Accident Report on Urea Synthesis Tower – Urgent Use

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“Investigation Report on the 3·21” Urea Synthesis Tower Explosion Incident http://bbs.hcbbs.com/viewthread.php?tid=440452&highlight=%C4%F2%CB%D8%BA%CF%B3%C9%CB%FE

“Investigation Report on the 3·21 Urea Synthesis Tower Explosion Incident. Update time: 2006-6-20 16:26:35. Views: 2308. Around 21:20 on March 21, 2005, an explosion occurred in the urea synthesis tower at Pingyin Luxi Chemical’s Third Fertilizer Factory Co., Ltd. The accident resulted in 4 deaths and 32 injuries, with direct economic losses amounting to approximately 7.8 million yuan as of March 28. On March 22, the People’s Government of Jinan established an investigation team for the ‘3·21’ explosion incident at the urea synthesis tower of Pingyin Luxi Chemical Third Fertilizer Factory Co., Ltd. This team was headed by the Jinan Work Safety Supervision Bureau, with participation from departments such as the Municipal Quality Supervision Bureau, the Supervision Bureau, the Federation of Trade Unions, the Public Security Bureau, and the People’s Government of Pingyin County. Experts from institutions such as the Shandong Antai Chemical Pressure Vessel Inspection Center, Jinan Petrochemical Design Institute, and Mingshui Fertilizer Factory were also invited to assist in the investigation. Sequence of the accident: On March 21, 2005, production was stable after the shift change; the ammonia synthesis capacity was 17.5 units, and the normal operating pressure for urea production was 0.75 MPa. At around 21:20, the urea synthesis tower suddenly exploded and caught fire. A massive fire broke out in the main framework of the entire urea production plant. The urea tower, which consisted of ten sections, was broken into three parts: the tenth section remained in place, connected to the foundation; the ninth section fell in the southwest direction toward the stairs on the second floor of the framework; while sections 1 through 8 were thrown about 86 meters to the northeast, landing in front of the gas production plant. This caused some of the pipes on the external piping frame – such as those for steam, soft water, and hydrogen extraction – to break, and these pipes fell to a depth of seven or eight meters underground. The intense shock wave generated by the explosion severely damaged the main framework of the urea plant, and destroyed most of the window and door glass in the production facility. Upon hearing the explosion at the copper washing station, the on-duty dispatcher realized that an accident had occurred, activated the emergency response plan, and used a walkie-talkie to issue an order for an emergency shutdown to the shift supervisor. They rushed to the large compressor station and instructed that when shutting down the compressor, shortcuts or venting should not be used to prevent accidents. After confirming that the compressor had completely stopped, the electrician in the main power distribution room was notified to turn off the Roots blower. At this time, the gas supply shift supervisor reported that all gas generation furnaces had been safely shut down, the water seals at the inlet and outlet of the gas holder had been sealed, and the power supply to the urea distribution room had been cut off. After the entire plant was shut down, the management of Pingyin Luxi Chemical’s Third Fertilizer Plant Co., Ltd. organized personnel to go to the main framework of the urea production workshop to carry out rescue and fire extinguishing efforts. At around 21:50, the fire in the main frame of the urea plant was brought under control. Around 22:00, the plant’s chemical defense unit assisted the fire department in completely extinguishing the fire. Since Pingyin Luxi Chemical Third Fertilizer Factory Co., Ltd. had developed an emergency response plan for such incidents and conducted regular drills, this plan was effectively put to use during the accident, thereby preventing it from spreading further. Investigation findings of the accident: 1. Site inspection results: The explosion caused the tower to break into three sections. The first section was the base, which fractured circumferentially above the ring weld of the tenth tube segment; it remained connected to the foundation at its original location. The base was severely deformed, tilting in a south-west direction at an angle of about 45° from the horizontal, with the concrete damaged and the rebar exposed. The fracture is located above the circumferential weld; the entire fracture surface is flat, with the section in the northeast direction being even flatter and containing numerous longitudinal cracks. In this area, the four layers of steel plates exhibit a completely flat fracture surface, and a shear lip oriented outward in the shear direction can be seen across the entire fracture surface. The shear at the fracture in the southwest direction is more severe, causing the stainless steel lining to bulge out in a trumpet-like shape. The welds on the lining surface and the surfacing layer on the head are bright white in color, while the lining surface is brownish-gray. No significant thinning was observed during the inspection of the brackets. The second part is the ninth tube section, which flies upward at a 45° angle southwestward for about 12.5 m and strikes the second floor of the control building in the southwest corner of the plant; it then cracks longitudinally in reverse direction. The crack surface passes through the location where the thermocouples are installed. In all layers except the lining plate, the areas near and below the thermocouple holes on the longitudinal fracture surfaces exhibit brittle planar fractures (with a total height of 80–100 cm). Numerous longitudinal open cracks can be seen near the main fracture surface, while the upper part of the longitudinal main fracture surface shows ductile oblique fractures, and no other cracks are present on the layers surrounding this fracture surface. The inner lining surface is grayish-black; the true metal color can be seen when wiped. The third section is the part above the eighth barrel joint; it weighs about 100 tons and is tilted 30° toward the northeast, flying approximately 86 meters. The main part of the fracture surface in this section is located above the circumferential weld. The entire cross-section is characterized by oblique fractures; the fracture surface flares out in a trumpet shape, with the outer edges curling outward. In the section located in the northeast direction of the fracture, the outer layer is mainly found below the circumferential weld, while the inner layer (excluding the lining plate) consists of three steel plates that have fractured above the circumferential weld ； From the northeast to the southwest, the fracture surface gradually transitions from the lower part of the weld to the upper part of the circumferential weld, and eventually reaches the base metal of the upper tube section in the southwest direction, where it creates a triangular tear in the base metal. Observation shows that the inner lining surface is brownish-gray, the welds are bright white, and no significant thinning was detected upon inspection of the brackets. After the explosion, most of the sieve plate supports welded to the lining inside the first and third sections of the tower were deformed, with the direction of deformation all pointing toward the fracture surface. Except for the first layer at the very top, which remains at the top of the third section inside the tower, all the other layers are ejected outside the tower.

2. Process parameters and record-keeping: Since the Chemical Machinery Factory of Nanjing Chemical Industry Company, Sinopec Group, did not provide the Third Fertilizer Plant of Luxi Chemical in Pingyin with the ‘Installation, Operation, and Maintenance Manual’ for the R-1102 urea synthesis tower, it also failed to specify any specific requirements for the operation of this tower. Therefore, the standards for measuring process parameters other than temperature and pressure can only be referred to the \"Production Technical Regulations for a Plant Producing 40,000 tons of urea per year using the Complete Aqueous Solution Cycle Method (Trial Version)\\", compiled by the Fertilizer Department of the former Ministry of Chemical Industry in November 1990 (published by the Chemical Industry Press; hereinafter referred to as the \"Urea Technical Regulations\"). Due to the fire and firefighting that took place at the accident site, some of the shift records have been damaged; therefore, these process records can only rely on records from the previous shift or more recent records as a reference. (1) The on-site automatic record sheets and manual record sheets are used to record the process parameters: ① Temperature: 188°C ± 2°C at the top of the tower, 178°C ± 1°C at the bottom of the tower℃ ； Pressure: 19.2～19.5 MPa ； The temperature and pressure are respectively lower than the design temperature of 195°C and the design pressure of 21.57 MPa for the urea synthesis tower ； It also meets the requirements specified in the \"Urea Technical Specifications\", with a top temperature of 188°C and a pressure of 19.6 MPa. ②The purity of CO2 gas entering the urea synthesis tower is 96.5–97% (V) ； It meets the requirement of ≥95.7% (V) as specified in the Urea Technical Specifications. ③Oxygen content in CO2 gas entering the urea synthesis tower: 0.7–0.8% (V) ； Slightly higher than the specified range of 0.4–0.6% in the Urea Technical Specifications. ④Cl‑ content in deionized water: generally 3.5–5 mg/L ； The Cl－ level was basically kept within ≤5mg/L as specified in the \"Urea Technical Specifications.\" A total of 1,896 test results (24 data points per day) from 79 days in 2005 were examined, and 1,223 of these values were within ≤5mg/L. ⑤Ni content at the outlet of the urea synthesis tower: 0.085–0.09 ppm ； It is below the 0.2 ppm threshold specified in the Urea Technical Specifications. ⑥H2S content in the feed gas: 0 mg/m3 ； It is below the value specified in the \"Urea Technical Specifications\": 10 mg/m3. (2) Process parameter analysis: ① The operating temperature, pressure of the urea synthesis tower, purity of CO2 fed into the tower, H2S content in the feed gas, and nickel content at the outlet of the urea synthesis tower are all within the specified limits. ②The oxygen content in the gas entering the urea synthesis tower is about 0.2 percentage points higher than the standard specified in the \"Urea Technical Specifications\". From what is known about other small nitrogen fertilizer plants, most of them increase the oxygen content in the CO2 gas in order to improve the corrosion resistance of the inner tubes in the urea towers. ③The Cl‑ level in the desalinated water is higher than the specified limit of ≤5mg/L in the \"Urea Technical Specifications\"; upon verification, 23.23% of the test data were above 5mg/L, while 12.26% were above 6mg/L. Based on the existing technical literature, it is generally believed that Cl- is not a sensitive agent for low-carbon steel; it only exerts an indirect effect on the stress corrosion of low-carbon steel in acidic environments. (3) Other aspects: ① At Pingyin Luxi Chemical’s Third Fertilizer Plant Co., Ltd., the urea production process relies on five-stage compressors manufactured by Shanghai Dalong. The fifth stage of these compressors uses oil-free lubrication, and the amount of oil to be used in each stage is not specified in the equipment manual. The oil drainage frequency is once every half hour in the third stage, and once every hour in the other stages. The records are clear, and the appropriate frequency of oil drainage plays a positive role in preventing or reducing the formation of an oil layer at the top of the urine tower. Machine oil grade 13 is used for lubricating the compressor cylinders; its flash point is 215°C, with actual values ranging from 222°C to 230°C. Under the normal temperature and pressure conditions in the synthesis tower, spontaneous combustion will not occur. Taking these two points into account, it is largely possible to rule out the possibility of an explosion caused by explosive gases accumulated at the top of the urine tower as a result of spontaneous combustion in the oil layer. ②The steam used for leak detection in the urea synthesis tower remained unobstructed until the accident occurred; the difference in ammonia content between the inlet and outlet of this detection steam was within the range of 0.002% to 0.007%, which is lower than the 0.01% threshold specified in the \"Urea Technical Specifications\". Prove that there was no leakage in the urine tower lining prior to the accident. ③By reviewing the \"Equipment Maintenance Records\" for the three months prior to the accident, it was found that the pressure regulation and temperature indication systems of the urea synthesis tower were in good condition. Only the temperature gauge at the bottom of the urea tower was serviced in February 2005; therefore, the possibility of overpressure resulting from a malfunction in the pressure regulation system, or excessive temperature leading to corrosion of the urea tower due to inaccurate readings, can be ruled out (there are no maintenance records for the period from March 12th to 21st, but there are records of routine instrument inspections with no abnormalities detected). ④The startup and shutdown plan for the urea system overhaul in 2004 met the requirements of the Urea Technical Specifications, but the temperature rise curve of the urea tower was damaged during the accident. ⑤The liquid ammonia storage tanks used by Pingyin Luxi Chemical’s Third Fertilizer Plant Co., Ltd. are Tank No. 1 and Tank No. 2, with capacities of 100 m3 each; the maximum allowable capacity for each tank is 50 tons. At the time of the accident, the inventory in Tank 1 was 35 tons, and that in Tank 2 was 42 tons. Since the liquid phase outlet is located at the bottom of the liquid ammonia tank, it is possible to rule out the risk of an explosion in the urea synthesis tower caused by air from the tank entering the urea system due to low inventory levels. ⑥Check the decarburization operation records: At the time of the accident, the liquid levels in both the flash tank and the decarburization tower were normal, which rules out the possibility that shift gas entered the urea system and caused an explosion in the urea tower. ⑦Pingyin Luxi Chemical’s Third Fertilizer Plant Co., Ltd. uses the method of introducing air at the inlet of the first stage of the carbon dioxide compressor to adjust the oxygen addition level. The raw gas analysis records and routine inspection records indicate that the oxygen content in the raw gas was normal at the time of the accident. ⑧Before the accident, production scheduling records showed that at 16:35, due to maintenance work on the compressors in the synthesis section, the number of operating compressors was reduced from 17.5 to 13.5 ； At 17:30, increased to 15 aircraft ； Fill to 17.5 units at 17:45 ； At 19:35, the boiler slag remover failed, and there was no record of weight reduction. Urea system: Load reduced to 0.6 MPa at 17:00 ； Fill to 0.75 MPa at 18:00. The increase and decrease records for the CO2 units in the urea system are as follows: at 16:00, Unit 1 had a pressure of 0.38 MPa (pressure at the outlet of Stage 1 of the CO2 unit; the same applies hereafter), while Unit 2 had a pressure of 0.37 MPa ； At 17:00, Unit 1 was at 0.33 and Unit 2 at 0.32 ； At 18:00, Unit 1 was at 0.36 and Unit 2 was at 0.36 ； At 19:00, Unit 1 was at 0.38 and Unit 2 at 0.37 ； At 20:00, Unit 1 was at 0.30 and Unit 2 at 0.32 ； At 17:00, Unit 1 was at 0.35 and Unit 2 was at 0.35. Up to the time of the accident, the maximum fluctuation in the urea system load was 17.3%. For this model of CO2 compressor, the adjustment of flow rate is achieved by controlling the inlet pressure and using one-way valves, so that the inlet pressure remains essentially constant. Therefore, with the inlet gas volume and oxygen content remaining essentially constant, the possibility of the mixture in the gas phase space of the urea tower entering the explosive range is extremely low (the records of the overall control operations for urea on the day of the accident on March 21st are damaged). ⑨The static grounding test results for the urea synthesis tower show a value of 0.5Ω, and the grounding resistance meets **the relevant regulations**. Therefore, the energy source that triggers a chemical explosion in the urea synthesis tower cannot be static electricity buildup. ⑩Evidence provided by the local meteorological department shows that the weather at the time of the accident was mainly overcast, with no thunderstorms or lightning. Therefore, weather factors will not serve as an initiating source for a chemical explosion in the urea synthesis tower.

Determination of the nature of the accident (I) Data analysis: The accident investigation team conducted a thorough analysis and study of the materials collected and seized at the scene. Based on interviews with the persons involved at the time of the accident, it was determined that: 1. The safety production responsibility system, safety production management systems, safety status assessment reports, applications for safety production licenses, emergency rescue plans, and other documents provided by the enterprise generally met the requirements stipulated in laws and regulations such as the \"Safety Production Law of the People’s Republic of China\" (hereinafter referred to as the \"Safety Production Law\"), the \"Regulations on the Safety Management of Hazardous Chemicals\", the \"Regulations on the Safety Supervision of Special Equipment\", and the \"Technical Specifications for Urea Synthesis Towers\". 2. The records and data collected on site showed no signs of abnormal conditions such as overheating or overpressure in the tower prior to the accident, indicating that the tower was operating normally. 3. Interviews with relevant personnel and the collected information indicate that no start-up, shutdown, or other abnormal operations occurred at this urine tower in the two months prior to the accident. 4. The results of the analysis regarding the operational conditions of the process, process records, safety and technical measures, as well as on-site sample tests show that prior to the accident, the operation of the tower and the parameters of the relevant media and materials were for the most part below or within the ranges specified in the tower’s design parameters or those set out in the \"Urea Technical Regulations\". 5. On-site inspection of the appearance of the accident tower: (1) After the accident, the passivation layer of the stainless steel lining of the tower was intact, with no signs of corrosion. (2) The three main material pipes at the bottom of the tower are significantly twisted and deformed, and the pressure gauges on these pipes are severely damaged in appearance. Upon inspection by the Jinan Metrology and Calibration Institute, it was confirmed that the internal structure of both pressure gauges was intact, and the damage was caused by external forces. (3) A large number of open cracks could be observed in the fractured section of the tower, and the section was flat. (II) Commissioning of the explosion cause analysis and report on the analysis findings 1. Selection of the cause analysis agency Since the analysis of the reasons for the explosion of this tower involves multiple disciplines such as design, manufacturing, materials, production, and process operations, in order to conduct a scientific analysis of the causes of the explosion in the urea synthesis tower, the accident investigation team decided, after careful consideration, to commission a reputable agency with **metrological certification and** laboratory accreditation to analyze and determine the causes of the accident. After verifying qualifications such as the **Accreditation Certificate** issued by the Iron and Steel Research Institute and the accreditation certificate from the **China National Laboratory Accreditation Committee**, and after seeking opinions from the equipment users and manufacturers, on April 3, 2005, the Iron and Steel Research Institute was commissioned to analyze and determine the causes of the explosion that occurred on March 21 in the urea synthesis tower at Pingyin Luxi Chemical Third Fertilizer Factory Co., Ltd., and to prepare a conclusive analysis report. 2. Determination of the cause of the accident: Based on the conclusions contained in the \"Failure Analysis Report on the Urea Synthesis Tower of Pingyin Luxi Chemical Third Fertilizer Factory Co., Ltd.\" prepared by the Iron and Steel Research Institute, the accident investigation team concluded as follows: (1) Through macroscopic and microscopic analysis of the fracture surfaces resulting from the explosion in the urea synthesis tower at Pingyin Fertilizer Factory, as well as samples taken from locations away from the fracture sites, no significant welding defects were found, indicating that the welding quality performed by the manufacturing factory was reliable. Fracture analysis showed that the urine tower already contained a large number of stress corrosion cracking cracks prior to the explosion, and many of these cracks occurred in the base metal area. The bottom ring sections with lower temperatures developed numerous and severe stress corrosion cracks, creating sections that pose a load-bearing risk. Fracture analysis shows that the crack beneath the thermocouple insertion tube, where longitudinal stress corrosion cracking is relatively severe, has exceeded the material’s tolerance limits, which could be a potential cause of explosive fracture ； Stress corrosion cracking occurs simultaneously in multiple layers of the laminates on the upper circumferential section of the lower ring weld of the burst tube section, resulting in almost no remaining net cross-sectional area for that section. As a consequence, the stress in that section exceeds the tensile strength of the material, causing the laminates in that section to break one after another; this represents another possible initiating cause for explosive fracture. Both types of fractures can cause rapid leakage of the medium inside the tower, leading to a disruption of the gas-liquid phase equilibrium within the tower, and ultimately resulting in a ’balance-disruption type vapor explosion’. ⑵All evidence shows that the stress corrosion cracking resulting from the explosion of the urine tower was concentrated in the lower-temperature areas where the leak-detection steam was present, such as below the thermocouple insertion tube, at the bottom sections of the tower, and on the upper side of the circumferential welds. This indicates that the main contributing factor to stress corrosion in this tower is the liquefaction of the leak-detection steam, along with the presence of stress-corrosion-sensitive elements in the liquidized water solution. Through electron probe analysis of the composition of inclusions in the stress corrosion crack gaps and plate gap areas within the cylinder welds, as well as analysis of the composition of the effluents from the vent holes in the tenth cylinder section, high levels of potassium and sodium ions were detected. Combined with the analysis of desalinated water and an investigation of the water treatment process, it can be inferred that this is due to the concentration of trace amounts of potassium and sodium ions in the steam. ⑶The actual structure of the leak detection holes in the R-1102 urine tower at Pingyin Fertilizer Factory, as manufactured by the factory itself, caused the leak detection steam to escape between the layers of the tower. This led to stress corrosion cracking occurring in multiple layers simultaneously, accelerating the rate of such cracking. This was the structural factor that contributed to this explosion accident. Meanwhile, the as-built drawings of the R-1102 urine tower provided by the manufacturer to Pingyin Fertilizer Plant in the \"Technical Documents and Materials\" do not show a structure for steam leak detection holes. Meanwhile, the number of layers and thicknesses indicated in the drawings are 8 mm (316L), 6 mm (Q235-A), 12 mm (16MnR), and 14×6 mm (15MnVR), for a total of 17 layers; whereas the actual structure consists of 8 mm (316L), 6 mm (Q235-A), 12 mm (16MnR), 9×8 mm (15MnVR), and 2×6 mm (15MnVR), for a total of 14 layers. There is a significant discrepancy between the two. ⑷An energy analysis of the urine tower explosion shows that the total work generated by the materials inside the tower, such as expansion work and boiling work after the tower cracked, amounts to more than 20 times the energy required for the explosion. Meanwhile, the energy produced by a complete hydrogen explosion in the gaseous phase of the tower is only about 45% of the energy needed for the tower’s explosion. Neither the review of process records, on-site inspections, nor fracture analysis revealed any evidence of a chemical explosion in the gas phase section of the urea tower. In summary, the leak detection pipes for the urea synthesis tower at Pingyin Fertilizer Factory, manufactured by the factory in question, are connected to the 16MnR plates using pipe threads. If these leak detection pipes are not properly sealed or become loose during use, leak detection steam can leak into the spaces between the plates of the urea tower. Alkali ions such as sodium in this steam get concentrated to high levels, resulting in severe stress corrosion cracking. As a consequence, the urea tower had numerous and serious stress corrosion cracks prior to the explosion. Calculations have shown that some of these stress corrosion cracks have reached or exceeded the allowable limits for the laminate material. Additionally, due to stress corrosion cracking in the circumferential fracture surfaces of the explosion chamber sections, the net stress in certain circumferential sections has reached or surpassed the material’s yield limit or even its tensile strength. Both of these conditions create the possibility of significant cracking in the urine tower, leading to severe leakage of the internal fluid, and ultimately resulting in the boiling and explosion of that fluid. According to energy calculations, the explosive energy generated by the boiling of the medium inside the urine tower is about 20 times that required for an explosion in the tower; it is also more than 40 times the energy of a hydrogen explosion, which represents the highest level of energy in the gaseous phase of the urine tower. Thus, all the conditions necessary for an explosion in the urine tower are met. Due to the use of steam leak detection, the actual structure of the leak detection holes in the R-1102 urea synthesis tower manufactured by the Chemical Machinery Factory of Nanjing Chemical Industry Company, Sinopec Group, for Pingyin Luxi Chemical Third Fertilizer Plant Co., Ltd., caused the leak detection steam to escape between the layers of the tower. This led to stress corrosion cracking occurring in multiple layers simultaneously, accelerating the rate of stress corrosion cracking in those layers – and this was the main cause of this accident. 3. Determination of the nature of the accident: Based on the \"Failure Analysis Report on the Urea Synthesis Tower of Pingyin Luxi Chemical Third Fertilizer Factory Co., Ltd.\" prepared by the Steel Research Institute, and through an examination of management systems, operating procedures, and original records; analysis of the products in use and the materials that leaked; as well as inspections and calibrations of pressure gauges and safety valves, the accident investigation team concluded that Sinopec Nanjing Chemical Industry Company’s Chemical Machinery Factory failed to inform the customer, when delivering the R-1102 urea synthesis tower to Pingyin Luxi Chemical Third Fertilizer Factory Co., Ltd., about the possibility that the actual structure of the tower’s leak detection holes might become loose during transportation, installation, and operation, thereby allowing leak detection steam to escape between the layers and leading to serious consequences. During transportation, installation, and use, the leak detection holes of the urea synthesis tower may become loose. This causes the leak detection steam to leak between the layers of the tower, resulting in stress corrosion cracking occurring in multiple layers simultaneously. In summary, this accident was a liability incident that resulted in severe casualties and property losses due to an explosion caused by the poor quality of the urea synthesis towers manufactured by the Chemical Machinery Factory of Nanjing Chemical Industry Company under Sinopec Group – issues related to both manufacturing and maintenance. Determination of accident liability: In accordance with the provisions of laws, regulations, and standards such as the Work Safety Law, the Regulations on the Safety Management of Hazardous Chemicals, the Regulations on the Safety Supervision of Special Equipment, the Technical Regulations for the Safety Supervision of Pressure Vessels, and the Technical Requirements for Urea Synthesis Towers, the investigation team, through investigation and analysis, has arrived at the following conclusions regarding the liability for this accident and suggestions for dealing with the relevant responsible parties: (1) Liability of the manufacturing unit: The Chemical Machinery Factory of Nanjing Chemical Industry Company, under Sinopec Group, possesses the qualifications for designing and manufacturing pressure vessels. The safety production management organization and safety production responsibility system are well-established. The following problems exist in the design and manufacturing of the explosive urea synthesis tower: ① The user was not informed about the changes made to the structure of the leak detection holes. ②The steam leakage detection hole structure of the urine tower is a proprietary technology of the enterprise; no safety assessment was conducted prior to its use in industrial production, nor was there any follow-up evaluation of the safety aspects for users, which violates Article 6 of the Interim Regulations on the Safety Supervision of Boilers and Pressure Vessels (State Council Order [1982] No. 22). ③The structural design of the steam leak detection holes in the urine tower was modified without going through the proper design change procedures, which violates Article 2.1.6 of the Implementing Rules of the Interim Regulations on the Safety Supervision of Boilers and Pressure Vessels (Laoguo [1982] No. 6). ④No installation operation manual was prepared, which violates the provisions of clause 7.3 of the \"Technical Requirements for Urine Synthesis Towers\" (GB9842-2004). ⑤The technical service department detected abnormal usage of the steam leakage detection holes and ventilation holes in the urine tower by its users, but failed to provide timely feedback; as a result, the quality assurance system did not function properly, in violation of Article 2.2.2 of the Detailed Rules for the Implementation of the Interim Regulations on the Safety Supervision of Boilers and Pressure Vessels (Labor Boiler [1982] No. 6). ⑥Under the pretext of worrying about the leakage of proprietary technology, only the production workshop was informed to proceed according to the new design; no part drawings showing the leak detection holes were provided to the users, and the archived as-built drawings and process record cards did not match the actual products. ⑦In the cylinder material substitution form, some of the main material laminates are substituted from δ=8mm to δ=6mm. The as-built drawings indicate that the urine tower has 14 floors × 6, whereas the actual structure consists of 11 floors with composite slabs; the actual structure does not match the as-built drawings. Based on the above analysis and the conclusions contained in the \"Failure Analysis Report on the Urea Synthesis Tower at Pingyin Luxi Chemical’s Third Fertilizer Plant\" issued by the Steel Research Institute, the Chemical Machinery Factory of Sinopec Nanjing Chemical Industry Company failed to fully recognize the serious safety consequences associated with steam leakage from the urea synthesis tower. In particular, it did not pay sufficient attention to the fact that the new design of the leakage detection holes might, due to improper installation and use, lead to accelerated steam penetration into the lining plates, resulting in more severe problems than those caused by welded leakage detection holes. The company also failed to fulfill its duty to inform the users properly, which led to steam leaking into the lining plates and causing severe stress corrosion – this was the direct cause of the explosion. The Chemical Machinery Factory of Nanjing Chemical Industry Company, under Sinopec Group, is directly responsible for this explosion accident. Liu Jinbao, the former director (now a member of the Party committee**) and the person in charge of the unit, bears leadership responsibility for this explosion accident ； As the main person in charge of the design and technology at the unit, Chief Engineer Chen Jianjun was responsible for altering the structural design of the steam leak detection holes; by failing to inform the users in a timely manner on the grounds of confidentiality, he is the direct culprit behind this accident and bears direct responsibility for the explosion. II) Determination of responsibilities of the using unit: Pingyin Luxi Chemical Third Fertilizer Factory Co., Ltd. has established a safety production management organization, and has formulated safety production responsibility systems, emergency rescue plans for accidents, safety regulations, process procedures, and operational guidelines for specific positions ； Major hazard sources have been identified, and corresponding monitoring measures have been implemented ； In December 2004, Liaocheng Zhengxin Safety Assessment Co., Ltd. conducted a safety assessment of the hazardous chemical production facilities, and on March 8, 2005, the \"Work Safety License\" was obtained ； All pressure vessels in use have passed inspections by the Jinan Boiler and Pressure Vessel Inspection Institute, and personnel performing special operations hold the required certificates ； The on-duty leaders at all levels and the operators provided proper on-site supervision and operation, and the on-site process performance data was generally normal. The investigation team believes that no direct cause linking the plant to the explosion of the urea synthesis tower has been identified at present. By reviewing the available documents, the accident investigation team found that the technical documentation provided by the manufacturer for the urea synthesis tower involved in the explosion was incomplete. From the time this urea synthesis tower was put into use until the explosion occurred, the necessary technical documents such as relevant drawings and operation manuals were not provided, indicating that Pingyin Luxi Chemical Third Fertilizer Factory Co., Ltd. had shortcomings in the management of equipment technical records. Recommendation (1): The explosion of the urea synthesis tower was caused by issues related to the quality of the tower itself, including its manufacturing and maintenance aspects; the Chemical Machinery Factory of Nanjing Chemical Industry Company under Sinopec Group served both as the design entity and the manufacturer. In accordance with Article 3.3.4 of the Implementing Rules of the Interim Regulations on the Safety Supervision of Boilers and Pressure Vessels, as well as Paragraph 4 of Article 25 of the Provisions on the Handling of Accidents Involving Special Equipment such as Boilers, Pressure Vessels, and Pressure Pipelines, it is recommended that the quality supervision authorities take action against the Chemical Machinery Factory of Sinopec Nanjing Chemical Industry Company in accordance with relevant regulations; meanwhile, administrative liability should be imposed on Liu Jinbao, the person in charge of the manufacturing unit, and Chen Jianjun, the chief engineer, in accordance with the law. (II) It is recommended that the quality supervision authorities instruct the Chemical Machinery Factory of Sinopec Nanjing Chemical Industry Company to carry out a comprehensive inspection of the urea synthesis towers with this type of steam leak detection hole structure that have been manufactured. The quality supervision department conducts comprehensive inspections to prevent similar accidents from occurring. Preventive measures: The ’3·21’ accident at Pingyin Luxi Chemical’s Third Fertilizer Plant Co., Ltd. resulted in severe casualties and substantial property losses, serving as a very sobering lesson. To earnestly learn from the lessons of this accident and prevent similar incidents from occurring, the following preventive measures are proposed: (1) The Chemical Machinery Factory of Nanjing Chemical Industry Company, under Sinopec Group, should thoroughly learn from the lessons of this accident, strictly adhere to relevant standards and regulations, strengthen accountability, give top priority to the quality of equipment and products in its operations, and strictly prevent substandard pressure vessel equipment from entering the market. (II) Pingyin Luxi Chemical’s Third Fertilizer Factory Co., Ltd. should learn from the lessons of this accident, be strict in the procurement of production equipment, and strengthen the management of equipment technical records. For the pressure vessel equipment purchased, it is necessary to thoroughly examine technical documents such as product design and manufacturing drawings as well as operation manuals, and to conduct regular shutdown inspections to ensure production safety. (III) Safety supervision and quality inspection departments shall strictly comply with the provisions of the Work Safety Law, the Regulations on the Safety Management of Hazardous Chemicals, and the Regulations on the Safety Supervision of Special Equipment. By learning from past mistakes, they should strengthen the safety management in hazardous chemical manufacturing enterprises as well as the safety oversight of special equipment, thereby ensuring safe production in high-risk industries. Investigation Team on the ’3·21’ Explosion in the Urea Synthesis Tower at Pingyin Luxi Chemical Third Fertilizer Factory Co., Ltd., Jinan City, December 31, 2005