What are the sources of pressure in pressure vessels?
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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 increased outside the vessel； Another category is those in which pressure is generated or increased within the container. 1. The pressure sources that generate or increase pressure outside a 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 external pressure sources on the container generally does not increase suddenly. 2. The pressure generated within a container is usually due to changes in the state of aggregation of the substances contained within it; the temperature of these substances rises sharply when heated, or chemical reactions occur that cause an increase in their volume. Such changes often take place when liquid or solid substances evaporate or decompose into gases as a result of heating. The gas that evaporates or decomposes experiences an **increase in volume; however, due to the limitations of 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 this 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 experience an increase in pressure due to rising 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 rises slightly, the increase in pressure 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, the control of their pressure must also be more stringent.
Why is it written in such a messy way? Sources of pressure in pressure vessels: 1. External input: Typical devices include steam cylinders and heat exchangers； 2. Internal generation: Typical equipment such as reaction vessels and cryogenic storage tanks ； Most people consider pressure vessels that generate internal pressure to be particularly dangerous, especially polymerization reactors; even a single mistake in operation can lead to a sudden increase in internal pressure, resulting in severe consequences. External inputs are generally under control by the gas supply source, so more safety devices are required to prevent overpressure. Personal opinion: There is no difference in terms of level of hazard; they are all the same. A kilogram of steam explosion is enough to kill a group of people. Although the pressure source for the steam cylinders comes from outside, if the pressure of the gas supply exceeds the capacity of the cylinders, an accident can still occur. Failing to check the safety devices related to the gas supply in these cylinders is actually even more dangerous. Something that occurs internally does not necessarily mean there will be problems; low-temperature storage tanks maintain a constant pressure over time, making them easier to manage than reaction vessels.
Classifying pressure vessels based on their level of danger is a mistake in itself; the approach adopted by the RBI does not actually ensure the safety of such equipment – it’s more like taking a risky gamble. Pressure vessels come in various forms and are complex in nature, and different types of equipment should be subject to appropriate management methods. The regulations establish categories for them, as well as classification levels and purposes. Currently, management has abolished the previous pressure levels and classification systems; pursuing management by category in an uncritical manner is essentially a gamble with risks. It is incorrect to manage things based solely on the criterion that Category 3 risks are greater than those in Category 2, which in turn are greater than those in Category 1. It’s all managed by people who don’t even have basic common sense; categories are used for comparison only when management systems are well-established and managers understand them properly. Are low-temperature storage tanks in Category 2 really less dangerous than spherical tanks in Category 3? In terms of management, more attention should be paid to the purpose and type of the equipment; only by implementing targeted management can accidents be effectively reduced. For example: 1. For low-temperature storage tanks, attention should be paid to their vacuum level, the condition of the external insulation, and the effectiveness of safety accessories ； 2. For the steam cylinders, attention should be paid to the pressure of the air supply and the effectiveness of the safety devices in the system ； 3. For the reaction vessel, attention should be paid to the sensitivity and effectiveness of reaction control, etc ； 4. When opening the door quickly, attention must be paid to the reliability of interlock devices, etc ； 5. For the air storage tank, it is necessary to check for any water accumulation and the effectiveness of safety accessories ； 6. Heat exchange equipment requires focused attention on specific parts depending on the medium; the focus points for steam-water heat exchange differ from those for water-water heat exchange. 7. Others: There are different considerations for towers, separators, etc.; it’s not possible to generalize.