Fatigue Performance of Cut and Rolled Pipe Thread Connections Installed in LDPE Plant

Threaded flange is commonly used as an alternative to a welded connection in a Low-Density Polyethylene (LDPE) plant, which is operated with very high pressure and high strength steel. ASME SA723-specified threaded pipe connections installed at a newly commissioned LDPE plant were found to be a mixture of machined (cut) thread and rolled thread pipe connections. However, rolled thread connections are standard industrial practice in LDPE plants. These pipe thread connections are designed for limited fatigue life as they are exposed to cyclic stresses during service. It is understood that cut thread components have inferior fatigue performance compared to rolled thread components. Therefore, the end user may experience challenges to the quality of the pipe thread connections’ fatigue performance and risk management concerning potential premature fatigue failure. This study aims to investigate the effect of the fabrication process, including thread type, on the fatigue performance of threaded pipe connections by carrying out a literature survey. In addition, laboratory mechanical testing (i.e., static tensile, dynamic fatigue, metallographic, and fractographic examinations) was undertaken to directly evaluate fatigue properties of specifically fabricated cut and rolled thread specimens. No significant difference in tensile properties was observed between cut and rolled thread samples; however, greater fatigue performance was achieved by rolled threads. Fatigue life at the testing condition of typical operating condition exceeded 100,000 cycles for both cut and rolled threads, which demonstrated relatively low risk of fatigue failure for both cut and rolled thread connections.