With the development and utilization of natural gas resources, in order to improve the environmental pollution caused by the burning of petroleum products, compressed natural gas (CNG) and liquefied natural gas (LNG) have been widely promoted and applied to meet the needs of natural gas fuels. However, natural gas has a fire and explosion hazard and must be given high priority.
　 1. Hazard characteristics of CNG and LNG
The main component of natural gas is methane, which is a Class I combustible gas. The fire hazard of Class A is 5% to 15% (volume fraction), the minimum ignition energy is only 0.28mJ, the burning speed is fast, and the combustion heat value is high (average calorific value). It is 33440kJ/m3), the relative density of air is 0.55, the diffusion coefficient is 0.196, it is very easy to burn and explode, and the diffusion ability is strong, and the fire spreads rapidly.
The hazards of CNG and LNG are mainly due to their flammable and explosive characteristics. Once a leak occurs, explosions and fires will occur at the same time, and the accident is difficult to control and rescue, and the loss will be very serious.
　 2. Fire accident prevention of CNG and LNG
In the process of pretreatment, liquefaction (or compression), storage, transportation, receiving, and gas station, the prevention of fire accidents is the key. Both CNG and LNG are volatile gases. When exposed to open flames, fires are likely to occur. When CNG is mixed with air, as long as the temperature reaches 650 °C, it will catch fire even if there is no fire source. Therefore, it is necessary to prevent leakage or volatilization of CNG and LNG. More importantly, keep CNG and LNG away from fire. There are a variety of ignition sources in operation: the equipment control system is a system that implements manual or automatic control of various equipments, potentially with electrical sparks; natural gas flows at high speed in the pipeline, which is prone to generate static electricity sources; improper use of tools in operation, or Friction, impact sparks, etc. caused by carelessness. The vast majority of CNG and LNG stations are located on the side of frequent traffic lanes with frequent traffic. The surrounding environment is complex and threatened by external ignition sources, such as flying fires near building chimneys, fires in nearby buildings, and frequent vehicles entering and leaving. Artificial pyrotechnics, lighter flames, mobile phone electromagnetic sparks, nailing shoes friction, impact sparks, static sparks generated by chemical fiber clothing, scattered sparks of firecrackers, lightning strikes, etc., can all become the ignition source of gas station fires. Therefore, it is necessary to strictly control the use of fire, prohibit fireworks and open flames, prevent friction from hitting and catching fire, and do not use electric welding or cutting during operation; the fire separation distance between compressor units and gas cylinder groups in CNG and LNG stations and surrounding buildings and structures should not be Less than (the specification of vehicle refueling and gas station design and construction) GB 50156-2002. The general layout of the gas station shall be carried out in accordance with the "Code for Fire Protection of Building Design" and "Code for Design of City Gas". Except for gas cylinders (gas storage wells), production buildings and necessary auxiliary facilities, it is not appropriate to arrange other buildings. Gas station production, office should be partitioned. The gas storage bottle group (gas storage well), compressor room, pressure regulating room, and gas filling machine in the gas station should be clearly separated and meet the spacing specified by the regulations.
3. CNG and LNG explosion accident prevention
The main component of natural gas is methane explosion limit of 5% to 15% (volume fraction), so if the CNG or LNG leaks or volatilizes in the air, the concentration reaches the explosion limit of methane, and it will explode in case of fire, causing incalculable loss. . The technology of the CNG filling station requires that the compressor of the filling station must be pressurized to 25 MPa or more in order to compress the natural gas into the cylinder. This is the highest pressure storage container for domestic combustible gases. If the cylinder quality or pressurization equipment does not meet the basic technical requirements, a slight oversight may result in an explosion or fire. The high pressure operation of the system is prone to overpressure, the system pressure exceeds the allowable pressure it can withstand, and eventually exceeds the strength limit of the equipment and accessories and explodes or partially bursts.
The classification of the explosion range of the explosion danger zone in the gas station shall be determined in accordance with the Code for Design and Construction of Automobile Refueling Stations GB 50156-2002. Use explosion-proof electrical equipment that is higher than or equal to the gas level of the corresponding work area in accordance with the provisions of GB 50058 for Design of Electrical Installations for Explosive and Fire Hazardous Environments. Mobile and portable appliances should be used with caution in explosion-hazardous areas. It is forbidden to use private connections and illegal use of electricity. In order to detect abnormal leakage of flammable gas in time, so that the staff can carry out leakage treatment as soon as possible to prevent or eliminate hidden dangers of explosion accidents, the gas station should be equipped with a flammable gas detection and alarm system. Combustible gas detectors should be installed in compressed gas storage tanks (sheds), natural gas pumps and compressor rooms (sheds). The alarm should be set centrally in the control room or in the duty room, and the operator can get an alarm in time. The selection and installation of combustible gas detectors and alarms shall comply with the relevant provisions of the national industry standard "Design Specification for Detection and Alarm of Combustible Gases and Toxic Gases in Petrochemical Enterprises" SH3063. The flammable gas detector alarm (high limit) setting value should be less than or equal to 25% of the flammable gas explosion lower limit concentration value.
Gas cylinders should use products that comply with relevant national regulations and standards. The gas station should use a large-volume gas storage bottle of the same specification and model. The large-capacity storage tank has the advantages of fewer bottle valves, less interfaces and high safety.
The gas storage tank (gas storage well) of the gas station should adopt open or semi-open reinforced concrete structure or steel structure, which is conducive to the diffusion and ventilation of flammable gas and increase the pressure relief ratio of the building. The roof should be adopted. Made of non-burning lightweight materials. When the gas storage bottle group (gas storage well) and the compressor, the pressure regulator, and the power distribution room are not able to meet the corresponding fire separation requirements, they should be separated by a reinforced concrete fire partition wall. The top of the partition wall should be better than the gas cylinder group. The height of the top of the (gas storage well) is 1m or more. The length of the partition wall should be the total length of the gas storage bottle group (gas storage well), and 2m and above should be added at both ends. The thickness of the partition wall should not be less than 0.2m, which can prevent each other when the accident occurs. influences. The firewall should be able to withstand certain explosion pressures.
In order to prevent the formation of explosive mixtures, rooms in the explosion hazardous area should be ventilated to prevent poisoning and explosions. When using natural ventilation, the total area of ​​the vents should not be less than 300cm2/m2 (ground), the vents should not be less than 2, and should be placed close to the easy accumulation of flammable gas, as uniform as possible, leaving no dead angle for flammable gas Can spread quickly. For buildings that may leak natural gas, the upward exhaust is dominant. When forced ventilation is used, the ventilation capacity of the ventilation equipment shall be calculated 15 times per hour during the operation of the process equipment, and 5 times per hour during the non-working period of the process equipment. The design pressure of all equipment, valves, pipes and fittings in the filling station should be 10% higher than the maximum working pressure and should not be lower than the initial working pressure of the safety valve under any circumstances.
In addition, in the case where the LNG leak encounters water (such as rainwater in the sump), due to the very high heat transfer rate between the water and the LNG, the LNG will violently boil with a loud sound and spray water mist. Causes an explosion of LNG vapor. This phenomenon is similar to the occurrence of water falling on a red-hot steel plate, which allows the water to evaporate immediately. To avoid this danger, the rainwater in the sump should be drained regularly.
　 1. Hazard characteristics of CNG and LNG
The main component of natural gas is methane, which is a Class I combustible gas. The fire hazard of Class A is 5% to 15% (volume fraction), the minimum ignition energy is only 0.28mJ, the burning speed is fast, and the combustion heat value is high (average calorific value). It is 33440kJ/m3), the relative density of air is 0.55, the diffusion coefficient is 0.196, it is very easy to burn and explode, and the diffusion ability is strong, and the fire spreads rapidly.
The hazards of CNG and LNG are mainly due to their flammable and explosive characteristics. Once a leak occurs, explosions and fires will occur at the same time, and the accident is difficult to control and rescue, and the loss will be very serious.
　 2. Fire accident prevention of CNG and LNG
In the process of pretreatment, liquefaction (or compression), storage, transportation, receiving, and gas station, the prevention of fire accidents is the key. Both CNG and LNG are volatile gases. When exposed to open flames, fires are likely to occur. When CNG is mixed with air, as long as the temperature reaches 650 °C, it will catch fire even if there is no fire source. Therefore, it is necessary to prevent leakage or volatilization of CNG and LNG. More importantly, keep CNG and LNG away from fire. There are a variety of ignition sources in operation: the equipment control system is a system that implements manual or automatic control of various equipments, potentially with electrical sparks; natural gas flows at high speed in the pipeline, which is prone to generate static electricity sources; improper use of tools in operation, or Friction, impact sparks, etc. caused by carelessness. The vast majority of CNG and LNG stations are located on the side of frequent traffic lanes with frequent traffic. The surrounding environment is complex and threatened by external ignition sources, such as flying fires near building chimneys, fires in nearby buildings, and frequent vehicles entering and leaving. Artificial pyrotechnics, lighter flames, mobile phone electromagnetic sparks, nailing shoes friction, impact sparks, static sparks generated by chemical fiber clothing, scattered sparks of firecrackers, lightning strikes, etc., can all become the ignition source of gas station fires. Therefore, it is necessary to strictly control the use of fire, prohibit fireworks and open flames, prevent friction from hitting and catching fire, and do not use electric welding or cutting during operation; the fire separation distance between compressor units and gas cylinder groups in CNG and LNG stations and surrounding buildings and structures should not be Less than (the specification of vehicle refueling and gas station design and construction) GB 50156-2002. The general layout of the gas station shall be carried out in accordance with the "Code for Fire Protection of Building Design" and "Code for Design of City Gas". Except for gas cylinders (gas storage wells), production buildings and necessary auxiliary facilities, it is not appropriate to arrange other buildings. Gas station production, office should be partitioned. The gas storage bottle group (gas storage well), compressor room, pressure regulating room, and gas filling machine in the gas station should be clearly separated and meet the spacing specified by the regulations.
3. CNG and LNG explosion accident prevention
The main component of natural gas is methane explosion limit of 5% to 15% (volume fraction), so if the CNG or LNG leaks or volatilizes in the air, the concentration reaches the explosion limit of methane, and it will explode in case of fire, causing incalculable loss. . The technology of the CNG filling station requires that the compressor of the filling station must be pressurized to 25 MPa or more in order to compress the natural gas into the cylinder. This is the highest pressure storage container for domestic combustible gases. If the cylinder quality or pressurization equipment does not meet the basic technical requirements, a slight oversight may result in an explosion or fire. The high pressure operation of the system is prone to overpressure, the system pressure exceeds the allowable pressure it can withstand, and eventually exceeds the strength limit of the equipment and accessories and explodes or partially bursts.
The classification of the explosion range of the explosion danger zone in the gas station shall be determined in accordance with the Code for Design and Construction of Automobile Refueling Stations GB 50156-2002. Use explosion-proof electrical equipment that is higher than or equal to the gas level of the corresponding work area in accordance with the provisions of GB 50058 for Design of Electrical Installations for Explosive and Fire Hazardous Environments. Mobile and portable appliances should be used with caution in explosion-hazardous areas. It is forbidden to use private connections and illegal use of electricity. In order to detect abnormal leakage of flammable gas in time, so that the staff can carry out leakage treatment as soon as possible to prevent or eliminate hidden dangers of explosion accidents, the gas station should be equipped with a flammable gas detection and alarm system. Combustible gas detectors should be installed in compressed gas storage tanks (sheds), natural gas pumps and compressor rooms (sheds). The alarm should be set centrally in the control room or in the duty room, and the operator can get an alarm in time. The selection and installation of combustible gas detectors and alarms shall comply with the relevant provisions of the national industry standard "Design Specification for Detection and Alarm of Combustible Gases and Toxic Gases in Petrochemical Enterprises" SH3063. The flammable gas detector alarm (high limit) setting value should be less than or equal to 25% of the flammable gas explosion lower limit concentration value.
Gas cylinders should use products that comply with relevant national regulations and standards. The gas station should use a large-volume gas storage bottle of the same specification and model. The large-capacity storage tank has the advantages of fewer bottle valves, less interfaces and high safety.
The gas storage tank (gas storage well) of the gas station should adopt open or semi-open reinforced concrete structure or steel structure, which is conducive to the diffusion and ventilation of flammable gas and increase the pressure relief ratio of the building. The roof should be adopted. Made of non-burning lightweight materials. When the gas storage bottle group (gas storage well) and the compressor, the pressure regulator, and the power distribution room are not able to meet the corresponding fire separation requirements, they should be separated by a reinforced concrete fire partition wall. The top of the partition wall should be better than the gas cylinder group. The height of the top of the (gas storage well) is 1m or more. The length of the partition wall should be the total length of the gas storage bottle group (gas storage well), and 2m and above should be added at both ends. The thickness of the partition wall should not be less than 0.2m, which can prevent each other when the accident occurs. influences. The firewall should be able to withstand certain explosion pressures.
In order to prevent the formation of explosive mixtures, rooms in the explosion hazardous area should be ventilated to prevent poisoning and explosions. When using natural ventilation, the total area of ​​the vents should not be less than 300cm2/m2 (ground), the vents should not be less than 2, and should be placed close to the easy accumulation of flammable gas, as uniform as possible, leaving no dead angle for flammable gas Can spread quickly. For buildings that may leak natural gas, the upward exhaust is dominant. When forced ventilation is used, the ventilation capacity of the ventilation equipment shall be calculated 15 times per hour during the operation of the process equipment, and 5 times per hour during the non-working period of the process equipment. The design pressure of all equipment, valves, pipes and fittings in the filling station should be 10% higher than the maximum working pressure and should not be lower than the initial working pressure of the safety valve under any circumstances.
In addition, in the case where the LNG leak encounters water (such as rainwater in the sump), due to the very high heat transfer rate between the water and the LNG, the LNG will violently boil with a loud sound and spray water mist. Causes an explosion of LNG vapor. This phenomenon is similar to the occurrence of water falling on a red-hot steel plate, which allows the water to evaporate immediately. To avoid this danger, the rainwater in the sump should be drained regularly.
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