A branch connection linking to a high-pressure steam attemperator has experienced a failure, similar to a previous incident. Observations on-site indicate periodic movement of the connecting header housing equipment J-3103, suggesting a fatigue failure. The objective of this analysis is to propose a permanent solution to prevent future cracks in the connecting branch by addressing the observed line movement.
Analysis
The approach involves a static stress analysis of the existing line followed by a dynamic analysis to identify potential vibration excitation mechanisms. This analysis assesses vibration modes and response amplitudes that contribute to the line failure. Based on dynamic insights, modifications to supports were recommended to restrict pipe movements while ensuring static stresses remain within allowable limits.
Results
Key conclusions drawn from the analysis include:
- The design temperature necessitates a relatively low allowable sustained stress for the material in use.
- Initial assessments showed sustained stresses exceeding allowable levels at design temperature, necessitating reduction via additional supports.
- Insufficient lateral restraints allowed significant dynamic movement, contributing to the failure.
- Implementing new supports and modifying existing ones substantially improved both dynamic and static behaviors.
- While uncertainties remain regarding boundary conditions affecting expansion stress checks near model boundaries, proposed support modifications are expected to enhance all static stresses (both sustained and expansion).
In summary, inadequate restraint of the system has led to low-frequency natural vibration modes easily excited by flow, underscoring the importance of comprehensive support strategies in preventing such failures.
By maintaining a detailed analysis and strategic modification approach, future occurrences of branch connection failures can be mitigated effectively.
