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The key to explosion protection in the piping network of an RTO exhaust gas collection and treatment system lies in the fact that the gas condensers in the reaction vessels, distillation (rectification) units or vacuum systems, as well as in the storage tanks or intermediate tanks, have sufficient condensation capacity. This ensures that the concentration of exhaust gases entering the RTO piping network remains below its lower explosive limit, and that reliable detection and alarm systems are in place when the concentration approaches this limit. How to determine that the condenser has sufficient condensation capacity and what reliable monitoring and alarm systems to use is a process safety technical issue that many of us have not explored thoroughly. To detect whether the exhaust gas concentration in the RTO pipeline network is below its lower explosive limit, people usually think of installing an online combustible gas detection system. Unfortunately, due to the complex composition of exhaust gases, high humidity, the presence of corrosive substances, and high concentrations of organic vapors, expensive online combustible gas detectors often have a short lifespan and are essentially useless in practice. In 2020, when we conducted an explosion-proof analysis and evaluation of the exhaust gas systems of two styrene storage tanks (one with a capacity of 5000 m3 and another with 8500 m3) at a chemical plant in the Ningbo petrochemical area, we found that at the flash point, the vapor concentration of the flammable liquid is close to its lower explosive limit. The storage of styrene in these tanks is carried out at temperatures below 30°C, with compressed air being used at the bottom to maintain the activity of the polymerization inhibitors; the vapor pipelines from these two tanks are directly connected to the main pipeline leading to the incinerator. When the temperature of the exhaust gas at the condenser outlet is below the flash point, its concentration is below its lower explosive limit, classifying it as an intrinsically safe exhaust gas. By installing temperature detection, alarm, and interlock devices at the outlet of the condenser, we can control any process deviations in the reaction system, vacuum pumping system, and condensation system. This allows us to prevent accidental flash explosions of exhaust gases in exhaust gas collection and treatment systems such as RTOs at their source. Taking styrene as an example: Styrene has a flash point of 32°C and an explosion lower limit of 1.1%. The saturated vapor pressure of styrene at 30°C is 1.0985 KPa, while the standard atmospheric pressure is 101.3 KPa; therefore, the styrene concentration in the gas phase space of the styrene storage tank is 1.0985/101.3 = 1.084%. The saturated vapor pressure of styrene at 32°C is 1.2328 KPa, and the styrene concentration in the gas space of the styrene storage tank is 1.2328/101.3 = 1.21%. It can be seen from the definitions of flash point and lower explosive limit as well as the above calculation examples that: ① According to the definitions, the flash point is the temperature below which combustion does not occur, while the lower explosive limit is the concentration below which an explosion does not take place. Since there is no strict distinction between combustion and explosion, for flammable liquids, we can consider the flash phenomenon at the flash point to be a type of explosion as well; the vapor concentration of a flammable liquid at its flash point represents its lower explosive limit. ② The temperature at which a flammable liquid reaches its lower explosion limit is not exactly the same as its flash point, but it is quite close; the discrepancy may be due to differences in testing methods and experimental errors. The condenser outlet temperature setting: theoretically, it should be below the flash point to prevent the formation of an explosive mixture ； However, since the flash point is a value determined through laboratory tests and is always subject to errors, it is safer to use the flash point minus 15°C as a basis for control (some also use a reduction of 9°C, depending on the specific manufacturing process). When conducting explosion-proof safety analysis on the exhaust gases at the outlet of the gas condensers in reaction vessels, distillation (rectification) units or vacuum systems, as well as storage tanks or intermediate tanks, we can consider the vapor concentration of a flammable liquid at its flash point to be its lower explosive limit. Considering safety margins and from an energy-saving perspective, we can set the condenser outlet temperature at the flash point minus 9°C as the process control value. For styrene storage tanks, it is advisable to keep the material temperature below 23°C. Some Class A flammable liquids have very low flash points; for example, the flash point of propane is -20°C. To cool propane exhaust gas to below -29°C, many companies may not have such cooling agents available. When the outlet temperature of the condenser cannot be kept below the flash point minus 9°C, the concentration of the exhaust gas at the condenser outlet exceeds its lower explosive limit; in the presence of air, and triggered by ignition sources such as static electricity, a flash explosion of the exhaust gas may occur. Under such circumstances, we should take further process safety measures for the exhaust gas at the condenser outlet, such as inerting, washing, and changing the exhaust gas treatment methods, in order to control the risks associated with exhaust gas collection and treatment. An illustrative analysis is as follows: Physical and chemical properties of the medium and corresponding safety measures. Propylene has a flash point of -20 °C and a lower explosive limit of 2.5% (V/V) ； It is soluble in water and organic solvents such as methanol, ethanol, ether, chloroform, and pyridine. ① Inerting: the system should be free of oxygen ； ② Taking advantage of the fact that waste gases such as acetone, furan, acetaldehyde, and propylene oxide are readily soluble in water, the waste gas at the condenser outlet is washed with water. This reduces the concentration of the waste gas below its lower explosive limit before it is discharged into waste gas collection and treatment systems such as RTOs. ③ Alternatively, the exhaust gas at the condenser outlet passes through a flame arrester or water seal before entering a specialized fresh air dilution device, such as a suction hood system, to rapidly reduce its concentration below the lower explosive limit, and then it is sent to waste gas collection and treatment systems such as an RTO. **Furan flash point: -20°C, lower explosive limit: 1.5% (V/V) ； Soluble in water, ethanol, ether, acetone, benzene, and most other organic solvents. Acetaldehyde: flash point -39°C, explosive limit 4.0% (V/V) ； It is miscible with water, ethanol, ether, chloroform, etc. Propylene oxide has a flash point of -37°C and an explosion limit of 2.3% (V/V); it is soluble in water as well as in organic solvents such as ethanol and ether. Ethyl acetate has a flash point of -4°C and an explosion limit of 2.2% (V/V) ； Slightly soluble in water, soluble in alcohol, ether, chloroform, and most other organic solvents. ① Inerting: the system should be free of oxygen ； ② The exhaust gas at the condenser outlet does not enter exhaust gas collection and treatment systems such as RTOs; instead, after passing through a water seal, it is sent to flares, boilers, heaters, etc. where it is burned as fuel. ③ Alternatively, the exhaust gas at the condenser outlet passes through a flame arrester or a water seal before entering a dedicated fresh air dilution device, such as a suction hood setup, to rapidly reduce its concentration below the lower explosive limit, and then it enters exhaust gas collection and treatment systems such as an RTO. Vinyl acetate flash point: -8°C; explosive limit: 2.6% (V/V) ； Slightly soluble in water, soluble in alcohol, ether, acetone, benzene, and chloroform. Benzene flash point: -11°C, explosion limit: 1.2% (V/V) ； Slightly soluble in water, miscible with ethanol, ether, propane, carbon tetrachloride, carbon disulfide, and acetic acid. Conclusion (1) The sudden gas explosions in waste gas collection and treatment systems such as RTOs are mainly caused by the discharge of waste gas with a concentration above the lower explosion limit into the pipeline network, which then leads to gas explosions due to static electricity (either from the pipes or from the rubber connectors of the exhaust fans) or open flames in furnaces like RTOs. The waste gas with a concentration higher than the lower explosion limit is mostly high-concentration waste gas resulting from condenser failures or insufficient condensation capacity, such as material backflow from reaction vessels or vacuum systems, as well as gases from reaction vessels, distillation (rectification) units or vacuum systems, storage tanks or intermediate tanks. (2) When conducting explosion-proof safety analysis on the exhaust gases from gas condensers in reaction vessels, distillation (rectification) units or vacuum systems, as well as storage tanks or intermediate tanks, considering that the vapor concentration of flammable liquids at their flash point is very close to their lower explosive limit, and taking into account safety margins and energy savings, the outlet temperature of the condenser can be set at the flash point minus 9°C as the process control value. A detection and alarm system for the outlet temperature of the condenser should also be installed, with high-limit alarms and interlock shutdown at even higher limits, in order to ensure the intrinsic safety of waste gas collection and treatment systems such as RTOs. (3) When constrained by conditions such as a very low flash point or the absence of suitable cryogenic refrigerants, that is, when the temperature at the outlet of the condenser cannot be kept below the flash point minus 9°C and the concentration of the exhaust gas at that outlet exceeds the lower explosive limit, such exhaust gas must not be directly fed into exhaust gas collection and treatment systems such as RTOs. Additional safety measures must be taken for the exhaust gas at the condenser outlet, such as inerting, washing with water, dilution with fresh air, or changing the method of exhaust gas treatment, in order to control the process safety risks associated with exhaust gas collection and treatment. (4) When conducting an explosion-proof safety assessment of waste gas collection and treatment systems such as RTOs, it is also necessary to conduct a comprehensive analysis of each waste gas source based on the \"three elements of fire and explosion\" principle, considering factors such as the process itself, the waste gas generation process and temperature control methods, flash point, the presence of air, the material used for waste gas pipelines, whether there is a risk of backfire in these pipelines, waste gas pretreatment methods, and the temperature of refrigerants. In particular, it is important to identify process deviations such as uncontrolled reactions, material surges, and condenser failures, in order to ensure the process safety of waste gas collection and treatment systems like RTOs.