(1) Chongqing Tianyuan Chemical General Factory "4.16" chlorine gas explosion accident
1) the basic profile of the enterprise
Chongqing Tianyuan Chemical General Factory was founded in 1939, 1956, the company joint venture for the local state-owned enterprises, the earliest of the chlorine-alkali enterprise It is one of the earliest chlor-alkali enterprises in China. It is subordinate to Chongqing Chemical Medicine Holding (Group) Company, with a production capacity of 60,000 tons/year of caustic soda, producing varieties of caustic soda, hydrochloric acid, liquid chlorine, carbon tetrachloride, trichlorohydrosilicon, ferric trichloride, chloroacetic acid, bleaching powder and sodium hypochlorite. There are now 2112 registered workers and 1942 on duty. 2003 industrial output value was 190.85 million yuan and sales income was 187.22 million yuan. With the accelerated pace of construction in the main urban areas, the plant was officially launched in 2003 to protect the environment overall relocation project.
2) Accident
Accident occurred before the daytime of April 15, 2004, the plant is in a normal state of production. 17:40 on the 15th, the plant chlorine and hydrogen plant refrigeration section of the liquefaction position to receive the main plant scheduling order to open the No. 1 chlorine condenser. 18:20, chlorine drying position found that the chlorine pump pressure is high, the No. 4 liquid chlorine tank liquid surface pipe in the frost. The operator on duty twice on the liquefaction post inspection, did not find the chlorine condenser any abnormalities, judging that the No. 4 storage tank liquid chlorine inlet pipe may be clogged, and then turn to the No. 5 liquid chlorine storage tanks (stop the No. 4 storage tanks) for liquefaction, the liquid surface pipe is not frost. 21:00, the duty personnel inspection of the No. 1 chlorine condenser and brine tanks, found that a large number of brine brine brine tanks of calcium chloride (CaCl2) reduction, there is a chlorine from the ammonia evaporator brine box The chlorine condenser was judged to be perforated, and about 4m3 of calcium chloride brine entered the liquid chlorine system.
After the discovery of the chlorine condenser perforation, the plant's main dispatch office quickly took the No. 1 chlorine condenser from the system disconnect, refrigeration emergency stop and other measures. And the No. 1 chlorine condenser shell calcium chloride brine through the brine pump inlet back discharge into the brine tank. No. 1 chlorine condenser residual chlorine and No. 1 liquid chlorine gas separator liquid chlorine discharged into the sewage tank.
At 23:30 on the 15th, the plant took measures to turn on the liquid chlorine packaging tail gas pump to extract the chlorine gas in the sewage tank to the bleaching device of sodium hypochlorite. at 0:48 on the 16th, in the process of pumping, the sewage tank exploded. at 1:33, the whole plant was stopped. at about 2:15, the No. 1 brine pump, which was 4 hours after the discharge of brine, exploded at rest, and the pump body was crushingly blown up.
After the dangerous situation, the plant promptly reported the chlorine condenser perforation and chlorine leakage accident to the Chemical Medicine Group, and made a report to the Municipal Safety Supervision Bureau and the municipal government duty room. In order to eliminate the risk of re-explosion and chlorine leakage of large quantities, Chongqing Municipality on the morning of the 16th to start the implementation of the emergency response plan, including rescue, evacuation of the masses, including the 16th at 9:00 set up a deputy mayor of Chongqing Municipal Chemical Plant as the general director of the "4.16" accident scene rescue headquarters, under the leadership of the command, and immediately Set up by the city inside and outside the relevant experts composed of experts, for the command of the risk decision-making to provide technical support.
The experts concluded that the key to eliminating the danger is to consume as much chlorine as possible and eliminate the danger of massive chlorine leakage. Command accordingly decided to take the natural decompression discharge discharge chlorine gas way, through the opening of ferric chloride, bleaching powder, sodium hypochlorite three chlorine production device, in a short period of time to reduce the total amount of chlorine in the source of danger; and then dissolve the residual nitrogen trichloride (NCl3) in the tanks with carbon tetrachloride; and finally use nitrogen to dissolve the nitrogen trichloride of carbon tetrachloride waste hydraulic pressure, in order to eliminate the risk of explosion. 10 o'clock or so according to the plant's decision to open the production of chlorine gas. Command's decision to turn on the chlorine production unit.
16th 17:30, the command held a meeting of all members, to study the next step in the disposal program and the evacuation of the masses that night. 17:57, the expert group is reporting to the command, to discuss the next step in the specific disposal program, suddenly heard two consecutive explosions, liquid chlorine tanks exploded violently, the meeting was interrupted.
According to the survey, the explosion made the No. 5, No. 6 liquid chlorine tank tank rupture disintegration and the formation of a 9m long, 4m wide, 2m deep crater. To the pit as the center, about 200m of the ground and structures on the ground with a large number of scattered explosive debris, the explosion resulted in the death of nine on-site disposable personnel in the line of duty, three people were injured.
Accident investigation team that the Tianyuan "4.16" explosion is the plant's liquid chlorine production process due to corrosion of the chlorine condenser perforation, resulting in a large number of ammonium-containing calcium chloride brine directly into the liquid chlorine system, generating a very dangerous nitrogen trichloride explosives. Nitrogen trichloride enrichment to reach the explosive concentration and start the accident chlorine treatment device vibration detonated nitrogen trichloride.
3) Causes of the accident
①Direct causes
A. Corrosion and perforation of the equipment led to the leakage of brine, which is an important cause of the formation and aggregation of nitrogen trichloride. According to the technical appraisal of Chongqing University and the analysis of experts, the cause of chlorine leakage and brine loss is the corrosion perforation of the column tube of chlorine condenser. Corrosion perforation is mainly due to five aspects: one is chlorine, liquid chlorine, calcium chloride cooling brine on the chlorine condenser there is a general corrosion effect; two is the water in the column tube on the corrosion of carbon steel; three is the column tube outside the brine due to the ionic potential difference on the pipe electrochemical corrosion and pitting corrosion; four is the column tube and the tube plate welded to the stress corrosion; five is the use of up to 8 years did not carry out the pressure test, so that the corrosion phenomenon failed to obvious corrosion and corrosion wear. The phenomenon was not detected in time before the obvious corrosion and corrosion perforation.
B. The investigation also learned that liquid chlorine production process will by-produce a very small amount of nitrogen trichloride. But through the sewage tank regular discharge, the use of dilute lye absorption can be avoided explosion. However, in 1992 and January 2004, the liquid chlorine refrigeration post of the evaporator system had a leakage, resulting in a large amount of ammonium into the brine, generated a high concentration of ammonium containing calcium chloride brine (calcium chloride brine by the extraction of the scene of the accident to determine the total amount of ammonia and ammonium ions in the brine for the 17.64g / L). Due to the corrosion and perforation of the No.1 chlorine condenser column tube, the calcium chloride brine with high concentration of ammonium entered the liquid chlorine system, generating about 486 kg (theoretical calculated value) of nitrogen trichloride explosives, which is more than 2,600 times of the normal production situation. It is the intrinsic cause of the successive explosions of the sewage tank and brine pump in the early morning of the 16th as well as the explosion that evolved during the rescue process in the afternoon of the 16th.
C. Nitrogen chloride enriched to reach explosive concentration and start the accident chlorine treatment device caused by vibration, is the direct cause of nitrogen trichloride explosion, the investigation confirmed that the plant on-site personnel without the consent of the command to speed up the speed of chlorine treatment, in the nitrogen trichloride enriched in the lack of understanding of the danger of explosion in the case of hasty, misjudgment, by virtue of the previous operation of the treatment experience, started the accident chlorine treatment device, the No. 4 chlorine treatment device, the first time in the morning of the 16th and the brine pump explosion. Accidental chlorine treatment device, No. 4, No. 5, No. 6 liquid chlorine storage tanks for pumping treatment. In the suction process, the accident chlorine treatment device water seal at the nitrogen trichloride due to contact with air and the first explosion, the explosion of the formation of a huge energy through the pipeline to the liquid chlorine tanks, stirring and vibration of the tanks of nitrogen trichloride, leading to No. 5, No. 6 liquid chlorine tanks within the nitrogen trichloride explosion.
②Indirect causes
A. Pressure vessel daily management is poor. Testing and inspection is not standardized, equipment renewal investment is insufficient. State Bureau of Quality and Technical Supervision, "Pressure Vessel Safety and Technical Supervision Regulations," Article 117 clearly stipulates that: "the use of pressure vessels, pressure vessels must be established by the management of technical files and unified custody," but the plant's equipment and technical files are incomplete, the last two years, no maintenance, repair, inspection records, pressure vessels and other equipment Management is chaotic. Pressure Vessel Safety Technology Supervision Regulations", Article 132, Article 133, respectively: "Pressure vessels are put into use for the first time after the use of internal and external inspection period, at least one pressure resistance test". But the plant and the Chongqing Chemical Energy Conservation measurement pressure vessel monitoring Institute did not according to the provisions of the first inspection of the pressure vessel and pressure test, in February 2002 for re-inspection, the two inspections have not been put forward pressure test requirements, but also did not do the pressure test. Resulting in equipment corrosion phenomenon in the obvious corrosion and corrosion perforation before timely detection, leaving a major accident hidden danger. The plant equipment aging phenomenon is very common, pressure vessels and other equipment corrosion is serious, equipment renewal investment is insufficient.
B. Production responsibility system is not in place, the production safety management force is weak. February 12, 2004, Chongqing Chemical Medicine Holding (Group) Company and the plant signed the production safety responsibility after the plant, the plant did not break down the target responsibility to the plant's various units and signed the safety target responsibility, did not implement the safety responsibility to the grass-roots level and the workplace, the responsibility for safety management is not in place. Safety management staffing is unreasonable, safety production management force is insufficient, Chongqing Chemical Medicine Holding (Group) Company in charge of leadership and plant managers and other safety production management personnel are not familiar with the safety management of the chemical industry.
C. Ineffective supervision and inspection of hidden accidents. Chongqing Tianyuan Chemical Factory on its own accident hidden dangers of poor rectification, especially the plant "2.14" hydrogen chloride leakage accident, caused by the city leaders attach great importance to the municipal party committee, the municipal government on the leadership of this important instructions, for this reason, the Chongqing Chemical Medical Holdings (Group) Company and the plant, although some measures taken, but not from the management to find out the causes of the accident and summarize the results. Management to find the cause of the accident and summarize the lessons learned in the accountability to take financial penalties instead of administrative sanctions, and thus did not allow the responsible personnel to learn from the deep lessons of the accident, rectification measures are not in place, supervision and inspection efforts are not enough, so that the problems in the safety aspects of effective rectification has not been done. After the "2.14" accident, the hydrochloric acid synthesis tail gas and carbon tetrachloride tail gas starvation monitoring system should have been added, but it was not equipped until the "4.16" accident.
D. The research on the mechanism and conditions of nitrogen trichloride explosion is immature, and the related safety technical regulations are not perfect. The relevant national authoritative experts in the "on the Chongqing Tianyuan Chemical Factory" 4.16 "accident analysis report on the cause of the comments" that: "At present, the domestic mechanism of nitrogen trichloride explosion, the explosion of the conditions of the lack of relevant technical information on how to avoid the explosion of nitrogen trichloride of the relevant technical standards is not yet perfect. Not perfect enough", "because of the high concentration of calcium chloride brine leaked into the liquid chlorine system, resulting in an explosion of the accident in China is still the first case". This shows that there are indeed great complexity, uncertainty and unpredictability in the handling of nitrogen trichloride in this accident. Therefore, this accident is because the existing technology in the chlor-alkali industry is difficult to predict, there is no precedent of the accident, the human factor does not play a leading role. At the same time, the national chlor-alkali industry has not yet on the calcium chloride brine ammonium content in the regular analysis of the provisions of the plant's calcium chloride brine for more than 10 years without replacement and testing, resulting in the brine of ammonium continued to enrich the conditions for the generation of a large number of nitrogen trichloride to create, and for the occurrence of explosions to lay a major potential hidden danger.
4) Measures to prevent such accidents
Based on the above analysis of the cause of the accident, the investigation team believes that the "4.16" accident is a responsible accident. Chongqing Tianyuan Chemical Plant "4.16" accident, leaving a deep, painful lessons, the chlor-alkali industry has a general warning.
A. The relevant personnel of the original chemical plant on the lack of monitoring of the operation of the chlorine condenser, the relevant personnel of the chlorine drying section of the night of April 15, chlorine transfer pump outlet pressure has been high and liquid chlorine tank liquid surface tube does not frost, the lack of timely and accurate judgment, failed to find the chlorine liquefaction system within a short period of time abnormalities, and ultimately due to the chlorine condenser chlorine tube leakage expansion, so a large number of frozen salt water into the chlorine liquefaction system, and the chlorine condenser. In the end, the leakage of the chlorine condenser chlorine tube expanded, causing a large amount of refrigerated salt water to enter the chlorine liquefaction system. This lesson should be seriously summarized. The enterprises concerned should take this as a warning.
B. At present, most of the chlor-alkali enterprises use liquid ammonia to indirectly cool calcium chloride brine in the traditional production process, and have not yet paid enough attention to the salt content of brine. It is necessary to monitor the ammonium content in the frozen brine or add an automatic alarm device.
C. Improve the safety management system and a variety of operating procedures and strictly enforced. Strengthen equipment management, accelerate the pace of equipment renewal, especially to strengthen the detection and management of pressure vessels and pressure piping to prevent leakage. For the key pressure vessels in use, should increase the safety accessory facilities and inspection, testing frequency, to reduce the safety hazards caused by equipment defects.
D. Further study the prevention and control technology of the domestic chlor-alkali enterprises about nitrogen trichloride, reduce the concentration of ammonium in the raw material salt and water source, and take corresponding measures to reduce the enrichment of nitrogen trichloride in the process of liquid chlorine production.
E. Try to use new refrigerant to replace the traditional process of liquid ammonia refrigeration, and improve the intrinsic safety level of liquid chlorine production.
F. Explore technically and form a safe, mature and reliable emergency plan and method for preventing and dealing with nitrogen trichloride as soon as possible, and popularize it in the chlor-alkali industry.
G. Intensify the in-depth study of nitrogen trichloride, completely clarify its physical and chemical properties, explosion mechanism, is the entire chlor-alkali industry has a fuller understanding of nitrogen trichloride.
H. Accelerate the main urban areas of chemical production enterprises, especially the major sources of danger and pollution sources of enterprises to relocate the pace of reducing chemical safety accidents on the harm to society and its negative impact.