In this article, we will see the oil mist lubrication and oil mist lubrication Working Principle.
Oil mist lubrication is the preferred method of bearing lubrication for equipment used in the environment conditions adversely affect bearing life due to the ingress of contaminants in the bearing housing. Oil mist lubrication will enhance the bearing reliability resulting
- High standards of lubrication quality control
- Prevention of ingress of water and other contaminants into the bearing housing
In Oil mist lubrication systems, the lube oil is atomized to the size of 2 μm to 7 μm in diameter. The oil mist size will depend upon the size of the bearing and heat generated in the bearing contact points (Load point).
The system will consist of a central mist generator, from the central mist generator lube oil send to various pumps located in the plant. The oil mist lubrication consists of a dry, low-pressure air of 50 mbar(g) or 0.725 psia. The normal concentration of oil mist in the distribution air is approximately 6 ppm.
Type of Oil mist lubrication
In industries they are two type of oil mist lubrication systems are used. They are
- Pure Oil Mist Lubrication
- Purge Oil Mist Lubrication
In the above Pure and Purge oil mist lubrication are widely used in pumps.
Major Component of Oil Mist Lubrication
The below-listed components for Oil mist lubrication are the minimum requirement. Some time will be change due to the Oil Mist Lubrication manufacture and purchaser requirements.
Oil mist generator
The oil mist generator is the heart of oil mist lubrication system. The oil mist will be generated by means of either venturi or vortex chamber. When compare with vortex, venturi type is widely used.
In the generator, the oil mist is generated by passing compressed air through a venturi or vortex. It will create low pressure and cause the oil to be siphoned from the reservoir. Due to this, the oil is introduced into the high-velocity air stream and it will be atomized.
Droplets size of larger than 7 μm is not easily transported by the dry air stream. Hence they are intercepted by a baffle and return to the reservoir.
Oil mist cabinet
The oil mist generator is enclosed in a stainless steel cabinet, which shall be freestanding and provided with a hinged door with robust closures. The cabinet is weatherproofed and vented. Usually, it is mounted at ground level.
The reclassifiers are used to control the flow of mist to each applicable points. It creates turbulence and causing the dry oil mist to coalesce and form wet oil mist. The reclassifiers are usually located at the ends of the branch lines in the mist manifold. The reclassified oil shall be fed to the individual bearing housings via feeder lines which shall be located within (maximum) 2.5 m (8 ft) long.
The sizing of reclassifiers are based on the bearing type, shaft speed and shaft diameter.
The air supplied to the mist generator shall be clean and dry. Usually, Instrument air shall be used for this application.
The air supplied to the oil mist system shall meet the following requirements:
- Air shall be free from liquids, toxic, corrosive and flammable gases
- The solids content shall be less than 0.1 g/m3 and the particle diameter shall not more than 2 to 3 μm
- The dew point at the supply pressure shall always lower than the lowest annual ambient temperature.
An air filter or coalesce filter shall be used at the air inlet supply. It shall be capable of removing 100% of particles of the size from 30 μm and larger.
An air regulator shall be provided to control the pressure of the air supply within the oil mist distribution system.
For oil mist lubrication systems that need to operate in colder climates. A thermostatically controlled electric air pre-heater will be used to maintain the air temperature above the dew point.
Oil Mist Lubrication Working Principle
The oil mist generators are usually designed for a capacity of 100 to 125 % of the rated requirements of the system. This requirement is usually jointly decided by the purchaser and manufacturer. If the design capacity is more than 125% care should be taken the oil mist lubrication system can meet the minimum operating condition.
The Principal shall provide a dry air supply to the oil mist console at a pressure range between 3 bar (g) (60 psig) and 8 bar (g) (100 psig) from an instrument air header. This air supply shall be via piping installed solely for the oil mist lubrication system. The before the air enters into the oil generator it will pass through the air filter to remove the impurities. Then via air pressure regulation to maintain the required pressure of oil mist generator and air heater to make the air dry (Refer Schematic diagram below).
In the generator, the oil mist is generated by passing the dry air through a venturi or vortex. It will create low pressure and cause the oil to be siphoned from the reservoir. Due to this, the oil is introduced into the high-velocity air stream and it will be atomized. The atomized oil enter the distribution network. The design of the distribution network shall ensure that a homogeneous mixture of air and oil reaches all equipment.
The distribution header is designed to maintain the sufficient pressure to impart a velocity to the oil mist. The flow is laminar throughout distribution header and it will be changed to turbulent flow at the oil mist distribution manifold.
The manifold consist of reclassifiers, view glass chamber, drain connection and push valve as shown in the figure. The manifold is used to collect the coalesced oil particles from the drop points and then the oil mist is injected into the bearing housing for bearing lubrication.
Advantages of Oil Mist Lubrication
•Positive pressure will prevent the ingression of contamination into the bearing housing.
• Bearing life will be extended from 150% to 180%.
Widely used in contamination/dirt environments
•Lubrication consumption is very less when compared to the conventional method.
•Oil change over is not required.
Due to less moving parts, maintenance cost is less.
Suitable oil shall be selected jointly by Purchaser and Manufacturer. As a minimum, the following factors to be considered for selecting the oil for mist lubrication:
• The viscosity of the oil at the operating temperature of the oil mist system.
• Surface tension of the oil
• Pour point relative to the Operating temperature.
• Wax formation tendency of the oil at low ambient temperature, to prevent blockage of the reclassifiers.