Failure Analysis of the Threaded Connection of the Top Inlet Pipe for the High-Pressure Polyethylene Reactor

Abstract

To investigate the thread failure issue of the top inlet pipe end of a high-pressure polyethylene reactor, a fluid–structure interaction (FSI) numerical model was developed for the reactor's top inlet pipe. Bidirectional FSI analysis revealed that, although fluid pressure pulsations are the primary cause of pipeline vibrations, asymmetric secondary flow at the double elbows induces out-of-plane structural vibrations, leading to an out-of-plane deviation of the crack locations at the threaded pipe end. A localized numerical model of the threaded straight pipe segment was developed, and the reaction forces and moments at the fixed end of the pipe segment were directly applied based on results from the FSI analysis to assess the very high cycle fatigue (VHCF) life of the structure. A parametric analysis was performed by replacing the real threads with a virtual thread structure, and the simulation results were refined to identify optimized reinforcement strategies for the inlet pipe segment. The results indicate that adding support at the end of the inlet elbow enhances the fatigue life by a factor of 4.35 relative to the original structure. Fractographic analysis using scanning electron microscopy revealed the presence of shallow nonmetallic inclusions at the crack initiation site, characterized by atypical “fish-eye” features. Based on a high-strength steel VHCF life prediction model, recommendations were provided to improve the fatigue life of the pipe segment by limiting the size of nonmetallic inclusions.