Despite being an accepted method of lubrication in countless reliability-focused process plants around the globe, questions about this technology continue to surface.
In early 2008, two widely read U.S. trade journals carried articles on grease lubrication of electric motors. Neither article mentioned oil mist lubrication, probably because the scope of the articles was, of course, grease lubrication. Failing to mention oil mist as a reasonable alternative for modern process plants, though, is somewhat like aiming an article about the repair of belted automobile tires at today’s drivers—as another publication recently did. Belted tires were great for a 1950 Chevrolet, but what savvy car owner is using them on his/her vehicle today?
Dry sump oil mist on electric motors is not new technology. In the mid-1960s, oil mist—a mixture of 200,000 volume parts of clean and dry plant or instrument air with one part of lubricating oil—gained acceptance as the ideal lubricant application method on rolling element electric motor bearings in several major United States oil refineries. Since then, this lubrication method has gained further acceptance at hundreds of reliability-focused process plants in this country and overseas. As of the late 1990s, many thousands of electric motors were being lubricated by dry sump oil mist (Fig. 1).
However, while taking at least some steps to become more profitable through increased equipment reliability, the majority of process plants have not yet abandoned their traditional costly repair-focus. Along these lines, questions and concerns relating to oil mist that had been answered decades ago are again surfacing today. The reasons are not always clear and may even be difficult to comprehend. Nevertheless, these questions are being asked and should be answered by open discussion.
This overview deals with considerations that have allowed oil mist lubrication to improve the reliability and energy efficiency of electric motors. It is not meant to dissuade plants from using grease lubrication, if that’s their choice and preference. But, it will discuss what’s out there if an operation is truly reliability-focused and wishes to understand proven best available technology.
In this series, particular emphasis will be placed on industry practices relating to oil mist lubrication of explosion- proof electric motors. The article is not intended to bypass compliance with regulatory edicts as they might relate to explosion-proof motors and might have to be adhered to regardless of merit. Be this as it may, these articles will be laying out the facts as they exist in 2008 and asking readers to draw the right conclusions for themselves.
Documented wide application range
For the past 44 years, empirical data have been employed to screen the applicability of oil mist. The influences of bearing size, speed and load have been recognized in an oil mist applicability formula, limiting the parameter “DNL” to values below 10E9, or 1,000,000,000. Here, D = bearing bore, mm; N = inner ring rpm; and L = load, lbs. An 80 mm electric motor bearing operating at 3600 rpm and a load of 600 lbs would thus have a DNL of 172,000,000—less than 18% of the allowable threshold value. The vast majority of electric motors equipped with rolling element bearings can thus be served by dry sump oil mist.
Although major grass-roots olefins plants commenced using oil mist on motors as small as 1 hp (0.75 kW) decades ago, the prevailing practice among smart reliability-focused users is to apply oil mist lube on horizontal motors, 10 kW and larger, and vertical motors of approximately 3 kW and larger, fitted with rolling element bearings. Note that oil mist serves not only to lubricate the bearings of operating machines, but also protects and preserves the bearings of non-operating (spare or standby) equipment. This is hugely important in humid and dust-laden (desert) environments.
In the 1960s, it was customary to apply oil mist near the center of the bearing housing, letting the excess mist vent to the atmosphere after passing through the bearings. More recently, and in accordance with the recommendations of API- 610 8th and later editions governing centrifugal pumps in the petrochemical and refining industries, the oil mist has been routed through the bearings. The oil mist enters at a convenient location between the bearing housing protector (bearing isolator or end seal) and bearing (see Fig. 2). The metering orifice (reclassifier) may or may not be incorporated in the end cap as shown here, although locating it close to the bearing is considered advantageous.
Originally intended for centrifugal pumps, the recommendations of API-610 have worked equally well for electric motors with rolling element bearings. The resulting diagonal through-flow route shown in Fig. 2 guarantees adequate lubrication, whereas oil mist per Fig. 3, entering at the top of the bearing housing and exiting directly below,might:
- Cause some of the oil mist to leave at the drain port, without first wetting the rolling elements (Ref. 1).
- Inadvertently be kept away from the rolling elements due to windage, or fan effects, generated by certain inclined bearing cage configurations.
It is acknowledged that a few “business-as-usual” oil mist users continue to be satisfied with routing the oil mist from the top of the bearing housing to the bottom of the same housing (Fig. 3). Nevertheless, it can be shown that highly-loaded bearings and bearings operating at high speeds must use the API-recommended routing of Fig. 2. A risk-averse user thus recognizes throughflow as one of the key ingredients of successful oil mist implementations. No less a company than Siemens A.G. has published technical bulletins showing oil mist as a superior technique for electric motors ranging in size from 18 to 3000 kW (Ref. 2).
It can be stated without reservation that through-flow oil mist addresses the above concerns and will accommodate all of the lubrication needs of electric motors furnished with rolling element bearings (see above).
Flow requirements explained
The required volume of oil mist is often translated into bearing-inches, or “BI’s.” A bearing-inch is the volume of oil mist needed to satisfy the demands of a row of rolling elements in a one-inch (~25 mm) bore diameter bearing. One BI assumes a rate of mist containing 0.01 fl. oz., or 0.3 ml, of oil per hour. Certain other factors may have to be considered to determine the needed oil mist flow. These are known to experienced oil mist providers and bearing manufacturers. The various factors also are well-documented in numerous references and include:
- Type of bearing… the different internal geometries of different types of contact (point contact at ball bearings and linear contacts at roller bearings), amount of sliding contacts (between rolling elements and raceways, cages, flanges or guide rings), angle of contact between rolling elements and raceways, and prevailing load on rolling elements. The most common bearing types in electrical motors are deep groove ball bearings, cylindrical roller bearings and, occasionally, angular contact ball bearings.
- Number of rows of rolling elements… multiple row bearing or paired bearing arrangements require a simple multiplier to quantify the volume of mist flow.
- Size of the bearings… related to the shaft diameter— inherent in the expression “bearing-inches.”
- The rotating speed… the influence of the rotating speed should not be considered as a linear function. It can be linear for a certain intermediate speed range, but at lower and higher speeds the oil requirements in the contact regions may behave differently.
- Bearing load conditions… (preload, minimum or even less than minimum load, heavy axial loads, etc.)
- Cage design… Different cage designs might affect mist flow in different ways. It has been reasoned that stamped (pressed) metal cages, polyamide cages or machined metal cages might produce different degrees of turbulence.
Fortunately, industrial experience shows that no further investigations are needed for bearings in the operating speed and size ranges encountered by motors driving process pumps.
As of 2008, several thousands of oil mist lubricated electric motors continue to operate flawlessly in reliability-focused user plants. Moreover, a 2004 survey of these plants confirmed that their procurement specifications for new installations and replacement motors require oil mist lubrication in sizes 15 hp and larger. The largest motor with oil mist lubricated rolling element bearings had a nameplate rating of 1250 hp (933 kW). Questions as to whether different rates of turbulence cause different amounts of oil to “plate out” on the various bearing components are thus of academic interest only.
Sealing and drainage issues
Although oil mist will not attack or degrade the winding insulation found on electric motors made since the mid- 1960s, mist entry and related sealing issues must be understood and merit discussion.
Regardless of motor type, i.e. TEFC, X-Proof or WP II, cable terminations should never be made with conventional electrician’s tape. The adhesive in this tape will last but a few days and become tacky to the point of unraveling. Instead of inferior products, competent motor manufacturers use a modified silicone system (“Radix”) that is highly resistant to oil mist. Radix has consistently outperformed the many other “almost equivalent” systems.
Similarly, and while it must always be pointed out that oil mist is neither a flammable nor explosive mixture, it would be prudent not to allow a visible plume of mist to escape from the junction box cover. The wire passage from the motor interior to the junction box should, therefore, be sealed with a product such as 3M Scotch-Cast Two-Part Epoxy potting compound to exclude oil mist from the junction box. As mentioned earlier, the volumetric ratio of oil to air is 0.000005. The weight ratio of oil to air is 0.00035. The lower volumetric explosive limits of heptane and hexane are approximately 0.01 (Ref. 3). Another source gives the lower explosive limit of oil in air at 0.035 by weight (Ref. 4).
TEFC vs. WP ll construction
On TEFC (totally enclosed, fan-cooled) motors, there are documented events of liquid oil filling the motor housing to the point of contact with the spinning rotor. Conventional wisdom to the contrary, there were no detrimental effects, and the motor could have run indefinitely! TEFC motors are suitable for oil mist lubrication by simply routing the oil mist through the bearing, as has been explained in Ref. 1 and numerous other references, including the more recent editions of API-610. No special internal sealing provisions are needed.
On weather-protected (WP II) motors, merely adding oil mist has often been done in the field, and occasionally even with the motor in operation. These on-the-run modifications have generally worked surprisingly well. In this instance, however, it was found important to lead the oil mist vent tubing away from regions influenced by the motor fan. Still, WP II electric motors do receive additional attention from reliability-focused users and knowledgeable motor manufacturers.
Air is constantly being forced through the windings and an oil film deposited on the windings could invite dirt accumulation to become objectionable. To reduce the risk of dirt accumulation, suitable means of sealing should be provided between the motor bearings and the motor interior. Since V-rings and other elastomeric shaft-contacting seals may be subject to wear, low-friction face-contacting seals based on mechanical seal technology are considered desirable. The axial closing force on these seals could be provided either by springs or small permanent magnets.
As is so often the case, the user has to make choices. Low friction axial seals (face seals) are offered by several manufacturers. Some of these may require machining of the cap, but long motor life and the avoidance of maintenance costs will make up for the added expense. Nevertheless, double V-rings using Nitrile or Viton elastomeric material should not be ruled out since they are considerably less expensive than face seals. Certain rotating labyrinth seal designs with axially contacting O-rings were introduced in early 2005 and offer another possible option.
Sealing to avoid stray mist
Even when still accepted by prevailing environmental regulations (e.g. OSHA or EPA), the regulatory and “good neighbor climate” will sooner or later force industry to curtail stray oil mist emissions. Of equal importance and to set the record straight, it must again be noted that state-of-art oil mist systems are now fully closed, i.e. are configured so as not to allow any mist to escape. In the late 1980s, the author collaborated with a California-based engineering contractor in the implementation of two plant-wide systems in Kentucky. As of 2008, these systems have continued to operate flawlessly and have even been expanded. The owner company has added another fully closed system at its refinery in Minnesota and is planning to convert existing, open systems to closed systems.
It should be noted that combining effective seals and a closed oil mist lubrication system is a proven solution. Application per Fig. 2 eliminates virtually all stray mist and oil leakage, but makes possible the recovery, subsequent purifi- cation and re-use of perhaps 97% of the oil. These recovery rates make the use of more expensive, superior-quality synthetic lubricants economically attractive. Needless to say, closed systems and oil mist-lubricated electric motors give reliability-focused users several important advantages:
- Compliance with actual and future environmental regulations
- Convincing proof that closed oil mist lubrication systems exist that won’t put avoidable stress on the environment
- The technical and economic justification to apply energysaving, long-lasting, high-performance synthetic oils
Optimized energy efficiency
To capture energy efficiency credits, lubricants with suitably low viscosity must be used in combination with the correct volume of mist. Moreover, and as mentioned above, low-friction seals are desired on WP II motors.
PAO and diester lubricants embody most of the properties needed for extended bearing life and greatest operating efficiency. These oils excel in the areas of bearing temperature and friction energy reduction. It is not difficult to show relatively rapid returns on investment for these lubricants, providing, of course, the system is closed and the lubricant re-used after filtration.
Again, very significant increases in bearing life and overall electric motor reliability have been repeatedly documented over the past four or five decades.
Contributing Editor Heinz Bloch is the author of 17 comprehensive textbooks and more than 340 other publications on machinery reliability and lubrication. He can be contacted at: firstname.lastname@example.org
1. Bloch, Heinz P., and Alan Budris, (2006), Pump User’s Handbook: Life Extension, Fairmont Press, Inc., Lilburn, GA, 30047; ISBN 0-88173-517-5, pp. 265-290.
2. Bloch, Heinz P., and Abdus Shamim, (1998), Oil Mist Lubrication: Practical Applications, Fairmont Press, Inc., Lilburn, GA, 30047; ISBN 0-88173-256-7, Fig. 9-7, p.109.
3. Shelton, Harold L., “Estimating the Lower Explosive Limits of Waste Vapors,” Environmental Engineering, May-June 1995, pp. 22-25.
4. Lilly, L.R.C., (1986), Diesel Engine Reference Book, Butterworth & Co, London, U.K., ISBN 0-408-00443-6, p. 21/3.