Although many types of lubrication methods are available for bearings in horizontal process pumps, the most widely used method is still oil sump lubrication.
The most common form of bearing lubrication is direct contact. As the shaft rotates, the rolling elements in the bearing ," typically steel balls ," make contact with a controlled level of oil.
Although some debate exists regarding the most effective depth of contact, the amount of contact between the rolling element and the oil is not generally considered a specific measurement. The most important considerations are speed, oil viscosity and load.
It is critical that an effective oil film be maintained between the rolling element and the race of the bearing. Only enough contact between the bearing and the oil surface as necessary to "load" the bearing with lubricant is required.
If the lubricant level is too high or too low, excessive heat will be generated, accelerating oil degradation and shortening bearing life. When the oil level is too high, a condition known as "churning" occurs. Similar to what occurs through the use of an egg beater, air is "whipped" into the oil. This, along with the induced heat, increases the oxidation rate, shortening the effective life of the oil.
When the oil level is too low, contact is not sufficient to lubricate the bearing or act as a heat sink to carry away the normal heat levels generated by the bearing.
Critical lubrication elements
The most critical elements of lubrication are quality and quantity. Without one, the other is affected negatively. The proper quantity of low-quality oil is no better than an insufficient quantity of high-quality oil. Plants should select the best oil for the application.
Quality. In basic terms, lubrication quality can be looked at in two ways: how the lubricant can become contaminated and how the lubricant can degrade. Although contamination is widely recognized for its effect on the quality of oil, degradation can be just as damaging to equipment. The leading causes of contamination are particulate matter, moisture, incompatible fluids and air entrainment. The leading causes of degradation are oxidation, heat and regular use.
Quantity. The proper quantity of oil is perhaps even more important than the quality of the oil. Oil sump lubrication does not require a specific level to be maintained for proper bearing "loading." However, if the oil level in the sump reaches critically low or high points, damaging conditions could occur. In a low-level operating condition, the bearing will not receive enough lubricant for proper film strength ," a precursor to surface contact, skidding and possible catastrophic failure. In a high-level operating condition, "churning" of the lubricant will occur, accelerating the oxidation rate as a result of excessive air and elevated temperatures.
New on-line monitoring devices measure water contamination by percentage or SRH.
Specially designed containers can decrease the likelihood of contaminated oil introduction.
Poor lubrication: the culprits
Contamination, oil degradation, oil starvation and excessive lubrication all can adversely affect oil sump lubrication.
Contamination. Particle contamination is possibly the most well known form of lubricant contamination. This form is considered the cause of component part wear, silting and surface fatigue. In a study performed by the National Research Council of Canada, nearly 85 percent of contaminant-related wear of components and surfaces was found to be particle induced. To make matters worse, particle contamination can create more particles ," and more wear.
Lower particle counts significantly extend the life expectancy of equipment. By reducing contamination levels from ISO 21/18 to ISO 11/8, plants can extend the life of a 50-gallon-per-minute (gpm) pump by a factor of seven.
Particle contamination can occur from ingress from the surroundings, improper cleaning of the bearing housing during maintenance cycles, or corrosion products from the high water content in the oil.
Water contamination can cause several problems. Because each type of oil has its own unique "safe" water level, the common practice of measuring parts per million (ppm) is not conclusive.
Significant differences exist among oils, beginning with mineral and synthetic bases. Additive packages, commonly referred to as "ad-pacs," also can make a difference in how much water an oil can hold before phase separation occurs ," and free water forms. Temperature also plays a major role in how much water oil can hold. Damaging levels of water, or "free water," begin to occur in some mineral-based oils between 400 ppm and 500 ppm at 140 Degrees F. Free water might form at 200 ppm at 125 Degrees F in the same oil.
Therefore, it is important to know the saturation point of an oil at a given temperature to begin to determine a valuable setpoint for effective lubrication maintenance. Sponges provide a simple way to illustrate this. Different sponges can hold varying amounts of water. A dense-cell sponge can hold more water than an open-cell sponge, even though both have the same cubic volume.
Oil degradation. The primary causes of oil degradation are high heat, air entrainment and the mixing of incompatible fluids. Increased viscosity (thickening) is one of the results of this degradation. This usually happens over time, and varies by the combination of these elements.
Viscosity is the single most important property of a lubricant. To more fully understand the significance of viscosity, it is necessary to understand how a lubricant works. The primary functions of a lubricant are to reduce friction and wear. To perform these functions, a protective oil film is required. The three basic oil film conditions are referred to as full film, elastohydrodynamic (EHD) and boundary layer.
The full-film condition denotes the presence of enough lubricant to ensure complete separation of the moving surfaces. It also is known as hydrodynamic full film. The EHD condition refers to a hydrodynamic film formed by applied pressure or load. It is predominantly found in rolling element bearings. Sometimes referred to as thin-film lubrication, the boundary layer condition is usually the result of an insufficient lubricant supply. Although lubrication is present, it does not exist in sufficient quantities to prevent metal-to-metal contact.
In rolling element bearings, for instance, the load on a roller causes the roller to move toward a stationary element, or a raceway. This load creates a pressure area that deforms elastically, resulting in a "Hertzian" contact area.
This pressure can go as high as 200,000 pounds per square inch (psi) ," compressing the lubricant into a thin film. The viscosity of the oil increases where this fluid film acts as a solid and allows the ball to roll without metal-to-metal contact. When the viscosity is "wrong," the load-carrying ability of the lubricant is affected negatively. Additionally, if the oil degrades to a point where it is too thick to penetrate between these surfaces, the oil supply might not be adequate to prevent sacrificial contact.
Oil starvation. The use of too little lubricant can be catastrophic. This is commonly the result of incorrect filling, incorrect oiler settings and unrecognized leakage. Without enough oil to prevent friction, "thermal runaway" can happen very quickly to a steel bearing.
As the temperature of the bearing increases, the ball and race both expand, creating an even tighter fit. This increases the temperature even more, and the cycle continues to a rapid, catastrophic failure.
A less obvious cause of oil starvation is high viscosity, resulting from oxidation or degradation, or improper oil selection. If the oil is too thick, it cannot penetrate the small clearances of a rolling element bearing, particularly at higher speeds.
Excessive lubrication. It is a common mistake to believe that more is better, especially when it comes to oil sump lubrication. Too much oil can adversely affect the operation of flinger rings, slingers and direct bearing contact. Churning can lead to higher operating temperatures, increased oxidation and reduced equipment efficiency.
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Chemical Processing Plant Case History The Albemarle chemical processing facility in Pasadena, Texas, increased pump reliability by improving oil sump lubrication. Beginning in 1998, a failure history was kept on an ITT Goulds 3196 MT process pump used in the production of bromide. Operating outdoors, the pump runs 24 hours a day/seven days a week and was suffering from a very high seal failure rate ," 11 occurrences per year, at an average rebuild cost of $3,345. After evaluating the failure mode, most often the mechanical seal, the plant determined the seal failure to be a symptom of other problems. Numerous bearing failures helped direct the attention to the moisture contamination of the oil. In 1999, the first change was implemented. The plant switched from mineral oil to synthetic oil for the superior demulsification characteristics. Some improvement was seen, but problems continued. In late 1999, bearing isolators and a sleeveless shaft for more rigidity were installed. Water contamination problems continued until desiccant cartridges were installed. Although the cartridges worked, they lasted only two to three weeks before becoming saturated. After the plant installed an automatic moisture removal unit in August of 2000, the oil samples showed no contamination. Seal failures were reduced from 11 per year in 1998 to five in 2000 and two in 2001. These seal failures are thought to be related to cavitation. The plant achieved a savings ," in rebuild costs alone to just one piece of equipment ," of $23,415 in just one year (2001). |
Three steps to improvement
By taking a three-step approach incorporating prevention, detection and correction, chemical facilities can vastly improve the reliability of lubrication ," and that of related equipment.
Step 1: prevention
Prevention of contamination. When specified properly, housing components, including oilers, seals and vents, can be very effective in preventing contamination. For many years, constant-level oilers have been essential in maintaining oil levels. Most of these are vented to the surrounding atmosphere, which can lead to contamination ingression to the housing sump.
By switching to a nonvented oiler, plants can reduce ingression significantly. Bearing housing seals, more often recognized as isolators, have been producing positive results in reducing oil leakage and contamination ingression.
Prevention of degradation. Lubricant life is reduced significantly when exposed to high operating temperatures. The oxidation rate of oil doubles every 18 Degrees F. This can be significant when considering pump operating temperatures are frequently near, or above, 140 Degrees F. By simply lowering the operating temperature of the oil to 122 Degrees F, plants could realize a 50 percent reduction in the rate of oxidation ," doubling the oil's effective life. The most basic methods to reduce (or maintain) oil operating temperatures include:
Using oil with the correct viscosity. Too high or too low of a viscosity will raise the oil's temperature.
Using quality oil. Do not buy cheap oil to save money ," it will end up costing you more.
Using the right amount of oil. Maintain proper oil levels ," too much or not enough will increase the oil's temperature.
Keeping the oil clean. Contaminated oil operates at a higher temperature than clean oil.
Prevention of improper quantity. Understanding and maintaining the proper quantity of lubricant comprise perhaps the easiest means of increasing lubrication life and effectiveness. Consult your equipment manufacturer for recommended oil levels, optimum lubricating equipment and preferred practices.
Prevention of excessive oil and starvation operating conditions can be easy and less time-consuming with the proper equipment and instruction. A general guideline is to maintain minimal contact with the lubricating element.
Step 2: detection
Lubricant sampling and testing. Evaluation of lubrication sampling and testing procedures is recommended. Routine oil sampling and changes can be costly and time-consuming. Every piece of equipment has its own unique set of circumstances, including age, operating environment, process fluid, speed, operating mode, temperature and history. A number of products and recommended practices are available to help increase the efficiency of oil sampling, testing and maintenance.
Visual indicators. Visual inspection is not the most reliable method of determining whether contamination is present or the oil has exceeded its useful life. However, visual indication can be used as an early warning to degrading conditions.
View ports mounted in the housing can be used for both quantity and quality checks. The "business card" analysis tool also can be an easy, low-cost method of checking oil quality. In this approach, a small amount of oil is placed on the back of a business card, or a similar type of paper. After a short period of time, the plant can evaluate the way the oil "wicks out" toward the edges of the card to determine the presence of contamination and degradation.
Monitoring devices. Newly introduced on-line monitoring devices measure water contamination by percentage, or saturated relative humidity (SRH). Continuous monitoring by this type of device prevents unnecessary sampling and oil changes. New technology "brings automatic understanding" to the safe operating levels of moisture in oil.
By maintaining water levels below the saturation point, hydrogen embrittlement of steel is prevented, minimizing or eliminating spalling and cracking on the steel ball bearing surfaces and increasing bearing life significantly. This level varies from oil to oil in terms of ppm, but is universal in terms of SRH. Only sensors specifically designed for this purpose can measure it.
Step 3: correction
Oil change and analysis. An evaluation of oil analysis lab results from routine sampling indicates that 25 percent of the samplings are performed unnecessarily. More than 60 percent are conducted after damage begins. This would indicate that more than 85 percent of oil changes/sampling are potentially performed at the wrong time.
It is important to learn what your problem is ," and when to look for it. For example, if you know your most common problem is water contamination, installation of moisture detection units will eliminate guesswork.
Storage and handling procedures. It is not uncommon for oil to be stored haphazardly in areas not specially designed for lubricant storage. This can lead to contamination of the oil and handling tools before the oil is even installed in the equipment.
It is important for plants to create a controlled atmosphere in which temperature and air quality can be controlled. Reduced inventory is recommended for cleaner, higher-quality oil. Evaluate how the oil is transported from the storage area to the equipment. Open storage containers and contaminated bottles, funnels and filling vessels increase the likelihood the equipment will be filled with contaminated oil. Specially designed containers are available commercially that will decrease the likelihood of introducing contaminated oil.
Maintenance procedures. Understanding how a constant-level oiler works is essential. Two leading causes of misuse are an improper level setting and the "second-shift syndrome." Review the instruction sheet provided with the oiler for a better understanding of how to adjust and set the device for proper use.
"Second-shift syndrome" results when maintenance personnel are instructed to keep the constant-level oiler reservoir of the oiler completely full. Tests have shown that frequent removal and replacement of the constant-level oiler reservoir result in an increased oil level in the equipment sump.
Another cause of improper quantity control is pump-filling methods. Frequently, the filler plug on the top of the housing is removed, and oil is introduced until the proper level is indicated in the view port. This leads to over-filling because much of the oil still is draining from the shaft in-line between the filler port and the sump.
Housing configuration. Close it up. Through proper configuration of nonvented oilers, housing seals, expansion chambers, vent replacements, desiccant dryers and monitoring devices, the pump housing environment can be maintained nearly effortlessly. For example, by installing a "closed system" consisting of a nonvented oiler and a desiccant oil dryer, plants can reduce oil changes from every six weeks to every three months. At an estimated cost of $30 per oil change, the payback on an installation cost of $72 can be realized in less than eight months (with two desiccant cartridge replacements).
Additional cost savings can be realized through increased mean time between maintenance (MTBM) and reduced oil disposal costs. Improper configurations of bearing housings are common and can contribute to high oil contamination levels. For example, tests have revealed that when a vent is used with certain types of labyrinth seals, the ingression rate of the air surrounding the pump increases ," as many as 10 times more than without a vent.
Contacting shaft seals such as lip seals and magnetic seals can minimize ingression significantly. However, pressure might increase without proper expansion chamber configuration. Technical support from the suppliers of these products is essential to ensure optimum performance and safe operation.
An automatic moisture removal unit can help reduce oil contamination significantly.
Conclusion
By timing oil changes correctly and selecting and installing oil sump lubrication systems properly, plants can reduce the frequency of oil changes significantly. In addition, associative costs, including those related to labor, used oil disposal, laboratory tests and equipment downtime frequently can be reduced.
Lubrication methods and maintenance often are overlooked or misunderstood as tools to increase equipment reliability and decrease maintenance costs. Don't just change your oil ," change the way you use it. CP
Rake has more than 21 years of experience in the research and development of industrial equipment. He holds a patent in lubrication technology and has four patents pending. He most recently held a position with Trico Manufacturing Corp., an industrial lubrication equipment engineering and manufacturing firm based in Pewaukee, Wis. Contact Trico at (800) 558-7008 or via e-mail at [email protected].