirt in the oil, and oil-ring debris in
the oil sump all cause bearing life expectancies to be substantially less. It
must be assumed that similar findings by other major users of ANSI