Study on failure mechanism of suspension lug elbow in an ethylene cracking pyrolyzer

Abstract

The cracking of a suspension lug elbow caused a shutdown in a certain petrochemical enterprise. In this study, the authors conducted physical and chemical analysis to investigate the cracking mechanism. Macroscopic observations indicated no signs of corrosion or thinning on the inner and outer surfaces of the head, and the cracks were distributed in the body of the head and the ear. Chemical composition analysis revealed that the main chemical components of the failed elbow met the standard requirements. Metallographic observations showed significant creep damage in the substrate material, and after long-term high-temperature service, both the room temperature and high-temperature mechanical properties of the failed elbow significantly deteriorated. The failure sample exhibit a fracture elongation of 0 %, and the high-temperature creep performance of the head material was only 6.868 h, both lower than the standard requirement of a fracture elongation of 4 % and 120 h of high-temperature creep performance. Fracture surface observation revealed obvious creep voids and fatigue striations, indicating severe creep damage and the influence of alternating loads on the head. Field observations indicated that the head experienced significant shaking inside the furnace. Therefore, simulation analysis suggested that the maximum stress coincided with the crack location, ranging from 121 MPa to 131 MPa, far exceeding the tensile strength at the operating temperature when the elbow was subjected to alternating loads generated by left–right and back-forth swinging. In conclusion, the failure of the suspension lug elbow was caused by creep-fatigue cracking. This study reveals that, even when the internal medium is in single gaseous phase, elbows are still susceptible to cracking under the combined effects of high temperature vibration and creep interaction. To avoid future failures, strengthening the monitoring of the cracking furnace operation, controlling the load appropriately, and preventing severe vibration and shaking of the furnace tube are recommended.