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This post was last edited by LQ198619 on 2017-6-6 19:25. Chapter 1: Definition of Pressure Vessels I. Definition of Pressure Vessels in a Broad Sense A vessel, as it is generally understood, is a structural element made up of materials that is used to hold substances. Put simply, the external casings of various equipment used in industries such as chemicals, petroleum refining, pharmaceuticals, and food production all fall under the category of containers. It goes without saying that all sealed containers under pressure are called pressure vessels, or vessels under stress. II. Pressure source of pressure vessels: The substance contained within a vessel or involved in reactions within it is referred to as the working medium. The working media of commonly used pressure vessels are various gases, water vapor, or liquids; therefore, we will focus here on the sources of pressure for gas media. Pressure sources can be divided into the generation or increase of gas pressure, which comes from either inside or outside the container. 1. When the gas pressure in a container is generated outside the container, the pressure source is usually a gas compressor or a steam boiler. Gas compressors are mainly divided into two categories: positive-displacement types (piston, screw, rotary, vane, etc.) and dynamic types (centrifugal, axial flow, mixed-flow, etc.). Positive-displacement gas compressors increase gas pressure by reducing the volume of the gas and thereby increasing its density. Speed-type gas compressors increase gas pressure by raising the flow velocity of the gas, thereby converting the gas’s kinetic energy into potential energy. A pressure vessel whose working medium is compressed gas has a maximum pressure that can be reached equal to the gas pressure at the outlet of the gas compressor (of course, except in cases where the temperature of the gas inside the vessel rises significantly or other physicochemical changes occur, leading to an increase in pressure). A steam boiler is a device that uses the heat generated by burning fuel to heat water and cause it to evaporate, thereby producing steam. This is because the volume of water vapor at the same pressure is more than 1,000 times that of saturated water. 2. When the gas pressure in a container is generated inside the container, the reasons include: a change in the aggregation state of the medium within the container ； The gas medium is heated inside the container, causing its temperature to rise sharply ； Chemical reactions that cause an increase in the volume of a medium inside a container, etc. Pressure is generated or increases due to a change in the aggregation state of the medium; this usually occurs when liquid or solid substances inside a container are heated (for example, as a result of an increase in the surrounding temperature or exothermic chemical reactions taking place within the container), evaporate, or decompose into gases. This leads to a significant expansion in volume. However, due to the limitations imposed by the container’s volume, the gas density increases greatly, thereby generating pressure inside the container or raising the existing gas pressure. For example, with sulfur dioxide, when the temperature is below –10.1°C (the standard boiling point), its vapor pressure in a sealed container is lower than atmospheric pressure; whereas when the temperature rises to 60°C, the liquid sulfur dioxide evaporates in large quantities, and its vapor pressure increases to 11.25 absolute atmospheres. Another example is the solid polymer polyoxymethylene, which undergoes ‘depolymerization’ when heated to turn into a gas; its volume increases by about 1065 times, and this results in very high gas pressure even within a sealed container. l It is generally rare for the pressure to be generated or significantly increased due to the heating of a gaseous medium inside a container. Only under special circumstances, when gas in a container absorbs a large amount of heat causing its temperature to rise significantly, will the pressure increase considerably. For example, in some gas containers that store gases prone to polymerization reactions (such as certain hydrocarbon storage tanks), under suitable conditions, monomeric gases can undergo localized polymerization reactions, generating a large amount of heat from this process. This heat heats the gas inside the container, causing its temperature to rise significantly and thus leading to a sharp increase in pressure; in some cases, this can result in the container bursting due to overpressure. There are many examples of pressure increases due to chemical reactions that cause an increase in the volume of the medium within a container; for instance, when calcium carbide is mixed with water, a chemical reaction takes place to produce acetylene gas, resulting in a significant increase in volume and thus high pressure inside a closed container. Another example is the reaction of electrolyzing water to produce hydrogen and oxygen; since 1 cubic meter of water can be broken down into 1,240 cubic meters of hydrogen and 620 cubic meters of oxygen, the volume increases by about 2,000 times, which results in very high pressure even in a sealed container. In commonly used pressure vessels, the gas pressure increases more outside the vessel and less inside it. But the latter is more dangerous and requires stricter pressure control. III. Boundaries of Pressure Vessels The pressure vessels discussed here refer primarily to those that are prone to accidents, whose consequences can be severe; such vessels require supervision by specialized agencies and must be manufactured and used in accordance with specified technical management standards. In other words, it is about establishing a boundary for pressure vessels – determining which ones should be treated as ordinary equipment and which ones as special equipment. Therefore, what is mentioned refers to pressure vessels treated as special equipment. 1. The factors to be considered when determining the boundaries of pressure vessels are mainly the likelihood of an accident occurring and the severity of the potential consequences of such an accident. Currently, there is no completely unified standard internationally regarding the scope of pressure vessels. Generally speaking, when a pressure vessel experiences an explosion, the degree of harm associated with it is related to factors such as the state of the working medium, the operating pressure, and the volume of the vessel. Pressure vessels with liquid as the working medium have very low compressibility of the liquid; therefore, when the vessel ruptures, the expansion work, or the energy released, is minimal, resulting in low hazard. Pressure vessels whose working medium is gas are particularly dangerous, as gases are highly compressible; when the vessel ruptures, the expansion energy, or the energy released in an instant, is very large, resulting in high levels of hazard. It can be seen that when the working medium is a liquid, even if the container ruptures, the level of hazard is relatively low; therefore, pressure vessels with such liquid media are generally not classified as pressure vessels that belong to the category of special equipment. It is worth noting that the liquids referred to here are those at room temperature, and do not include those whose maximum operating temperature is higher than their standard boiling point (i.e., the boiling point under standard atmospheric pressure). 2. The Regulations on Safety Supervision of Special Equipment and the Relevant Standards provide definitions for pressure vessels. IV. Basic requirements for pressure vessels 1. Strength: The ability of metal to resist permanent deformation and fracture. Common strength criteria include yield strength and tensile strength. Strength refers to the ability to resist plastic (permanent) deformation under pressure, and it is a key issue related to safety. 2. Stiffness: Rigidity refers to the absence of unacceptable elastic deformation under external forces (during manufacturing, transportation, installation, and use), such as in flanges (seals) and tube sheets ； 3. Stability: The ability to prevent sudden loss of its original shape under external forces, such as in the case of external pressure and vacuum containers. 4. Durability 5. Sealing performance V. Process parameters of pressure vessels The process parameters of pressure vessels are determined by the requirements of the manufacturing process, and they serve as the main basis for the design of pressure vessels as well as for their safe operation. The main process parameters of pressure vessels are pressure and temperature. (1) Pressure: Here, it mainly refers to the pressure of the working medium in the pressure vessel, that is, the main load that the pressure vessel has to bear during operation. The pressure during the operation of a pressure vessel is measured using a pressure gauge, and the pressure value displayed on the gauge is gauge pressure. In the specifications for various pressure vessels, concepts such as operating pressure, maximum operating pressure, and design pressure frequently appear; their definitions are outlined below. 1. Work pressure. Operating pressure, also known as process pressure, refers to the pressure at the top of a vessel during normal operational conditions (i.e., excluding the hydrostatic pressure of the liquid). 2. Maximum operating pressure. It refers to the maximum gauge pressure that may occur at the top of the container during operational processes (i.e., excluding the hydrostatic pressure of the liquid). When the pressure exceeds this value, the safety device on the container will activate. The determination of the maximum operating pressure of a container is related to the working medium. 3. Design pressure. It refers to the pressure used at the corresponding design temperature to determine the calculated wall thickness of the vessel and the dimensions of its components. It is generally not less than the maximum operating pressure; due to different perspectives on the issue, various standards may have slightly different principles for selecting the design pressure. The code stipulates that the design pressure of a container should be slightly higher than the highest operating pressure it experiences during use. Containers equipped with safety devices must have a design pressure that is not less than the opening pressure or burst pressure of those safety devices. (II) Temperature 1. Medium temperature refers to the temperature of the working medium inside the container, which can be measured using temperature measuring instruments. 2. Design temperature ； The design temperature of a pressure vessel differs from the temperature that its internal medium might reach; it is the maximum or minimum temperature that the shell or component metals may attain under the corresponding design pressure during normal operation of the vessel. The Design Regulations specify the following rules for selecting the design temperature: (1) When different temperatures may occur in various parts of the container during operation, the expected different temperatures can be used as the design temperatures for those respective parts. (2) For containers with internal insulation, wall temperature calculations should be performed, or the measured wall temperature of containers under similar operating conditions shall be used as the design temperature.