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Preface: The sources of pressure in pressure vessels can be divided into two categories: one is where the pressure is generated or increases outside the vessel； Another category is those in which pressure is generated or increased within the container. 1. Outside the container: Pressure sources that generate or increase pressure outside the container are generally gas compressors or steam boilers. A compressor increases gas pressure through mechanical means. Positive displacement compressors increase the pressure of a gas by reducing its volume and thereby increasing its density; piston or screw compressors are examples of this type. Speed-type compressors increase the pressure of gas by raising its flow velocity and converting its kinetic energy into static pressure energy, such as centrifugal or axial flow compressors. Therefore, for pressure vessels whose working medium is compressed gas, the maximum operating pressure generally does not exceed the compressor outlet pressure. A steam boiler heats water and turns it into steam. The specific volume of steam is much larger than that of water; for example, when water at normal pressure is converted into saturated steam at the same pressure, its volume increases by about 1,700 times. When water evaporates in a sealed pot, the steam pressure continues to increase until it reaches the exhaust pressure of the boiler. Therefore, for pressure vessels whose working medium is water vapor, their maximum operating pressure is also limited to the exhaust pressure of the boiler. If the steam pressure required by the pressure vessel is lower than the exhaust pressure of the boiler, the pressure can be reduced using a steam pressure reducing valve. The pressure generated by a external pressure source in the container generally does not increase suddenly. 2. Pressure within the container: The pressure generated within a pressure vessel is usually due to changes in the state of aggregation of the substances contained within it. This can occur when the temperature of these substances rises sharply as a result of heating, or when chemical reactions take place that cause an increase in the volume of these substances; in such cases, liquid or solid substances evaporate or decompose into gases. The gas that evaporates or decomposes increases in volume; however, due to the limitations imposed by the container’s capacity, this leads to an increase in the pressure of the gas inside the container. For example, liquid ammonia has a saturated vapor pressure of 0.4244 MPa (absolute pressure) at 0°C, and this pressure rises to 1.534 MPa (absolute pressure) at 40°C. Certain polymer compounds are solid by nature; if they undergo \"depolymerization\" when heated and turn into gaseous monomer molecules, the pressure rises sharply due to the increase in volume. For example, the specific volume of solid polyoxymethylene is about 0.7 L/kg; when it depolymerizes into gaseous formaldehyde, its specific volume is 746 L/kg, meaning the volume increases by approximately 1,065 times. If such a change in aggregation state occurs inside a sealed container, very high gas pressure will be generated. It is relatively rare for gas to generate or increase pressure due to an increase in temperature. For an ideal gas, when the volume of a given mass of gas remains constant, for every 1°C increase in temperature, the pressure increases by only 1/273 of its pressure at 0°C. Therefore, when the gas temperature increases slightly, the pressure increase is not significant. However, if the gas temperature rises sharply for some special reason, its pressure will still increase significantly. For example, in a container holding certain hydrocarbons prone to polymerization reactions, such reactions occur under appropriate conditions, generating a large amount of heat from polymerization; as a result, the temperature inside the container rises significantly and the pressure increases greatly as well. If a chemical reaction that increases volume takes place in a sealed container, and the reaction gets out of control with the resulting products not being able to be removed in time, the pressure inside the container will increase. A common example is the reaction in which calcium carbide reacts with water to produce acetylene; calcium carbide is a solid, water is a liquid, and the product of this reaction, acetylene, is a gas. As the reaction proceeds, the volume of this gas increases rapidly. If this reaction takes place in a sealed container with the exhaust pipe not opened, the pressure inside the container will rise rapidly, potentially leading to an overpressure explosion. In general, the pressure of the medium in pressure vessels is generated outside the vessel, that is, compressors or steam boilers serve as the sources of pressure. Pressure vessels that generate pressure inside the vessel are used less frequently, but since the pressure in such vessels is created within them, they pose a greater risk; therefore, pressure control for them must also be more stringent.