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Chapter 9 Safe Operation of Pressure Vessels 1. Operating Characteristics of Pressure Vessels Pressure vessels are special types of equipment that operate under specific conditions. These specific conditions refer mainly to the physical properties of the media such as pressure and temperature, as well as the chemical properties of these media. (1) Physical properties of the medium: These refer to the physical characteristics of the medium related to pressure and temperature, such as thermal expansion and contraction, as well as the three physical states (gas, liquid, solid). (2) Chemical properties of the medium: mainly referring to flammability and explosiveness, toxicity, corrosivity, etc. The flammability and explosivity characteristics can be classified into four types: combustible, highly flammable, inert, and oxidizing. Toxic substances can be categorized as extremely hazardous, highly hazardous, moderately hazardous, and slightly hazardous. 2. The basic requirements for pressure vessels include strength, stiffness, stability, durability, sealing performance, and so on. 3. Failure of pressure vessels (1). Definition of failure: The failure of pressure vessels includes explosions, ruptures, and leaks, as well as excessive deformation, expansion, local bulging, severe corrosion, the formation of large cracks, fatigue propagation or corrosion propagation of cracks, excessive creep deformation at high temperatures, deformation due to imbalances in the vessel’s geometric shape, and deformation resulting from the long-term use of metal materials. Therefore, any situation in which a container fails to perform its intended function due to safety issues is considered a failure. (2). Failure modes of pressure vessels: The failure modes of pressure vessels are generally classified into ductile fracture, brittle fracture, fatigue fracture, corrosion fracture, creep fracture, and composite fracture. (3) Introduction to Container Rupture and Explosion and Their Hazards ①. Definition of explosion: In a broad sense, an explosion refers to an extremely rapid process of physical or chemical energy release. In this process, the system’s internal potential energy is converted into mechanical energy as well as radiation in the form of light and heat. When a pressure vessel ruptures, the high-pressure gas inside is freed from the constraints of the vessel’s casing, expands rapidly, and releases its internal energy at very high speeds. This is what is commonly referred to as a physical explosion. ②The hazards of container rupture and explosion: When a container ruptures, the energy released as a result of gas expansion causes the container to crack further, and the container or its fragmented pieces are propelled at high speeds in all directions, leading to injuries or damage to surrounding equipment. On the other hand, a larger portion of it does work on the surrounding air, generating shock waves. In addition to being able to directly injure people, these shock waves can also destroy buildings such as factories, causing even greater damage. If the working medium inside the container is a toxic gas, then upon the rupture of the container, large amounts of this toxic gas will spread into the surrounding area, causing air pollution and potentially creating a large area affected by poisoning. What’s worse, the container holds flammable liquefied gases; when the container ruptures, these gases immediately evaporate and mix with the surrounding air to form an explosive mixture. In the presence of sparks generated by debris hitting equipment or static electricity resulting from high-speed air currents, a chemical explosion occurs immediately – what is commonly referred to as a secondary explosion of the container. The high-temperature gases it generates spread around, causing the flammable materials in the vicinity to catch fire, which results in a large-scale fire area. ③. During a gas explosion, the shock wave pressure: When a pressure vessel ruptures due to a gas explosion, aside from a small portion of the energy being used for further tearing of the vessel and ejecting the vessel or its fragments, most of that energy is converted into shock waves. Near the center of the explosion, the overpressure ΔP on the shock wave front can reach over one megapascal. Under such high pressure, buildings will be destroyed, and equipment, pipelines, and the like will suffer severe damage. Even shock waves within 0.1 MPa can cause significant damage; a pressure excess of 0.005 MPa is sufficient to shatter window and door glass. ④Other hazards caused by the rupture and explosion of containers: A. The destructive effect of fragments – When a pressure container ruptures, the reaction force from the high-speed expulsion of gas can push the entire container shell in the direction opposite to the rupture; some shells may break into pieces of varying sizes, with those fragments scattering in all directions. These fragments with higher speeds or greater masses possess significant kinetic energy as they fly, and can also cause considerable damage. B. Pressure vessels containing toxic liquefied gases, in which the toxic substance in the hazard zone upon rupture is the liquefied gas, can experience violent steam explosions when they break, as already discussed above. Many of the liquefied gases stored in pressure vessels are toxic substances, such as liquid ammonia, liquid chlorine, sulfur dioxide, nitrogen dioxide, hydrocyanic acid, etc. When containers holding these toxic liquefied gases rupture, large amounts of the liquid turn into vapor and spread through the air, creating large areas of toxicity, a phenomenon that is not uncommon either domestically or internationally. For example, in January 1951, an ammonia receiver used in a small refrigerator at an ice-making plant in Oita Prefecture, Japan, had a diameter of only 560 mm and a volume of 0.6 m3. A steam explosion occurred due to cracks in the head weld, resulting in a highly concentrated ammonia gas filling an area with a diameter of 80 meters. As the container was placed by the road, most of the passersby were poisoned, resulting in 6 deaths and 11 people suffering from minor to severe injuries. C. In the combustion zone when combustible liquefied gas containers rupture, there are such pressure vessels, especially large storage tanks, that contain combustible liquefied gases such as liquefied petroleum gas. When these containers rupture, the liquefied gas inside evaporates in large quantities and mixes with the surrounding air; under appropriate conditions, combustion and explosion can occur outside the container, resulting in serious fire accidents. Fires caused by the rupture of storage tanks for flammable liquefied gases are extremely dangerous and result in huge losses; such incidents occur quite frequently. When a civilian liquefied petroleum gas cylinder (15kg) ruptures, its combustion range can reach at least 20 meters. The rupture and explosion of a 10T liquefied petroleum gas storage tank can result in a combustion range of at least 170m.