Failure Analysis of an Elbow Tube Break in a Pyrolysis Furnace

The pyrolysis furnace, a critical component in a pyrolysis unit, inevitably faces operational challenges during its use. This study investigates a case of pyrolysis furnace failure, particularly focusing on an occurrence at the 90 degrees lug elbow and furnace tube weld. The failure, characterized by a comprehensive fracture of the furnace tube in the circumferential direction along the weld vicinity, transpired within a timeframe significantly shorter than one-third of the design life. To unravel the root cause, a series of experiments was conducted on a sample extracted from the failed tube. These experiments, comprising visual inspection, chemical composition analysis, metallographic examination, microstructure analysis, fracture scanning electron microscopy, and energy spectrum analysis, collectively aimed at a comprehensive understanding of the failure mechanisms. The results disclosed that the fracture between the lug elbow and the inlet pipe stemmed from the presence of porosity and inclusions in the butt weld. The initiation of cracks was traced to the pores and inclusions in the fusion line of the inner wall of the pyrolysis tube, extending to connect with the pores in the heat-affected zone on the side of the pyrolysis tube parent material. Subsequently, under the influence of high temperature and stress, the cracks propagated, crept, and expanded along the circumference of the pyrolysis tube parent material until the final fracture occurred. In light of these findings, practical recommendations are proposed to mitigate the risk of similar failures in the future.