Collapse of air separation equipment at a steel plant in Jiangsu

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In July 2009, an accident occurred at the air separation unit of Xin Sanzhou Steel Co., Ltd. in Wuxi City, Jiangsu Province. On July 12, a regional air division detected cold leakage and frosting near the V18 pipeline valve on the outer wall of the cold box, at a height of over 10 meters above the ground, and promptly arranged for the unit to be shut down for maintenance. On the 12th, the cryogenic liquid in the fractionation tower was drained, and from 10:00 on the 13th to 15:30 on the 14th, heating treatment (compressed air purging) was carried out. It was decided to proceed with the sand removal operation without fully confirming whether the temperature inside the cold box had dropped to safe operating levels. At 6:00 on July 15, the local branch began the sand removal operation. To speed up this process, instead of using the Ø320mm sand removal opening originally provided in the cold box, a 600 × 800mm sand removal opening was created at the bottom of the cold box using gas cutting. Shortly after the sand-scraping operation began, the cold box suddenly broke apart and collapsed in the middle; equipment inside the cold box, such as the fractionation column, Column I for crude argon, and Column II for crude argon, also broke and collapsed at the same time. More than 2,000 cubic meters of perlite inside the columns was ejected outward in a very short period of time. Two workers on site died from asphyxiation due to inhaling large amounts of perlite, another person died after being hit in the head by debris that fell down, and 8 people were injured. Fortunately in this accident, the collapse did not occur near the cryogenic storage tanks located at the bottom of the tower; had it occurred just a few degrees further away, those storage tanks would have been affected, resulting in even greater losses. ²The possibility of a chemical explosion was ruled out, based on three pieces of evidence: ²First, according to interviews with those involved in the accident, no explosion sound was heard at the time of the incident, and the breaking and collapse of the cryogenic tank and tower did not occur instantly, but over a relatively short period of time. ² II. No traces of a chemical explosion were found at the scene. ² III. Judging from the break points of the collapsed fractionation column and the crude argon column, there are no signs of explosion such as outward deformation. ²The direct cause of the accident as determined by the investigation was that although the manhole cover on the top of the cold box was opened prior to sand removal for evacuation and heating, it was not fully confirmed during the sand removal process whether the temperature inside the cold box met the safety requirements. During the sand removal operation, the low-temperature liquid gas inside the cold box, when exposed to external air that enters the cold box, absorbs heat and vaporizes rapidly, causing expansion. (Low-temperature liquid gas expands by nearly 800 times when it turns into a gas at normal temperatures, which leads to a sudden increase in pressure within the cold box. The resulting pressure cannot be released entirely through the sand removal openings at the top and bottom, thus putting stress on the walls of the cold box and causing them to rupture. A large amount of perlite sand is ejected from the cracks, the cold box breaks in two, and towers such as the fractionation tower also break and collapse.) The equipment inside the cryostat is designed individually to ensure stability, and other comprehensive factors related to the stability of the cryostat are not taken into account during the design of each individual piece of equipment. In the event of a destabilization or collapse of the cold box, it can affect related special equipment, including tower-type pressure vessels and pressure pipelines, leading to secondary accidents. ²According to online searches, sand removal accidents in the cold boxes of air separation towers have occurred frequently in China. For example: In 2003, a sandblasting accident occurred during maintenance work at the gas production plant of Ma’anshan Iron and Steel Company in Anhui Province, resulting in equipment deformation and broken pipes. ²2. In 2006, a sandblasting accident occurred during maintenance work at the Shuigang Oxygen Plant in Guizhou Province; 27 workers were buried under piles of sand at the scene, and 7 of them were female workers who lost their lives. Most of these accidents occur during maintenance when perlite sprays out while the cold box is opened, resulting in suffocation of personnel and damage to equipment.