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Why Closed Oil Mist Lubrication Systems Are Worth Considering

It is not uncommon to receive queries about “closed” plant-wide oil mist lubrication. Years ago, the writer of a question began his email with a commendable statement that summarized what oil mist does: “Oil mist lubricates operating equipment, protects and preserves standby equipment, and provides superior lubrication to electric motors at little incremental cost.” But next, he inadvertently divulged an interesting confluence of partial recollections and misremembered anecdotes as he questioned: “Wouldn’t collecting the oil after it had made its once-through travel through a bearing (a) possibly require oil sampling and oil changes at times; (b) not protect/preserve standby equipment; and (c) require motors to continue to need inferior grease lubrication?

"It should be pointed out that closed oil mist systems have been in successful use since the early 1980's"

Before answering this question, it should be pointed out that closed oil mist systems have been in successful use since the early 1980s. In any typical oil mist system---closed or open---a small volume of oil mist passes through the bearing. In a closed system and after the volume of oil has done its work, it is converted (actually, coalesced), collected, and manually pumped back into the oil supply header. Because this header has a slightly negative return slope, gravity assists as the oil flows through a fine filter. From the filter, it simply trickles back into its original supply tank or some other reservoir.

Figure 1 depicts a closed oil mist system with separate supply and oil mist return headers. This article, though, illustrates a slightly different system, namely one where the supply header doubles as an oil return header. However, in either system, a small (painted blue in figure) oil collecting tank is located near the outboard end of each drive motor, as shown in Figure 1. After wetting out and thus lightly coating or lubricating the various motor and pump bearings, the spent oil is carried off in its instrument-quality “carrier air” in the form of small atomized oil globules. These atomized oil globules and much larger drops of coalesced liquid oil are piped into the nearest small blue tank. Whenever a quart or liter of oil has accumulated, a plant operator strokes the small, built-in piston pump (visualize a bicycle tire pump). In single header systems, the oil is pushed upward in a vertical pipe and into the slightly sloped supply header. After slowly flowing back and then leaving this sloped header, the oil re-enters a vertical pipe and is led to a small oil reservoir inside an oil mist cabinet of the type and configuration shown in either Figure 1 or the midsize oil mist cabinet shown in Figure 2.

Figure 1: The oil mist console in the foreground contains a small oil reservoir and an oil mist generator nozzle (an ejector-like stationary part) where instrument air and lube oil meet; the dark oil mist return header ends at the coalescing and collecting vessel, far left

Figure 2: A medium-sized oil mist console is shown on the left; an oil return tank with its associated blower is positioned on the right

With regard to the positive displacement pumps built inside small, blue, collecting tanks, they may have to be stroked every three weeks. Depending on the number, style and size of the bearings in a specific pump set, it may take three to six weeks before a quart of oil is collected in one of the blue tanks.

When tracing the pipes in Figure 1, note that this version of a widely used closed oil mist system has an oil return header separate from the oil supply header. Oil and oil mist from the blue box are fed into one of two sloped headers. This would be the return header, with a pressure near or slightly below atmospheric. The header pressure, either slightly positive or negative, is produced by the small blower located at the top of the freestanding, stainless steel tank at the extreme left of Figure 1.

However, this blower, which is also shown on top of a similar stainless steel tank in Figure 2, serves two important purposes: (a) It develops a suction pressure below that in the oil mist supply header and, (b) Crinkled wire mesh, securely fastened near the periphery of the impeller discharge vanes, causes any remaining atomized oil droplets to coalesce. It then travels through a filter into the collecting tank and, from there, a fractional horsepower electric pump sends the clean oil into the oil mist cabinet’s small reservoir for reuse.

Are Frequent Oil Sampling and Oil Changes Needed?

Returning back to the inquiry, the first part indicates uncertainty regarding the need to periodically change the oil and sampling of the oil. Oil changes are not necessary because the oil is ultraclean. The oil in the bearing housing sumps of conventionally lubricated process pumps is reintroduced to the bearings hundreds of times per hour without first being filtered. Quite obviously, reusing filtered oil from a collecting tank is much more desirable than reusing unfiltered oil from a small, possibly dust and water contaminated, traditionally lubricated bearing housing or oil sump.

The use of premium synthetic oils expands upon the answer. As previously noted, Figure 1 simulates an entire closed oil mist system with associated piping. Chances are that a reliability-focused fluid machinery owner would use a superior synthetic oil and, for peace of mind, would probably do so in closed loop oil mist systems as well. Analyzing a small oil sample once per year, these owner-users would likely find the oil well within the range of its original specification. It would then be pumped through a filter from the collecting vessel (far left in Figure 1 and to the right of Figure 2) into a small oil reservoir located inside the oil mist console depicted in the foreground of Figure 1. Premium synthetic oils in closed oil mist systems would probably be replaced after 10 years of circulation.

In contrast, more frequent oil replacement and recycling would be considered for certain, much less expensive, mineral oil formulations. Mineral oils tend to oxidize more readily when encountering an occasional hot bearing. Accordingly, these oils should be analyzed and recycled more frequently. The spent oil is sent to a waste oil recycling facility or mixed with the fuel used in boilers and furnaces. In neither case would the lube oil go to a landfill.

Is Standby Equipment Protected/Preserved?

In response to the second part of the question, the answer is based on six decades of well-established experience. Plant-wide oil mist supplies are never turned off. Thus, come rain or shine, an oil mist system serves to protect the non-running (i.e., standby) equipment as oil mist travels through the bearings and toward the bearing housing drain port. Without this preservation method, standby equipment will be exposed to an elevated risk of oil being wiped off due to vibration transmitted from the adjacent running equipment. Moreover, without this preservation method, the corrosion risks of bearings in non-running equipment are far greater. The rate of corrosion is a function of ingress and egress of air (i.e., the “breathing” action) of the affected bearing housings. This ingress-egress volume of ambient air is orders of magnitude greater with unprotected bearing housings, yet it cannot possibly occur in bearing housings filled with oil mist at about 0.1 to 0.25 psi over atmospheric pressure. Best in class companies can point to well over 40 years of success and experience with oil mist protection on many thousands of pumps and motors.

Do Motors Still Require Inferior Grease Lubrication?

With regard to the last part of the question, all motors with rolling element bearings will benefit from the application of pure oil mist. Close to 50,000 electric motors have been so lubricated over the past 40 years. Chapters in 10 or more books and dozens of papers have been written on the subject. The claim that motors would require inferior grease lubrication is factually incorrect. All motors with epoxy insulation and irradiation cross-linked polymeric cable terminations can be safely lubricated with oil mist. For owner-operators with plant-wide oil mist systems, maintenance-intensive grease lubrication is truly a thing of the past. And whenever electric motor lubrication by oil mist is included in cost justification calculations, payback will be considerably shorter than with grease lubrication.


If the three questions were indeed asked by an oil mist user, one would be inclined to assume that entrenched traditions are at work. Perhaps an uninformed staffer chose not to become familiarized with the very simple underlying scientific principles governing oil mist or words got mangled in translation.

Whatever the case may be, trustworthy answers are available. Seek out oil mist experts, read oil mist papers written by experts in oil mist technology, or attend conferences, seminars, or lectures that address oil mist lubrication.

Sadly, one such oil mist lubrication expert, Don Ehlert, succumbed to cancer in October 2019. He had ably assisted in compiling material for a book on optimized equipment lubrication, oil mist technology and storage preservation. Scheduled for release in late 2019, Optimized Equipment Lubrication, Oil Mist Technology, and Storage Preservation Best Practices ( will include highlights of his many decades of practical expertise with plant-wide oil mist systems. His knowledge of the subject was unmatched, and he will be missed by many, both in the U.S. and in countries around the world. The book will be dedicated to his memory.

Heinz Bloch

Heinz P. Bloch is a professional engineer with offices in West Des Moines, Iowa. He advises process and power plants worldwide on reliability improvement and maintenance cost reduction opportunities. Heinz is the author of 17 full-length texts and over 400 papers and technical articles. His most recent texts include "A Practical Guide to Compressor Technology" (2006, John Wiley & Sons, NY, ISBN 0-471-727930-8); "Pump User's Handbook: Life Extension," (2006, Fairmont Publishing Company, Lilburn, ISBN 0-88173-517-5) and "Machinery Uptime Improvement," (2006, Elsevier-Butterworth-Heinemann, Stoneham, MA, ISBN 0-7506-7725-2)

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