The conclusion of a two-part primer on optimal ways of lubricating the bearings in process pumps.
The first part of this two-part article focused on lubrication fundamentals, pump housing designs, and the importance of proper quantity and quality of lubricant. This second part will cover “Best Practices” on how to maintain the quantity and quality of lubrication.
Bearings that are properly lubricated with minimal contamination will operate at lower temperatures and for longer periods of time. Extending oil change intervals while increasing the overall mean time between failures (MBTF) is achievable and results in significant savings.
There are numerous means available for ensuring that the proper quality and quantity of lubrication is maintained on process pumps. Using one or more in combination may be the correct solution. Understanding the pumps’ components, as well as the surrounding environment, is critical for applying the correct and most economical lubrication management system.
Maintaining the proper quantity of lubricant is perhaps the easiest means of increasing lubrication life and effectiveness. Consult with your equipment manufacturer for recommended oil levels, optimum lubricating equipment, and preferred practices.
A general guideline is to maintain minimal contact with the lubricating element. Rolling element bearings should not be submerged more than one-half the diameter of the rolling element (ball) at the deepest point of submersion in static condition. Oil rings are more dependent on the shaft speed relative to the depth of submersion, but a good rule-of-thumb is to use 3/8-in at the deepest point. Flinger disks are less susceptible to problems of over-lubrication since they are attached directly to the rotating shaft.
One of the most widely used methods of maintaining the proper level lubricant in a bearing housing is the constant level oiler (see Figure 1). 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 (bulls-eyes) can also be used to verify proper oil level.
Figure 1. Cross Section of a typical pump with a constant level oiler.
Improving Quality – Contamination
Pressure differentials between the equipment housing and surrounding atmosphere is 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 the surrounding environment is “breathed” into the oil sump through vents, seals, and oilers.
Housing components – including oilers, seals, and vents – can be very effective in preventing contamination when specified properly. 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.
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. As a softer material, the rings will wear and generate particle debris. These small particles can penetrate the critical area between the rolling elements and the raceway, which results in abrasive wear.
Bearing isolators are used to prevent lubricant leakage and contaminant ingress. Labyrinth type bearing isolators are the most widely used on horizontal pumps (see Figure 2). Bearing isolators allow increased pressure created by normal pump operation to vent through the seal. They have proven to be very effective at reducing contamination ingression. The rotor and stator are not in contact, so venting occurs and wear is prevented. The life of the seal is prolonged.
Figure 2. A typical bearing isolator.
Lip seals can also be very good at preventing contamination. However, with the contacting type design, eventual wear to the seal allows for contamination ingression and oil leakages. Lip seals need to be replaced more frequently than do bearing isolators or face seals. Face seals are used to prevent damage to bearings from 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 decrease the possibility of water or other contaminants entering into the bearing housing. Proper care should be taken during the pump rebuild process to ensure any contamination is properly removed.