Oil mist systems are incredibly simple and orders of magnitude more dependable than traditional oil application methods. Still, the practice of allowing excess oil mist to escape into the atmosphere has come under scrutiny. As a result, it is time to revisit closed systems that avoid polluting the air.
Some closed systems have been in highly successful service since the mid-1980s and unquestionably represent best available technology. While the purpose of this article is to provide you with a relevant experience update on closed oil mist systems, we wish to first summarize the overall advantages of oil mist:
- Almost completely free of maintenance and fully self-checking. Users don’t have to rely on operators or maintenance workers to check and fill housings with oil.
- Better lubricant properties because the oil on the bearings is always new.
- Lower operating temperatures are routinely obtained. Reductions typically range from 10 to 20 degrees Fahrenheit.
- Reduced power requirements (typically 3 percent) since the bearings operate in a thin film of oil instead of plowing through a pool of oil.
- Oil mist is applied without using oil rings. Oil rings are subject to abrasive wear if the shaft system is not absolutely parallel.
Figure 1. Old-style (Non-API Type) Oil Mist Introduced at Midpoint of Bearing Housing
Background on Oil Mist
Oil mist lubrication has been used in petrochemical plants (ExxonMobil, Shell and many others) since the 1960s. The extended mean time between failure (MTBF) benefits of oil mist over traditional oil sump lubrication have been well-documented, and oil mist was included in the venerable API-610 pump standard, seventh edition (1989).
In describing the basic oil misting process, we first note that no liquid oil is placed in the bearing housing. Instead, an oil mist generator with no moving parts creates the mist in a central plant that serves all of the facility’s process pumps within approximately a 600-foot radius. The mist is a mixture of microscopic (less than 3 microns) oil droplets combined with clean air at a ratio of about 1:200,000. The mist is moved out to the pumps via 2-inch pipe headers at a low pressure (less than 1 psi) and at low velocity (less than 7 feet per second). At each process pump, the oil mist passes through a nozzle or reclassifier. Here, the mist velocity is greatly increased; this causes the droplets to collide and coalesce into larger liquid drops of oil. While in motion, the various bearing components contribute to the number of collisions and aid in doing the coalescing more efficiently.
Figure 2. Closed Oil Mist System Supplying Continuous Lubrication to Pumps and Drivers (Source: Lubrication Systems Company)
Comparing Open and Closed Systems
In old-style, “open” oil mist systems (Figure 1), the air/oil mixture fills the bearing housing, and much of it contacts the bearings. A portion of the coalesced droplets takes a straight path through the bearing housing; this portion of the coalesced oil and stray mist passes through a hole near the bottom of the housing and can be collected for disposal at the drain location in Figure 1. With these old-style configurations, much oil mist also will escape at the two unsealed regions where many pump shafts protrude through the bearing housing.
Efforts to simply provide effective bearing housing seals at the ends labeled “oil mist out” in Figure 1 had unexpected consequences for inexperienced users. Oil mist functions by letting oil mist globules coat (actually wet-out or coalesce on) the bearing components. To provide continuous oil replenishment on bearing surfaces, the oil mist must flow and cannot be dead-ended. Tight-sealing bearing protector seals very often caused dead-ending.
Irrespective of prevailing or legislated clean-air requirements, an environmentally conscious user shouldn’t allow continual releases of oil mist into the atmosphere. It is worth mentioning that oil mist releases are low enough to stay within the worker health guidelines of many industrialized nations. Still, from a housekeeping viewpoint, it is clearly advantageous not to have smudges of oil on the ground around every pump. Also, from a wastewater processing viewpoint, oil spills end up in an oily-water sewer, and it takes money to extract that oil before the water can be discharged. An open oil mist system, thus, does not represent best available technology.
The authors recommend using technology found in closed oil mist systems (Figure 2). Such systems incorporate a collecting tank (shown near the far left in Figure 2) to which a return header system is connected. A small blower is provided at the top of the collecting tank, and the suction effect of this small blower causes stray mist to be pulled into the tank. Inserted in the blower is a coalescer maze. Coalesced oil droplets fall out and the oil can be reused. Virtually oil-free air is vented to the atmosphere.
Favorable Experience Ascertained
The use of face-type bearing housing seals and oil mist routing (per Figure 3) make closed systems possible. Forward-looking oil mist users have discontinued “old” mist application since the mid-1970s and have enjoyed decades of superior experience with the routing shown in Figure 3. The API standards (starting with API 610, eighth edition, released in 2000) have recommended oil mist introduction into the region between the bearing and the bearing housing protector seal.
Best-technology oil-mist systems recover 99 percent of the lube oil and best protect physical plant as well as environmental assets. Closed systems emit no oil mist into the environment and, for years, have been used by refineries and petrochemical plants concerned about the environment.
In late 2003, one of the authors visited eight petroleum refineries located in the U.S. Gulf Coast region. During these visits, it was discovered that oil-mist lubrication was the predominant method of lubricating pumps throughout the U.S. refining industry. An equipment sales specialist with more than 20 years of experience as a refinery reliability engineer estimated that oil mist was being used by 24 out of the 30 refineries in or near Port Arthur, Texas. He believed that about 80 percent of the pumps in each facility were lubricated by an oil mist system. A West Coast consulting engineer with a background in refinery engineering estimated that approximately 50 percent of all U.S. refineries are using oil mist.
Many of these refineries have now employed closed-system oil mist technology for a few decades and are considering this application method an unqualified success. They consider closed oil mist systems a competitive advantage and have fully endorsed the application routines illustrated in Figure 3. Moreover, these users are doing their part toward achieving a cleaner environment while imparting reliability to their rotating equipment assets.
Figure 3. API 610-compliant Oil Mist Application at Locations Between the Bearings and Isolators (Source: AESSEAL plc)
Bibliography and Suggested Reading
Bloch, Heinz P.; “Practical Lubrication for Industrial Facilities”, Fairmont Publishing Company, Lilburn, Ga., second edition, 2009, (ISBN 0-88173-579-5)
Bloch, Heinz P., and Alan Budris; “Pump User’s Handbook: Life Extension,” Fairmont Publishing Company, Lilburn, Ga., first edition, 2004, ISBN 0-88173-452-7; second edition, 2006, ISBN 0-88173-517-5; third edition, 2010, ISBN 0-88173-627-9
Bloch, Heinz P., and Abdus Shamim; “Oil Mist Lubrication: Practical Applications”, Fairmont Publishing Company, Lilburn, Ga., 1998, ISBN 0-88173-256-7
About the Authors
Heinz Bloch works as a consultant for Process Machinery Consulting. He is the author of more than 400 technical papers and similar publications. He has written 17 books on practical machinery management and oil mist lubrication published by major engineering publishers. To learn more, e-mail Heinz at hpbloch@mchsi.com or visit www.heinzbloch.com.
Chris Rehmann is the North American operations marketing manager for AESSEAL, a designer and manufacturer of mechanical seals and engineered systems. He previously worked for Schlumberger, an oilfield engineering firm, for 15 years, holding positions in field engineering, technical sales and management in the United States, the Middle East and Asia-Pacific. He can be reached at chris.rehmann@aesseal.com or 865-531-0192.
