Plant-wide oil mist systems have been in use in numerous reliability-minded refineries and petrochemical plants since the mid-1960s. The 8th (2000) and subsequent editions of the API-610 Standard for centrifugal pumps also have described advantageous application parameters for oil mist. In the United States, Canada, South America, the Middle East and Pacific Rim countries, oil mist lubrication has matured to the point where major design contractors now are specifying these types of plant-wide systems quite extensively.
Oil mist is easily controlled & appliedModern plants use oil mist as the lube application of choice. Plant-wide pipe headers distribute the mist to a wide variety of users. Oil mist is easily produced in an oil mist generator console (Fig. 1) and its flow to bearings is not difficult to control. Flow, of course, is a function of orifice ("reclassifier") size and piping ("header") pressure. Unless plugged by an unsuitable (e.g., an elevated pour point) lubricant, reclassifiers have a fixed flow area that is selected based on bearing size criteria.
Depending on make and system provider, header pressures range from 20"-35" (500-890 mm) of H2O. Modern units are provided with controls and instrumentation that will maintain these settings without difficulty. It should be noted, however, that mixing ratios—typically 160,000 to about 200,000 volumes of air per volume of oil—are frequently incorrect on oldstyle mist generators that incorporate gaskets and O-rings in the mixing head, unless these elastomers have been periodically replaced or properly serviced.
Comparing plants with non-optimized mist entry (Fig. 2) to equipment bearing housings with their modern optimized counterparts (Fig. 3), lubricant and air consumption are about 40% less for plants that have implemented the superior mist entry and vent locations of Fig. 3. This has been reported in the cited references and is implied in API-610 8th Edition (2000) and later standards.
Forward-looking plants have used the API method, i.e. Fig. 3, since the mid-1970s. These plants had recognized that mist entering at locations far from the bearings could have difficulty overcoming bearing windage effects. Windage is most often produced by the diagonally-oriented ball cages in angular contact bearings. If such windage were produced by the left row of the thrust bearing in Fig. 2, the mist would take the preferential path straight to the vent exit at the bottom of the bearing housing and insufficient amounts of oil mist would reach the bearing rolling elements.
A larger quantity of oil mist or specially designed "directional" reclassifiers will be needed with certain bearing types unless the API method is used. This latter method will overcome windage, the flow-induced action induced by the skewed cages.
Environmental & health concerns
For decades, environmental and health concerns related to oil mist have been addressed by using oil formulations that are neither toxic nor carcinogenic. Such formulations are available to responsible users. Appropriate lubricants also have been formulated for minimum stray mist emissions. These, too, are readily available to responsible users.
Stray mist emissions can be kept to very low values by installing suitable magnetically-closed dual-face bearing housing seals (Fig. 4, also Ref.1). Unlike old-style labyrinth or other housing seals that allow highly undesirable communication between housing interior and ambient air, face-type devices seal off this contamination route.
Closed oil mist systems also are available—and have been since first being applied in the Swiss textile industry in the late 1950s. Today, closed systems are in use at several U.S. petrochemical plants. They allow an estimated 99% of the lube oil to be recovered and reused. Closed systems emit no oil mist into the environment and are available to environmentally conscious users.
Header temperature & size
Temperature never has been an issue for properly designed systems. Once a mist or aerosol of suitably low particle size has been produced—and particle size is influenced by the temperature constancy of both air and oil in the static mixing head—the oil mist will migrate to all points of application in non-insulated headers at low velocity.
Ambient temperature has little influence on mist quality and effectiveness. Mist temperatures in headers have ranged from well below freezing in North America to over 122 F (50 C) in the Middle East. Regardless of geographic location, conscientiously engineered systems will incorporate both oil and air heaters, since these are needed to maintain constant and optimized air/oil mixing ratios. The heaters must have low-watt density (low power input per square inch of surface area) in order to prevent overheating of the oil. Users that try to save money by omitting heaters or using undersized headers will not be able to realize the greatest life cycle cost benefits from their assets.
Using undersized headers may increase the flow velocity to the point where the small oil globules suspended in the carrier air experience too many collisions. They may thus agglomerate into droplets large enough to fall out of suspension, causing excessively lean mist to arrive at the point to be lubricated.
Wet sump ("purge mist") vs. dry sump ("pure mist")
In the wet sump method, a liquid oil level is maintained and the mist fills the housing space above the liquid oil. Wet sump (also called "purge" mist) is essentially "old technology"— and primarily used with sleeve bearing-equipped pumps and blowers(Figs. 5 and 6).
Dry sump oil mist describes the application method whereby no liquid oil level is maintained in the bearing housing. (This principle was illustrated earlier in Figs. 2 and 3.) Pumps lubricated in dry-sump fashion are depicted in Figs. 7 and 8. Here, lubrication is provided entirely by oil mist migrating through the bearing.
The application of dry sump oil mist is advantageous for a number of reasons. Among these, we find lower bearing temperatures, the presence of nothing but uncontaminated oil mist and the exclusion of external contaminants. However, one important, but often overlooked, reason involves oil rings (Fig. 9)—or rather the fact that no oil rings are used with this application method.
Oil rings often represent outdated 18th century technology as they were developed for slow-speed machinery during the Industrial Revolution. Elimination of oil rings is one of the many keys to improved reliability of virtually any type or style of bearing. Oil rings are known to have journal surface velocity limitations, sometimes as low as 2000 fpm, or 10 m/s. So as not to "run downhill," which might cause the rings to make frictional contact and slow down, ring-lubricated shaft systems would have to be installed with near-perfect horizontal orientation.
Furthermore, frictional contact often results in abrasive wear and the wear products certainly contaminate the oil. Oil rings will malfunction unless they are machined concentric within close tolerances. They suffer from limitations in allowable depth of immersion and, to operate as intended, need narrowly defined and controlled oil viscosity.
Experience with modern oil mist systems
Actual statistics from a world-scale facility convey an accurate picture of the value of properly applied oil mist technology. This petrochemical plant went on-stream in 1978 with 17 oil mist systems providing dry sump oil mist to virtually every one of the many hundreds of pumps and electric motors in the facility. As stated previously, with the dry sump ("pure") method per current API recommendation, the oil mist is introduced at a location that guarantees its flow through the bearings and to an appropriate vent location. There are neither oil rings nor any other provisions for the introduction of liquid oil on pumps and motors with rolling element bearings at the plant.
Over a period of 14 years, one qualified contract worker serviced these systems by visiting the plant one day each month. In this 14-year time period, there was only one single malfunction; it involved a defective float switch in one of the 17 systems. The incident caused a string of pumps to operate (and operate without inducing even one bearing failure!) for eight hours. In 1992, the combined availability and reliability of oil mist systems at this U.S. Gulf Coast plant was calculated to be 99.99962%.
Being aware of the relative unreliability of conventional lubricant application methods involving oil rings and certain constant level lubricators (Fig. 10), knowledgeable reliability professionals can attest to the utility and overall advantages of properly engineered dry sump oil mist systems. Certainly, the known advantages of properly engineered oil mist systems far outweigh the actual or perceived disadvantages. It is unfortunate that much information to the contrary is either anecdotal or pertains to systems that were not correctly designed, installed, maintained and/or upgraded as new technology became available.
Only dry-sump applications will lubricate, preserve and protect both operating and stand-by rolling element bearings. At all times, only fresh oil will reach the bearings. In many instances, bearing operating temperatures with dry sump oil mist lubrication are 10 or even 20 F degrees (6 or 12 C degrees) lower than with wet sump lubrication.
Industry experience with dry-sump oil mist systems is well documented [Refs. 1, 2 & 3] and its superiority over both conventionally applied liquid oil and wet sump oil mist lube applications has been solidly established.
Regrettably, there are still entire plants that try to get by on wet sump oil mist. Wet sump lubrication makes economic sense on sleeve bearings only. Here, its only function is the exclusion of atmospheric contaminants. It does so by existing at a pressure slightly above that of the surrounding ambient air. Often, the wet sump oil level is expected to be maintained by an externally mounted constant level lubricator. However, due to the slight pressurization, and on bearing housings equipped with traditional open-to-atmosphere constant level lubricators [Ref. 2], the oil level in the bearing housing will now be below the oil level in the lubricator. Keep in mind that fully pressure-balanced constant level lubricators will be more reliable than many other wet sump lubrication alternatives available today.