- Buyer's Guide
More and more plants have goals focused toward extending the mean time between repairs (MTBR) for their rotating equipment, which includes centrifugal pumps for use in petroleum, petrochemical and natural gas industries. Maximizing the lubrication effectiveness in API process pumps will be a big contributor toward meeting this goal. In this article, you will learn about optimal ways of lubricating the bearings in API process pumps, including data on discs vs. oil ring lubrication, maintaining the proper level and contamination control. Bearings that are properly lubricated with minimal contamination will operate at lower temperatures and for longer periods of time.
This article is focused on horizontal centrifugal API (American Petroleum Industry) process pumps but many of the topics/concepts can also apply to ANSI pumps. There have been various studies done on why rolling element bearings fail prematurely, and consistently, the No. 1 cause can be attributed to poor lubrication. One particular study stated that 50 percent of damage is caused by defective lubrication. Poor or defective lubrication can be classified as:
• Incorrect lubricant
• Incorrect quantity of lubricant
• Contaminated lubricant
• Lubricant degradation
Figure 1: Typical Bearing Failure Causes
The API Standard 610 requires a minimum bearing life (L10) of 25,000 hours with continuous operation and rated conditions and at least 16,000 hours at maximum loads and speed. The L10 life is the number of revolutions that 90 percent of a group of identical bearings under identical conditions will endure before the first sign of fatigue failure occurs. If we assume 80 percent run time per year (292 days per year), the minimum expected bearing L10 life would be 3.5 and 2.2 years, respectively.
As cited above, poor lubrication will cause 50 percent of these bearings to fail before any signs of fatigue occur. These failures can occur within a few hours after installation, after one year, or just before fatigue. At what point prior to fatigue do these bearings fail can be extended by following the 5 R’s of Lubrication: Right lubricant, Right quantity, Right location, Right time and Right quality.
Many companies are requesting L10 life ratings of 40,000 hours (5.7 years) when purchasing new pumps, but it is still important to address poor lubrication practices.
Anti-friction bearings in process pumps can either be grease, mineral oil or synthetic oil lubricated. The primary purpose of oil, or the oil constituent of grease, is to separate the roller elements and raceway contact surfaces, lubricate the sliding surfaces within the bearings, and provide corrosion protection and cooling.
Viscosity is the single most important property of a lubricant. Use of the correct viscosity lubricant for the speed, operating temperature and load ensures the development of a full oil film between rotating parts. When the incorrect viscosity is used, the load-carrying ability of the lubricant is negatively affected. The oil degrades to a point where it is too thick to penetrate between the surfaces and the oil supply may not be adequate to prevent sacrificial contact. Viscosity is influenced by load, temperature, water, contaminants and chemical change. The OEM operation manual should be consulted for recommendations on viscosity, but it is also important to measure the oil sump operating temperature since viscosity decreases as temperature increases.
Table 1: SKF Recommendation for Ball Bearings in Pumps
Oil in process pumps is typically an ISO grade 46, 68 or 100. These numbers relate to the Kinematic viscosity in centistokes. The oil is usually hydrocarbon oil, although synthetic oils are sometimes used for specific lubrication applications. The viscosity of synthetic oil is less sensitive to temperature changes and more widely used when temperature fluctuations exist. If temperature also exceeds 100 degrees Celsius (212 Fahrenheit), a synthetic is recommended as the oxidation rate of mineral oil accelerates faster at higher temperatures.
Incorrect Quantity of Lubricant
Selecting the right lubricant for the application was the first step, and the next is to determine the correct quantity is initially applied and maintained. It is important to understand the design of the bearing housing assembly and, more specifically, the designed oil level.
Oil Level – Design
The most common types of methods for lubricating rolling element bearings in horizontal process pumps are:
• Oil splash (direct contact, rings or flingers)
• Pure oil mist
• Purge oil mist
The use of grease is primarily limited to lower horsepower pumps where the parameters are in the size and speed range of rolling element bearings. Oil splash lubrication is the most common method utilized. Oil splash designs include direct contact, oil rings, flinger discs or combinations of each.
In direct contact, as the shaft rotates, the rolling elements in the bearing make contact with a level of oil. The bearings should not be submerged in the oil more than one-half the diameter of the lowest rolling element or ball (Figure 2). Oil rings are utilized where speed or loads are factors and the oil is not in direct contact with the bearing. Oil rings make contact with the oil and provide splash type lubrication without direct bearing contact. Flinger discs are similar to oil rings in that the oil is not in direct contact with the bearing. The discs make contact with the oil and provide splash type of lubrication. Oil rings are more dependent on the shaft speed relative to the depth of submersion, but a good rule of thumb to use is to use three-eighths of an inch at the deepest point. Flinger disks are less susceptible to problems of over-lubrication since they are attached directly to the rotating shaft and they should also submerge about 3/8 inch into the oil. A combination design would incorporate a metallic disc and direct contact. The bearings directly contact the oil and the disc provides additional splash lubrication for cooling.
Figure 2: Pump Cross Section – Oil Splash/Direct Contact
The basic concept of the oil mist lubrication system is dispersion of an oil aerosol into the bearing housing. Air atomizes the oil into particle sizes of one to three microns. Airflow transports these small oil particles through a piping system into the pump housing which flows through bearings. It is a centralized type of low-pressure lubrication system. In pure mist lubrication, the oil/air mist is fed under pressure to the housing. There is no reservoir of oil in the housing and oil rings are not used. Purge mist lubrication utilizes the same principles of pure mist, but a reservoir of oil in the housing exists. A slinger/flinger disc or oil rings can also be used to provide splash lubrication.
In a low-level operating condition, the bearing will not receive enough lubricant necessary for proper film strength – a precursor to surface contact, skidding and possible catastrophic failure. Without enough oil to prevent friction, thermal runaway can happen quickly to a steel bearing. As the temperature of the bearing increases, the ball and race both expand, which creates an even tighter fit. This increases the temperature even more, and the cycle continues to a rapid, catastrophic failure.
A low level of oil will affect all types of oil splash lubrication. In direct contact, there will be insufficient film strength and rings or discs may not be able to pick up enough oil to satisfactorily lubricate the bearings.
In a high-level operating condition, churning of the lubricant will occur, accelerating the oxidation rate due to excessive air and elevated temperatures. It is a common mistake to believe that more is better – especially when it comes to oil sump lubrication. Too much oil can affect the operation of oil rings, flingers and direct bearing contact. Another result of high lubricant levels is leaking seals. If the oil level is too high, the ring will become submerged and no longer sling the oil. Flinger discs are less susceptible to this as they are directly attached to the shaft.
Maintaining Proper Level
Oil sump lubrication does not require that a specific level be maintained for proper bearing load – only that oil levels do not reach critically low or high points (Figure 3).
Figure 3.Typical Oil Level Conditions
Maintaining the proper quantity of lubricant is perhaps the easiest means of increasing lubrication life and effectiveness. Consult with your equipment manufacturer or the operation manual for recommended oil levels, optimum lubricating equipment and preferred practices. The majority of equipment will have an external marking on proper oil level that is either cast into the housing or a tag is applied.
One of the most widely used methods of maintaining the proper level lubricant in a bearing housing is the constant level oiler (Figure 2). The constant level oiler replenishes oil lost by leakage through seals, vents and various connections, and plugs in the bearing housing. Once the proper level has been set, replacing the oil in the reservoir is the only required maintenance. View ports (bullseyes) can also be used to verify proper oil level.
Constant level oilers have a “control point” that must align with the proper oil level of the equipment. The oiler is installed on the equipment and oil is filled to the proper level. All constant level oilers require air to function properly. If the oil level within the sump lowers, the seal at the control point is broken, allowing air to enter the reservoir, displacing the oil until the seal is re-established. If the constant level oiler is set correctly and there is oil in the reservoir, the equipment will always have the optimum oil level within the sump.
The quality of lubrication is affected by contamination, which is a large contributor to premature bearing failures. The major types of contaminants are particulate, moisture, incompatible fluids and air entrainment. Particles impede lubricant performance and further localize pressure on components causing denting, fatigue, spalling and abrasion to the surface of mating surfaces. Water will affect the lubricant’s ability to provide a proper fluid film, causing premature failure and excessive wear. Corrosion, cavitation, and premature oxidation and filter plugging of the oil are other symptoms of water contamination. Air contamination affects oil compressibility, causes poor heat transfer, film strength loss, oxidation and cavitations.
The sources of these contaminants are:
• Generated contamination
• External ingression of contaminants
• Maintenance induced
Particle contamination can be generated during the break-in or during operation of the pump. Oil rings are typically made of bronze and are sensitive to horizontality of the shaft, speed and oil levels. They can tend to skip or hang up due to these sensitivities and make contact with other components. The rings will wear, being a softer material, and particle debris is generated. These small particles can penetrate the critical area between the rolling elements and the raceway resulting in abrasive wear.
External Ingression of Contaminants
Pressure differentials between the equipment housing and surrounding atmosphere are a leading cause of contamination ingression. Pump operation where housing temperature fluctuations occur during frequent on/off running conditions, process fluid temperature changes, outdoor use and air flow over the pump create this atmospheric exchange as pressure is equalized. During this air exchange, contamination (dirt, water, etc.) from surrounding environment is “breathed” into the oil sump through vents, seals and oilers.
Housing components – including oilers, seals and vents – when specified properly can be very effective in preventing contamination. For many years, constant level oilers were used to maintain oil levels. Most of these were vented to the surrounding atmosphere, which can lead to contamination ingression to the housing sump. By switching to a non-vented oiler and removing vent plugs, ingression can be significantly reduced. Bearing isolators are used to prevent lubricant leakage and contaminant ingress. Labyrinth-type bearing isolators are most widely used on horizontal pumps. Bearing isolators allow increased pressure created by normal pump operation to vent through the seal and have proved to be very effective at reducing contamination ingression. The rotor and stator are not in contact, which allows for the venting to occur while preventing wear – prolonging the life of the seal. Magnetic or face seals are used to prevent damage to bearings due to contamination and lubricant leakage. Face seals are characterized by optically flat stationary and rotating faces loaded together by magnetic force or springs.
Contamination can exist in the oil prior to being put into the equipment. It cannot be assumed that new oil is clean. Proper storage of oil and proper dispensing containers will also decrease the possibility of water or other contaminants from entering into the bearing housing. Proper care should be taken during the pump rebuild process to ensure any contamination is properly removed.
All lubricants will degrade over time, requiring the oil to be changed. The frequency of these changes can be extended by maintaining the quality of the lubricant.
Elevated operating temperatures are a major contributor of oil oxidation. Combined with air, particulate and water contamination, the chain reaction of oil oxidation begins. Additives are affected first, followed by the basestock, which leads to machine and component surface wear and fatigue. For every 8 degrees C (18 F) increase in oil operating temperature, the oxidation rate doubles. This can be significant when considering pump operating temperatures are frequently near, or above, 60 C (140 F). By simply lowering the operating temperature of the oil to 50 C (122 F), a 50 percent reduction in the rate of oxidation would be realized – doubling the effective life of the oil.
The most basic methods to reduce (or maintain) lower oil operating temperatures are:
• Use the correct viscosity oil.
• Use quality oil.
• Use the right amount of oil.
• Keep the oil clean.
Air entrainment is a primary source of oxygen in the oxidation failure of oil. New oil can contain as much as 10 percent air at atmospheric pressure. Splash-type bearing housings utilizing flinger rings or slingers are all aeration-prone applications. Excessive aeration has a negative effect on acid number (AN), oil color, film strength and viscosity. In addition, air entrainment can lead to accelerated surface corrosion, higher operating temperatures and oil varnishing.
Operating temperatures can vary with each type of lubrication method. The graph below was based on laboratory testing measuring operating temperature of the oil sump from startup until the temperature leveled off. Two tests were run using ISO 68 weight oil and operating speed of 3,600 rpm. One test had the oil level at mid-ball of the lowest rolling element and the other had the oil level dropped below and a flexible flinger disc was installed to provide splash lubrication. The flexible flinger disc operating temperature was 9 F lower than the direct contact operating temperature. As stated above, this reduction in temperature results in a 25 percent decrease in rate of oxidation.
Graph 1. Temperature vs. Run Time
The importance of proper lubrication in process pumps is well known, but achieving it is not always easy. It is important to start with the basics:
Understanding the pumps’ components as well as the surrounding environment is critical for applying the correct and most economical lubrication management system. Obtaining the designed L10 rating of the bearing can be obtainable by eliminating poor lubrication.
• Brandlein, Eschmann, Hasbargen, Weigand. “Ball and Roller Bearings” 3rd Edition
• Bloch, Budris. “Pump Users Handbook”