An experienced catalytic flash explosion casualty incident

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IV. Casualties Caused by Flash Explosions in the Catalytic Fractionation Tower 1. Accident details: On October 6, N years ago, an abnormal operation occurred in tower T201 of our company’s catalytic unit; analysis confirmed that a middle tray had fallen off, leading to an emergency shutdown of the unit. The reaction system operates in single-vessel fluidization; after purging the distillation tower, it is isolated as a single unit, while the absorption and stabilization systems maintain pressure. At 10:00 on October 8, ventilation was enabled through all manholes in T201; at 13:30, team members and personnel from the maintenance workshop were assigned to carry out the dismantling of the trays, starting from the bottom up. Two maintenance workers were responsible for dismantling each tray, while two team members were tasked with overseeing the process and transporting the trays. At 14:00, I began to check the status of each manhole from bottom to top. Around 14:18, I reached the manhole in the 13th-floor passage (the location where T201’s top components were being removed). At that time, a worker named Huang was inside the tower, removing tower components, while Wang was lying in the manhole, handing tools to Huang inside the tower. I told Wang to get up and I got down into the manhole to inspect the situation inside. When I stuck my head into the manhole, I noticed an unpleasant smell; I then pulled myself back out and planned to call a safety officer to conduct an inspection. At that moment, Wang leaned his head into the manhole again. Just as he did that, there was a loud noise, the distillation tower started shaking violently, and an explosion occurred inside the tower. 2. Accident losses: The repair worker, Mr. Wang, collapsed and leaned against my legs; he was bleeding heavily from his face and head. I carried him to room T201 in order to get him emergency medical care, but due to the severity of his injuries, he unfortunately passed away. The equipment operator who was resting against the protective railing on that level had his head struck by the blast wind coming out of the manhole, hitting him against the railing; he lost consciousness on the spot. He was standing to the side of the manhole at the time, and the blast wind caused burns to his left earlobe as well as to the hair in that area. At the top manhole of T201, a team member named Zhang had one leg extended into the manhole, ready to enter; the blast wind pushed his leg against the edges of the manhole, resulting in a severe fracture. The personnel inside the lower manholes had not yet entered, and one officer suffered minor facial injuries due to the rust particles ejected by the blast. Huang, one of the workers inside the tower, suffered burns to his neck and wrists, which were exposed outside his work clothes. All the trays inside T201 were damaged and deformed. The accident resulted in 1 death, 2 serious injuries, and 3 minor injuries, with losses amounting to approximately 2 million yuan. 3. Accident cause analysis: Upon inspection and analysis, it was determined that the accident occurred because liquefied gas entered T201, where it mixed with air to form explosive gases, which then ignited upon contact with an open flame, resulting in an explosion. After the emergency shutdown, the workshop purged the distillation system; once the purging was completed successfully, blind flanges were installed on various side lines. Since the diameter of the oil and gas lines at the top of the tower was large and it was difficult to install blind flanges there, the workshop arranged to close the 8 valves at the inlet and outlet of the air cooler, and used blind flanges to isolate the reflux tank R201 at the top of the tower from the pipelines of other systems. During the blindflanging work, a small φ32mm pipeline was overlooked; this pipeline was part of the residue recycling line in the liquefied gas tank area and had not been in use for many years. The three valves along its length were all closed, and after the accident, inspections of these three valves revealed leaks in each of them. At around 10:00 on the morning shift on October 8, the liquefied gas tank farm took advantage of the temporary shutdown of the catalytic unit to carry out a liquid residue removal operation on the liquefied gas with excessive C5 content in tank C. Some of this liquefied gas residue, along with the liquefied gas itself, entered R201 via the residue reprocessing line; this was the source of energy that led to the accident. At that time, all the manholes in the fractionation tower were opened, creating a siphon effect in the oil and gas pipelines. After the liquefied gas vaporized in R201, the gas phase flowed via water cooling and air cooling to T201. Of the 8 air cooling valves, 7 were closed; one valve was difficult to operate due to its confined location, and the operator who went to close it managed to close about 4/5 of it before giving up, which was the main reason for the formation of the siphon effect. At around 10:00 on the 8th, the tank transfer operation in the liquefied gas tank area began. It is estimated that the residual liquid and liquefied gas that seeped in through the leaky valve along the residual liquid reprocessing line reached R201 between 12:30 and 13:00. Back then, safety management was rather lax; after opening the T201 manhole for ventilation, a test was conducted around 11:00 and it came back positive, but no further test was carried out before workers started working in the afternoon. At that time, T201 was under emergency repair; to save time, steam supply was stopped before the end of the night shift on the 8th. The morning shift then proceeded to inject fresh water into the tower through the water supply line via pump B201 in order to force cooling. During the maintenance work, once it was confirmed that the conditions inside the tower were satisfactory, a non-explosion-proof portable lamp (an incandescent lamp with a metal protective mesh) was brought into the tower. Due to the poor ventilation in the tower, there was a strong odor, and water droplets kept falling. The maintenance worker used a respirator with a long tube while working inside the tower, so he did not detect the smell of liquefied gas in time. When liquefied gas entered T201 and mixed with other gases, creating an explosive mixture, the water droplets fell on the incandescent lamp, causing it to explode and thus creating a source of fire that led to the flash explosion. 4. Lessons from the accident: Carelessness in carrying out the isolation using a blind flange at R201; the mistaken belief that, since the liquefied gas residue reprocessing line had not been in use for many years and multiple valves were closed, there would be no risk of leakage – these were the direct causes of this accident ； When closing the air-cooled valve, the operation is abandoned before it is fully closed due to the inconvenient positioning of the valve ； To meet tight deadlines and accelerate progress, purging and ventilation were not carried out in accordance with the standard procedures ； Lack of safety awareness; explosion-proof electrical equipment was not used as required ； Lack of proper monitoring of confined spaces is one of the main causes of accidents.