The tower was constructed from thick plates of type ASTM A516 Grade 70 steel per ASME Section VIIIpressure vesselcode. The vessel was tall, in diameter.
History
Beginning in 1970 the vessel served as an amineabsorber tower, used to strip hydrogen sulfide from a process stream of propane and butane. Inspection identified hydrogen blisters and laminations and in 1974 several meters of the lower shell section were completely replaced using manual metal arc welding. After welding, no post weld heat treatment, a vital process that reduces the residual stresses formed during welding and restores the macro structure of the steel, was performed. Two years later, in 1976, a Monel liner was installed into the bottom head to reduce corrosion; however, the previously repaired section was not covered by the liner. Prior to the explosion, an operator tried to close off the main inlet valve to stop gas leaking from a horizontal crack the vessel. The crack grew to and the operator initiated evacuation. The crack continued to grow and a significant quantity of flammable gas was released as company fire fighters gathered at the scene. The gas ignited, fatally wounding personnel, damaging the refinery, and launching the upper portion of the vessel to from its original location. Failure analysis of the vessel segments included the following non-destructive testing methods.
Thickness measurements using a micrometer showed that the wall thickness for both sections was well within the allowances for pressure vessels.
The cause of cracking did not become clear until metallographic results were combined with stress corrosion cracking and hydrogen embrittlement tests, followed by 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.
Ruptured vessel
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 because the toughness of the vessel steel had been reduced by hydrogen embrittlement.
