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 oil-mist lubrication.
| Fig. 1. A modern, large-scale oil-mist console.|
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 motors.
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 mist.
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 application was |
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 globules.
| Fig. 3. Mid-sized oil-mist console serving 10 to |
20 pump sets in a refinery (left). A collecting
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 components.
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 housing.
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 mist.)5
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 service factor.
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 bearing.
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 minutes.
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 perform flawlessly.
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 older-style applications.
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 bearings |
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 end.
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 large electric |
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. HP
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.
||Don Ehlert 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 systems. |