Machinery Lubrication magazine published by Noria Corporation
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safe when the pump is in service, the present oil can be drained while such pumps are on-line and running. Many superior synthetics are compatible with the oil presently used in a particular pump. Still, compatibility must be verified. Of course, there are certain pump bearing or lube degradation problems that have nothing to do with the lubricant type. In those instances, nothing will be gained by changing over to better oils. Lubricants for Oil-Mist Systems Pure oil-mist lubrication eliminates the need for oil rings or flinger discs. No liquid oil sumps are maintained in the bearing housings, hence the term "dry sump" is often used to describe modern oil-mist lubrication. ISO VG 68 and VG 100 mineral or synthetic oils are used, although properly formulated ISO VG 32 synthetics (but not mineral oils) will serve the majority of pump bearings and also virtually all types of rollingelement bearings in electric motors. Decades of experience on thousands of pumps and Figure 5. This chart shows how changes in lube application, oil type and electric motors attest to the viability and cost effeclube viscosity tend to affect percentage reductions in bearing friction. (Ref. E. Villavicencio) tiveness of modern plant-wide oil-mist systems. Typical payback periods when using oil mist on problem Micro-cracks in bearing surfaces can cause increased noise pumps have generally been less than one year. and vibration. Suitable high-film-strength oils will fill these Bearing friction can be reduced by switching to different micro-cracks. This then lowers noise intensity and reduces oils, going with a different lube application method or switching vibration severity. High-film-strength lubricants also lessen the probability of lube both lube application method and oil type. Five different modificaoil darkening during the running-in period of bearings with brass or tions were closely examined in a cooperative effort involving bronze cages. There have been reported instances of high frictional a multi-national lube contact during the initial run-in period of the copper-containing oil producer and a bearing material recommended by API-610. If the net axial thrust action on prominent manufacturer. The one of the two back-to-back oriented bearings causes it to become results were plotted, unloaded, it may skid. The risk of lube oils darkening during the run-in period of such and percentage reducpumps is reduced through the use of high-film-strength synthetic tions in bearing friction on the lubes. To be fair, this risk could also be reduced by insisting on displayed vertical scale in Figure impeccable installation techniques and the selection of bearings with cages made of advanced high-performance polymers. The 5. There are also advanAPI-610 stipulation of using thrust bearings with particular load temperature tages associated with angles and brass cages represents a compromise, which is especially attractive for plants that place the desire to standardize different oils and different lube applicaahead of failure-avoidance concerns. Whatever the differential cost of a quart of high-film-strength tion methods (see synthetic, it is insignificant compared to the value of an avoided Figure 6). Remember, by failure incident on critical, non-spared refinery pumps. Therefore, consistently staying critically important pumps, pumps in high-temperature service and Figure 6. This chart illustrates how changes in lube application, oil type and lube acceptable pumps that have failed more often than others in the plant's pump within viscosity affect bearing temperature. population should be lubricated with high-film-strength synthetic ranges of dimensional, (Ref. E. Villavicencio) oils. Their higher cost is easily justified by four to six times conven- material composition, tional oil replacement (draining) intervals and by keeping the oil fabrication-specific and application-related parameters, you can free from contamination. Oil contamination is effectively avoided avoid process pump failures. Of course, operations, maintenance and project engineering must cooperate. Still, adherence to sound by installing advanced bearing protector seals. On pumps where a problem is in progress, changing to a supe- specifications is not difficult once a proper mindset is cultivated. rior synthetic is highly recommended. If access to the sump drain is The difficulty is in cultivating the mindset. www.machinerylubrication.com | January - February 2013 | 15