Here is what oil mist lubrication is

June 1, 2020
TMI Staff & Contributors

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Oil mist is a centralized lubrication system that continuously and efficiently atomizes oil into small particles and then conveys and delivers the correct amount of lubricant to bearings and metal surfaces which improves the lubrication process and extends machinery life. The oil mist particles when generated are nominally in the 1 to 3 micron range and are referred to as dry mist.

This article contains excerpts from the paper, "Getting the facts on oil mist lubrication" by Don Ehlert of Lubrication System Co. at the Middle Eastern Turbomachinery Symposium.

Dry mist particles are too small for lubrication but are easily transported, via air flow, throughout the piping header system. The mist travels at a velocity of 7.3 meters per second (24 feet per second) in a continuous laminar flow at a pressure of 500 MM (20” water column or .74 PSI). Oil mist can be referred to as the ultimate oil filter. The dry mist or small particles of oil, 1 to 3 micron, are physically too small to carry water or particulates throughout the piping system. The larger and or heavier particles that could possibly carry contaminates fall out of suspension either in the reservoir or in the piping system. Therefore only fresh clean oil is delivered to the bearings for lubrication.

Contaminated oil is not re-circulated through the bearings as with a sump system (6). The mist is generated by passing air through a vortex or venture chamber where it picks up oil with a vacuum, siphoning effect, and introduces it into the high velocity air stream where the oil is atomized into the small particles, thus creation of the oil mist.

A pressure drop occurs as the air passes through the vortex or venturi which creates the mist header pressure of 500MM (20” of water column). This air oil mixture is known as oil mist; it is not a VOC, Volatile Organic Compound or a vapor, it is an aerosol that consists of 1 part of oil to 200,000 equal parts of air. This mixture is well below the lean limits of flammability (7) and is non explosive.

Regarding toxicity; the American Conference of Government Industrial Hygienists; gives a threshold limit value of 5mg/m3 for a normal 8 hour workday. Simply, this means that oil mist in concentrations found around oil mist lubricated machinery in typical open air HPI facilities is not a health hazard. The particle size distribution of the oil mist is an important characteristic for successful lubrication. A large volume of very small mist particles (< 1.5 microns) can lead to inadequate lubrication, as particles of this size will not coalesce to allow wetting. Conversely, a large volume of large mist particles (> 3 microns) will fall out of suspension prior to reaching the application points.

Careful selection of the correct oil coupled with the design advantages of the vortex mist head can assure the mist distribution necessary for the system. Additionally, the velocity of oil mist travel through the distribution piping is important for proper lubrication. High mist velocity will cause the smaller particles to coalesce and wet out after impinging on the piping walls. Low mist velocity will not support the flow of oil mist particles in suspension.

Regardless of the control of particle size distribution and mist velocity, some level of oil will coalesce in the piping. The modern oil mist system installation is designed to accommodate this coalesced oil, returning it to the generator, unlike some of the problematic layouts of the past. The header pipe, normally field routed, is supported, braced and sloped to facilitate the return of liquid oil to the oil mist generator (OMG) where it is recycled in the systems operation. The drop pipe is connected on the top side of the header and extends up over and down to the mist distribution manifold (MDM) where the reclassifiers are located in a horizontal plane. This piping arrangement prevents liquid oil from draining down to the MDM’s and plugging the reclassifiers.

The MDM’s are now equipped with a sight glass below the reclassifiers to allow a visual of liquid oil collecting with a push button drain valve that allows for the collected oil to be drained off, either to an oily water sewer cup or oil collection device. This arrangement allows only the oil mist to flow through the reclassifier and into the bearing to be lubricated. A fresh clean oil supply that continuously lubricates the bearings and pressurizes the bearing housing to eliminate the intrusion of air borne contaminates. Oil mist lubricates operating equipment and preserves/protects idled or standby equipment all from the same system.