輕油裂解工場碳鋼氫氣管線斷裂分析

Fracture Analysis of Carbon Steel Pipe under Hydrogen Atmosphere in the Naphtha Crack Plant

本文探討輕油裂解工場碳鋼氫氣管線斷裂原因，該管線操作溫度約為260℃（500°F），壓力約為32 kgf/cm2 （455 psig），使用16年。管線共有四處發生斷裂，所有斷口皆在銲道熱影響區，其斷口平整且未有塑性變形現象。成分分析結果顯示碳鋼管線符合ASTM A53 Gr.B規範要求。微觀觀察發現裂紋周圍有脫碳現象，且裂紋大都起源於銲道與母材間之銲接不完美外型處，此處具有相當程度之應力集中效應。根據API 941 Nelson Curves經驗曲線，本段氫氣管線之製程條件接近氫攻擊曲線之臨界值，推測該管線斷裂主因為氫攻擊發生於銲道不完美外型處所致。

This paper investigated the causes of the fractures carried out on a carbon steel pipe under hydrogen atmosphere in a naphtha crack plant. The 16-year pipe line was operated at ~ 260 ℃ (500 °F) and ~ 32 kgf/cm2 (455 psig) normally. There were four fractured sites that were all at heat affect zone along the pipe line. The cross-sections of the fractured pipes were flat and no plastic deformation phenomenon by visual observation. The GDS analysis showed the composition of the carbon steel pipe is according with ASTM A53 Gr.B,. There are decarburization phenomena around the fissures by optical microscope observation. The origin of the cracks initiated mostly from poor welding place. It is affected by the stress concentration along the weld and base metals. Besides, the hydrogen pipe was operated very close to the hydrogen attack limit according to Nelson Curves of API 941. We conjectured the pipe was fractured due to hydrogen attack at the imperfect weld inside the pipe wall.