Oil Mist Lubrication Working Principle

In this article, we will learn about the Oil Mist Lubrication Working Principle

What is Oil Mist Lubrication?

Oil mist lubrication is the preferred method of bearing lubrication for Rotating Equipments (ie., Pumps, Compressor, Turbine, blowers, fan) used in the environmental conditions adversely affect bearing lifetime due to 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
Improve the bearing life
Avoid premature failure of bearings

In Oil mist lubrication systems, the lube oil is atomized at the size of 2 μm to 7 μm in diameter. The oil mist size depends upon the size of the bearing and heat generated in the bearing contact points (ie., Load point).

The system consists of a central mist generator, from the central mist generator lube oil send to various Rotating equipment located in the plant. The oil mist lubrication consists of 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, two types 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 widely used for Rotating equipment.

Major Component of

The below-listed components for Oil mist lubrication are the minimum requirement. Sometimes it may change due to the Oil Mist Lubrication manufacture and purchaser requirements.

Oil Mist Generator

The oil mist generator is the main component of the oil mist lubrication system. The oil mist generated by means of either a venturi or vortex chamber. When compare with vortex, venturi type has widely used.

The compressed air passes through a venturi or vortex, it creates low pressure and the oil siphoned from the reservoir. Due to this, the oil has introduced into the high-velocity air stream and its atomized.

The dry air system not able to transport oil droplets size of larger than 7 μm. Hence baffles are used to restrict the larger oil droplets and return to the reservoir.

Oil Mist Cabinet

The oil mist generator has enclosed in a stainless steel cabinet, which shall be freestanding and provided with a hinged door with robust closures. The cabinet is weatherproof and vent arrangement provided. Usually, the cabinet mounted at ground level.

Re-classifier

The re-classifier used to control the flow of oil mist to each applicable point. It creates turbulence and causing the oil mist in dry air to coalesce with each other and form wet oil mist. The re-classifier 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) 2.5m (8 ft) long.

The sizing of re-classifier shall be based on the bearing type, shaft speed and shaft diameter.

Air

The air supplied to the mist generator shall be clean and dry. Usually, Instrument air is 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.

Air filter

The inlet air supply passes through the air filter or coalesces filter. Normally the air filter capable of removing 100% of particles of size from 30 μm and above.

Air regulator

An air regulator is provided to control the pressure of the air supply to the oil mist distribution system.

Air pre-heater

If the oil mist lubrication system needs to operate in colder climatic condition. A thermostatically controlled electric air pre-heater used to maintain the air temperature above the dew point.

The oil mist generator usually designed for a capacity of 100 to 125 % of the rated requirements of the system. Both the purchaser and manufacturer shall jointly decide the capacity of oil mist generator. If the design capacity is more than 125% of rated capacity, then care should be taken that the selected oil mist lubrication system can able to meet the minimum operating condition.

The 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 through piping installed solely for the oil mist lubrication system. Before the air enters the oil generator, it passes through the air filter to remove the impurities. Then via air pressure regulation to maintain the required pressure of the oil mist generator and air heater to make the air dry.

Either a venturi or vortex chamber used to generate the oil mist. Compressed air passes through a venturi or vortex, it creates low pressure and the oil siphoned from the reservoir. Due to this, the oil has introduced into the high-velocity air stream and it’s atomized.

The atomized oil enters 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 designed to maintain the sufficient pressure to impart a velocity to the oil mist. The flow is laminar throughout distribution header and its changes to turbulent flow at the oil mist distribution manifold by re-classifier.

The manifold consists of re-classifier, view glass chamber, drain connection and push valve as shown in the figure. The manifold used to collect the coalesced oil particles from the drop points and then the oil mist 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 has extended from 150% to 180% of design life.
Widely used in contamination/dirt environments
Very less lubrication consumption when compared with the conventional method.
Frequent oil change over not required.
Due to less moving parts, less maintenance cost.

Oil selection

Both Purchaser and Manufacturer shall jointly select the suitable oil for equipment. As a minimum, the following factors shall be considered for selecting the oil for mist lubrication:

The viscosity of the oil at the operating temperature of the oil mist system.
The 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 re-classifiers.