Explosion accident of air separation unit at home and abroad

On January 4, 1961, an air separation tower of a 4000m3/h air separation equipment in the former Federal Republic of Germany exploded, killing 15 people and seriously damaging the equipment and buildings.

On November 23, 1973, the typical malignant explosion of the 3350m3/h air separation unit in Anshan Iron and Steel Oxyoxygen Plant occurred outside the tower, which also caused the explosion of the air separation base inside the tower. The equipment was destroyed in many places, and the production was resumed after six months of maintenance.

On July 27, 1986, the 3200m3/h air separation equipment of Qianjin Chemical Plant of Yanshan Petrochemical Company made a loud noise, and the whole equipment became a pile of ruins.

On November 1, 1992, an explosion occurred in the 150m3/h air separation tower of the oxygen station of Lanzhou Petrochemical Machinery Plant, resulting in the death of one person and the abandonment of the air separation tower.

On July 25, 1993, the main condensing evaporator of 150m3/h air separation tower of Jinchuan Nonferrous Metals Company in Gansu Province suffered a crushing explosion, which killed one person on the spot and scrapped the air separation tower.

On March 2, 1996, the 6000m3/h air separation equipment in Xinyu Iron and Steel Plant of Jiangxi Province, under the condition that no abnormal symptoms were found, the plate fin main condensing evaporator suddenly exploded, and the equipment was seriously damaged. The blast wave shattered the glass of the surrounding buildings.

On July 18, 1996, the main cooling of the 10000m3/h air separation equipment in the air separation branch of Harbin Gasification Plant exploded, and the main cooling and upper tower were scrapped.

On May 16, 1997, a malignant explosion occurred in the 6000m3/h air separation tower of Fushun ethylene chemical plant in Liaoning Province. The equipment and plant were seriously damaged,4 people died,4 people were seriously injured and 27 people were slightly injured.

On December 25, 1997, a set of 81760m3/h air separation equipment of Malaysia Bintulu Shell Oil Company suffered a malignant explosion. The explosion started in the main condensation evaporator and expanded to the tower body; The lower tower is pressed into the ground; The upper tower and the main cold were blown 750 meters away; Window frames shattered within 5km and flying metal smashed oil and kerosene tanks, sparking the fire.

August 21, 2000, Jiangxi Pingxiang Iron and Steel Company oxygen plant 1500m3/h air separation unit maintenance site of an explosion accident, resulting in 22 deaths, 7 serious injuries, 17 minor injuries.

On July 7, 2003, when the 10000m3/h air separation equipment of Shanghai Cosco Chemical Co., Ltd. was preparing to hoist the upper column and the upper section of the coarse argon column, there was a loud noise, and the two-layer plastic color strip on the upper seal of the lower section of the coarse argon column was torn to pieces by the air waves.

On August 22, 2003, the 20,00m3 /h air separation equipment in the oxygen plant of Maanshan Iron and Steel Corporation deflagrate occurred during installation, and people were ejected and 35% burned. After rescue, they escaped from the emergency.

On September 17, 2003, during the installation of the 10000m3/h air separation equipment of Lengshui Iron and Steel Company in Hunan, a blast of gas burst out, and the welder was knocked out and fell off the platform. He died in the rescue.

On April 10, 2017, an air separation accident occurred in Shenhua with 4 million tons of coal-to-oil

The explosion occurred at 5:45 PM on July 19, 2019, in the air separation unit C of the Yima Gasification Plant of Henan Energy and Chemical Group in Sanmenxia, Henan Province. The explosion resulted in 15 deaths, 15 serious injuries and 256 hospitalizations.

In recent years, with the large-scale of air separation equipment, the explosive energy of air separation equipment is also increasing. From the principle of explosion, air separation equipment can be divided into physical explosion and chemical explosion. Chemical explosions cause more harm than physical explosions.

Causes of physical explosion of air separation equipment are as follows:

1. A large amount of high temperature gas enters the fractionation tower containing cryogenic liquid, and the cryogenic liquid rapidly vaporizes, resulting in increased pressure in the fractionation tower, slow pressure relief speed of the safety valve, and deformation and rupture of the air separation tower.

2, air separation, cold box is memory is cryogenic liquid of fractionating column filled with thousands of cubic perlite insulation materials, fractionating column leakage fault occurs, will have a lot of cryogenic liquid, perlite in high temperature gas, cryogenic liquid evaporation sharply, the ratio of cold box burst, a large number of spray into the surrounding, perlite terminology called sand blasting or hydraulic blasting.

The causes of chemical explosion in air separation equipment are as follows:

1. 1% liquid oxygen is not discharged in time, and the hydrocarbon accumulation in liquid oxygen reaches the standard. The total hydrocarbon in liquid oxygen, especially acetylene, will exceed the standard reaction, causing chemical explosion. When acetylene in liquid oxygen exceeds 0.5PPm or the total hydrocarbon content exceeds 300PPm, spontaneous combustion explosion may occur.

2. The expansion secret sealing gas pipeline is blocked, and the lubricating oil of the expander bearing seeps into the air side through the oil seal, and is carried into the upper tower by the expanding air, resulting in the total hydrocarbon content in the main cold liquid oxygen at the bottom of the upper tower exceeding the standard.

3, molecular sieve carbon dioxide analyzer after failure, and molecular sieves are used, overtemperature, regeneration, into the free water, feed poisoning and other reasons, cannot be completely adsorbed carbon dioxide and total hydrocarbon, hydrocarbon through the molecular sieve into the fractionating column, caused by the bottom liquid empty on the bottom and the cold liquid oxygen total hydrocarbon content exceeds bid.

4, for the free end bearing within the suction duct of air compressor, the free end bearing seal trachea disconnect or congestion, of the formation of negative pressure in the suction duct will place within the bearing, lubricating oil suction air, molecular sieve poisoning, caused by the total hydrocarbons in the air will be through the molecular sieve, into the fractionating column, caused by the bottom liquid empty on the bottom and the cold liquid oxygen total hydrocarbon content exceeds bid.

5. Because chemical plants or chemical vehicles release heterocyclic hydrocarbon 1#, heterocyclic hydrocarbon 2#, crude phenol, light crude benzene, sulfur, ammonium sulfate and other gases near the air compressor inlet, the air contains a large number of total hydrocarbons. Air compressor inhalation of high content of total hydrocarbons in the air, will cause the total hydrocarbons will pass through the molecular sieve, into the fractionation tower, resulting in the liquid at the bottom of the lower tower and the main cold liquid oxygen at the bottom of the tower total hydrocarbon content exceeds the standard.

 

 

In view of the above dangerous factors, corresponding control measures should be formulated for oxygen production:

1. The inlet valve of the air separation tower must be operated slowly, and the speed of hot air entering the tower should be gradually adjusted according to the pressure change. After stopping, be sure to close the valve into the main heat exchanger.

2. When the liquid leakage fault occurs in the tower, stop the car in time and open the sand loading port on the top of the tower to discharge the pressure of the cold box. When the leakage situation is serious, evacuate the surrounding people to avoid being buried by pearlescent sand asphyxiation.

3, 1% liquid oxygen emissions should be timely increased according to the test indicators, the total hydrocarbon analyzer should be regularly effective, to ensure the accuracy of the data.

4. The expansion secret sealing gas should be paid more attention to to avoid entering the wet air in the maintenance state, causing icing and blocking.

5, molecular sieve after carbon dioxide analyzer should be regular effect, to ensure sensitive and accurate data. Molecular sieve is strictly prohibited from overuse, overtemperature use, insufficient regeneration, into free water, oil poisoning and other accidents. Once the occurrence of molecular sieve after carbon dioxide exceed the standard phenomenon, immediately stop the operation of air separation equipment, regenerate molecular sieve.

6. The sealing air pipe of the free end bearing must be unblocked, and the maintenance equipment shall not be dismantled or broken.

7. Volatile chemical products such as gasoline, paint, rubber and water shall not exist near the suction inlet of the air compressor or in the westerly wind room. Vehicles of chemical products shall not stay or disperse near the suction inlet of the compressor. Once the chemical product leakage occurs near the suction inlet of the air compressor, the air separation equipment will stop immediately and clean up the chemical product leakage before opening the air separation equipment.

Risk factors for

External risk factors of air separation equipment

Thunder and lightning

Lightning phenomenon is one of the common natural phenomena in nature. Because of its uncertainty, instantaneous and strong discharge, lightning can cause serious impact on all electrical equipment and pose serious threat to the normal production and safe operation of air separation equipment. Lightning strikes can cause grid swings or power outages. This will cause power equipment such as compressors, pumps, etc., outage or damage; When the oil pump is out of operation, it is easy to cause the bearing failure of the high-speed expander due to the lack of forced lubrication, and even the accident of burning the tile. The shutdown of the compressor leads to the interruption of the raw air conveying to the rectifying tower, causing serious consequences; Lightning strike can cause the damage of the inductive DC proximity switch of the molecular sieve, causing the electric heater of the molecular sieve to be unable to start due to interlocking; Lightning strike can also cause damage to the electronic and electrical equipment of the air separation device, and the central control system can be paralyzed, and then the air separation equipment can be stopped, leading to the stop of the subsequent production. In serious cases, unexpected accidents can be caused and the consequences are unimaginable.

oil

Air separation equipment mainly uses turbine oil and lubricating oil. The flash point (opening) of the turbine oil is more than 195℃, which belongs to Class C fire risk flammable liquid. Once the oil pipeline of the turbocharged turbine expander leaks, it will cause fire and explosion in case of high heat or open fire. Lubricant flash point (opening) ≥230℃, belongs to the Class C fire risk of combustible liquid, once the oil pipeline leakage, high heat or open fire, will also cause fire and explosion.

Internal risk factors of air separation equipment

Risk factors of chemical explosion

From the analysis of most cases of air separation equipment explosion, chemical explosion is the majority. There are three main factors to form chemical explosion: one is combustible, the other is combustible, the third is the source of ignition. Therefore, the internal risk factors of air separation equipment can be divided into the above three aspects.

fuel

In air separation equipment, the combustible material is mainly explosive and dangerous impurity such as hydrocarbon or oil. The raw material air contains a certain amount of hydrocarbons, which have a very low flash point and a wide explosion limit. Excessive accumulation of carbon and oxygen compounds in the air separation device in the production process is easy to cause explosion if there is a detonating source. A large number of studies have shown that acetylene is the most important factor in the harmful impurities of air separation equipment. When the lubricating oil of piston air compressor and expander is too much, some oil droplets or oil mist may enter the rectifying tower with compressed air. When the pressure of ordinary lubricating oil is 7MPa and the temperature is higher than 150℃, it is easy to crack into light fraction. Its boiling point is much lower than the original lubricating oil, and it is easy to gasify and mix with oxygen. After the maintenance of air separation equipment, oil pollution is also easy to be left in the equipment.

oxidizer

Oxygen and liquid oxygen are combustion-supporting substances, which are class-B fire risk substances. They are one of the basic elements of combustible combustion and explosion. They can oxidize most active substances and form explosive mixtures with combustible substances such as acetylene and methane. Liquid oxygen is a combustible for chemical explosion in air separation equipment. When the concentration of combustibles in the air separation equipment reaches the explosion condition, the combustibles liquid oxygen or gas oxygen are prone to chemical explosion in the presence of the detonating source. Liquid oxygen is one of the essential conditions for the chemical explosion of the air separation equipment, but it is also one of the main products of the production equipment. Therefore, the problem of chemical explosion prevention of air separation equipment mainly lies in the combustible and detonating source.

Set the source

The main sources of detonation are: explosive impurity solid particles friction with each other or with the wall friction; Electrostatic discharge; Pressure pulse caused by gas wave shock, fluid shock or cavitation causes local pressure to be high and temperature to rise; The presence of particularly strong chemically active substances increases the explosive sensitivity of combustible material mixtures in liquid oxygen. The following impurities risk factors can create a detonating source.

Carbon dioxide

When the liquid oxygen contains a small amount of ice particles and solid carbon dioxide, it will produce static charge. If the content of carbon dioxide is raised to 200-300* 104%, the electrostatic potential generated can reach 3000V. At the same time, solid carbon dioxide can block the channel of liquid oxygen and lead to "dead-end boiling", so that the concentration of carbon and oxygen compounds in liquid oxygen increases continuously. After reaching the explosive concentration, an explosion will occur once there is a detonating source. The main reasons for the high C02 content are as follows: molecular sieve is crushed due to long-term use of sinking or airflow impact, molecular sieve adsorber adsorption bed gap, airflow short circuit; Molecular sieve has a strong adsorption capacity for specific gases, but it has a suitable temperature work area, intake temperature beyond this work area, will lead to molecular sieve adsorption capacity of C02 decreased.

Nitrous oxide

Nitrous oxide does not belong to flammable components, only the existence of nitrous oxide does not result in a major security incidents, but it has high boiling point, low volatility, low solubility, belongs to the blocking component, once in the cold for some reason nitrous oxide in solid state after precipitation, easy to form a "dry evaporate" or "dead end" boiling and hydrocarbon accumulation, after reach explosion concentration, once detonation source exists that can produce explosion. The common adsorbents (alumina, molecular sieves and silica gel) can only partially adsorb nitrous oxide.

Liquid ozone

Liquid ozone (O3) is a kind of dark blue liquid, belongs to the strong chemical characteristics of the material, under normal conditions, the liquid gasification, decomposition, so that the partial pressure of oxygen sharply increased, increased the explosive sensitivity of the mixture of flammable substances in liquid oxygen, in the outbreak rate of 100%, the energy required to detonate the general decline of 30% ~ 45%. In the production process, when liquid oxygen passes through the valve of air separation tower, it is subjected to friction and airflow impact for a long time. Under the condition of electrostatic action generated, a small part of liquid oxygen can be turned into liquid ozone.

Solid dust

Solid dust is harmful to the safety of air separation equipment. Light blockage heat exchanger channel, reduce heat transfer efficiency, blockage rectification tower plate, reduce product purity and yield; If the main cold plate oxygen channel is blocked, the concentration of hydrocarbon impurities in liquid oxygen and the accumulation of other harmful impurities in liquid oxygen are accelerated. It is a kind of electrostatic discharge detonating source, will cause the main cold explosion. Solid dust mainly comes from the following aspects:

The air filter does not filter the dust in the atmosphere and make it enter the air separation tower with the air. The aluminum rubber powder of the air separation heating system drier enters the air separation tower with the air; Powder produced by silica gel adsorber with liquid air and liquid oxygen into the tower and the main cooling; Alumina powder produced by aluminum alloy pipes or containers in the air separation tower enters the main cooling air separation equipment due to corrosion and aging; Careless manufacturing, installation and maintenance may cause dust, metal powder or pearlite to enter the container or pipe, and eventually into the main cold.

Physical explosion risk factors

According to Appendix I of Pressure Vessel Safety Technical Supervision Regulations, the design pressure (P) of pressure vessels can be divided into four pressure grades: low pressure 0.1Mpa ≤ p < 1.6Mpa, medium pressure 1.6Mpa ≤ p <10MP, A high pressure 10 Mpa≤ p < 100Mpa, and ultra-high pressure p> 100Mpa. In air separation equipment, the highest working pressure of many devices will be in the high pressure section. If the pressure of these devices exceeds the design allowable value or the pressure gauge fails, there will be the danger of cracks, breakage and explosion. In addition, the gas pressure pipeline can also exist in a similar danger.

Risk factors of air compressor

The main performance of air compressor risk factors

1. Risk factors of oil lubrication air compressor

Early air separation equipment used piston compressors, whose cylinders were lubricated with mechanical oil. The cylinder oil of the air compressor is easy to produce carbon formation at high temperature, which makes the effective flow diameter of the air discharge pipeline shrink gradually and the flow rate increase continuously. When the flow rate exceeds the limit, the energy generated by the friction of the air stream ignites the carbon formation, which will lead to the explosion of the pipeline.

The cylinder oil or light fraction of the air compressor is brought into the molecular sieve purifier with the air flow, which will cause molecular sieve poisoning, lower adsorption capacity and incomplete adsorption of carbon dioxide. It not only blocks the plate-fin heat exchanger, affects the operation cycle, but also increases the carbon dioxide in the liquid oxygen, and gradually precipitates into ice-like solid and frictions with the inner wall of the condensing evaporator, generating static electricity.

2. Hazard factors of excessive shaft position

Under normal operation, the axial forces on both sides of the rotor impeller of the centrifugal compressor cancel each other out. The unbalanced part is reduced by the balance plate to reduce the axial push, and the rest is borne by the thrust bearing. When the axial force is increased, or the thrust bearing is damaged, and other factors will lead to serious deviation of the shaft displacement.

Preventive measures for risk factors

Strengthening the management of air separation equipment

Regular cleaning

When running for 2 years or longer, the distillation tower and the liquid oxygen circulation system should be cleaned and degreased. The main cooling unit should be soaked for 8 h. After cleaning, it should be thoroughly blown away with air of sufficient pressure, and then fully heated and dried.

The unit resistance of liquid oxygen is large, which is easy to generate static electricity. It can generate thousands of volts of static electricity when it is not grounded. At the same time, lightning strikes are also a great threat to air separation equipment, so it is necessary to check the grounding of air separation device regularly.

Prevent oil from entering

If the oil is brought into the air separation device, it will pollute the adsorbent and affect the adsorption of acetylene. Therefore, Roots fan which is easy to make the air with oil should be cancelled and the overhaul and maintenance of the expansion machine should be strengthened.

Strengthening the management of calcium carbide slag

The residual acetylene in calcium carbide slag is very serious to air pollution, especially in rainy days, so it should be strictly managed and buried far underground.

Strengthen operation, maintenance and management

To remove harmful impurities to be careful link; Instruments and meters used for monitoring should be checked regularly; Super-cycle operation should be careful to stop in time for heating and blowing; Strictly abide by the process discipline, put an end to illegal operations, strictly implement the "four not let go".

Strengthen the purification of the front end of the equipment

Strengthen raw material air quality control

The oxygen production area should be in the upwind direction all the year round, more than 300m away from the acetylene generating station, away from harmful gas sources, strengthen the control of raw air quality, once the pollution is serious, corresponding measures should be taken.

Remove harmful substances to prevent the accumulation of hydrocarbons

Give full play to the function of liquid-air and liquid-oxygen adsorber to remove harmful impurities, strictly change the adsorber on schedule and control the heating and regeneration temperature to improve the adsorption efficiency; 1% liquid oxygen of the product is discharged from the main cold to remove hydrocarbons; Large heating of air separation is carried out regularly to remove residual carbon dioxide and carbon oxide impurities accumulated in the heat exchanger and rectification tower; The liquid oxygen pump is put into operation for a long time. The adsorption effect of nitrous oxide by molecular sieve is not good. A layer of 5A molecular sieve can be added in the molecular sieve adsorber.

Establish a sound monitoring system and alarm system

Adopt high, fine and sharp detection instrument to realize online and offline monitoring of harmful impurities in air separation gas source and equipment, including acetylene, methane, total carbon, carbon dioxide, nitrous oxide and other harmful substances. The air separation equipment is equipped with corresponding alarm system, so that when the environment deteriorates, the early warning system and effective measures can be started to control the harmful substances within the standard. Monitor the oil quality and content of the lubricating oil to ensure sufficient viscosity and stability, and ensure that the air at the outlet of the air compressor does not carry oil.

conclusion

There are many dangerous factors of air separation equipment. "Hidden danger is in open fire, and prevention is better than disaster relief". The work of preventing these unsafe factors should not be slacking off, and any hidden danger should not be let go. Firstly, it is required to take technical measures to control the content of hydrocarbon burning carbon and oxygen compounds in liquid oxygen to ensure that the indicators are within the required control range. The second is to strengthen the control of the source of the explosion and increase monitoring measures, while strengthening management and plugging loopholes, only in this way can the accident be avoided.