Welcome to the field of reliability engineering. Most schools do not teach oil mist lubrication. So many new engineers meet this technology for the first time on the job. This paper gives you the basics. Read it, and you will understand oil mist lubrication well enough to ask the right questions.
The Four Rs of Lubrication
Good lubrication follows four simple rules. We call them the four Rs: the right oil, the right amount, the right place, and the right time. Most lubrication failures happen because a bearing is missing one of these four Rs. Oil mist is popular because it delivers all four Rs automatically, every minute a bearing runs… and sits idle!
Two Methods: Pure Mist and Purge Mist
Oil mist works in two ways. In pure mist, oil mist is the only source of lubrication. This method works well for rolling element bearings in pumps and motors. In purge mist, the equipment still uses an oil sump. The oil mist adds a layer of protection on top of the sump. Purge mist works well for sleeve bearings and the gears inside a gearbox. Know which method your equipment uses before you inspect it or troubleshoot it.
The Numbers Every New Engineer Should Remember
Keep these benchmarks in mind. They will help you judge whether a system is performing well:
- A properly designed oil mist system can reduce bearing failures by up to 90%.
- Typical pump MTBF (mean time between failures) with oil mist runs 4 to 6 years. The best-in-class facilities report over 10 years.
- Oil mist lowers bearing operating temperature by 25°F to 35°F compared to an oil sump. Every 10°F drop adds about 11% to bearing life.
- Dual injection systems, required under API-610 8th Edition, use up to 75% less oil than older center-injection designs.
- Mechanical seal failures can drop by 35% to 50%.
Three Rules That Prevent Most Problems
Most oil mist problems trace back to one of three design errors. Learn these three rules early in your career.
Reclassifier placement. Place the reclassifier within 6.6 feet (2 meters) of the bearing. Beyond this distance, the oil particles fall out of the mist before they reach the bearing.
Vent and drain sizing. Size the vent and drain port at 3 to 4 times the diameter of the reclassifier orifice. An undersized vent creates back-pressure. This back-pressure restricts mist flow and can even cause a false-high reading in an oil level sight glass.
Oil selection. Use paraffin-free oil only, preferably a synthetic. Because oil mist lubrication reduces oil usage, going to a premium synthetic is very cost effective.
What Oil Mist Cannot Do
Oil mist is not a cure-all. It cannot repair a bearing that is already damaged. If a bearing fails shortly after a conversion to oil mist, the bearing was likely already compromised before the conversion. Oil mist also depends on good distribution design. Most of the reliability complaints about oil mist over the years trace back to bad installations by inexperienced pipefitters: sagging headers, bottom-connected drop pipes, or a poorly placed reclassifier, not the technology itself.
Learn from the Father of Oil Mist
Heinz P. Bloch is widely regarded as the father of oil mist lubrication. Over his long career, he wrote four books and many technical papers on oil mist, all drawn from decades of hands-on experience in the field. Much of that research is available online. As a new reliability engineer, you have a wealth of proven knowledge at your fingertips. Start with his work. It will answer many of your questions before you ever need to ask them.
Conclusion
Oil mist lubrication has a long track record. Learn the four Rs, know your pure mist from your purge mist, remember the key numbers, and watch for the three common design errors. This foundation will serve you well throughout your career.
I am always happy to answer questions from anyone who is new to oil mist. Feel free to reach out.
Sources and Further Reading
- Guidelines for Oil Mist Lubrication — Machinery Lubrication
- Oil Mist Lubrication for Pumps and Electric Motors — Machinery Lubrication
- Towne, C. (1994). Oil Mist Lubrication for the Petrochemical Industry — Texas A&M Turbomachinery Laboratories