The first of a two-part primer on best practices for maintaining quantity and quality of lubrication.
More and more plants have goals focused towards extending the mean time between repairs (MTBR) for their rotating equipment which includes centrifugal pumps. Maximizing the lubrication effectiveness in process pumps will be a big contributor towards meeting this goal. In this two part article, you will learn about optimal ways of lubricating the bearings in process pumps that include:
- Lubrication Fundamentals
- Typical Pump Designs for Providing Lubricant to the Bearing
- Quantity of Lubricant
- Quality of Lubricant
- Best Practices – Improving Quantity
- Best Practices – Improving Quality
Bearings that are properly lubricated with minimal contamination will operate at lower temperatures and for longer periods of time. There have been various studies done on why rolling element bearings fail prematurely, and consistently the number one cause can be attributed to poor lubrication. One particular study stated that 50% of damage is caused by defective lubrication (Figure 1). 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 primary functions of the lubricant in rotating equipment are:
- Minimize or eliminate friction – Separate moving parts
- Wear control – Reduce abrasive wear
- Corrosion control – Protects surfaces from corrosive substances
- Temperature control – Absorbs and transforms heat
- Contamination control – Prevention of dirt and wear debris damage
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 and loads 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 changes. 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 are typically an ISO grade 32, 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-deg C (212-deg F), a synthetic is recommended as the oxidation rate of mineral oil accelerates faster at higher temperatures.
Methods of Lubrication
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. Grease is usually lithium, with a normal viscosity of 100 centistokes and typically has a maximum operating temperature of 121-deg C (250-deg F), but limited to a service temperature of 93-deg C (200-deg F). To prevent the loss of grease, shielded bearings may be used. Shielded bearings may be limited to a maximum operating temperature of 52-deg C (125-deg F).
The most common form of bearing lubrication is direct contact. As the shaft rotates, the rolling elements in the bearing make contact with a level of oil. Since 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 surface of the oil as necessary to provide the bearing with lubricant is required. If the level of lubricant is too high or too low, excessive heat will be generated, accelerating the degradation of the oil and shortening the life of the bearing.
Oil rings are one option with oil splash lubrication. In cases where speed or loads are factors, the oil is not in contact with bearings due to elevated temperatures experienced. Oil rings make contact with the oil and provides splash type lubrication without direct bearing contact. Proper level is important in a splash type as well. If the oil level is too high, the ring will become submerged, reducing its ability to splash oil to the bearings. If the level is too low, the ring may not be able to pick up enough oil to satisfactorily lubricate the bearings.