In industries like petroleum and chemicals, pressure vessels are essential pieces of equipment, widely used in processes such as storage, reaction, separation, and transportation. With long production lines, many different types of vessels, and their close interconnections, the number of pressure vessels in use can be quite large. Additionally, many production processes are continuous, meaning pressure vessels need to handle extreme working conditions to keep everything running safely and efficiently. As production processes evolve and automation increases, the reliability of these vessels becomes even more important. To meet the ever-changing demands, pressure vessels face a number of design and operational challenges.
Working Environment and Load Requirements of Pressure Vessels
The design and use of pressure vessels must consider a variety of complex working environments and load demands. First, pressure vessels must withstand different types of pressure, including internal pressure, external pressure, and the pressure differences caused by temperature fluctuations. In the petroleum and chemical industries, these vessels often operate under high working pressures. In some cases, pressure vessels might face pressure changes, especially during intermittent operations or frequent media changes, which places higher demands on their safety and stability.
Second, pressure vessels often need to endure extreme temperature variations. Some vessels must operate in very cold environments as low as -20°C, while others need to handle high temperatures above 450°C. Furthermore, some vessels experience frequent temperature changes during operation. In processes that alternate between hot and cold conditions, the effects of thermal expansion and contraction can have a big impact on the vessel structure. Designers need to carefully consider these temperature swings when selecting materials and calculating wall thickness.
Effects of Medium Properties on Pressure Vessels
Pressure vessels contain various types of media, some of which can be corrosive, especially in the chemical and petroleum industries. Certain media inside the vessel can corrode the inner walls, reducing the vessel's lifespan and even causing it to rupture. That's why it's important to select materials that are resistant to corrosion by taking into account the chemical properties of the media. Additionally, some of the media may be flammable, explosive, or toxic, requiring strict sealing, safety design, and operational standards to prevent leaks, fires, or explosions. Comprehensive safety and monitoring systems must be in place to ensure the vessel's safe operation.
Structural Loads on Pressure Vessels
During operation, pressure vessels not only need to withstand internal pressure but also face external loads.
Static liquid pressure: The pressure exerted by the liquid inside the vessel.
Weight of the vessel itself: Including the weight of the vessel body and internal equipment.
Weight of auxiliary equipment and pipelines: The gravitational load of any auxiliary equipment, pipes, ladders, and platforms attached to the vessel.
Environmental loads: Wind, snow, and seismic forces, especially for tall equipment installed outdoors.
Temperature differential loads: In processes that alternate between hot and cold, temperature differences cause the vessel to expand or contract, adding pressure inside and increasing stress on the equipment.
On top of that, pressure vessels can be affected by vibration loads, impact reactions (such as fluid impacts), and other external factors. To ensure stable operation under all these conditions, it's crucial to perform a thorough strength and stress analysis during the design phase.
Operating Parameters of Pressure Vessels
The safety and stability of pressure vessels depend closely on their operating parameters, including working pressure, design pressure, test pressure, and temperature. Key parameters are as follows.
Working pressure: The maximum pressure the vessel can safely handle during normal use. The actual operating pressure shouldn't exceed this limit.
Design pressure: The pressure the vessel is designed to withstand, usually determined alongside the design temperature to ensure it can handle the pressure under typical working conditions.
Test pressure: The pressure applied to the vessel during testing to verify its strength and sealing performance.
Design temperature: The temperature the vessel is designed to handle during normal operation, which should not be lower than the highest temperature it might experience during use.
Additionally, the design pressure should always be at least as high as the vessel's "working pressure", and special safety relief devices should be included, especially for specialized vessels like those used for liquefied gas. All parameters must comply with strict industry standards.
Stress Analysis of Pressure Vessels
During operation, pressure vessels mainly face the following types of stress.
Stress from internal and external pressure: The primary source of stress in pressure vessels, where the shell experiences tensile stress due to internal and external pressure, causing material deformation. This must be carefully considered in the design, with appropriate materials and thicknesses selected.
Stress from weight: The self-weight of the vessel, internal media, and any external equipment add stress to the vessel's structure.
Stress from temperature differences: When operating in high or low temperatures, the material of the vessel expands or contracts due to thermal effects, leading to uneven stress, particularly at joints. Designers need to account for these temperature-induced stresses.
Stress from external loads: External factors like wind, snow, seismic activity, and torque from pipelines can also add stress to the vessel.
Operational and Maintenance Requirements
The safe operation of pressure vessels depends not only on their design and manufacturing quality but also on proper operation and regular maintenance. Operators need to be highly skilled and responsible, strictly following operating procedures. Since pressure and temperature changes inside the vessel can happen suddenly, any operational mistakes or equipment failures could lead to serious accidents, including explosions. Regular inspections are necessary to ensure everything is functioning correctly, and any issues should be addressed immediately to avoid accidents.
Conclusion
Pressure vessels are vital in industries like petroleum and chemicals, and their safety is crucial for smooth production. As process requirements continue to grow, pressure vessels are faced with more complex and demanding working conditions. To ensure stable operation under all types of stress and extreme environments, it's important that the design, manufacturing, operation, and maintenance of pressure vessels meet strict standards. By using solid design practices, selecting the right materials, and implementing thorough safety monitoring systems, we can ensure the safe, stable, and efficient operation of pressure vessels in production processes.
