Oil Mist Lubrication - Seminar Report

INTRODUCTION

The Oil Mist principle was developed by a bearing manufacturer in Europe during the 1930s. The problem that nurtured this development was the inability to satisfactorily lubricate high-speed spindle bearings on grinders and similar equipment. In 1948, oil mist was brought to the United States in the steel industry. Since'the late 1950% oil mist has been servicing rotating equipment 'in the refining industry. The first use of oil mist in the petrochemical industry was in the Gulf Coast due to the high humidity and condensation resulting in bearing failures. The speed of these bearing was too high for grease lubrication, and liquid oil generated too much heat through fluid friction, necessitating an expensive recirculating system. Continuous thin-film lubrication with Oil Mist provided a solution. The purging and slight cooling effects of the carrier air gave additional benefits. The Oil Mist generator resulted later from this development and used a small amount of air to produce a dense concentration of small oil particles. About 97% of these particles could be transmitted to the bearings without condensing in the piping, regardless of the distance of the bearings from the Oil Mist generator itself.

Oil Mist is an aerosol with an appearance of smoke or fog. Oil Mist systems are made up of several simple but effective components, which can supply the proper lubrication for electric motor bearings. The main components of an Oil Mist system are the generator head or venturi, reclassifiers, vents and drains. Passing high velocity air over an orifice that pulls oil into an air stream creates the oil mist.

Later the technology was extended in a limited fashion to the petrochemical industry. Initial applications were for process pump lubrication. A great deal of caution was exercised with the new technology and while progress was made, it was slow. Experience in recent years has shown that many of the early concerns were not really applicable and that oil mist lubrication is a very efficient and reliable method for attaining optimum bearing life.

HOW MODERN OIL MIST IS GENERATED  

Modern oil mist is generated by the introduction of liquid oil and air into the sonic velocities at the neck of a vortex chamber (shown here). There the oil is pulverized into a homogenous form creating oil particles or droplets of 1 to 3 microns within the air flow

This oil suspension in dry instrument air is a mixture if 1 part oil to 200,000 parts air or 5 ppm oil to air. In this form the mist has the characteristics of a “dry mist” a lean stable mixture which can be conveyed horizontally up to 600 feet. Incidentally, ambient temperatures do not affect oil mist during the 7 minutes maximum it takes for the oil mist to arrive at the bearings furthest from the generator.

It must be made clear that in order to achieve the maximum results the cleanliness of the air and the oil must be assured before the generation of the oil mist in the Vortex chamber. For this, when necessary, additional filters should be applied both for the air and the oil. For the oil filters a minimum efficiency of β5 = 200 is recommended.

WHAT IS AN OIL MIST SYSTEM AND HOW DOES IT WORK?

Oil mist is formed of oil particles from 1.0-3.0 microns suspended in an air current, consisting of 1 part oil to 200,000 parts air. This mixture is not a volatile organic compound (VOC); therefore there is no risk of explosion or combustion. There are two main oil mist applications, pure mist for lubrication and purge mist for preservation. The equipment and its operating conditions determine the type of mist application that should be used. Due to its advantages, whenever possible, pure mist is recommended to lubricate rotating equipment.

Oil Mist is a centralized system in which the energy of compressed gas, usually air taken from the plant supply, is used to atomize oil. Oil is then conveyed by the air in a low pressure distribution system to multiple points of lubricant application.

The compressed air is passed through a venturi. Oil, siphoned from a reservoir by the air flow, is atomized into a fine spray. Baffles downstream from the venturi nozzle causing the larger oil particles to coalesce and return to the reservoir. The remaining air-oil-mixture is Oil Mist. Oil Mist contains oil particles (droplets) averaging about 1-1/2 microns in diameter (.00006 inches), which can be conveyed through distribution piping (mist manifold), at velocities up to 24 feet per second, to application fittings (mist fittings which meter oil to bearing housings). Air-borne oil particles are then "wetted out" by impinging upon bearing surfaces rotating at sufficient speed to cause adherence and the formation of larger drop sizes. Because there are no moving parts in the basis Oil Mist generator system, and because the system pressure is very low (manifold pressures from 5 to 40 inches of water), it is a reliable lubrication method. Also, the system can be interlocked with machine operation or an alarm system to ensure proper functioning. Note that, even when malfunctions occur, most bearings will operate for hours on an existing film of lubricant.

OIL MIST DISTRIBUTION SYSTEM

Once generated, the mist is transported through a pipe, or distribution header. The header is normally 2 inch NPT threaded galvanized steel pipe , schedule 40. The header must be installed with a slope in the direction of the generating console, this allows greater efficiency in oil consumption, since particles that get condensed in the pipe, by contact among them or contact with the walls of the pipe, are returned to the reservoir of the console to be re-circulated. The slope gradient will depend on the length of the header the viscosity of the oil and the temperature. Figure contain some percentages of recommended inclination which depend on the viscosity and temperature of the oil.

In practice, usually the length of pipe is set on the header which will have the required slope. Normally, in long headers (greater than 90 m), the first 15 meters will have a slope in the direction of the generating console, since most of the condensation occurs in this area. If the lines are smaller (30-45 m), the complete header can be inclined towards the console. To reduce the quantity as well as the severity of impacts between particles, the speed of flow within the header must be around 8m/s (26 ft/s) this assures a laminar flow and reduces the amount of condensed mist.

TYPES OF OIL MIST

Oil mist can be applied in two forms: pure mist (dry sump oil mist) or purge mist (wet sump oil mist). With purge mist applications, the oil mist is injected into the vacant space above the liquid oil bath, primarily in gear boxes with journal (plain) bearings. This is done to exclude water and other contaminants. With pure mist applications, the mist replaces the liquid oil bath in the rolling element bearing housing where there is no other lubricant being supplied to the bearings. 

Ø  PURE OIL MIST:

Pure oil mist is often referred to as dry sump oil mist, which indicates that no oil or grease is maintained in the bearing housing being serviced. The bearings are lubricated directly and continuously by a supply of fresh, condensed clean oil. Coalesced oil is obtained by the introduction of turbulence into the feed mist stream within an enclosed space. Additional condensing action is provided by the rotation of the bearing elements. The mist application fittings (reclassifiers) are individually sized to each bearing or the equipment chamber. This is necessary to feed the appropriate amount of lubricant to the each of the various sizes and types of bearings found within a drive/load string. While pure oil mist is the primary means of lubrication for operating equipment it also acts to preserve and protect the rolling elements within stand-by or stored equipment.

Ø  PURGE OIL MIST:

Purge oil mist is often referred to as wet sump oil mist, which indicates that an oil level is maintained in the equipment being serviced. The oil mist is applied on top of the oil sump which creates a slight positive pressure that prevents thermal cycling of the cavity and eliminates the intrusion of airborne contaminates. Purge oil mist is not the primary means of lubrication for operating equipment but it is extremely effective in preventing water build up and corrosion in stand-by, idled and stored equipment.

The equipment outfitted for purge oil mist is typically gearboxes, turbines with sleeve bearings, motors with sleeve bearings and oil reservoirs of lube oil systems.The continuous film of oil that is applied to internal machined surfaces prevents corrosion from forming and causing premature equipment failure.

The oil mist injection point must be above the oil sump with an unobstructed flow path to a vent, also above the oil level. This can be accomplished by using two separate openings on top of the housing, or by using a vent fill assembly in a single opening. The vent fill assembly incorporates a mist injection tube, connection for venting and a removal cap for adding oil to the sump. The pressure above and below the oil level must be balanced by using an oil level sight assembly or a constant level oiler with balance line. An overflow tube incorporated in the oil level sight assembly and constant level oiler is adjusted to allow excessive oil buildup to overflow to the collection container and to vent the bearing housing. The top of the overflow tube should be set ¼” above the desired oil level.

ADVANTAGES OF OIL MIST LUBRICATION

ü  Reduced friction losses in bearings.

ü  Cooler operating bearings. 

ü  Longer bearing life.

ü  Exclusion of external contaminants.

ü  Reduced lubricant consumption. 

ü  Elimination of lubricant misapplication. 

ü  Increased Reliability.

ü  Energy efficiency.

ü  Reduced  operating  temperatures.

ü  Reduction in friction coefficients.

ü  Increase in the nominal life of bearings.

ü  Reduced Vibrations.           

BENEFITS OF OIL MIST LUBRICATION  

There are numerous direct benefits of oil mist, not the least of which is that it reduces bearing failures by 90% with pure mist. A bearing manufacturer says that the bearing L10 life is extended by a factor of 6 by oil mist.Oil mist reduces lubricant consumption by as much as 40% for once through, open total loss systems. If a closed loop system with the application of the seals mentioned below is used, the oil consumption is reduced by considerably much more. This reduction is achieved because the oil is precisely metered to each bearing: the right amount in the right place at the right time. This reduction in oil consumption represents a very strong argument for switching from mineral oils (about $3 per gallon) to synthetic oils (about $18 per gallon) which results in an even longer service life both for the oil and for the equipment, this provides an increase in energy efficiency through the additional reduction in friction provided by these oils.

Because there is no liquid oil level and so no liquid friction, operating temperatures are about 10° F- 20° F cooler. One user reported 90° F cooler on a crude charge pump unit. According to bearing manufacturers, there is an inverse relation between bearing life and temperatures: the higher the temperature, the shorter the life. Another collateral benefit is up to a 3% reduction in energy consumption. In virtually all hot oil pumps equipped with bearing cooling jackets, the cooling water can be deleted. This is beneficial because cooling water can actually decrease bearing life. Cooling water shrinks the outer race while the heat conducted down the shaft expands the inner race, making the bearing run hotter due to the increase in the friction caused by the deformations of the housing and consequently of the track of the outer ring. 

WHERE DOES IT APPLY?

Oil Mist is applied to the centrifugal pumps, electric motors, gear boxes, fans and blowers of continuous process plants; such as petroleum refineries, gas and power generating plants and textile, steel and paper mills etc.

Centrifugal pumps:

The pumping of fluids is the most important task within the processes of a refinery, petrochemical plant and in general, any process plant where production depends on the operation of centrifugal pumps. Due to this, a high degree of equipment reliability is required, which is only obtained by the use of the best available lubrication and maintenance techniques. 

Lubrication by pure oil mist is the best-proven method of lubrication for centrifugal pumps and electric motors; this is the reason why its application to the total equipment population in a plant is necessary to achieve a greater degree of reliability and energy efficiency.

Electric Motors:

Modern pure mist lubrication is a proven, environmentally friendly, energy efficient and economically viable method of lubrication for electric motors in process plants.

* The application of mist to explosion proof motors is still a subject that requires agreement, though in several refineries it has been noted that applying mist to these motors does not involve risk, nevertheless, some manufacturers specifications and some maintenance departments do not guarantee the equipment if modifications are carried out to install oil mist.

Gear Boxes:

Applying purge mist avoids the entrance of humidity and polluting agents to the interior of the gear box due to the slight positive pressure created with the injection of oil mist.

LIMITATION OF OIL MIST LUBRICATION

While mist systems have dramatically reduced maintenance and operating problems, establishing flow rates in a system has proved troublesome and the following details commonly need attention:

The air stream in feed lines must be kept laminar, below approximately 24 feet/second, because turbulence causes oil particles to impact the pipe wall and be removed from the air stream before reaching delivery points. At abnormally low velocities, on the other hand, oil droplets may also settle out prematurely.

Performance is sensitive to temperature. Even when not required by viscosity considerations, heaters are often employed to stabilize the oil/air ratio under widely varying ambient temperatures. When used, air heaters are usually accompanied by oil reservoir heaters.

Spray mist involves environmental hazards. Vent lines are needed at lubrication points for collecting stray mist which has not been classified. OSHA requirements state that in an eight-hour period, a person can be exposed to no more than five milligrams of oil per cubic meter of air.

CONCLUSION

Oil mist is a proven technology that demonstrates every day around the world that it significantly reduces bearing failures, reduces maintenance costs, improves machinery availability, reduces energy consumption, reduces the need for warehoused repair parts, reduces life cycle costs, and reduces the stress of operating a plant. It not only improves machinery reliability but frees up operators and maintenance personnel to perform their primary functions, to put on spec product out the door and to help make the company a profitable business. Oil mist is discretionary but there is probably not another thing that a plant may do that can achieve these level reliability improvements than as can oil mist lubrication. It is also an effective method for lubricating electric motor bearings. Studies have shown significant increases in bearing life compared to grease lubrication.