Although receiving input from both OEM and a highly experienced oil mist provider, the gentleman expressed concern with certain issues that arose in the course of his research. We want to share his questions and some of our answers.
Blowers Operate at a Range of Pressures
The positive displacement blowers at this engineer's plant are of the type illustrated in Figure 1. They move air ranging in pressure from slightly negative suction to about 15 psig positive discharge, as is typical for polyethylene powder or pellet transfer. His machines come in different sizes, but are predominantly 10, 12, and 14-inch (shaft center to shaft center) rotary lobe units. The drives at his facility include direct-drive motors, gearboxes, and belt arrangements. He noted that some were splash-lube only, while some were forcedfeed lubricated.
When the blowers were installed many years ago, there was, at that time, an understanding between the oil mist provider and user-owner that pure mist (dry sump) might later be retrofitted to the blower bearings. A wet sump (purge mist) arrangement was considered appropriate on the timing gear/oil sump side of the smaller splash-lubricated units. In his research, the user-owner found industry references to oil mist application on small blowers at other facilities; however, the engineer was now trying to determine if it was acceptable to eliminate forced lubrication on the larger units. Although not disallowing it, the blower manufacturer apparently knew of no users that had eliminated forcedfeed lubrication.
Information supplied by the blower manufacturer showed a general preference for forced feed lubrication when a certain pressure rise (and, therefore, differential temperature) through the blower is exceeded. However, the engineer didn't think he had found a suitable reference on the temperature limitations of oil mist applications. He also struggled with locating case histories where elevated-temperature applications are featured (with the exception of slow-turning bearings for rollers, etc.) He knew that removing oil flooding actually decreases the frictional heat generated within the bearing; but, in the case of his more heavily loaded bearings on larger blowers, he thought that it might be the process (air) discharge temperature that outweighed frictional heat generation concerns.
He quite correctly noted that getting away from forced-feed lubrication would greatly simplify the oil system, eliminating a pump, cooler, piping, thermostatic valves, and pressure switches. With other than forced feed lubrication, he thought he would lose the benefit offered by even a "marginal filtration" strainer. However, if he could not remove the forced lube requirement, he would not expect much advantage of oil mist purge as opposed to simple nitrogen purges on these units. He acknowledged that some beneficial oil-wetting of components in standby equipment might occur, but noted that, on centrifugal pumps, his plant used oil mist with nitrogen as the carrier fluid.
The Quest for Case Histories
And so, he raised several specific questions he hoped we might comment on. First, he inquired if we were aware of any users that had switched to oil mist where forced lube was previously used on blowers or similar equipment. Our answer was affirmative, and confirmed that of the oil mist provider: oil mist lube on blowers exists and has been used quite successfully on many sizes for several decades. Ref. 1 gives the shaft speed, bearing size and load values for which oil mist lubrication was designed, irrespective of machine type. In fact, oil mist is viable at speeds and velocities much higher than those encountered here. There will certainly be no problem applying this technology to any machine type with rolling element bearings including, of course, the blower sizes used at his plant.
As regards a strainer (or the loss of it), we reasoned that, by the time contaminants or other debris particles are large enough to get caught in a strainer, a myriad of smaller particles will have passed through and will have severely damaged the machine's components. Our view is that strainers are only inserted in piping to catch hard hats and beer bottles left by the construction crew. Contaminant intrusion must be prevented by other means. Intrusion can be managed by advanced bearing housing protector seals -- a separate subject and one well worth studying. (For now, let's just remember that a serious professional examines how components operate and questions configurations that violate the precepts of sound engineering and the science of elastomeric materials behavior. Some products stand accused of such violations, and at least two of the cited references shed much more light on the issue).
Next, the engineer asked for a case history that we might comment on. Well, we know the oil mist provider has access to such case histories and would be pleased to share them with any prospective client. The general configuration of Roots-type positive displacement blowers is depicted in Figure 2; it highlights where and how both pure mist and dry sump oil mist are applied on the same blower. Unfortunately, users are often reluctant to share their success stories with the public and we may have to depend on others to do the explaining. The bearings on the gear side are splash lubricated from the action of the oilcontacting gears. The space above the liquid oil is not vented. Venting would lead to mist flow and would cause more mist to coalesce (become reclassified into liquid oil) and the oil level would rise. By not venting the gear side, one avoids adding to environmental pollution and avoids having to actually remove oil from the sump. Dry sump (pure) oil mist lubrication is used on the input side of the blower.
As to our opinion on using nitrogen purge instead of oil mist: In this application, oil mist is probably much less expensive than nitrogen. Moreover, it imparts a "lubricity" bonus that will be important for non-running equipment under conditions of vibration being transmitted from neighboring running machines. Those are just two of the reasons why we would stay with oil mist.
Also, the engineer had been led to believe that his plant was "stuck with" purge mist on the sump/gear side of the blower. Yet, he said, in his studies he had come across some general references to dry lubrication film development on warmer (i.e. 80ºC+) gear meshes utilizing oil mist, and was now wondering if these kinds of studies might offer challenges to his thinking. It wasn't clear to him when it would be reasonable to use dry lube films on the timing gears shown on the left side of Figure 2. As to the oil mist lubricant path, he expressed the view that, at high speeds, windage could be a problem (windage is the fan effect generated by an angularly oriented cage in rolling element bearings). He observed that the bearings on the timing gear side of the blower are not contained in a separate housing. In our answer, we noted that dry lube films are often used for speed reducing gears in motorized hand drills. We then advised against experimentation with dry film lubrication in the engineer's reliability-focused industrial setting and on Roots-type blowers.
Some More General Answers Worth Pondering
Dry sump oil mist (properly applied) is a real attractive lubrication method for rolling element bearings in virtually all industries. Only oil jet lubrication (a superior form of forced lube) is technically better. However, oil jet lube would be more expensive and probably difficult to cost-justify for the positive displacement blowers at issue here.
In short, it is well known that many Roots-type blowers are lubricated by oil mist. However, first and foremost, the dry sump ("pure") oil mist is intended for rolling element (erroneously called "anti-friction") bearings and not, in the case of these blowers, for sleeve (or "plain") bearings or gears. Dry film lubrication development is strictly a function of oil type and temperature. For Roots-type blow-ers, dry film lube sounds like a research project with lots of variables. And so, we would stay clear of that, for now.
The oil mist must be ported through the bearings at the input side of the blowers. Except for "bearings only" regions where dry sump oil mist greatly excels over purge mist, wet sump (purge mist) will help considerably by preventing contaminant entry into the bearing and gear housing. Thus, purge mist should be used on the gear side of the illustration in Figure 2, but the mist routing may not be the same on different physical layouts or other housing configurations.
Of critical importance to dry sump oil mist lubrication is the mist entry point. If applied per API-610 (8th through 10th Edition), oil mist will protect bearings better than anything short of an oil jet impinging on the rolling elements. While oil mist may still work even if not applied per latest API-610 recommendations, a facility that disregards these straightforward recommendations loses its claims to be reliability-focused.
When utilizing an appropriate synthetic oil (mist) for both bearings and gears, the maximum allowable temperature is usually set by the bearing metal and bearing-internal clearance considerations (230ºF), and not by the lubricant temperature. The temperature permitted by several available high performance synthetic lubricants certainly exceeds 300ºF.
Reviewing Internal Sealing Provisions On Roots-type blowers, process pressure considerations rarely enter the picture. However, that statement assumes that internal sealing provisions (sealing between a bearing region and the adjacent pressurized process environment) are made with reliability focus and forethought. Internal sealing may differ for various machine sizes or internal features. Chances are that seal upgrades are feasible and merit inclusion in a pre-purchase review. That kind of up-front review is done by Bestof- Class companies and is neatly described in books and articles, courses, and technical seminars 2). For the past 43 years (since 1965), smart companies have allocated money for these reviews by factoring the cost into the project budget. As to the cost of such reviews, these companies have claimed benefit- to-cost ratios of 100-to-1 and higher. Sad to say, the not-so-smart companies still base their budgets on buying the cheapest possible equipment and then wonder why their maintenance costs are high and equipment reliability is low. Could it be that one only gets what one pays for?
Experience surveys are fine. Nevertheless, we should shy away from blindly trusting what someone said, regardless of whether they had good, or bad, or just so-so experiences. Sometimes, Many people's word-of-mouth experience may not be relevant at all. Most personnel don't know the difference between a typical oil mist fitting, a spray fitting (for slow speeds), a condensing fitting (for sliding ways) and a directed oil mist fitting (used to overcome windage). They probably don't perform root cause failure analysis (RCFA) and a neverending stream of repeat failures at their plant attests to the veracity of our suspicion. Any feedback or opinions must be linked to several variables and would mandate that we knew these variables. In any event, there is never a substitute for understanding how parts work, and how they fail.
So, a simple review of the cross-sectional configurations of the blowers in question is really necessary. Anything short of such a review is just guesswork - no value added, just risk added. As long as the review is done diligently, the owner-operator will prosper. If he "saves" the relative pittance that such a review will cost by not performing it, his facility will never reach the degree of low failure incidence that others enjoy. In other words, we believe the failure risk far outweighs the cost of an up-front analysis and our engineer is to be commended for asking some of his questions up front.
But there's also some criticism due. We will perhaps never understand why seemingly modern facilities claim they cannot afford funding the few hours it takes a competent advisor to teach the underlying fundamentals. Out of maybe a hundred questions that reach us in a year's time, at least ninety are asked out of context. For us to quickly answer them is, ultimately, not a productive endeavor. What's worse is that it very often leads to defensiveness on the part of the questioner.
Virtually all plants have training budgets, but certainly not all of them spend their training funds wisely. Truly teaching an understanding as to how machines function and malfunction would pay huge dividends. That's as true for oil mist on blowers as it is for thoroughly analyzing bearing housing (bearing protector) seals where half the O-ring is contacted by a groove in the stationary part and the other half is contacted by a groove in the rotating part. Did you ever wonder what happens to the O-ring at slow-roll, or when there is axial movement of the two parts relative to each other 3? When would we consider retrofitting a dual-face magnetic seal4? Now that is real food for thought! Once we upgrade to oil mist lubrication, we might as well do the whole job and do it right.
References:
1. Bloch, Heinz P. and Abdus Shamim; "Oil- Mist Lubrication Handbook"--Practical Applications", (1998), Fairmont Press, Lilburn, GA, 30047 (ISBN 0-88173-256-7)
2. Bloch, Heinz P.; "Improving Machinery Reliability", (1998), Third Edition, Gulf Publishing Company, Houston, TX, 77520 (ISBN 0-88415-661-3)
3. Bloch, Heinz P.; "Counting Interventions Instead of MTBF," (Hydrocarbon Processing, October 2007)
4. Bloch, Heinz P.; "Consider Dual Magnetic Hermetic Sealing Devices for Equipment In Modern Refineries," (Pumps & Systems, September 2004)
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)
