Pumps & Systems, August 2008
Progressive maintenance practices are vital to maximize pump life, minimize downtime and avoid costly repairs and lost production. It is good practice to plan the detailed groundwork of maintenance programs, along with contingencies for various maintenance events to minimize downtime.
Good maintenance programs should encompass maintenance activities for all possible failure modes, such as wear, seal damage and bearing failure. Bearing maintenance is one opportunity in pump maintenance optimization since bearings are vital components in a pump rotating system. Additionally, bearing technologies can significantly reduce pump maintenance. State-of-the-art, severe duty material technologies can yield superior wear resistance and longer fatigue life.
Centrifugal pumps, the most common industrial pumps in the world, come in a variety of configurations and sizes. Figure 1 shows a common overhung impeller centrifugal pump. This system permits the shaft to rotate freely while minimizing mechanical loss. It typically includes two or three bearings. The inboard side (non-drive end) bearing supports part of the radial load caused by the cantilevered impeller and is able to float axially to allow for thermal expansion of the shaft. The outboard side (drive end) bearing also supports the radial load, but it is fixed to keep axial shaft movement to a minimum. This maintains an acceptable clearance between the impeller and housing.
For these bearings to continuously perform their function, it is important to know how they commonly fail and what to do to prevent or predict that failure. This opens the door toward consistent reliability, potentially extending the life of the pump bearing system beyond conventional standards.
Figure 1. Diagram of typical overhung impeller centrifugal pump
Instead of being reactionary, maintenance should be predictive, preventive and proactive. We will discuss pump bearing failure modes to help improve and develop the preventive and proactive approach of pump bearing maintenance.
Causes for Bearing Failure
The most common causes for bearing failure are contamination and poor lubrication; however, excessive loading and bearing damage prior to operation-during assembly or handling-can also be culprits. In many cases, clear indications point to the root cause, so try common solutions first.
When more than one failure mode exists, more thought must be used to determine the sequence of failure. This is not always easy because there are multiple possible causes for every failure mode. Use what is known about the application and reference the pump manufacturer's specifications and recommendations to eliminate as many situations as possible. Good failure mode analysis pays dividends and should support the development of maintenance strategies.
Figure 2 explores some common bearing failure modes.
Contamination usually occurs in three forms: dust/dirt, metallic particles and fluids, such as water or chemical solutions. It is highly recommended to keep the bearing in its packaging until it is ready for installation. This eliminates the chance for dust particles in the air to stick to open type bearings, which typically have a rust preventive oil coating. When handling an unpacked bearing, beware of dirt, grease or other contaminants on hands, rags and surfaces that can make their way into the pump or the raceways of the bearing.
Once contaminants enter the internal components of a bearing, a chain reaction starts. When a rolling element runs over small particles, it creates dents that result in high, localized stresses on the raceway and the rolling element. This will lead to further denting, fatigue cracks, spalling and eventually early bearing failure. Once spalling occurs, small metallic particles from the bearing can enter the oil and lead to further damage. It is just as important to make sure that the lubricating oil is kept clean as it is to ensure contaminated oil is not supplied to the pump. Transfer oil in clean containers and avoid using dirty rags when handling funnels or replacing caps.
In some applications, fluid contamination can be a big issue. Seals are important to keep the lubrication in and the pumped medium out. Regularly check any joints or connections that could allow water or contaminants into key mechanical locations of the pump. If contamination might be an issue, regular lubrication analysis can be performed.
To check for dirt or metallic particles, run the pump to mix the lubricant and take a sample from the top of the reservoir. To check for water contamination, let the lubricant settle and take a sample from the bottom of the oil sump to see if water is present. If fluid contamination exists, follow the pump manufacturer's oil servicing instructions for ensuring contaminated oil is replaced with clean oil and filled to the correct level. Bearing life can be extended on problematic applications where contamination is difficult to control with specialized bearing material heat treatments that offer superior wear resistance.