Oil-mist lubrication has been
applied successfully to pumps and motors in the hydrocarbon
processing industry (HPI) since the late 1960s.1
Many large-scale installations have served industry well all
over the world (Fig. 1). As of 2011, the US and Canadian refining industries alone had over
1,500 large-scale oil-mist systems in operation. Of the more
than 60,000 machines lubricated by these systems, around half
were purchased with oil mist as the intended lube application
method. The other half had been originally furnished with
conventional oil lubrication and was later converted to
Fig. 1. A modern, large-scale
Since overhung process pumps and
their electric-motor drivers are the most common machines in
the HPI, one can expect pumps and motors to predominate among
equipment lubricated by oil mist. However, it is important to
note that numerous other machine types also benefit from oil
mist; these include gearboxes, blowers, turbines and
pillow-block bearings, to name a few. After a brief recap and
introduction, this article will highlight strategies for
lubrication system conversion for pumps and electric
Why oil mist?
In and of itself, oil-mist
lubrication does not cure or prevent every
conceivable lubrication-related failure of rotating equipment.
Oil mist will not heal a compromised bearing, and a
pre-existing defect can culminate in a bearing failure.
However, properly applied oil mist will indeed extend bearing
life when compared to most alternative lubrication methods.
The main reason for this life
extension is that oil mist provides an ultra-clean environment.
Introduced into a bearing housing at slightly higher than
atmospheric pressure, oil mist precludes the entry of
atmospheric contaminants into running, as well as non-running,
machinery. Oil mist lubricates, preserves and protects;
large-scale oil-mist systems also require far less maintenance than do virtually all of
the traditional methods. Risk-inducing components or items that
often require considerable maintenance attention (such as
oil-slinger rings and constant-level lubricators) are
completely eliminated in equipment lubricated by pure oil
History of use
Since the late 1970s, many papers
and several books have been written and published about
oil-mist lubrication.26 These references
remind the reader that much of the early reluctance to use oil
mist was attributable to plugging issues caused by
wax-containing paraffinic oils. There also were failures due to
not following proper pipe installation procedures. With modern
installation practices and the use of non-paraffinic oils,
plugging events have become a thing of the past.
Less-than-optimum oil-mist delivery
methods presented another hurdle for early oil-mist
applications on pumps. In old systems, a single reclassifier
was typically mounted at or near the center of the bearing
housing; venting took place through the bearing housing shaft
protrusion (Fig. 2) or at the housing end seals. Bearing
windage (a fan effect), unbalanced housing-internal pressures,
and housing-internal passageways also could cause mist to
bypass certain bearings.2
Fig. 2. Old-style oil-mist
wasteful and imprecise.
These unanticipated conditions led
to occasional bearing failures that were primarily observed on
double-row thrust bearings. Fortunately, competent oil-mist
system suppliers have been successful in pointing out these
oversights. Their modern oil-mist systems are virtually
trouble-free and require less maintenance than does any other
method of lube application.
Pure oil-mist application
Although oil mist occasionally is
used to simply purge the vapor space above liquid lubricants in
bearing housings, the experience-tested conversion strategies
described in this article pertain exclusively to the more
prevalent pure oil mist applied to rolling-element
bearings in modern hydrocarbon processing plants.
Pure mist, sometimes described as
feeling dry to the touch, is produced (or
generated) at the oil-mist generator module (Fig.
3, left side). It initially exists as atomized, transportable,
small-size globules. These sub-micron-sized, fog-like globules
are conveyed via a piping or header system to the various
bearing locations. A reclassifier fitting near the bearing
converts the oil mist to large, wet-to-the-touch oil
Fig. 3. Mid-sized oil-mist console
serving 10 to
20 pump sets in a refinery (left). A
vessel (right) allows total environmental
compliance and resource-friendliness of this
closed oil-mist system.
Another way to describe the
dry oil mist is an oil fog that ultimately has to
pass through a reclassifier, which essentially is an orifice
fitting. This orifice fitting must be located no farther than 2
meters (6.6 feet) from the bearing to be lubricated.
Reclassifiers create a turbulent region where mist velocity
increases. At high velocities, the small, atomized mist
particles contact each other and then combine into larger
droplets. Larger droplets are heavy and cannot remain suspended
in the carrier air. They become wet mistlarge
enough to plate-out and fully coat rolling bearing
The preferred through-flow mist
application method shown in Fig. 4 was adopted by the American
Petroleum Institute (API) a number of years ago. Here, oil mist
passes through the rolling elements from side to side or top to
bottom. Oil-mist through-flow will ensure proper lubrication of
the bearings. At low flow or no flow, oil globules would
ultimately fall out of suspension, and only carrier air
deprived of oil would then surround the bearings.
Fig. 4. Through-flow oil-mist
application per the
current edition of API-610. A modern bearing
protector seal is shown at each end of this
After the oil mist flows through
the bearing, it continues to travel toward the vent or drain
(outlet) shown in Fig. 4. Should the vent/drain/outlet port be
too small, it would create back pressure in the housing that
could restrict the flow through the bearing. The decreased flow
into and through the bearings would, of course, reduce the
amount of oil actually reaching the bearings. Experience shows
that vent/drain port diameters should be three to four times
larger than the diameter of the reclassifier or orifice.
Adherence to this general rule and to related parameters should
be closely monitored. The rule takes on greater importance when
more than two reclassifiers are feeding into a common bearing
Process pumps that comply with
current editions of API-610, both overhung and between bearing
styles, are designed for oil-mist flow through the bearings
(Fig. 4) prior to exiting the housing. Modern systems are
closed; i.e., the coalesced lubricating oil or spent oil mist
leaving at the drain is returned to a collecting vessel (Fig.
3, right side). The through-flow arrangement of Fig. 4 is more
important on multiple-row bearings; however, through-flow is
not always necessary with single-row bearings in applications
such as electric motors, where the load is constant and the
thrust is not usually excessive.
Oil-mist retrofitting to pumps
Overhung and between-bearing pumps
are fairly easy to retrofit or convert to pure oil mist in the
field. Such retrofitting is entirely possible without equipment
shutdown since the bearing housings are essentially
self-contained and are not contacted by process fluids.
Draining the original liquid lubricant and applying oil mist
are accomplished by removing drain plugs; these can be left
open or connected to a common drain header. The oil mist
inlet/supply tubing is then attached to the bearing housing or
to the housing end caps.
Again, pure mist is used in
rolling-element bearings at reliability-focused plants, whereas
purge mist is often used on machines equipped with sliding, or
sleeve-type, bearings. (It should be noted that,
with purge mist, additional components are required to maintain
proper levels of liquid oil and to contain the oil
When lubricating rotating equipment
with pure mist, no liquid oil level is maintained in the
bearing housing. The oil sump is empty; it is a dry sump. Power
input (driver hp), shaft speed (rpm), bearing housing internal
configuration and bearing housing seal selection are of
interest and will govern the selection of reclassifier size.
When the driver is 150 kW (200 hp) and above, and when the rpm
is 3,000 or higher, the reclassifiers should be sized for a
heavy service factor. When below 150 kW (200 hp) and at 3,000
rpm or less, the reclassifiers may be sized for a moderate
Older API-style and many ANSI pumps
typically have a single oil refill port located at the bearing
housing top. This is where oil mist can be applied with a
single reclassifier sized to serve both the thrust and radial
bearings. With this application, the oil mist creates pressure
in the central housing; the pressure then forces the mist
through bearings and housing seals. The oil mist provides
lubrication for the outboard rolling element of the
multiple-row bearing, as well as for the single-row radial
bearing found in typical pumps. Many of the older bearing
housing internal configurations benefit from plugging
drain-back ports that connect the two bearing faces. This
precautionary plugging2,5 ensures that oil mist will
pass through the bearings, thereby removing the remote
possibility of oil mist circumventing or bypassing the
In a situation where older bearing
housings are present and oil mist is applied per the
now-superseded method shown in Fig. 2, the low-point vent/drain
would require a defined restriction to ascertain that the oil
mist passes through the bearings before exiting via the housing
seals. Although simple-fixed, rotating-labyrinth and even-lip
seals are often retrofitted on bearing housings of the type
depicted in Fig. 2, blocking the mist exit at the shaft
protrusions may impede through-flow and deprive lube supply to
the outer element on multiple-row bearings. This makes it all
the more important to consider implementing the through-flow
scheme shown in Fig. 4.
Debate has centered on whether oil
rings (slinger rings) should remain in place or be removed when
converting to pure oil mist. Best practice calls for lifting or
suspending these rings until the pump is taken to the shop for
any reason.6 At that time, the oil rings should be
removed. Some sources report that oil rings pose no threat to
the application of pure oil mist; they believe the additional
labor to remove them is unnecessary.
The same sources have observed that
increases in noise and vibration readings likely occur when the
rings are run dry after conversion to pure mist. Since noise
and vibration are forms of energy, experienced reliability professionals are
encouraged to draw the right conclusions from the various
claims and counter-claims. A recent book2 argues in
favor of foregoing oil rings altogether.
Field conversions from conventional
oil sump to pure oil mist involve taking bearing temperature
and vibration readings before, during and after draining the
oil. Observing and recording bearing temperatures and vibration
amplitudes prior to draining the oil provide a baseline
reference. Readings should be taken immediately after the oil
is drained and then again every five minutes for at least 20
Typically, bearing temperatures
will drop 10°F to 20°F (6°C to 12°C) in the
first 10 minutes. This indicates that the bearings are in good
condition, and one should not expect any issues to arise. If
the temperature holds steady or increases, conventional lube
application may need to be resumed, and a high-grade synthetic
lubricant of suitable viscosity should be chosen.
As a standard precaution, the
occasional pump experiencing a bearing temperature increase
should be scheduled for bearing replacement. Chances are that
bearing life was already compromised or the pump impeller
produces high axial thrust loads. Load and temperature are
interrelated, and a different reclassifier may be needed.
In the event that bearing
temperatures escalate, or if the pump is operating at 3,000 rpm
and the driver is 150 kW (200 hp) or greater, the bearing
housing may require two lubrication points so as to provide
sufficient lubrication to the thrust bearing. This modification
also may be performed in the field without taking the pump to
the repair shop. It would, however, require that a small
(1⁄8-inch national pipe thread) port be tapped into the
bearing end cap to allow for another reclassifier to provide
lubrication directly to the thrust bearing. This dual-orifice
arrangement would resolve lubrication issues.
Should bearing failure persist
regardless of the dual-port provision, other pump-related
issues should be investigated. As the pump goes into the repair
shop for work, it would be wise to remove the oil (slinger)
rings, plug the drain-back ports, and tap both end caps to
accept oil-mist reclassifiers. Being certain that the bearing
housing seals are containing the oil mist, configuring the mist
path per the schematic in Fig. 4, and allowing excess mist to
vent only at the bottom port will make an oil-mist application
Older-model between-bearing pumps
commonly have the oil inlet and drain ports on the same plane,
which does not favor the flow of oil mist through the rolling
elements (Fig. 2). Until a shop modification to Fig. 4 is made,
a directional reclassifier must be used in Fig. 2
configurations. Directional reclassifiers extend down into the
bearing housing; the orifice opening is directed at
the midpoint of the top rolling element. The relative proximity
to the top rolling elementtypically 12 mm (1⁄2
inch)allows the mist to overcome bearing windage effects.
Directed reclassifiers are important on double- or multiple-row
bearings arranged for applications per Fig. 2. At times, the
bearing housing may require minor modifications to accommodate
directional reclassifiers. On cylindrical, spherical and
tapered roller bearings, additional measures are necessary to
ensure proper oil-mist application.
Due to the high frictional load
typically acting at the end of the rollers, care must be taken
to ensure that ample oil mist is directed precisely at the
shoulders of the bearing element. At all times, the
through-flow application shown in Fig. 4 will be superior to
Retrofitting oil mist to electric motors
Electric motor construction is not governed by
detailed industry standards such as API. As a result, oil-mist
conversions for motors, although not complicated, require more
labor than do conversion efforts for process pumps. However,
motor manufacturers are making significant strides to meet user
demands and requirements for oil-mist applications. In fact,
some manufacturers have motor designs that inherently
accommodate oil mist with little to no extra work. Over 40
years ago, one foresighted manufacturer in New Jersey simply
stamped its standard totally enclosed fan-cooled (TEFC)
electric motors as Suitable for oil-mist
lubrication.6 Many decades of experience
proved that these motors could last just as long as products
advertised by other manufacturers as being specifically
engineered for oil mist.
TEFC motors (Figs. 5 and 6) that
are grease lubricated are excellent candidates for pure oil
mist. Typically, 15 kW (20 hp) and greater show significant
payback values for oil-mist conversion. For motors not
manufactured specifically to accommodate oil mist, several
factors must be considered during the conversion.
Fig. 5. Oil mist applied to motor
featuring magnetic face seals.
Fig. 6. A typical oil-mist
application on a
standard electric motor.
First, all grease must be removed
from the bearing cavity and the inlet/outlet lines prior to
applying oil mist. Since the low-pressure oil mist is not
capable of displacing grease lodged in motor bearing cavities,
grease removal is accomplished by briefly applying an
intermittent air-gun blast from the plants air supply
header. Next, winding epoxy and lead wire insulation must be
confirmed as compatible with the oil. Most epoxies and
insulation materials used since 1980 have no compatibility
issues; however, this deserves to be reconfirmed.
Also, the internal porting to the
junction box (where the lead wires enter) must be well sealed
with a potting compound to prevent oil mist from entering the
box. Additionally, as oil mist will enter the housing around
the rotor, and since running the motor will cause the mist to
wet out and settle in the lower housing of the motor, a case
drain (Fig. 6) must be installed in the condensate drain plug
port on the coupling end of the motor. Finally, the fan-end
condensate drain plug must be removed and replaced with a
standard plug to prevent mist leakage and venting at the fan
Oil-mist lubrication is a proven
technology when systems are properly installed and mist is
properly applied to rotating equipment. Many systems have been
performing in the HPI for 40 years and have consistently
provided cost-justified results. Closed-loop oil-mist systems
represent the best available technology5 for
lubrication, preservation and protection of process equipment.
Electric motor lubrication (Fig. 7) is always an integral part
of any plantwide lubrication strategy.
Fig. 7. Oil-mist application on a
Oil-mist systems low maintenance requirements, reduced
operating expenses and improved rotating equipment reliability
have resulted in maintenance credits as high as 95% over
conventional lubrication in some major HPI facilities.2,3 Their
versatilitywhich includes flawless operation in all
climates, a lack of temperature limits, prevention of airborne
contaminants, and lubrication for operating equipment that
protects and preserves standby equipmentmakes these
systems valuable to reliability-focused users.
Note: Figs. 1, 3,
6 and 7 are provided courtesy of Lubrication Systems Co. Figs.
2, 4 and 5 are provided courtesy of AESSEAL Inc.
1 Towne, C. A., Practical Experience
with Oil-Mist Lubrication, ASLE Preprint No. 82-4C-1,
ASLE annual meeting, Cincinnati, Ohio, 1982.
2 Bloch, H. P., Pump Wisdom: Problem Solving for
Operators and Specialists, John Wiley & Sons, Hoboken,
New Jersey, 2011.
3 Bloch, H. P., Large-Scale Application of
Pure Oil Mist in Petrochemical Plants, ASME
Paper 80-C12/LUB-25, 1980.
4 Towne, C. A. and D. J. Sheppard, Oil-Mist
Lubrication for Electric MotorsWhere It Stands
Today, IEEE Transactions on Industry Applications, Vol.
IA 22, No. 6, November/December 1986.
5 Rehman, C. and H. P. Bloch, Is Closed
Oil-Mist Lubrication the Best Available Technology? Machinery
Lubrication, November/December 2010.
6 Bloch, H. P. and A. Shamim, Oil Mist
Application: Practical Applications, Fairmont Press,
Lilburn, Georgia, 1998.
is the manager for EPC sales at Lubrication Systems Co.,
a Colfax company, located in Houston, Texas. Since
joining LSC in 1984, he has held positions in equipment
assembly, field maintenance, field installation, field
management, sales and sales management. He has been
instrumental with the development of oil-mist-related
products and accessories for special applications. He
also provides technical support to oil-mist users
worldwide. Among his current responsibilities are
technical training, quotations and sales presentations to
both domestic and foreign engineering firms and user
companies. Prior to his employment with LSC, Mr. Ehlert
spent time in the US Navy, providing maintenance and operations
support on aircraft hydraulic and flight control