Production Technology: [Question of the Day] Sharing accident cases related to air separation – 10-11-30 [Tuesday]

Seven female workers died in the “sand pile” accident/~*t`-y
K 1. Account of the accident According to the Guizhou Metropolitan Daily, at 9:51 a.m. on December 6, 2009, during maintenance work on the oxygen production unit No. 3 at a steel company in Guizhou, a large amount of perlite suddenly erupted; 27 workers who were loading sand in front of the tower were buried under the pile of sand. After rescue efforts, 20 people were saved and treated at the outpatient department of Shuigang General Hospital, while 7 female workers lost their lives in this accident. On that morning, when the reporters arrived at the scene, they saw hundreds of migrant workers gathered outside the entrance of the oxygen plant, anxiously waiting for news from their relatives. About 100 meters away from the entrance, in front of the oxygen production tower No. 3, there was a pile of snow-white perlite sand that reached a height of 4.5 meters. According to survivor Ryū Chūken, at around 9:30 a.m. that day, nearly a hundred migrant workers were working at the site; some used tools to remove debris from inside the tower, while others wrapped up the debris in packaging bags. After a loud explosion, white dust was ejected, burying those workers who did not have time to escape, while the survivors ran away in all directions. According to the head of the safety department at the steel company responsible for emergency response on site, by 12:51, all seven victims’ bodies had been found; it is reported that all seven victims were women. The contractor in charge of this project has been taken into custody by the police. M J6z|k%iY 2. Accident analysis: The cause was a leak in the air separation system; when the manhole was opened, large amounts of liquid at low temperature vaporized, causing a large quantity of perlite to be ejected from the cryogenic tank and covering the personnel who were trying to repair the tower on site. Several such sandblasting accidents have occurred in the country; equipment inside a tower at one factory was severely damaged as a result, but it seems no casualties were reported. During routine maintenance of air separation systems and when removing the perlite sand, if a liquid leak is detected in the system in advance, all the manhole covers at the top should be opened first. If there is no urgency to remove the sand, sealed gas or other methods can be used to heat the perlite sand, thereby vaporizing the liquid stored therein. Even if not all of the liquid can be vaporized, reducing its amount will still help to decrease the amount of sand that is sprayed. If repairs need to be carried out urgently and the amount of liquid leakage is not significant, the bottom perlite discharge outlet should be quickly removed while keeping the upper manhole cover open; at the same time, personnel should evacuate to a safe distance. This way, even if perlite is ejected, the safety of the people can still be ensured. If the amount of liquid leakage is large, safety must come first: extend the heating time of the pearlescent sand to ensure that no liquid remains, and only then remove the sand in order to protect both personnel and equipment. 3. Lessons learned: Problems like those involving pearlescent sand are, in air separation operations, the most likely and dangerous issues aside from main cooling system explosions. While people pay attention to main cooling system explosions and take measures to address them, problems such as those with pearlescent sand are often neglected, which leads to accidents. It is necessary to strictly follow the operating procedures. Sand removal is not possible without stopping the machine and without heating the main tower. Cause analysis and handling of the “nitrogen blockage” accident in the 18,000 Nm3/h air separation unit’s crude argon column 1. Accident sequence Around 3 p.m. on the evening shift of June 20, 2005, while the operators were increasing the load on the 18,000 Nm3/h air separation equipment, they adjusted the operating conditions of the main column by gradually increasing the opening degree of the liquid nitrogen control valve V3 from 57% to 62%. After about 30 minutes, the purity of the oxygen product began to decline; within a few minutes thereafter, the value indicated on table AI705, which measures the purity of the crude argon in the crude argon column, also started to drop. Although the operators promptly took action to address these fluctuations by reducing the amount of oxygen extracted and releasing the nitrogen at the top of the crude argon column through valve V712, they were unable to restore normal operating conditions. Eventually, the argon purity as indicated by AI705 dropped from 99.9% to below 80%, while the purity of the oxygen product dropped from 99.7% to 93%. It was not until 5:30 p.m. that the oxygen purity reached acceptable levels, and it was not until 8:30 p.m. that the operating conditions of the argon system were fully restored to normal. 2. Analysis of the accident cause: The main reason for this \"nitrogen blockage\" incident in the crude argon column was that the operator mistakenly increased the opening degree of the liquid nitrogen control valve V3 to an excessive level, resulting in too low oxygen purity in the upper column, a significant shift downward of the argon-rich zone in that column, and an excess nitrogen content in the argon fraction. 2.1 Reasons for the low oxygen purity of the product There are two reasons for the low oxygen purity of the product: on one hand, it is due to the operator mistakenly increasing the opening degree of valve V3, which increased the amount of liquid flowing downstream in the upper tower ； At this point, with the amount of vapor rising remaining relatively stable, the reflux ratio in the upper column will increase; that is, when there is more cold liquid flowing downward or less vapor rising, the gas-liquid mixture temperature will inevitably lean toward the side of the cooler liquid ； As a result, the temperature rise of the liquid flowing downward is small, and less liquid evaporates; consequently, less nitrogen is released from the liquid through evaporation. This means that the oxygen concentration in the liquid leaving the tray increases more slowly ； This is the case for each tray, and as a result, the oxygen concentration of the liquid obtained at the bottom of the upper column is low. Another reason is that, when dealing with the \"nitrogen plug\" condition in the crude argon column, the operator reduced the amount of argon fraction taken out from the column too much (the flow rate of the argon fraction was lowered from 18,500 m3/h to around 13,000 m3/h). The reflux ratio in crude argon Column I was not properly controlled, resulting in an excessive amount of liquid flowing back from the bottom of Column I to the upper column. This in turn caused the liquid oxygen concentration at the bottom of the upper column to drop too low, leading to a product oxygen purity of 93%. 2.2 Causes of the “nitrogen plug” in the crude argon column The direct cause of the “nitrogen plug” in the crude argon column is an excessive nitrogen content in the argon fraction entering the column; under normal operating conditions, this nitrogen content should not exceed 0.1% (the typical composition of the argon fraction is: 9–10% argon, 90–91% oxygen, and 0–0.06% nitrogen) ； The sampling point for the argon fraction in the upper column remains constant; as the argon-rich zone in the upper column moves up and down along the tray, the composition of the material at the argon fraction sampling point also changes accordingly. When the oxygen purity in the upper column dropped too low, it caused the argon-rich zone in that column to shift significantly downward, resulting in an excessive nitrogen content in the argon fraction – which directly led to the \"nitrogen plug\" accident in this crude argon column. 3. Methods for handling the incident: At around 3:30, when the operators noticed that the oxygen purity value, as indicated by AI705 on the analysis sheet for the purity of crude argon in Tower II, began to decline, they immediately concluded that a nitrogen blockage was occurring. They took the following actions: (1) They gradually increased the opening of the crude argon vent valve V712 to boost the amount of crude argon being removed; the flow rate rose from 700 m3/h to around 1000 m3/h. However, the purity of crude argon in Tower II continued to drop, and at a rapid pace. As a result, the operators promptly closed the valve V705 that controlled the flow of crude argon to the refined argon tower, thereby stopping its operation. They then continued to increase the opening of V712 to ensure that all the crude argon was vented outside the tower, thereby removing the nitrogen from the top of Tower II as quickly as possible. Once the value of AI705 stopped declining and started to rise, they gradually reduced the amount of crude argon being removed, until the flow rate returned to the normal range of 500 m3/h to 750 m3/h. (2) Appropriately reduce the amount of oxygen taken from the product, and at the same time adjust valve V3 for liquid nitrogen according to the purity of the liquid air in the lower column; properly control the reflux ratios of the upper and lower columns to maintain a stable level of liquid oxygen in the main cooler, thereby ensuring that the purity of the product oxygen meets the required standards as soon as possible. (3) Appropriately reduce the amount of argon fraction taken from the upper column, but avoid excessive changes in this amount; properly control the reflux ratio of Column I for crude argon to prevent too much liquid from the bottom of this column from flowing back to the upper column, which could further deteriorate the operating conditions there and lead to a drop in oxygen purity. (4) Appropriately reduce the amount of expanded air fed into the tower, thereby increasing the vapor-liquid ratio in the upper section of the tower above the expanded air inlet. This enhances the component separation capability, increases the argon content in the argon fraction, and reduces the nitrogen content. (5) Open the valve that allows air to return from the argon storage tank under pressure to the pure argon tower. At the same time, use the liquid nitrogen feed valve V706 in the condenser of the pure argon tower and the vent valve V751 for releasing excess gas from the tower to maintain a pressure inside the tower between 20–40 KPa. This prevents outside humid air from entering the tower, thus avoiding the risk of blockage in the pure argon tower. By 6:30, when the purity of the crude argon reached 98.5%, the refined argon column was put into operation. At 8:30, when the oxygen content in the crude argon from Column II was less than 2.0 ppm as indicated by the AIAS704 value, and the nitrogen content in the refined argon was less than 3 ppm as indicated by the AE706 value, the liquid argon could be fed into the storage tank, at which point the operation of the argon system returned to normal.

Does a shutdown caused by instrument air leakage count as such? Once, the instrument air was used improperly here; since the isolation valve, which wasn’t used frequently, needed to be opened to supply instrument air to another location, the outlet for instrument air there wasn’t closed, which caused a sharp drop in air pressure and led to an immediate shutdown of the system. . . .

What’s the title? Why can’t I see the content?

1. The electric furnace in the purification system did not start, and the control personnel were not aware of this, which resulted in excessive levels of carbon dioxide at the purification outlet; fortunately, it did not lead to a major shutdown. 2. Chemical dosing in the circulating water system results in the formation of large amounts of foam, which affects the water supply to the purification adsorbent. 3. There is no pressure equalization during the start-up of purification; when the process is set to automatic mode, this causes stress on the adsorbent bed