Contents
Root Cause Analysis of Rolls Bearings Failure in Vertical Raw Mill: A Case Study
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Introduction
In the cement industry, the reliability and efficiency of equipment are critical to maintaining smooth operations. However, when equipment failures occur, they can lead to significant downtime and financial losses. This post delves into a specific case study of Rolls Bearings failure in a Vertical Raw Mill at the Kollenbach plant in Germany. The analysis aims to uncover the root causes of the failure and provide actionable recommendations for preventing similar incidents in the future.
Understanding the Case
On Thursday, September 22nd, at 6:39 AM, the Raw Mill at the Kollenbach plant was shut down for a scheduled preventive maintenance (PM) day, a routine task carried out every four weeks. During the inspection, large metal particles were found trapped in two magnetic bolts inside the rolls, leading to the conclusion that the bearings had failed. The subsequent repair took 80 hours, during which all the liners were replaced, and the kiln was stopped for 24.5 hours due to the lack of raw material.
The rolls that failed were installed during the last major repair in February 2005 and were brand new, fabricated and assembled by Gebr. Pfeiffer. The failure of these rolls was unexpected, raising concerns about the potential causes and the measures that could have been taken to predict or prevent the failure.
Investigating the Failure
The failure was analyzed from two perspectives: the mechanical problem that led to the breakdown and the failure to predict this issue. Let’s explore the key aspects of the investigation.
- External Object in the Raw Mill
- During the inspection, no apparent damage was found on the surfaces of the rolls, the table, or the mill internals, suggesting that the failure was not due to an external object entering the mill. This ruled out the possibility of an external object being the root cause of the failure.
- Mechanical Failure
- The investigation revealed a catastrophic failure of the bearings, particularly on the yoke side of the rolls, where significant damage was observed. The outer race of the bearings showed signs of spinning, a clear indication of severe mechanical stress.
- Practical Advice: Regular monitoring and inspection of the bearings are crucial to detect early signs of wear and tear. Implementing vibration analysis and monitoring systems can help identify abnormalities before they lead to catastrophic failures.
- Defective Assembly and Installation
- No apparent defects in the assembly or installation were noticed during the investigation. The installation process was straightforward and adhered to standard procedures. However, the design of the inside bearing (yoke side) had been changed in the new set of rollers installed during the last major repair, which could have contributed to the failure.
- Practical Advice: When implementing design changes, it is essential to conduct thorough testing and validation to ensure that the new components are compatible with the existing system. Involving third-party experts to review the design and installation can provide an additional layer of assurance.
- Vibration Issues
- The Vertical Raw Mill was known to run with high vibration levels at the rolls. However, no significant changes in vibration levels were observed in the six months leading up to the failure, suggesting that vibration was not the direct cause of the breakdown.
- Practical Advice: While vibration may not have been the direct cause in this case, it is important to regularly monitor vibration levels as part of a comprehensive maintenance program. Abnormal vibrations can indicate underlying issues that need to be addressed promptly.
- Seals Failure
- The investigation revealed a significant sealing problem, as large amounts of silica were found in the oil analysis of all three rolls. The roll with the most severe damage (Roll 1) had the highest silica content, while the roll with no apparent damage had the lowest silica content.
- Practical Advice: Ensuring the integrity of seals is critical in preventing contamination of the lubrication system. Regular inspection and replacement of seals, coupled with proper oil filtration, can help maintain the performance and longevity of the bearings.
- Defective Bearings
- The SKF bearings used in the rolls were identified as a potential root cause of the failure. The investigation noted that the design of the inside bearing (yoke side) had been changed during the last major repair, which could have contributed to the mechanical failure.
- Practical Advice: Collaborate closely with bearing manufacturers to ensure that the selected bearings are suitable for the specific application and operating conditions. It may be necessary to conduct joint testing and analysis to validate the performance of the bearings in real-world conditions.
- Oil Characteristics
- The oil used in the rolls complied with the manufacturer’s recommendations (ISO 1000). However, the presence of silica in the oil indicated that the seals were not effectively preventing contamination.
- Practical Advice: In addition to using the recommended oil, it is important to regularly monitor the oil quality and perform oil analysis to detect any contaminants. Implementing an oil re-circulation and filtration system can help maintain the cleanliness and effectiveness of the lubrication.
Root Causes & Recommendations
After analyzing all the evidence, several root causes were identified:
- Dust Seals Design
- The design of the dust seals was inadequate, as evidenced by the presence of silica inside the rolls. The damage to the bearings was more severe on the yoke side (seal side) than on the outside (self-aligning bearings), indicating that the seals were not effectively protecting the bearings.
- Recommendation: Request a review of the seal arrangement design by Gebr. Pfeiffer and implement improvements during the next repair. Consider installing an oil re-circulation system with parallel filtration to enhance the protection of the bearings.
- Predictive Maintenance Gaps
- The frequency of oil analysis was insufficient, and there was no continuous temperature monitoring in place, which hindered the ability to predict the failure.
- Recommendation: Increase the frequency of oil analysis to at least quarterly and implement continuous temperature monitoring for each roll. This will allow for more accurate and timely detection of potential issues.
- Design and Installation Review
- The changes made to the bearing design during the last major repair were not thoroughly validated, which may have contributed to the failure.
- Recommendation: Engage with the bearing manufacturer to review the design changes and conduct a thorough analysis of the bearing performance. Ensure that all future design changes are rigorously tested and validated before implementation.
Conclusion
The failure of the Rolls Bearings in the Vertical Raw Mill at the Kollenbach plant serves as a valuable case study for the cement industry. By conducting a thorough root cause analysis, we can identify the key factors that contributed to the failure and implement corrective actions to prevent similar incidents in the future.
By addressing issues related to seal design, predictive maintenance, and bearing performance, the plant can enhance the reliability and efficiency of its equipment, reducing downtime and improving overall productivity.
Sources
- SKF Bearing Analysis – https://www.skf.com/
- Oil Filtration Systems – https://www.pall.com/
- Vibration Monitoring Techniques – https://www.emerson.com/
- Preventive Maintenance Strategies – https://www.plantengineering.com/
- Gebr. Pfeiffer Equipment – https://www.gebr-pfeiffer.com/
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