A survey on oil mist usage in U.S. refineries found that almost 50 percent of the refineries use oil mist lubrication extensively. The survey was based on large-scale systems with a fully monitored oil mist generator (OMG), which serves multiple pumps and drivers. The average pump population per system is 30 to 50 pumps and drivers, usually pure mist or dry sump applications.
Major oil companies were the first to adopt and use oil mist. Now many of the smaller and even the independent refineries recognize the benefits of oil mist lubrication and are using it to achieve improved reliability with their smaller workforces.
What is Oil Mist and How it Works
Oil mist is an aerosol. It is not a Volatile Organic Compound (VOC) or a vapor. It is a mixture of one part oil to 200,000 equal parts of air. It is a lean mixture that will not support combustion and will not explode. The appearance of oil mist resembles cigarette smoke or steam drifting from a pump or motor through a vent line. Using oil mist lubrication, a typical application in the hydrocarbon processing industry will reduce oil consumption up to 40 percent over the traditional oil sump method of lubrication.
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Oil Mist System
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Oil mist is generated by passing high velocity air over or through an orifice that pulls oil into the air stream. The high velocity air shatters the oil into particle sizes of one to three microns, thus the resemblance to cigarette smoke. Airflow transports these small oil particles through a piping system to the equipment to be lubricated.
Prior to being applied to a bearing, the small particles of oil are passed through an orifice, reclassifier or mist fitting, causing the small particles to impinge on each other and grow in size. The heavier particles are then large enough to wet the surfaces and provide adequate lubrication for most rolling element bearings. It is excellent lubrication for bearings operating at 1,800 to 3,600 rpm and it is often the preferred method of lubrication for bearings operating in the 10,000 to 15,000 rpm range where splash lubrication is ineffective.
Because an oil mist system has no moving parts, there is little chance of failure. The only requirements for generating oil mist are clean air and clean oil. The electric monitors control and help maintain a constant mist density. Many systems without electrical devices are in operation, and may be serving critical equipment. There are numerous systems in refineries serving 30 to 50 pumps in pure mist applications that do not have the redundant or backup units.
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Pure Oil Mist
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Currently, there is only one OSHA regulation concerning oil mist: “Oil mist cannot contain more than five mg of oil per cubic meter of air that a person can be exposed to in an eight-hour period.” At this time, there are no other laws, regulations or mandates from the government controlling the use of oil mist lubrication.
Benefits of Oil Mist Lubrication
With the hydrocarbon processing industry (HPI) consolidating to reduce cost, the trend to improve machinery reliability is a top priority. The companies using oil mist lubrication see the mean-time-between-repair (MTBR) increase from 24 to 36 months up to 48 to 60 months. With the average repair cost per pump between $5,000 and $10,000, the reduced repair costs attributed to oil mist lubrication can have a dramatic impact on a refinery’s bottom line.
Oil mist lubrication provides a number of important benefits, including:
- Reduced bearing failures
- Decreased manpower for lubrication tasks
- Elimination of oil changes
- Reduced fire risk
- Improved mechanical seal life
The reduction of the work force in many plants has stimulated the discussion of oil mist in maintenance meetings. The oil mist system allows the reduced staff to spend less time on the tasks of lubrication and more time on high priority tasks, while lubrication is provided in the right amount at the right time. Other possible cost credits include the reduction in fire incidents, mechanical seal failures and exposure to hazardous chemicals.
Maintenance people often ask how oil mist can reduce mechanical seal failures when oil mist is not directly applied to the seals. It is common for an oil mist salesman to hear “We don’t have bearing failures. Mechanical seal failures are more common.” Consider this: With the bearing attached to the same shaft as the mechanical seal, and because everything affects the mechanical seal, it is safe to assume that when bearings last longer mechanical seals will last longer. The same is true when pump failures cause exposure to chemicals; by reducing the number of pump failures, the chance of exposure is reduced.
One of the most favorable benefits of pure oil mist lubrication is that it reduces the operating temperature of the bearing it lubricates, typically 20ºF to 35ºF (-6ºC to 2ºC). Heat is generated by friction created from rolling the bearing through an oil sump at a high speed. When the oil sump is drained away, there is no oil to generate heat and no sump in which to retain it, so the temperature drops. With every 10ºF (5.6ºC) reduction in temperature, the bearing fatigue life is increased by 11 percent.
See Results of Oil Mist Case Study
Oil mist lubrication, while often superior to the traditional oil sump lubrication, not only lubricates equipment during operation but it also protects and preserves internal components (bearings and gears) when equipment is idle. Idle or spare equipment is subject to surface corrosion on internal components due to the ingress of atmospheric moisture, and the possibility of fretting wear caused by ground vibration.
Idle equipment will experience thermal cycling, or breathing contaminated air as temperature changes, many times throughout a 24-hour period. This causes the surrounding atmosphere to enter the bearing or gear cavity. The atmosphere may be laden with moisture, dust or even acid fumes, depending upon the contaminant within the environment. The atmospheric contaminants accumulate on machine surfaces, causing corrosion and reducing equipment life.
In contrast, oil mist builds a slight positive pressure, approximately 0.25 inches water, in the cavity. This pressure prevents the intrusion of the airborne contaminants, and internal surfaces and components are coated with a protective film of oil. The oil film on internal components guards against corrosion, and protects against fretting wear by creating a thin film of separation.
Many facilities now use oil mist storage systems to protect and preserve equipment stored in warehouses. Large-scale storage yards with oil mist are also used in grass-roots plants during the construction phase to protect equipment over extended periods of time. Equipment such as pumps, motors, turbines, diesel engines, large lube oil skids, vertical sump pumps and gearboxes may be stored on a single mist system.
A refinery built in Thailand a few years ago used oil mist storage and experienced only one failure during plant startup. Conversely, refineries not using oil mist storage may experience numerous failures during equipment testing and startup.
Most Common Equipment Applications and How to Apply Oil Mist
The most common application for oil mist is the typical overhung process pump. Pumps are the workhorses in the HPI and are subjected to harsh chemicals, extreme temperatures, long intervals between oil changes, high thrust loads and other conditions for which they were not designed.
For years, maintenance/reliability engineers have experimented with how to apply oil mist, and have been pushing the pump manufacturers to adapt their equipment for oil mist. API 610 8th Edition Pump Standards now embrace oil mist lubrication. Standards of applying oil mist are gaining ground and making the process even more successful.
Motors are an excellent opportunity for oil mist lubrication, but because the rotors, stators and bearings are housed together, it is not an easy application. Oil in the windings is not the problem; it’s the housekeeping situation that makes the application less desirable on motors. In recent years, motor manufacturers have responded to the market’s wishes by researching the use of oil mist lubrication on motors.
As a result, housekeeping has greatly improved. Oil mist is a vast improvement over the traditional grease lubrication found on most motors. With oil mist, the bearing receives a fresh, clean, continuous supply of oil versus a single greasing at the factory or repair shop and infrequent applications of lubricant thereafter.
Pillow block bearings are also candidates for oil mist lubrication because they are normally lubricated with grease or a small oil sump. Grease and the oil baths both retain heat which increases operating temperature. Or, the small lubricant supply may become contaminated from the environment. Both heat and contaminants reduce bearing life.
Two common problems with grease lubrication are overlubrication and underlubrication. Applying the right amount of lubricant at the right time is difficult unless the grease is applied with an automatic system. Set up to control the frequency and amount of grease applied, the automated system can eliminate human error. Mist provides a constant supply of lubricant to properly lubricate the bearings.
Most pumps, motors and pillow block bearings with rolling element bearings have oil mist applied as pure mist or dry sump lubrication. Ideally, oil mist is applied in the upper quadrant on one side of the bearing and exits through a vent or drain from a low point on the opposite side. This means that the oil mist is the only source of lubrication, because there is no oil sump or grease to provide lubrication. Lubricating bearings with oily air may sound risky, but in many applications it has proven itself superior to the oil sump and grease lubrication.
Another way to apply oil mist is purge mist or wet sump lubrication. This requires an oil sump to be maintained in the bearing housing. The oil mist is applied as a purge above the oil level to prevent the intrusion of airborne contaminates. This method is commonly used on sleeve or plain bearings and in gearboxes. Small steam turbines with sleeve bearings are excellent applications due to the steam in the vicinity of the bearing housings. Cooling tower gearboxes also benefit from purge mist or wet sump oil mist due to the harsh (wet) environment around the gearbox.
Sleeve bearings and gears have too much surface area requiring lubrication to make them candidates for pure mist. Although pure mist has been utilized on sleeve bearings and gears, applying oil mist in a purge mist application is easier and cheaper. Many of the benefits of pure mist are not available from a purge mist system.
Limitations and Disadvantages of Oil Mist Lubrication
The main limitations and/or disadvantages of oil mist lubrication are the lack of knowledge about the operation and the application of oil mist. Because airflow delivers the oil mist (oil particles) to the point of lubrication, the suspension of the oil particles is important. Gravity has an effect on the oil particles, causing them to settle if they do not arrive at the point of lubrication within approximately five minutes. With the mist traveling at a velocity of 20 ft./sec., it can easily reach from the point of origin to the point of lubrication.
Distance is the easy part when designing an oil mist system. Achieving the proper pipe slope and pipe diameters is also critical to receiving benefits from the system. Pipe fitters and instrument personnel must route pipe and tubing according to the oil mist system’s requirements. Therefore, choose the installers of your oil mist system carefully.
Users need to be aware of and avoid overzealous application of oil mist. It is a common belief that “if a little oil does a little good, then a lot of oil will do a lot of good.” Oversizing of the reclassifier or mist fitting to improve lubrication has little or no impact on improving bearing life.
In fact, when oil is seen dripping from the equipment, it is due to improper sizing of the reclassifiers and not calibrating the OMG to the designed oil consumption. Oversized reclassifiers and overzealous application can cause a system that would normally consume one gallon of oil per day to consume three or four gallons of oil per day.
Cost/Benefit Comparison to Other Single-Point Lubrication Devices
As a rule, automated lubrication systems have established their value to industry. Although there may be areas of overlap, one type of system generally cannot be used in place of another. The exception to this rule is the air/oil system commonly used in place of a grease system. The most common types of automated systems are grease, circulating oil, oil mist, air/oil and the high-pressure compressor systems.
Grease is used in several applications, ranging from the manually operated grease gun to the injector system, the single-line system and the dual-line system. The grease gun is the most common application method. It is typically preferred when only a few points require lubrication or when management does not want to spend the money for an automatic system.
A grease gun delivers grease in three ways:the right amount, too much and too little. Too much and too little are not good for the equipment. Getting the right amount of grease requires someone well-trained in knowing how much grease is required on each piece of equipment and how to apply it. A grease gun is the least expensive method of applying grease, but how does it affect machinery reliability?
Injector systems are common on air-cooled heat exchangers and sump pumps found in most refineries. Single-line and dual-line systems can serve these same applications; the main differences are features and pricing.
Circulation oil systems have also established their place, not overlapping where other systems can be used. The air/oil system, common to the steel industry, does not have much application in the HPI.
When comparing lubricating systems, an oil mist system is by far the easiest system to maintain; repairs are relatively simple. Maintenance normally consists of changing filters and doing walk-down inspections to look for broken or leaking lines. Repairs often involve a simple replacement of components in the oil mist unit or replacing a broken line.
The least expensive way to lubricate process pumps is with an oil sump to get started, but considering the total life cycle cost, the oil sump leaves a lot to be desired.
These systems vary in price from a few hundred dollars to $100,000 or more, depending on the number of lube points being served. The bottom line is: When reliability is of utmost importance and when your business relies on rotating equipment and bearings that require lubrication, an automated system may be required.
Reference:
1. Nelik, Lev. (1993). Bearing Life Extension and Reliability Features of Modern ANSI Pumps. Second International Conference on Improving Pump Reliability in Petroleum Refineries and Chemical and Natural Gas Plants.
