The environmental friendliness of oil mist lubrication technology

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One of the main reasons for the resistance to the widespread use of oil mist lubrication technology in petrochemical systems in the mid-20th century was the concern over the potential harm to operators’ health caused by the emission of oil mist. The commonly used oil mist lubrication systems at present are of the open type; as dictated by the technical characteristics of oil mist lubrication, gas must flow, which inevitably leads to oil mist carrying oil droplets escaping into the atmosphere. In determining the properties of oil mist for lubrication purposes, it is necessary to take into account both the need to ensure good lubricating performance and delivery characteristics by keeping the oil mist particles as small as possible, as well as environmental considerations, which mean that the oil mist particles should not be too small. When concluding that small-particle oil mist is suitable for oil mist transportation, a contradictory requirement is to reduce such small-particle oil mist to meet environmental protection standards. Here is a set of studies that conducted comparative tests on various oils containing special additives, with the aim of finding ways to reduce the micron-sized particles in the oil mist without having a negative impact on its lubricating properties. The lubricant selected is derived from commonly used oils available on the market, with some additives added. By classifying them by type, the effect of these additives on reducing particles in discrete oil mists was determined. The objects and conditions for testing were 77.5 mm diameter radial thrust ball bearings, with a load of 125 kg and a rotation speed of 2400 r/min; the oil mist nozzle used for these bearings had a capacity of 43 cm3/c. Measure the mass-size distribution of oil mist particles for each oil before and after passing through the test bearing. Appropriate additives can be added to the lubricating oil to reduce the micron-sized particles in the escaping oil mist, without affecting the performance of the oil mist lubrication system. The Occupational Safety and Health Standards PART 1910.1000–Air Pollutants, currently implemented by the U.S. Occupational Safety and Health Administration, sets forth regulations regarding air pollutants in employees’ work environments; the maximum allowable concentrations of these pollutants are determined based on the type of pollutant and the duration of exposure by employees to that polluted environment. The maximum allowable concentration for exposure to a mineral oil mist environment is 5 mg (oil) per m³ of air, based on an 8-hour time-weighted average (TWA). The oil mist lubrication technique used keeps the oil mist enclosed before it enters the pump, with a concentration of 5 g/m³ in this enclosed pipeline. The developers of oil mist lubrication conducted an experiment: for a 0.18 SCFM nozzle in the oil mist system, the maximum concentration achieved when air was blown directly into the atmosphere at a distance of 6–12 inches was 2.0 mg/m³. Since each nozzle is connected to the pump chamber, the oil mist undergoes further separation; therefore, this condition represents the extreme limit. Another study measured the values at the pump shaft leakage points and at the outlet of the oil mist purification system (the outlet of the collection tank), and found them to be below 0.5 mg/m³; this indicates that the pollution caused by oil mist emissions is within acceptable standards. The verification method involved calibrating the lubricating oil that is released along with oil mist in actual applications. By comparing the amount of lubricating oil recovered over 19 days from 8 pumps that were either shut down or in use, with the total amount of lubricating oil that entered the pumps along with the oil mist, the following results were obtained: the total consumption of lubricating oil during the calibration period was 0.0352 L/h, while the total amount of lubricating oil recovered during the same period was 0.0132 L/h; thus, the recovery rate of lubricating oil was 88.64% ； The lubricant leakage rate is 11.56%, and the amount of lubricant that leaks is 0.004 L/h. The total amount of oil mist during the calibration period is 5606.74 L/h, which results in an oil mist concentration in the exhaust gas of 0.64 mg/m³. This result is very similar to the researchers’ findings. Since the height of the exhaust outlet generally does not exceed 0.5 m, the exhaust oil mist disperses and dilutes immediately once it enters the atmosphere, resulting in a concentration that is below the specified values, thus meeting the limits set out in the relevant regulations.