Explosion Accidents of Air Separation Plants at Home and Abroad
On January 4, 1961, the air separation tower of a 4000m3/h air separation plant in the former Federal Republic of Germany exploded, killing 15 people and severely damaging equipment and buildings.
On November 23, 1973, a typical vicious explosion of the 3350m3/h air separation unit of Anshan Iron and Steel Oxygen Plant occurred outside the tower, which also caused the explosion of the air separation base in the tower. The equipment was damaged in many places, and production resumed after 6 months of maintenance.
On July 27, 1986, the 3200m3/h air separation plant of Yanshan Petrochemical Company's advance chemical plant made a loud noise, and the whole plant was turned into ruins.
On November 1, 1992, an explosion occurred in the 150m3/h air separation tower of the oxygen generating station of Lanzhou Petrochemical Machinery Plant, causing 1 death and the air separation tower was abandoned.
On July 25, 1993, the main condensing evaporator of the 150m3/h air separation tower of Jinchuan Nonferrous Metals Company in Gansu Province suffered a crushing explosion, killing 1 person on the spot, and the air separation tower was scrapped.
On March 2, 1996, in the 6000m3/h air separation plant of Jiangxi Xinyu Iron and Steel Plant, when no abnormal symptoms were found, the plate-fin type main condensing evaporator suddenly exploded and the equipment was seriously damaged. The shock wave shattered the glass of surrounding buildings.
On July 18, 1996, the main refrigeration of the 10,000m3/h air separation plant of the Harbin Gasification Plant Air Separation Plant exploded, and the main refrigeration and the upper tower were scrapped.
On May 16, 1997, a vicious explosion occurred in the 6000m3/h air separation tower of Fushun Ethylene Chemical Plant in Liaoning Province. The equipment plant was severely damaged, with 4 dead, 4 seriously injured and 27 lightly injured.
On December 25, 1997, a vicious explosion occurred in an 81760m3/h air separation plant of the Shell Petroleum Company in Bintulu, Malaysia. The explosion started from the main condensing evaporator and expanded to the tower body; the lower tower was pressed into the ground; the upper tower and the main cold were blown 750 meters away; the window frame was broken within 5 kilometers, and the splashing metal smashed the oil tank and The kerosene tank sparked the fire.
On August 21, 2000, an explosion occurred at the maintenance site of the 1500m3/h air separation unit of the oxygen production plant of Jiangxi Pingxiang Iron and Steel Company, resulting in 22 deaths, 7 serious injuries and 17 minor injuries.
On July 7, 2003, when the 10,000m3/h air separation plant of Shanghai COSCO Chemical was preparing to hoist the upper and upper sections of the thick argon column, a loud noise was heard. Fragments.
On August 22, 2003, the 20.00m3/h air separation plant in Maanshan Iron and Steel Oxygen Plant deflagrated during the installation process, and 35% of the personnel were ejected and burned. After rescue, they escaped from the emergency.
On September 17, 2003, during the installation of the 10,000m3/h air separation plant of Hunan Lengshui Iron and Steel Company, an air wave suddenly erupted, and the welder was knocked down and fell off the platform. He died in the rescue.
On April 10, 2017, a 4 million ton coal-to-liquid air separation accident occurred in Shenhua
The explosion occurred at 5:45 pm on July 19, 2019, at Unit C of the Air Separation Plant of Yima Gasification Plant of Henan Energy and Chemical Group, Sanmenxia City, Henan Province. The explosion killed 15 people, seriously injured 15 and hospitalized 256.
In recent years, with the enlargement of air separation plants, the explosion energy of air separation plants is also increasing. From the perspective of the principle of explosion, air separation plants can be divided into physical explosions and chemical explosions. Chemical explosions do more damage than physical explosions.
The reasons for the physical explosion of the air separation plant are as follows:
1. A large amount of high-temperature gas enters the fractionation tower containing low-temperature liquid, and the low-temperature liquid vaporizes rapidly, causing the pressure in the fractionation tower to rise, the pressure relief speed of the safety valve is slow, and the air separation is deformed and ruptured.
2. Air separation and cold box are the memory that the low-temperature liquid of the fractionation tower is filled with thousands of cubic perlite insulation materials. If the fractionation tower leaks and fails, a large amount of low-temperature liquid will be produced. Perlite is in the high-temperature gas, and the low-temperature liquid will evaporate rapidly, and the cold box will rapidly evaporate. Bursting ratio, spraying in large quantities into the surrounding, perlite term is called sandblasting or hydroblasting.
The reasons for chemical explosions in air separation plants are as follows:
1. 1% liquid oxygen is not discharged in time, and the hydrocarbon accumulation in liquid oxygen reaches the standard. Total hydrocarbons in liquid oxygen, especially acetylene, will react beyond the standard, causing chemical explosions. When the acetylene in liquid oxygen exceeds 0.5PPm or the total hydrocarbon content exceeds 300PPm, spontaneous combustion and explosion may occur.
2. The gas pipeline of the expansion seal is blocked, and the lubricating oil of the expander bearing penetrates into the air side through the oil seal, and is carried into the upper tower by the expanding air, causing the total hydrocarbon content of the cold liquid oxygen at the bottom of the upper tower to exceed the standard.
3. After the molecular sieve carbon dioxide analyzer fails, the molecular sieve cannot fully absorb carbon dioxide and total hydrocarbons due to the use of molecular sieve, over-temperature, regeneration, free water, feed poisoning, etc. Bottom and cold liquid oxygen total hydrocarbon content over list price caused.
4. For the free end bearing in the suction pipe of the air compressor, the sealing air pipe of the free end bearing is disconnected or blocked, and the negative pressure formed in the suction pipe will be placed in the bearing, the lubricating oil will inhale the air, and the molecular sieve will be poisoned, resulting in total hydrocarbons in the air. It will pass through the molecular sieve and enter the fractionation tower, causing the bottom liquid to be empty and the total hydrocarbon content of low temperature liquid oxygen to exceed the standard.
5. Due to the release of heterocyclic hydrocarbon 1#, heterocyclic hydrocarbon 2#, crude phenol, light crude benzene, sulfur, ammonium sulfate and other gases near the inlet of the air compressor from chemical plants or chemical vehicles, the air contains a large amount of total hydrocarbons. The high content of total hydrocarbons in the air inhaled by the air compressor will cause the total hydrocarbons to pass through the molecular sieve and enter the fractionation tower, causing the total hydrocarbon content of the liquid at the bottom of the lower tower and the main cooling liquid oxygen tower at the bottom to exceed the standard.
In view of the above risk factors, corresponding oxygen production control measures should be formulated:
1. The air inlet valve of the air separation tower must be operated at a slow speed, and the speed of the hot air entering the tower should be gradually adjusted according to the pressure change. After shutdown, be sure to close the valve entering the main heat exchanger.
2. When there is a liquid leakage failure in the tower, stop in time, open the sand loading port on the top of the tower, and discharge the pressure in the cold box. When the leakage is serious, evacuate the surrounding people to avoid being suffocated by pearlescent sand and buried.
3. The discharge of liquid oxygen should be increased by 1% in time according to the detection index, and the total hydrocarbon analyzer should be effective on a regular basis to ensure the accuracy of the data.
4. Pay more attention to the expansion and sealing gas to avoid entering wet air in the maintenance state, causing ice blockage.
5. The carbon dioxide analyzer after molecular sieve should take effect on a regular basis to ensure sensitive and accurate data. Excessive use of molecular sieves, over-temperature use, insufficient regeneration, entry of free water, oil poisoning and other accidents are strictly prohibited. Once the carbon dioxide exceeds the standard after the molecular sieve, the operation of the air separation equipment should be stopped immediately, and the molecular sieve should be regenerated.
6. The sealed air pipe of the free end bearing must be unblocked, and the maintenance equipment must not be disassembled or damaged.
7. There should be no volatile chemical products such as gasoline, paint, rubber, water, etc. near the suction port of the air compressor or in the west wind chamber. Chemical product vehicles must not stop or disperse near the compressor suction. Once a chemical product leak occurs near the suction port of the air compressor, the air separation plant will immediately shut down, clean up the leaked chemical product, and then turn on the air separation plant.
risk factors
External Risk Factors for Air Separation Plants
thunderbolt
Lightning phenomenon is one of the common natural phenomena in nature. Due to its uncertainty, transient nature and strong discharge, lightning will cause serious impact on all electrical equipment and pose a serious threat to the normal production and safe operation of air separation plants. Lightning strikes can cause grid fluctuations or blackouts. This will lead to power failure or damage to power equipment such as compressors and pumps; when the oil pump stops running, due to the lack of forced lubrication, it is easy to cause the bearing failure of the high-speed expander, or even a tile burning accident. The shutdown of the compressor will lead to the interruption of raw gas delivery to the rectifying tower, which will cause serious consequences; lightning strikes will damage the inductive DC proximity switch of the molecular sieve, resulting in the failure of the molecular sieve electric heater to start the interlock; lightning strikes will also damage the electrical and electronic equipment of the air separation plant. Cause damage, paralyze the central control system, and then shut down the air separation plant, resulting in the stop of subsequent production. In severe cases, accidents will occur with unimaginable consequences.
Oil
Air separation plants mainly use turbine oil and lubricating oil. The flash point (opening degree) of turbine oil is greater than 195°C, which belongs to Class C fire hazard flammable liquid. Once the oil circuit of the turbocharger turbo expander leaks, it will cause fire and explosion in case of high heat or open flame. The flash point (opening) of the lubricant is ≥ 230°C, which is a Class C flammable liquid fire hazard. Once the oil pipeline leaks, high heat or open flames, it will also cause fire and explosion.
Internal Risk Factors of Air Separation Plants
chemical explosion hazards
From the analysis of most cases of air separation plant explosions, chemical explosions account for the majority. There are three main factors for the formation of chemical explosions: one is combustibles, the other is combustibles, and the third is ignition sources. Therefore, the internal risk factors of air separation plants can be divided into the above three aspects.
fuel
In air separation plants, combustibles are mainly explosive and dangerous impurities such as hydrocarbons or oils. The raw air contains a certain amount of hydrocarbons, which have a low flash point and a wide explosion limit. Excessive accumulation of carbon and oxygen compounds in the air separation unit during the production process, if there is an explosion source, it is easy to cause an explosion. A large number of studies have shown that acetylene is the most important factor in the harmful impurities of air separation equipment. When there is too much lubricating oil in the piston air compressor and expander, some oil droplets or oil mist may enter the distillation column with the compressed air. The pressure of ordinary lubricating oil is 7MPa, and when the temperature is higher than 150℃, it is easy to crack into light fractions. Its boiling point is much lower than the original lubricating oil, easy to gasify and mix with oxygen. After the air separation plant is repaired, oil stains are likely to remain in the equipment.
Oxidizer
Oxygen and liquid oxygen are combustion-supporting substances and are classified as Class B fire hazard substances. They are one of the basic elements for the combustion and explosion of combustibles. They can oxidize most reactive substances and form explosive mixtures with flammable substances such as acetylene and methane. Liquid oxygen is a combustible chemical explosion in air separation equipment. When the concentration of combustibles in the air separation plant reaches the explosion condition, the combustible liquid oxygen or gas oxygen is prone to chemical explosion in the presence of the detonating source. Liquid oxygen is one of the necessary conditions for chemical explosions in air separation plants, and it is also one of the main products of production equipment. so,
set source
The main sources of detonation are: explosive impurity solid particles rubbing against each other or with the wall surface; electrostatic discharge; pressure pulse caused by air wave impact, fluid impact or cavitation, which makes the local pressure high and temperature rises; the presence of particularly strong chemically active substances Increased explosion susceptibility of combustible material mixtures in liquid oxygen. The following impurity risk factors can create detonation sources.
carbon dioxide
When liquid oxygen contains small amounts of ice particles and solid carbon dioxide, electrostatic charges are created. If the content of carbon dioxide is increased to 200-300*104%, the electrostatic potential energy generated will reach 3000V. At the same time, solid carbon dioxide will block the channels of liquid oxygen, resulting in "dead boiling", which will increase the concentration of carbon oxides in liquid oxygen. After reaching the explosive concentration, an explosion will occur as soon as there is an initiating source. The main reasons for the high CO2 content are: the molecular sieve is crushed due to long-term use of subsidence or airflow impact, the gap between the adsorption beds of the molecular sieve adsorber, and the short circuit of the airflow; the molecular sieve has a strong adsorption capacity for specific gases,
nitrous oxide
Nitrous oxide is not a flammable component, but the existence of nitrous oxide will not cause major safety accidents, but it has a high boiling point, low volatility, and low solubility, and is a blocking component. Dinitrogen is solid after precipitation, and it is easy to form "dry evaporation" or "dead corner" boiling and hydrocarbon accumulation. After reaching an explosive concentration, an explosion will occur once a detonation source exists. Common adsorbents (alumina, molecular sieves, and silica gel) only partially adsorb nitrous oxide.
liquid ozone
Liquid ozone (O3) is a dark blue liquid with strong chemical properties. Under normal circumstances, the gasification and decomposition of the liquid state sharply increases the partial pressure of oxygen, increasing the explosion sensitivity of the mixture in liquid oxygen. When the explosion rate is 100%, the energy required for detonation generally decreases by 30% to 45%. During the production process, when liquid oxygen passes through the valve of the air separation tower, it is subjected to friction and airflow impact for a long time. A small fraction of the liquid oxygen can be converted into liquid ozone under conditions that generate static electricity.
solid dust
Solid dust endangers the safety of air separation plants. Lightly block heat exchanger channels, reduce heat exchange efficiency, block rectification trays, and reduce product purity and yield; if the main cold plate oxygen channel is blocked, the concentration of hydrocarbon impurities in liquid oxygen and other harmful impurities in liquid oxygen accumulation will accelerate. It is an electrostatic discharge detonation source that causes a major cold burst. Solid dust mainly comes from the following aspects:
The air filter does not filter the dust in the atmosphere, so that it enters the air separation tower with the air. The aluminum rubber powder of the dryer of the air separation heating system enters the air separation tower with the air; the powder produced by the silica gel adsorber enters the tower together with liquid air and liquid oxygen for main cooling; the oxidation caused by the aluminum alloy pipes or containers in the air separation tower Aluminum powder enters the main cooling air separation plant due to corrosion and aging; careless manufacturing, installation and maintenance may cause dust, metal powder or pearlite to enter the container or pipeline, and finally enter the main cooling.
Physical explosion hazards
According to Appendix 1 of the Regulations on Safety and Technical Supervision of Pressure Vessels, the design pressure (P) of pressure vessels can be divided into four pressure levels: low pressure 0.1Mpa≤p<1.6Mpa, medium pressure 1.6Mpa≤p<10MP, A high pressure pressure 10Mpa ≤p<100Mpa, ultra-high pressure p>100Mpa. In an air separation plant, the highest working pressure of many units will be in the high pressure section. If the pressure of these devices exceeds the design allowable value or the pressure gauge fails, there is a risk of rupture, breakage and explosion. In addition, gas pressure piping can present similar hazards.
Risk factors for air compressors
Air compressor main performance risk factors
1. Dangerous factors of oil-lubricated air compressors
Early air separation plants used piston compressors whose cylinders were lubricated with mechanical oil. The cylinder oil of the air compressor is prone to carbon deposition at high temperatures, which gradually reduces the effective flow path of the exhaust pipe and increases the flow rate. When the flow rate exceeds the limit, the energy generated by the friction of the airflow can ignite the carbon deposit, which can cause the pipe to explode.
The cylinder oil or light fractions of the air compressor are brought into the molecular sieve purifier with the airflow, which will cause molecular sieve poisoning, reduce the adsorption capacity, and incompletely adsorb carbon dioxide. It not only blocks the plate-fin heat exchanger and affects the operation cycle, but also increases the carbon dioxide in the liquid oxygen, which gradually precipitates into an ice-like solid and rubs against the inner wall of the condensing evaporator to generate static electricity.
2. Hazardous factors of excessive axial position
During normal operation, the axial forces on both sides of the centrifugal compressor rotor impeller cancel each other out. The unbalanced part is reduced by the balance plate to reduce the axial thrust, and the rest is borne by the thrust bearing. When the axial force increases, or the thrust bearing is damaged and other factors, the shaft displacement will be seriously deviated.
Precautions against risk factors
Strengthen the management of air separation equipment
regular cleaning
When running for more than 2 years, the distillation tower and liquid oxygen circulation system should be cleaned and degreasing. The main cooling unit should be soaked for 8 hours. After cleaning, it should be thoroughly blown away with sufficient pressure air, and then fully heated and dried.
The unit resistance of liquid oxygen is large, and it is easy to generate static electricity. Thousands of volts of static electricity can be generated when not grounded. At the same time, the threat of lightning strikes to the air separation plant is also great, so it is necessary to regularly check the grounding of the air separation plant.
prevent oil from entering
If oil enters the air separation unit, it will contaminate the adsorbent and affect the adsorption of acetylene. Therefore, the roots blower that easily makes the air oily should be cancelled, and the overhaul and maintenance of the expander should be strengthened.
Strengthen the management of carbide slag
The residual acetylene in the carbide slag is very serious to air pollution, especially in cloudy and rainy days, it should be strictly managed and buried in a far place underground.
Strengthen operation and maintenance management
Care must be taken to remove harmful impurities; instruments and meters used for monitoring should be checked regularly; over-cycle operation should pay attention to stop heating and air blowing in time; strictly abide by process discipline, prevent illegal operations, and strictly implement the "four do not let go".
Strengthen the purification of the front end of the equipment
Strengthen raw material air quality control
The oxygen production area is in the upwind direction all the year round, more than 300m away from the acetylene power station, away from harmful gas sources, and strengthen the control of the original air quality. Once the pollution is serious, corresponding measures should be taken.
Removes harmful substances and prevents the accumulation of hydrocarbons
Give full play to the role of the liquid-gas-liquid-oxygen adsorber in removing harmful impurities, replace the adsorber strictly on schedule, control the heating regeneration temperature, and improve the adsorption efficiency; 1% of the product liquid oxygen is discharged from the main cooling to remove hydrocarbons; air separation is carried out regularly. Large heating to remove residual carbon dioxide and carbon oxide impurities accumulated in heat exchangers and rectification towers; liquid oxygen pumps have been put into operation for a long time. Molecular sieves have a poor adsorption effect on nitrous oxide. A layer of 5A molecular sieve can be added to the molecular sieve adsorber.
Establish a complete monitoring system and alarm system
High-precision detection instruments are used to realize online and offline monitoring of harmful impurities in air separation gas sources and equipment, including acetylene, methane, total carbon, carbon dioxide, nitrous oxide and other harmful substances. The air separation plant is equipped with a corresponding alarm system. When the environment deteriorates, the early warning system and effective measures can be activated to control the harmful substances within the standard range. Monitor the oil quality and content of the lubricating oil, ensure sufficient viscosity and stability, and ensure that the air at the outlet of the air compressor is free of oil.
in conclusion
There are many risk factors for air separation plants. "The hidden danger is in the open fire, and prevention is not as good as disaster relief." The work of preventing these unsafe factors cannot be slack, and any hidden dangers cannot be let go. First of all, it is necessary to take technical measures to control the content of hydrocarbon combustion carbon oxides in liquid oxygen to ensure that various indicators are within the required control range. The second is to strengthen the control of the source of the explosion, increase monitoring measures, and at the same time strengthen management and plug leaks, so as to avoid the occurrence of accidents.
