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Retrofitting process pumps
Bearing isolators as an alternative to lip seals
Nearly all rolling element bearings fail long before attaining the
design life published by the bearing manufacturers. The use of
non-contacting bearing isolators in process pumps can eliminate failures
by keeping the lubricant contaminant-free and maintaining an adequate
supply of lubricant in contact with the bearings. Moreover, since the
return on investment (ROI) on the replacement of lip seals with bearing
isolators in pumps is normally greater than 100%, the cost of purchasing
and installing them is paid back over a year’s time.
Bearing failure is a leading cause of unplanned downtime in process
industries such as pulp and paper, chemical processing and automotive
machining. Production losses can be staggering, sometimes measuring in the
hundreds of thousands of euros per minute! Lost production, combined with
expenditures for maintenance costs and parts, can represent millions of
euros in unrealized profits.
What is now commonly known as a “bearing isolator” was invented in 1977
(patent #4,022,479). Until then, bearings were minimally “protected” by
rubber seals or open labyrinths that were short-lived or inadequate in
their ability to withstand contamination.
Lubrication Basics
The quality and quantity of lubricant interacting with rolling element
bearings is crucial to the reliability and longevity of rotating
equipment, as is a basic understanding of bearing lubrication concepts.
Rotating equipment used in industrial processes depends upon smooth and
reliable bearing performance. Bearings and other components require
lubricants with specific properties to meet their individual needs. A
lubricant is any material introduced between two relatively moving
surfaces so that the friction between them is reduced. Tribology, the
science of lubrication, deals with the design, friction, wear and
lubrication of interacting surfaces in relative motion (as in bearings or
gears). Rolling element bearings require an elastohydrodynamic film
between the rotating members for proper lubrication. That’s the theory. An
example of lubrication theory: If two steel shafts, rotating in opposite
directions at high speed, are brought together, end-to-end, under
hydraulic pressure, friction instantly makes the shafts glow red hot and
even melt. This technique is used in friction welding. If, on the other
hand, the shafts are not forced into contact and are held apart by as
little as one ten-thousandths of (0.0001) of a cm, there will be no
friction and no temperature increase. This simple principle of separation,
by a pure uncontaminated viscous lubricant, is the basis for dependable,
predictable, bearing performance. The result is no contact, low heat and
no wear. Viscosity is also an important characteristic of a lubricant. It
is the measure of a liquid’s resistance to flow. If, for example, a
bearing’s ball or roller is turning inside an outer race, the viscous
lubricant causes the rolling member to “hydro-plane”, and metal to metal
contact is prevented. Generally, a lubricant is selected with the minimum
viscosity necessary to provide adequate separation of the working elements
of the bearing. Excess viscosity will result in friction and heat
generation within the bearing.
Various Types of Lubricants
Almost all industrial lubricants are based on animal, vegetable or mineral
oils. Mineral oils, usually petroleum based, are graded light, medium and
heavy, according to their viscosity. They contain about 85% hydrogen and
about 12% carbon (hydrocarbons). The remaining 3% of the content differs
widely to adapt the mineral oil lubricant to specialized industrial
functions. Machine, turbine, engine, spindle, compressor, cylinder and
gear oils are examples of specifically designed mineral oil derived
lubricants. Greases are sometimes more convenient to use than liquid oil
lubricants. They are, however, in high speed or severe duty applications,
somewhat of a compromise. They do not provide the cooling effects of an
oil-type liquid lubricant. Greases are solid to semi-solid products that
contain thickening agents, usually in the form of metallic soaps.
Typically, they are about 80% oil. The remainder is the thickening agent.
Only the oil portion lubricates. Lime, sodium, clay and lithium thickened
greases are the most common.
Additives and Alternatives
Generally, conventional petroleum-derived lubricating oils deteriorate and
break down (oxidize) after a few months of use. Formulators have said that
in as little as 2.400 hours of use, even the finest lubricating oils
deteriorate below minimum lubricant specifications.
Anti-oxidants (corrosion inhibitors) and extreme-pressure additives are
commonly used to augment the properties of mineral oils for lubricants. In
addition, such diverse substances as anti-foam, anti-bacteria, detergent,
colorant and odor-masking additives are used, to name a few. It is the
eventual loss of these additives that make frequent oil changes necessary.
As an alternative to mineral oils, synthetic lubricants have been
developed that have proven to be vastly superior to conventional petroleum
products. Petroleum lubricants are a blend of large, viscous molecules and
distillates from the crude that are thin and unstable. Synthetics are
uniform and homogeneous in nature. They do not evaporate or oxidize. Many
synthetic lubricants, properly kept uncontaminated, will perform under
normal operating conditions for two years or longer. This extended
operating range negates the objection to the higher initial cost of the
synthetic product.
Bearing Life
Projections
The L-10 life projection for the # 6309 bearing, commonly used in medium
sized ANSI pumps, is 443.322 hours, or 50,57 years. By definition, this
life projection is expected to be exceeded by 90% of bearings. The loading
is what would be expected running a 2 x 3 x 10 pump at 10% of its best
efficiency point (BEP). Even though the bearing is quite heavily loaded,
the expected life is off-scale compared to real world experience. Why the
huge disparity? The fatigue life calculation assumes perfect lubrication.
Unless the bearing is run with lubricant that is of sufficient quantity to
adequately keep the rolling elements from contacting each other and of
perfect quality (uncontaminated), the theoretical results will never be
attained. In other words, such pristine conditions exist only in a bearing
test laboratory. Or do they?
Bridge the Gap
In order to realize the benefits of long, reliable bearing life, it is
necessary to replicate those conditions that allowed the bearings to
perform so spectacularly in the lab. Primarily, we are concerned about the
bearings in process pumps of the run-of-the-mill variety. It has been
estimated that more than 3.000.000 of these pumps exist in North America
alone. Common ANSI or “split case” pumps are relatively simple machines
with two or three bearings each. In the majority of cases the bearings are
adequately sized, if not oversized, to accommodate a shaft designed to
have minimum runout for a mechanical seal. Generally, the bearing
selection is not the limiting factor for determining useful bearing life.
Misalignment can be a problem, since it may be responsible for radially
loading the pump and driver bearings, and thereby accelerating wear and
premature bearing failure. In industrial applications, lubricants are by
far the major cause of bearing deterioration. As previously discussed, the
lubricant must be retained in the bearing enclosure and remain dry and
uncontaminated to perform to its full potential. The technology to
accomplish this end is conveniently available.
Bearing isolators
Bearing isolators of the non-contact, non-wearing, vapor blocking variety
have been proven to absolutely protect the bearing (lubricant) enclosure
from loss of lubricant and, at the same time, totally prevent the entrance
of contamination such as moisture, dust or chemical fumes. They are not a
temporary fix. Good ones last for a decade or more. Most new process duty
pumps are equipped with bearing isolators or some type of labyrinth style
bearing protection device when they are shipped from the factory. Some
carry a three year power frame warranty. However, the real productivity
gains will be realized when the already installed base of process pumps is
retrofitted and brought up to current standards of performance and
reliability. Bearing isolators are readily available from authorized
distributors, pump manufacturers and from bearing isolator manufacturers
directly. The most reliable bearing isolators are of metallic construction
and are nearly always available for same-day shipment. Since the return on
investment (ROI) on the replacement of lip seals with bearing isolators in
pumps is normally greater than 100%, the cost of purchasing and installing
them is paid back over a year’s time. The costs of unplanned downtime,
production losses and emergency maintenance costs are added benefits, and
they are not included in the stated ROI. Most bearing isolators are
performance guaranteed, so there is no risk to the user.
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