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Oil mist lubrication has been successfully used for pumps and their electric motor drivers in petrochemical plants since the 1960s. The basic process description is as follows: First, no liquid oil is placed in the bearing housing. Instead, an entirely static oil mist generator creates the oil mist in a central module that services all pumps within a radius of approximately 600 to 1,000 feet.
The mist is a mixture of atomised (less than three-micron) oil droplets mixed with air in a ratio of about 1:200,000. The mist, which has the appearance of light smoke, is moved out to the pumps via two-inch or four-inch piping in a low-pressure (less than 1 psi, approximately 5 to 6 kPa), low-velocity (less than 7 ft/s, about 2 m/s) process. At the pump, the oil mist is inserted into each pump bearing housing through a small-diameter application fitting or reclassifier, which greatly increases the mist velocity, causing the droplets to collide and coalesce into larger, liquid drops of oil.
This air/oil mixture passes into the housing, where some of it contacts and lubricates the bearings, while much of it falls straight through to the bottom of the housing. The coalesced oil finally passes through a hole in the bottom of the housing and is collected for disposal (Figure 1).
The advantages of oil mist lubrication include:
Almost completely maintenance-free and self-checking. It is no longer necessary to rely on operators to check and fill housings with oil.
Better lubricant properties, because the oil on the bearings is always new.
Lower operating temperatures, typically 10ﾰF and often 20ﾰF (approximately 6ﾰC to 12ﾰC).
Reduced power requirements, because the bearings operate in a thin film of oil instead of plowing through a pool of oil.
Protection of non running (standby) pumps. Maintaining an oil fog environment at all times greatly reduces bearing degradation and the resulting risk of premature failure.
The MTBF benefits of oil mist over oil traditional sump lubrication of rolling element bearings have been well-documented, and the oil mist lubrication option was included beginning with the API-610 Seventh Edition in 1989.