Lubrication expert Heinz Bloch has the following to say about ‘Lubricating rolling element bearings using oil mist’:
Kindly note that the recent article by Ron Carlson (Turbomachinery International, August 2013) contains a very serious error. Its author states that “wet sump oil mist is the most cost-effective method of bearing lubrication.” I know of upwards of 60 articles and at least 6 major textbooks that explain why wet sump, as of about 1975, has been superseded by dry sump oil mist.
As a reliability professional, let me assure you that the few (if any) installations that presently use wet sump are far more maintenance-intensive than the ones using dry sump. In terms of cost justification and bearing life/reliability, wet sump applications cannot compare to dry sump oil mist. Wet sump applications generally use slinger rings which, unless installed on absolutely horizontal shaft systems, will often abrade and seriously contaminate the lube oil in the (liquid oil) sump. With wet sump, one requires special constant level lubricators (very sensitive to adjustment errors) and relatively frequent oil replacement.
In early 1977 – over 36 years ago – I published an article on dry sump oil mist lubrication for electric motors in Hydrocarbon Processing Magazine. At that time, forward-looking petrochemical plants already had years of experience with dry sump oil mist lubrication on many thousands of process pumps. These facilities were then beginning to turn their attention to electric motor bearing lubrication. With dry sump, there are no oil rings and no levels to maintain. The article is unique because I would not change a single word in it. In contrast, I would make numerous changes (or updates) on many other pages I have published over the years.
Finally, as of 2013, an estimated 130,000 process pumps and 26,000 electric motors are lubricated with dry sump oil mist. Wet sump advocacy is 40 years behind the times and I was amazed to see it written up in Turbomachinery International.
Lubrication terms may sound similar, but could refer to different application methods. In essence:
- Wet sump lubrication is the term often used for conventional lubrication, whereby liquid oil partially fills the bearing housing. It implies that air (at ambient pressure) fills the bearing housing’s remaining, non-wetted, volume.
- Wet sump oil mist lubrication indicates conventional lubrication with liquid oil partially filling the bearing housing. However, in this case, slightly pressurized oil mist fills the remaining non-wetted bearing housing volume. Such pressurization prevents the entry of dirt or moisture-laden air. Any particles coming off an oil ring will still contaminate the oil; oil must be changed on a preventive maintenance schedule.
- With either wet sump or wet sump oil mist lubrication, oil rings (slinger rings) are often used to lift oil into the bearings. Unless the shaft system is absolutely level and ring concentricity, ring immersion, and lubricant viscosity are closely controlled, oil rings (slinger rings) can malfunction and adversely affect equipment life.
- Dry sump oil mist (often called pure oil mist) refers to a bearing housing filled entirely with oil mist. No (liquid) oil level exists in bearing housings so lubricated. Because this pure oil mist exists at a pressure slightly higher than that of the surrounding ambient air, the entry of dirt or moisture-laden air is precluded. Dry sump oil mist eliminates the need for oil rings (slingers) and also eliminates the need for constant-level lubricators and/or sight glasses. This oil application method has typically avoided 60-90 percent of traditional maintenance costs associated with oil changes, level control expertise (qualified labor), etc.
A number of books and technical papers have been authored by prominent mechanical seal manufacturers (and others) since 1970. This documentation shows bearings and mechanical seals at Best-of-Class oil refineries reaching 10 to 11 years of installed life (MTBR, mean-time-between-replacement). A six-year life, as mentioned in TMI’s August 2013 article, might be considered average for process pump seals and bearings at facilities using non-optimized lubricant application.
Technology-current cost justifications include maintenance cost savings and life extensions for electric motor lubrication, standby protection of pumps and motors, open air storage preservation yards, fire avoidance credits, and even the use of plastic-plugged oil mist-filled lateral branch lines which serve as pneumatic fire detectors. Cost justifications as short as 10 or 12 months have been achieved by knowledgeable industry experts for plant- or unit-wide dry sump oil mist systems.
Given below is Carlson’s response to Heinz Bloch’s comments on wet sump lubrication:
My proposition that wet sump lubrication (WSL) of pumps is more cost effective than oil mist is based on my experience that WSL provides acceptable reliability. By that, I mean the seals fail before the bearings with a plant wide MTBF of approximately 6 years. This is premised on replacing the bearings at the same time as the seals, that a pump is suited for WSL, has been properly installed and maintained.
Modern pump design includes grooved slinger ring retainers that axially position the rings and prevent them from “walking” on the shaft. This means the absolutely horizontal requirement is relaxed; not eliminated. Slinger rings are offered made of molded fiber-reinforced plastic and are of the correct shape and size to avoid being abraded and thus avoid contaminating the oil. Sight glasses can be installed on the bearing housing drains to monitor for water or particulate contamination.
Perhaps the mechanics I have had the fortune to work with are exceptionally skilled, but setting the constant level oilers did not pose a problem for them. They could even supervise a technical support person (me) to set them correctly. Using stable synthetic oils enables the oil change interval to be extended so that it can be incorporated into the annual P.M. at little additional time. If the optimum change interval is determined to be less than the yearly P.M., operators were trained to change the oil (non-union plant).
The reliability achieved by taking all of these steps lead to the plant not experiencing a lubrication related bearing failure in over ten years. This level of reliability made it very difficult to justify the approximately $500,000 investment to install an oil mist system.
Mr. Bloch did point to additional benefits, should you choose to install an oil mist system. Misting electric motors and purge misting general purpose steam turbines and year boxes will extend their bearing life at little additional cost.
