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

• DOI: 10.1115/1.4068435
• Publication Date: 2025-04-11T14:11:46Z
• Authors: Bingjun Gao; Hao Wang; Tong Wang; Chuanzhi Wang

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, causing the crack locations at the threaded pipe end to deviate out of plane. A localized numerical model of the straight pipe segment with threads was established, 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 corrected to explore optimized reinforcement strategies for the inlet pipe segment. The results indicate that adding support at the end of the inlet elbow increases fatigue life by 4.35 times compared to the original structure. Fractographic analysis using scanning electron microscopy revealed the presence of shallow non-metallic inclusions at the crack initiation site, characterized by atypical ‘fish-eye’ features. Based on a high-strength steel VHCF life prediction model, recommendations were proposed to enhance the fatigue life of the pipe segment by controlling the size of non-metallic inclusions.