A comprehensive vibration analysis was conducted on a water feed line operating in a slug flow regime. Slug flow, characterized by large intermittent fluid slugs within the pipeline, can exert significant dynamic forces on piping systems, potentially leading to structural failures. A recent leakage incident at an elbow joint highlighted issues related to fatigue, with a temporary fix applied using a clamp and sealant.
To address the root cause of the problem and ensure a robust solution, Dynaflow Research Group (DRG) was engaged to perform a dynamic analysis of the piping and its supports. The urgency of the analysis was underscored by the presence of Hydrogen Cyanide (HCN) in the liquid, necessitating a swift and effective resolution. By optimizing the support arrangement, the impact of slug flow on the system was significantly reduced.
Analysis
The initial analysis was based on existing drawings, photographs, and field observations of the support configuration. A simulation of unbalanced loads, resulting from the passage of five slugs, was carried out using CAESAR II piping analysis software. The system’s response was examined, and the mode shapes of the eigenmodes were compared with observed vibrations. Stress analysis revealed several locations where material stresses approached the endurance limit of the piping material.
To mitigate these stress amplitudes, several modifications to the support arrangement were proposed. These adjustments effectively eliminated the eigenmodes that contributed most significantly to the system’s response, thus reducing dynamic stress amplitudes and extending the fatigue life of the system.
Although some changes were no longer critical for bringing dynamic stress levels below the endurance limit, they were recommended to further enhance system integrity under slug flow conditions. These additional recommendations included minimizing support clearances and modifying trunnion supports. Each proposed modification was also evaluated for its impact on the system’s static flexibility.
Results
The vibration analysis led to essential recommendations for reducing dynamic stress amplitudes to levels well below the endurance limit. Furthermore, additional modifications were suggested to further diminish the impact of slug loads. While these additional changes were not critical for meeting endurance limits, they contributed to improving the overall dynamic behavior and system integrity in a slug flow regime. The recommended modifications involved refining support clearances and adapting trunnion supports, all of which were assessed for static acceptability.
By implementing these recommendations, the system’s performance and durability under slug flow conditions were significantly enhanced, ensuring a more reliable operation in the presence of HCN.
