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On July 23, 1984, an explosion followed by a fire took place at a Union Oil petroleum refinery in Romeoville, Illinois, outside Chicago, killing 17 people and causing major property damage. This vessel was an amine absorber tower used to strip hydrogen sulfide from a process stream of propane and butane. The vessel was tall, in diameter, and made from thick plates of type ASTM A516 Grade 70 steel. The explosive force had been able to propel the upper of the vessel a distance of from its original location, while the base remained at the center of the fire that followed after the explosion. Testing of the vessel segments began with non-destructive testing methods. i) Magnetic particle inspection revealed large number of cracks confined mainly to the inner surfaces along the welds between Courses 1 and 2 of the vessel and between Courses 2 and 3. ii) Ultrasonic measurements then detected clear indications of separation of laminate to layers confined to Course 1. iii) Thickness measurements, made with a micrometer, showed that Courses 1 and 2 had wall thickness well within the allowances for pressure vessels. So these tests showed that all initial and replacement components satisfied the industry standard specifications. The cause of failure did not become clear until metallographic results were combined with stress corrosion cracking and hydrogen embrittlement tests, followed by a fracture mechanics analysis. It appeared that an already existing crack had extended through more than 90% of the wall thickness and was about 800 mm in length. Further, it was determined that hydrogen embrittlement had reduced the fracture resistance of the steel by more than half. The vessel had been put into service in 1970 and had undergone several repairs and modifications before the July 1984 incident. The vessel was fractured along a path that was weakened by extensive cracking adjacent to a repair weld joining a replacement section to the original vessel. These pre-existing cracks initiated in areas of hard microstructure known to be susceptible to hydrogen stress cracking. This hard microstructure formed during the repair welding of the replacement section. The cracks grew through the vessel wall as a result of hydrogen pressure cracking. When the depth of the largest of these pre-existing cracks exceeded 90% to 95% of the wall thickness, the remaining thin ligament of steel in the cracked section ruptured and leakage occurred. This crack caused a complete fracture of the vessel circumference at the operating stress level of only 35 MPa (roughly 10% of the rated strength of the steel) because the toughness of the vessel steel had been reduced by hydrogen embrittlement. ==References== 1. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「1984 Romeoville petroleum refinery disaster」の詳細全文を読む スポンサード リンク
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