Guest Column
08/25/2017
European Oil Refinery “X” was about to commission a plant-wide oil mist lubrication system, i.e., a setup that combines 200,000 volumes of clean, dry carrier air with one volume of lubricating oil. The resulting oil mist or “oil fog” flows in suitably sized headers and branch pipes to the many pump and electric motor bearings in modern “best-of-class” user plants. Although oil mist lubrication has been successfully used at hundreds of refineries, the lead reliability engineer (RE) at Refinery X had two questions.
Oil Mist for Plain Bearings
His first question referred to an old research article that dealt with the use of oil mist in plain bearings. The article explained how, to be applied to plain bearings, the mist must be converted (reclassified) to air-free liquid oil and applied approximately 10 to 30 millimeters upstream of the plain bearing. The liquid oil is then allowed to flow into the sleeve bearing. The RE was considering oil mist in plain steam turbine bearings after first converting the mist into large oil droplets, or even an oil spray. Oil mist supply pipes were already installed for purge mist, and he correctly surmised that it would be better to avoid the traditional oil ring and sump arrangement if possible. Accordingly, he inquired if he could extract (reclassify) most of the fresh oil from the oil mist and use the oil for lubricating the turbine’s plain bearings. The leftover lean oil mist would still be in the bearing housing and keep out dust and moisture-laden ambient air. In our answer we mentioned that reclassified oil mist was well researched as a means of lubricating plain bearings in machine tools in the late 1950s. But there are more reliable ways to lubricate plain bearings in small- to mid-sized steam turbines in the hydrocarbon processing industry (HPI). In plain bearings and with the use of conversion (reclassifying) fittings, owners have to consider all the influencing factors around these bearings. Heat removal is one of the primary reasons for the oil. Hence, the flow rate of this reclassified liquid oil must equal the originally specified mass flow. Determining equivalency would be required. An oil mist feed for application and reclassification at plain bearings is cost-justified in relatively few cases.Are There Temperature Limits?
The RE’s second question related to Reference 2, where he noted our words about the “myth” of a limiting product temperature when applying pure oil mist to rolling element bearings in process pumps. The RE said, “Our specification allows installing pure mist in pumps with product temperatures ranging from 120 C to 250 C, as long as a fan is installed on the pump shaft. Above 250 C the specification disallows pure mist and, instead, requires purge mist (wet sump). The very existence of a temperature limit set by certain design contractors is something I have difficulty understanding. At present, I am attempting to maximize the use of pure oil mist (dry sump) in the plant instead of the purge oil mist (wet sump) that had been widely used at our facility in the past.” We agreed and confirmed that the world’s best refineries use pure oil mist on many of their thousands of rolling element bearings. Except for a few applications in small steam turbines, or as a floating “blanket” in the space above the conventional oil level in bearing housings with sleeve-bearings (plain bearings), one does not use purge mist. Pure oil mist (dry sump) excels in rolling element bearings and has been in use at refineries owned by BP, Exxon, Chevron, Shell and others for more than 40 years. So that specification at this RE’s plant makes no sense. It will be easy to verify 40 years of good experience in pipe-still bottoms pumps and other hot services with pumpage/product temperatures as high as 400 C. Years ago, there was a preference to move ambient air across the bearing housing. Since then, however, synthetic lubricants have become widely available. In closed oil mist systems, the coalesced oil is collected after it has passed through the bearings toward a drain port near the low point of the bearing housing. Moreover, refineries have applied pure oil mist in conjunction with appropriate synthetic lubricants (Poly-Alpha-Olefin “PAO” or PAO/Dibasic Ester–based mixtures) for decades, and no cooling off is needed. Therefore, fans are no longer used.What if There Are Hot Bearings?
If hot bearings are encountered with oil mist, we view this as a sure sign of overloaded bearings, incorrectly installed bearings, incorrect bearing fit or incorrect lubricant viscosity. When bearing temperatures exceed 210 F, remember all failures of rolling element bearings in process pumps are attributable to one of four contributors: force, reactive environment, time and temperature, or FRETT. Still, the RE wrote he would be grateful for another point of view. In his opinion, his employer could decide to install pure oil mist regardless of fluid temperatures. While he (correctly) reasoned that pure oil mist was the right lubricant application mode in pumps where field measurements of the bearing operating temperature did not exceed 190 F (88 C), he wanted assurances about pure mist working well in high temperature pumps. Our full answer again confirmed there are many hundreds of pumps in service with pumping temperatures of 740 F (approximately 400 C). They are lubricated with pure oil mist and some of these pumps have been in highly successful service for well over 40 years. Many of these are in operation for six or seven years before being dismantled for precautionary inspections.
Image 1. A circulating oil system can be used to upgrade to superior bearing lubrication in mid-size machines (Courtesy of AESSEAL, Inc, Rotherham, UK, and Rockford, TN)
Read more articles by Heinz Bloch here.