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Union Oil amine absorber tower

On the evening of Monday 23 July 1984, the Union Oil Co refinery near Lemont, Illinois, USA was seriously damaged by an explosion and fire. Seventeen people working at the refinery were killed and the property damage was estimated to be over $100 million (see Fig. 1). The explosion was caused by the ignition of a large cloud of flammable gas (a mixture of propane and butane) which had leaked from a ruptured amine-absorber pressure vessel.

An operator working near the absorber tower noticed gas escaping from a horizontal crack about 150mm long near the bottom of the vessel and tried to close off the main inlet valve. The crack grew to 600mm and he initiated evacuation of the area. As the company fire fighters arrived, the absorber tower cracked further and a large amount of gas was released. The gas ignited in a massive explosion which sent the upper part of the tower into the air, landing over a kilometre away. The explosion was felt over 20 kilometres away and the blaze which followed sent flames 150m into the sky.

The absorber tower first went into service in 1970. It was a cylindrical vessel 2.6m in diameter and of overall height 16.8m. The shell section consisted of six courses of 25mm thick ASTM A516 Grade 70 steel. These were joined by full penetration submerged arc welds in the as-welded condition. The vessel, built to ASME Section VIII, was designed to strip H2S from the propane/butane gas mixture passing through it. Monoethanolamine (MEA) was fed through the tower as part of this process. The operating conditions were 1.4N/mm 2 internal pressure at 38°C. The environment in the tower was corrosive.

Soon after the amine absorber tower entered service, hydrogen blisters were found in the lower two courses of the shell and laminations were detected in the steel. The growth of hydrogen blisters continued and in 1974 the second course of the tower was replaced on site using manual metal arc welding with no preheat or post-weld heat-treatment (PWHT). In 1976 a Monel liner to reduce corrosion was fitted in the bottom head and first course of the tower but it did not cover the repair section.

The investigation into the failure found that the tower fractured at the circumferential weld between the replacement ring and the lower course. Four large cracks in the heat affected zone (HAZ) had been present prior to the failure, originating at the inner surface of the tower and extending almost through the wall thickness. About 35% of the vessel circumference was affected. The location of the first leak observed corresponded to one of these HAZ cracks which was approximately 800mm long.

Microhardnesses measured in the HAZ near the surface exceeded 29 HRC and peak hardnesses of 40 to 48 HRC were found near the fusion line. These facts, taken with the in- section appearance of the pre-existing cracks (straight in the HAZ near the surface and then zig-zagging through the base material at the limit of the HAZ), pointed to the cracks initiating by hydrogen cracking and then progressing by hydrogen-induced stepwise cracking (HISC). Tests according to a NACE standard procedure confirmed that the material was susceptible to HISC.

The fracture ran around the HAZ of the circumferential weld at right angles to the axial stress of 35N/mm 2. The fact that this stress level was so low and the crack did not change directions to run in a direction perpendicular to the higher hoop stress, indicated very low toughness material in the HAZ. Charpy V notch tests of the replacement course material and the weld between the replacement course and the upper part of the tower showed the weld metal and HAZ to have superior notch toughness to the base material. (20J transition temperatures: 0°C for parent plate, -51°C for weld metal, -40°C for HAZ). Fracture toughness tests measuring crack tip opening displacement (CTOD) in the HAZ material gave much greater critical CTOD values than the applied CTOD in the tower at the time of failure, estimated ignoring any residual stresses, as 0.064mm. Tests on hydrogen charged specimens did, however, reveal much reduced CTOD fracture toughness values in the range of approximately 0.070-0.080mm at 38°C. A later fracture mechanics assessment of the tower found that when residual stresses were taken into account, failure was predicted at the level of CTOD measured in non-hydrogen charged specimens.

Taking all of these findings into account, it can be concluded that this failure occurred because the welding procedure used when replacing a section of the vessel caused the formation of a hard microstructure in the HAZ of the weld. This hard region was susceptible to hydrogen assisted cracking resulting in growth of large cracks in the vessel. The uncracked material in the vicinity of the existing cracks had low toughness due to hydrogen embrittlement and failed at the applied CTOD in the vessel arising from the operating pressure and residual stresses associated with the weld.

For operation in corrosive conditions, the control of weld properties is critical. Welding procedures, particularly for field repair welds, need to be formulated to avoid the formation of high hardness microstructures for service in hydrogen environments. The significant contribution of welding residual stresses to the applied CTOD at a flaw present in a structure must not be overlooked.

This is a case history taken from Report 632/1998 .

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 Site of Union Oil amine absorber tower failure
Site of Union Oil amine absorber tower failure

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