On a certain day 19: 30, a major explosion occurred in the nitration workshop of TNT production line of a factory in a certain province, causing serious casualties and huge property losses.

TNT is a high explosive, which is formed by nitration of toluene with nitric acid and sulfur. There are four dangers in the process of nitrification: combustion, explosion, corrosion and poisoning. The nitration reaction is divided into three stages: the first stage nitration is from toluene to mononitrotoluene (MNT), which is completed by four nitration machines in parallel; The second stage of nitration is from nitrotoluene to dinitrotoluene (DNT), which is completed by two nitrating machines in parallel. The three-stage nitration from dinitrotoluene to trinitrotoluene was completed by connecting 1 1 nitrators in series.

The chemical reaction formula is as follows:

ch 3c 6h 5+HNO 3+(h2so 4)→ch 3c 6h 4(NO2)+H2O

ch 3c 6h 4(NO2)+HNO 3+(h2so 4)→ch 3c 6h 4(NO2)2 deca H2O

ch 3c 6h 4(NO2)2+HNO 3+(h2so 4)→ch 3c 6h 2(NO2)3+H2O

Three-stage nitration is much more difficult than two-stage nitration, which requires a long reaction time and multiple nitrators in series, and the mixed acid concentration of nitrate and sulfur is high and controlled at a higher temperature, which is dangerous in production. This catastrophic explosion accident started from Unit 2 (code III_2+) in Area 3.

The nitration workshop where the accident occurred was composed of three actually connected workshops. The middle is a 9m×40 m× 15 m reinforced concrete three-story building with dome roof; The east and west sides are 8 m×40 m and 12 m× 40 m respectively. Most nitrifying bacteria are located in the west wing, and the physical and chemical analysis room is located in the east wing. The whole nitrification workshop is located in an explosion-proof earth embankment with a height of 3 m, which is closed around, and workers can only enter and exit through culverts. After the explosion, the workshop and more than 40 sets of equipment disappeared, leaving a pot bottom pit about 40 meters deep and 7 meters deep in Fiona Fang, with 2.7 meters of water at the bottom.

The explosion not only destroyed the workshop, but also seriously damaged the refining and packaging workshop, air compressor station and branch company, and also seriously affected the adjacent branch company. The third factory located in the west of the explosion center, the fifth factory in the south, the sixth factory in the north and the thermal power plant were all seriously damaged within 600 m from the explosion center; /kloc-within 0/200m, some buildings were damaged, and all doors and windows were shattered; The door and window glass within 3000 m is partially broken due to vibration. Nearly a thousand trees around the explosion center were cut off, washed down, or cut off half of the crown by shock waves.

Explosive flying objects-broken walls and equipment fragments, mostly fall within a radius of 300 m, and a few flying objects fall far away. For example, a steel shaft with a length of 800 mm and a diameter of 80mm falls to1685 m; A reinforced concrete block with a weight of 10 t (the residue of the vault of the original nitrification workshop) was thrown to the southeast direction of 487 m, breaking the main water supply line of a 400mm cast iron pipe buried 2m underground, and the water overflowed into a river. A cement wall slag with a weight of 10 kg flew to 3 1m and smashed through the roof of the sanitary towel production workshop of No.3 factory, causing serious injuries to two female workers indoors.

According to statistics, this accident killed 17 people, seriously injured 13 people and slightly injured 94 people. Abandoned buildings are about 50,000 ㎡, seriously damaged by 58,000 ㎡, and generally damaged by 6,543.8+0.76 million ㎡; The equipment was damaged by 95 1 set, and the direct economic loss was 22.666 million yuan. In addition, due to the suspension of production and reconstruction, the indirect loss is even greater.

According to the calculation of the amount of explosives in production equipment, the relationship between the damage degree of buildings and the overpressure of shock waves, and the estimation of the shape and size of the explosion pit, it is determined that the amount of explosives exploded in this accident is about 40t TNT equivalent.

Second, the cause analysis of the accident

1. Cause of the accident

After the accident, the joint investigation team composed of the competent department of the enterprise, the labor department of the government and the labor protection department of the trade union is responsible for investigating the accident. Because of this accident, all the original workshops and equipment were blown up, and the construction site became a big and deep water pit. Therefore, although the experts of the investigation team conducted repeated investigations on the scene of the accident, they found few physical evidence and incomplete instruments and records, which made it very difficult to determine the origin and analyze the cause of the accident. Fortunately, among the 34 workers on duty, 17 survived. After repeated inquiries, they provided the production situation before the accident and some phenomena at the time of the accident, which was very helpful to the investigation progress. Based on the oral records of the parties concerned, the investigation team verified many relevant drawings and materials, made some simulation tests, and made serious analysis and discussion from the aspects of process technology, production management, equipment status, raw material quality, production operation, etc., and finally determined and confirmed the cause of the accident-that is, the separator of Unit 2 in Section 3 was the first equipment to explode. The main basis is:

(1) Oral statements of the parties. The operator of III-2+ machine reported that samples of nitrate and waste acid were taken from the production equipment at 19: 00, sent to the physical and chemical analysis room, and returned to their posts at about 19: 15. When the separator of ш-2+machine was found to be smoking, he opened the rainwater separator device and the cooling water bypass valve of nitrator to cool down as required, and then went to the instrument control room to look for it.

(2) the testimony of the monitor. The monitor admitted that the operator of III-2+ machine reported to him about 19: 15, so he led two other workers to the nitration workshop. Seeing that the separator of the III-2+ machine was smoking, he directed the workers to open the circulating valve in front of the machine and add concentrated sulfuric acid to further cool down. However, this measure did not work. The smoke in the workshop was choking, so they and others retreated to the door of the workshop. Then they saw the flame spouting from between the separator mouth and the upper cover, thinking it was "bad", and immediately ran outside the explosion-proof earth embankment. Just out of the culvert, it exploded with a bang behind it.

(3) circumstantial evidence of relevant personnel. The operator of the III- 10+ machine confirmed that he had sent samples back from the analysis room on 19: 15, and when he saw smoke coming from the separator of the III-2+ machine, he went over and asked the operator of the III-2+ machine, "Is the temperature high?" Answer: "Not too high". He returned to his post. Later, I saw the monitor directing several workers to take measures to cool down and suppress smoke. But the smoke was getting bigger and bigger, so he retreated outside the workshop. As soon as he saw the fire, he ran out of the explosion-proof earth embankment from the nearby culvert.

(4) physical evidence. Some fragments of comprehensive records were found from the collapsed instrument control room. The data displayed after the subsidy was restored proved that the nitric acid concentration of the three-stage nitrification machine was too high at around 19: 00 that day. According to the technical specification, the nitric acid concentration of ш-2+machine is 1.0% ~ 3.5%, while the record is 7.9%. According to the process regulations, III-4+ and III-7+ machines account for 2. 0% ~ 4.0%, and the recorded III-5+ machine accounts for 12.6%, which is two or three times higher than the process regulations. This led to process confusion and the lowest freezing point moved forward. The most violent machine was III-2+, which provided conclusive material evidence for the first smoke, fire and explosion of III-2+.

(5) From the crater shape analysis. According to the crater surveying and mapping, the deepest contour line is in the shape of a sole, and the mouth is pear-shaped, and its main axis forms an angle of about 5 with the main axis of the nitrating machine, indicating that the initiation origin is several machines before three-stage nitration. According to the smoking and fire phenomena mentioned by the workers, it is determined that this is a ш-2+ machine. It exploded first, and its shock wave caused various machines to move to varying degrees in the future, and then a martyrdom explosion occurred. Although each machine exploded almost at the same time, the regular displacement before the explosion tilted the crater.

2. The cause of the accident

The investigation team used fault tree analysis to find the cause of the accident, which was very effective. According to the sequence and causality, experts draw the program block diagram of the conditions that may cause combustion and explosion accidents in the nitrification process. It shows the logical relationship between the factors that caused the accident. Then, find out the status and influence degree of various factors item by item, eliminate irrelevant factors, keep relevant factors, and further explore relevant factors until the cause of the accident is confirmed.

After analyzing the chart item by item, some irrelevant factors, such as water leakage in cooling coils, interruption or shortage of cooling water, agitator failure, instrument failure, impurities in raw materials, etc. , excluded, leaving a few related factors, can sort out two "accident causal chain", as shown in figure 5- 1.

In the first "accident causal chain", the key points are incorrect feed ratio and chaotic process conditions, which are caused by high nitric acid concentration. At this time, the nitration reaction is fierce, and the reactants with insufficient reaction in the nitration machine are lifted to the separator to continue the reaction, and there is neither cooling coils nor stirring device in the separator, which is easy to cause nitrate to decompose and catch fire due to local overheating. After investigation, this phenomenon occurred before the accident. On the day of the accident, the leakage of nitric acid valve of ш-6+and III-7+ machine was found in day shift production. 16: 30 After the second shift takes over, 17: 00 will be repaired by the instrument worker. However, the nitric acid leaked into the nitration system made the concentration of nitric acid in the reaction solution too high, and the nitric acid content in ш-2+machine reached 7.09.

In the second "accident causal chain", the key point is that when the reaction liquid comes into contact with unexpected combustible substances, such as cotton yarn, lubricating oil, rubber gloves or rubber washers, it will have a strong oxidation reaction with nitric acid in mixed acid, producing smoke and catching fire. After careful investigation. No cotton yarn fell before the accident. But further investigation found that. The filler between the separator rim and the upper cover is asbestos rope that does not meet the technical requirements. Contact with high temperature and high concentration nitric acid mixed acid. The fire may have caused the accident. As mentioned above, workers add a lot of concentrated sulfuric acid to the machine to cool down and suppress smoke, which increases the chance of acid contacting asbestos rope.

Asbestos rope that does not meet the technical requirements. After investigation, it was replaced during a month of equipment overhaul. Usually, the asbestos rope is nonflammable, but the residual asbestos rope found at the scene of the explosion accident and the residual asbestos rope used in the small craft warehouse can be ignited with matches. The analysis and inspection by the labor safety and health supervision station in this province proved that this kind of asbestos rope only contains 50% asbestos, and the rest is combustible fiber and grease. In order to prove the function of this asbestos rope and nitric acid mixed acid, the investigation team specially made a simulation test, which proved that this asbestos rope reacted violently with nitric acid mixed acid with specified concentration in the process, releasing a lot of yellow smoke, and the temperature rose from 1 10℃ to 150℃. The use of this asbestos rope may completely lead to nitrate fire. However, the asbestos rope that meets the technical requirements is used for the control test, and there is almost no response.

The investigation team also found the main way for the fire to spread and expand after the III-2+ separator caught fire. First, the fire spreads through the smoke exhaust pipe; The second is to conduct the fire through the low wooden house panel.

From fire to explosion. The main reason is that emergency and safe feeding measures were not taken in time. According to the regulations, the nitrating machine should have three sets of safety feeding devices, which are remote control, automatic and manual. In case of fire, the safety feeding devices can be opened in time to put the materials into the safety pool. However, this factory is an old factory with a long history, backward technology, outdated equipment, low factory building, low degree of production automation and poor intrinsic safety conditions. There is no automatic safety feeding device on the nitrating machine, and the operators and squad leaders did not manually feed in time after the fire broke out. So that it changed from fire to explosion.

To sum up, the cause of this accident can be summarized as follows: the accident was caused by the leakage of nitric acid valves of III-6+ and III-7+ machines, which led to the high nitric acid content in the nitrification system and the lowest freezing point moving forward. The III-2+ machine reacted violently and smoked. At this time, the nitrate in the separator of III-2+ machine caught fire because the high temperature and high concentration nitric acid mixed with asbestos rope (containing a lot of combustible fibers and grease) did not meet the process requirements. It may also be due to the intense reaction and local overheating in the separator, which leads to the decomposition and fire of nitrate. After the fire broke out, the nitrifying machine had poor intrinsic safety conditions, and there was no automatic feeding device or manual feeding by workers, so the fire turned into an explosion. At the same time, this accident has a lot to do with the loopholes in factory management, and the leaders have not paid enough attention to safety; There are many problems in production technology and equipment, and the solutions are ineffective; Workers have poor labor discipline and leave their posts without authorization; Coupled with the use of asbestos rope that does not meet the technical requirements. Therefore, this catastrophic explosion accident is a liability accident with poor intrinsic safety conditions.

Third, the accident responsibility division and handling

1. Direct responsibility

(1) Niu, an operator of No.2 nitrifier in the third section, opened the deluge valve and the bypass cooling water valve to cool down after discovering that the separator of No.2 nitrifier in the third section was smoking. However, after discovering that the separator was on fire, he ran out of the scene without taking the key measure of discharging materials into the safety pool, causing the fire to spread and causing an explosion. Therefore, Niu should be directly responsible for this accident. After research, he was dismissed from the factory and kept in the factory for observation. It is suggested that he be handed over to judicial organs for criminal responsibility.

(2) After learning that the separator of Unit 2 in Section 2 was smoking, the shift supervisor Zhang instructed the workers to take some cooling measures, but when the separator caught fire, he did not urge the mechanic to turn on the safety discharge switch of the nitrator, nor did he take other remedial measures, but shouted for evacuation, so that everyone fled the scene. He should also bear the main responsibility for the accident. After research, he was expelled from the factory and kept in the factory for observation. It is suggested that he be handed over to judicial organs for criminal responsibility.

2. Indirect liabilities

(1) Zuowei Liu, director of No.2 Factory, the organizer of production and the first person in charge of production safety in No.2 Factory, did not seriously implement the principle of "five simultaneities of production safety". TNT production line stopped production at the end of one year and resumed production in February of the following year 1. Due to insufficient preparation, the production, technology and equipment were abnormal for a long time, and the production was stopped for 7 times after 9 days. Single machine stopped frequently, and valves, gaskets and cooling pipes were replaced and repaired many times. He did not pay enough attention to these problems and did not solve them; And interrupted the night cadre duty system without authorization; The workers in the second branch factory were lax in labor discipline and left their posts seriously, which was not corrected in time. Therefore, he is mainly responsible for the accident. After research, he was dismissed from the post of factory director and kept in the factory for observation.

(2) Kim, the director of the General Factory, only served as the director 15 days, but the explosion caused heavy casualties and huge property losses, which caused bad political influence. As the first person responsible for safety production in the general factory, he should take indirect leadership responsibility for this accident. After research, administrative demerit was given.

(3) Li, the deputy director in charge of production and safety of the general factory, as the person in charge of production and safety of the enterprise, has certain leadership responsibilities.

(4) During the accident investigation, it was found that the filler used between the separator and the nitrification hood was flammable asbestos rope, which was one of the main reasons for the fire of the separator. The person who caused this kind of asbestos rope to be used is indirectly responsible for this accident, and should be further investigated and given disciplinary action.

(5) The TNT production line of this factory was built during the Japanese puppet regime. After liberation, although it has been improved many times, the situation of backward technology, outdated equipment, low level of automatic control and serious hidden dangers of accidents has become increasingly prominent. The factory and the competent department have reported to the superior many times, demanding safety technical transformation. However, the renovation plan was not approved until three years before the accident, and the investment was not approved until 1 year before the accident. Just as the new production line was under construction and the old production line was about to be retired, a serious explosion accident occurred. Therefore, the superior company and the superior departments concerned should also bear certain responsibilities for this accident.

Four, the accident lessons and preventive measures

We should draw the following lessons from this catastrophic explosion accident:

1. In terms of facilities and technology,

(1) The dangerous goods production workshop shall meet the requirements of fire and explosion prevention. In the workshop with explosion accident, the nitrification production line is mainly arranged in the west building of brick-wood structure, and the separator cover is only 1.7 m away from the wooden house panel, making the wooden house panel a fire conductor. In addition, the main workshop of nitration workshop adopts reinforced concrete heavy roof, which causes large flying objects to smash the surrounding buildings during explosion, injuring people and causing secondary disasters.

(2) In order to improve the intrinsic safety and automation level of dangerous goods production equipment, not only the production equipment should have perfect safety protection devices, such as automatic alarm and automatic feeding, but also the number of on-site operators should be reduced as much as possible.

(3) The process layout of dangerous goods production workshop should be orderly and easy to operate. Conducive to safe evacuation. However, the workshop where the accident occurred has dense equipment and vertical and horizontal pipes, which makes it inconvenient for workers to operate from the iron ladder. It is also not conducive to evacuation.

(4) The dangerous goods workshop must have enough safe distance from the surrounding buildings. This incident caused such huge casualties and property losses, which is directly related to the unreasonable layout of the factory, insufficient safety distance and the dilapidated of most workshops.

2. In production and safety management.

(1) The production of dangerous goods should have strict process equipment management. Before the accident in the nitration workshop, the equipment broke down many times, and the valves and gaskets were replaced and repaired many times, which led to frequent start-up and shutdown, resulting in process disorder, but the management cadres and technicians did not deal with it in time, which buried the hidden danger of the accident. In the future, we should operate in strict accordance with the technical conditions stipulated in the process, reduce process fluctuation, try our best to improve the equipment integrity rate, and reduce or even eliminate the hidden dangers of accidents.

(2) There should be strict labor discipline in the production of dangerous goods, and it is strictly forbidden to go on duty. According to the survey, six of the 34 workers left their posts within half an hour of the accident.

(3) Regularly carry out safety awareness education and drills on workers' anti-accident ability (emergency handling ability of accident symptoms). Before the accident, both the workers and the squad leader were in a hurry and fled the scene without taking artificial feeding measures in time, so that the fire turned into an explosion.

(4) Production of auxiliary materials. For example, the fire and acid resistance of asbestos rope can be tested before it can be used in production.

(5) Leading cadres should organize and direct production to achieve "five simultaneities of safety production", that is, safety work and production work should be planned, arranged, inspected, summarized and assessed at the same time. To some extent, this accident is related to the factory leaders' failure to achieve "five simultaneities of safety production".