Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/902206
4 | November - December 2017 | www . machinerylubrication.com sick people. Any time a lubricant is oxidized (e.g., a viscosity increase, dark opaque color, elevated acid number, pungent odor, etc.), a complete system flush is required to purge the infection. Oil analysis can help schedule oil changes well in advance of base oil oxida- tion. Although less common, a similar problem occurs when a system is infected with microbial contamina- tion (bacteria or fungi). One of the advantages of an oil change is the oppor- tunity to expunge ghost riders from your oil. Ghost riders are the microscopic hard particles that accumu- late in oil over time. ey are smaller than the pore size of the onboard oil filter. If you are using a 10-micron oil filter, particles smaller than 10 microns will circulate freely and unabated by filtration. e filter selectively removes only the particles larger than 10 microns. As new particles ingress during normal operation, the small ghost riders continue to grow in popula- tion until the oil is changed. e problem with these ghost riders is that they can do many harmful things to machines. For instance, anywhere there is elastohydrodynamic lubrication (e.g., rolling-el- ement bearings), these small particles can readily bridge the working clearance, damaging bearing surfaces in the load zone. Where boundary lubrication exists (starts, stops, slow-moving surfaces, misalignment, starved oil conditions, etc.), the ghost riders will cause the vast majority of the damage (three-body abrasion and surface fatigue). e following are additional benefits of an FVOC versus an SVOC: Machine Devarnishing New oil usually has a relatively high impurity- holding capacity (IHC) compared to used oil. is means when new oil enters a machine, some of the varnish and deposits from the previous oil can be cleaned away quickly. is devarnishing occurs when the oil has residual IHC aided by a higher operating temperature. Much of this can be lost due to an SVOC. To learn more about IHC, read the article titled "What is Your Oil's Impurity-holding Capacity (IHC)?" at MachineryLubrication.com. Better Demulsibility Most lubricants should demulsify water rapidly to mitigate damage to the oil and the machine. Solid and dissolved impurities of nearly all types build up in used oil and can inhibit demulsibility. ese are passed on to the blended new and used oil after an SVOC. Improved Air-handling Ability For the same reasons an FVOC is beneficial for demul- sibility, it is also good for a lubricant's efficient air release and low foam tendency/stability. Longer Filter Life e impact of soft contaminants (sludge, varnish insolubles, oxides, dead additives, etc.) on filter life can be substantial. Soft contaminants are the product of oil aging. As such, they are purged with FVOCs but less so with SVOCs. Good Oil Analysis Baseline Another disadvantage of an SVOC relates to oil anal- ysis. Good oil analysis needs a reliable and consistent new oil baseline. Most oil analysis alarms are set as an offset to this baseline. If the baseline has been corrupted by blending new oil with old oil, so too is the effectiveness (precision) of the oil analysis data interpretation. AS I SEE IT MACHINE OR SYSTEM SVOC Percentage of remaining old oil after the tank is drained and refilled with new oil. This is machine dependent. DOUBLE OIL CHANGE Approximate percentage of old oil remaining after draining and refilling the tank, followed by oil circulation. The drain-and-fill is then repeated. SVOC FOLLOWED BY LINE PURGE Approximate percentage of old oil remaining after performing an SVOC, followed by diverting old line oil to waste. A 60% 36% 12% B 40% 16% 8% C 30% 9% 6% D 20% 4% 4% E 10% 1% 2% F 5% 0.25% 0.5% Figure 2. The amount of used oil remain- ing in a system after performing an SVOC, a double oil change and an SVOC followed by a line purge (refer to the narrative on the following page) 44% of lubrication professionals perform short-volume or partial oil changes at their plant, based on a recent survey at MachineryLubrication.com