Classification and characteristics of media in pressure vessels

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Classification and Characteristics of Media in Pressure Vessels I. Classification of Media 1. Classification by flammability (1) Explosive limit: Combustible gases, vapors of combustible liquids, or mixtures of combustible dusts with air can explode, but not at any proportion; rather, within a specific concentration range, different combustible substances have their own particular concentration ranges. This fixed range is commonly referred to as the explosion range or explosive limit of that substance, and it is usually expressed as the volume percentage of combustible gases, vapors of combustible liquids, or combustible dusts in air. The lowest concentration at which an explosion can occur is called the lower explosive limit, while the highest concentration is known as the upper explosive limit. The wider the range of the explosion limit, the lower the lower explosion limit, and the greater the explosion hazard. (2) Minimum ignition energy: The minimum ignition energy refers to the smallest amount of energy required to cause a combustible mixture to burn and explode. The lower the minimum ignition energy value, the easier it is for that substance to be ignited. (3) Flash ignition and flash point: When a mixture of the saturated vapor of a flammable liquid and air comes into contact with a flame, sparks can be generated, leading to instantaneous combustion. This phenomenon is known as flash ignition, and the temperature at which this combustion occurs is called the flash point. The lower the flash point, the more likely the chemical is to cause combustion and explosion. (4) Ignition point: The lowest temperature at which a combustible material, in the presence of sufficient air, catches fire upon contact with a flame, and can continue to burn even after the flame is removed, is known as the ignition point or burning point of that material. (5) Flammable media: Flammable media refer to gases that, when mixed with air, have an explosion lower limit of less than 10%, or a difference between their explosion upper and lower limits of 20% or more. Substances such as monomethylamine, ethane, ethylene, chloromethane, ethylene oxide, cyclopropane, hydrogen, butane, trimethylamine, butadiene, butylene, propane, propylene, methane, semi-water gas, and carbon monoxide are all flammable materials.     2. Classification by toxicity level: The toxicity level of chemical substances in pressure vessels is determined in accordance with the provisions of HG 20660 \"Classification of Toxicity Hazards and Explosion Risks of Chemical Substances in Pressure Vessels\". In the absence of such provisions, the toxicity level is determined based on the following principles: (1) Extremely hazardous (Level I): Maximum allowable concentration < 0.1 mg/m3; (2) Highly hazardous (Level II): Maximum allowable concentration 0.1–1.0 mg/m3; (3) Moderately hazardous (Level III): Maximum allowable concentration 1.0–10 mg/m3; (4) Slightly hazardous (Level IV): Maximum allowable concentration ≥ 10 mg/m3. For example, fluorine, hydrofluoric acid, phosgene, hydrogen fluoride, carbonyl fluoride, and chlorine are all extremely or highly hazardous substances, while sulfur dioxide, ammonia, carbon monoxide, vinyl chloride, methanol, ethylene oxide, ethylene sulfide, carbon disulfide, acetylene, and hydrogen sulfide are moderately hazardous substances. II. Properties of common media in pressure vessels There are a wide variety of media contained within pressure vessels in chemical manufacturing. Many of these media possess flammable, explosive, toxic, and harmful properties. Especially, chemical production is generally carried out under special conditions such as high temperature, high pressure, deep cold, and vacuum. These conditions can change as a result of variations in the manufacturing process or external factors, which can easily lead to overheating, overpressure, leakage, and other issues, thereby causing fires and explosions. Therefore, pressure vessel operators should understand and be familiar with the properties of the media contained within the pressure vessels in their respective positions, as this is crucial for the safe operation of the pressure vessels and for accident prevention. Here is a brief introduction to the properties of several common media. 1. Hydrogen (H2) Boiling point: -252.8℃ ； Autoignition temperature: 570°C in air, 560°C in oxygen℃ ； Explosion limit: 4–74.5% in air, 4–94% in oxygen. Hydrogen is a colorless, odorless, and tasteless flammable and explosive gas. It is also an asphyxiant gas and a strong reducing agent; it can react chemically with many substances to form various hydrides. 2. Ammonia (NH3) Boiling point: -33.35℃ ； Autoignition temperature: 651°C in air. Explosive limit: 15–27% in air, 14–79% in oxygen. The toxic concentration is 0.2 mg/L. Ammonia is a colorless gas with a strong, pungent odor, making it easy to detect in the event of a leak. Ammonia can irritate the eyes and respiratory tract, causing tearing and severe coughing, as well as inflaming and reddening the mucous membranes of the respiratory tract; it is highly toxic to the human body. 3. Boiling point of hydrogen sulfide (H2S): -60.35℃ ； Autoignition point: 260℃ ； Flame limit: 4–44% in air. Hydrogen sulfide is a harmful gas with a foul odor and is highly toxic. When the hydrogen sulfide concentration in the air is ≥1 mg/L, it can cause immediate poisoning, followed by spasms, loss of consciousness, and rapid death. Hydrogen sulfide is also a flammable gas. Hydrogen sulfide and oxidizers can cause dangerous chemical reactions, even leading to explosions. 4. Boiling point of ethylene oxide (C2H4O): 10.3℃ ； Autoignition temperature: 430°C in air, flash point < 17.8℃ ； Flame limit: 3–100% in air. A mixture of ethylene oxide and air can form an explosive mixture, which can catch fire or explode when exposed to an open flame or high heat. Ethylene oxide is toxic; it has an inhibitory effect on the central nervous system and can cause irritation to the skin and mucous membranes. Ethylene oxide easily generates static electricity. 5. Methylamine (CH5N) boiling point: -6.4℃ ； Auto-ignition temperature: 430°C in air; flash point: 0℃ ； Explosion limit: 4.95~20.75% in air. Methylamine is highly irritating to the eyes and respiratory tract; frequent inhalation can cause central nervous system paralysis, and the maximum allowable concentration in the air is 5 mg/m3. 6. Methanol (C2H5OH) Boiling point: 64.8℃ ； Auto-ignition temperature: 385°C in air; flash point < 11.11℃ ； Explosion limit: 6.7~36.0% in air. Methanol is toxic; ingestion can lead to blindness or even death. The maximum allowable concentration in the air is 50 mg/m3. 7. Benzene (C6H12) boiling point: 80.1℃ ； Autoignition temperature: 562.2°C in air, flash point –11℃ ； Explosion limit: 1.3~7.1% in air. Benzene is volatile and toxic; the maximum allowable concentration in the air is 80 mg/m3. 8. Density of chlorine gas (Cl2) (under standard conditions): 3.214 Kg/m3 ； Boiling point: -34.6℃ ； Melting point: -120°C. Chlorine is a grass-green gas with a pungent odor and high toxicity. Chlorine is a reactive chemical element that readily combines with other elements: it reacts with water to form hydrochloric acid and hypochlorous acid. 9. Formaldehyde (CH2O), boiling point: -19.44℃ ； Flash point: 85℃ ； Auto-ignition temperature: 430°C, ； Explosion limit: 7.0~7.3%. Formaldehyde is corrosive, irritating, and asphyxiating; inhaling large amounts of its vapors can cause acute poisoning, with the maximum allowable concentration in the air being 3 mg/m3. 10. Boiling point of ethanol (C2H5OH): 78.32℃ ； Flash point 12.78℃ ； Autoignition temperature: 423°C, ； Explosion limit: 3.3~19.0%. Ethanol is flammable, and industrial ethanol contains other toxic substances. Consuming alcohol containing 50–100 grams of ethanol at once can cause poisoning, which may be fatal in severe cases. 11. Carbon monoxide is a highly toxic, colorless, flammable gas. Its density under standard conditions is 1.25 Kg/m3. Its explosive range in air is 1.25%–75%, while in oxygen it is 15.5%–93.3%. The harm it causes to the human body is not easily detectable. 12. Liquefied petroleum gas: It is a mixture composed mainly of propane, propylene, n-butane, isobutane, and other components. It is a flammable medium; in its gaseous state it is heavier than air, with a density 1.5 to 2 times that of air.