Design and Manufacturing of Cryogenic Pressure Vessel

Cryogenic pressure vessels, as critical equipment in cryogenic industrial processes, are widely used in air separation, liquefied gas storage, chemical reactions, and other fields. These vessels operate under extremely low temperatures and high-pressure environments, necessitating long-term structural safety and reliability. Therefore, the design and manufacturing of cryogenic pressure vessels involve multiple technical requirements, including material selection, structural design, welding techniques, and heat treatment processes. This article will delve into the key technologies of their design and manufacturing, covering working principles, design requirements, material selection, welding processes, and safety design.

Principles and Failure Modes of Cryogenic Pressure Vessels

The working environment of cryogenic pressure vessels requires them to withstand pressure at temperatures below ambient and effectively isolate cryogenic media. The toughness and plasticity of steel significantly decrease at low temperatures, while brittleness increases. Consequently, the primary failure mode of cryogenic pressure vessels is brittle fracture. This type of fracture typically occurs under low stress at low temperatures and is often sudden, potentially causing significant production losses.

To avoid brittle fracture, the low-temperature performance of materials must be rigorously considered during design. Especially at lower design temperatures, the tensile strength and toughness of materials decrease, making sudden fractures more likely. These fractures are often accompanied by noticeable crack propagation. Therefore, material selection, welding quality, and heat treatment processes must be specifically designed and required for low-temperature environments.

Temperature and Material for Cryogenic Pressure Vessels

The design temperature of a cryogenic pressure vessel is a crucial factor in determining material selection and design specifications. Based on the operating temperature, cryogenic pressure vessels are categorized into different types, each requiring different materials.

Design temperature below -20°C: For vessels with design temperatures below -20°C, commonly used materials include carbon steel and low-alloy steel. 16MnDR steel is a representative material often used for outdoor storage tanks, such as air and nitrogen tanks.

Design temperature below -40°C but not lower than -196°C: Materials for such vessels typically include Ni-series low-temperature steels, such as 09MnNiDR, 08Ni3DR, and 06Ni9DR. These steels are suitable for liquid ammonia equipment and other cryogenic vessels in lower temperature environments.

Design temperature below -196°C but not lower than -273°C: For extremely low-temperature environments, austenitic stainless steel (e.g., S30408) is commonly used. This type of stainless steel maintains good mechanical properties at ultra-low temperatures.

Structural Design of Cryogenic Pressure Vessels

The structural design of cryogenic pressure vessels must fully consider the impact of temperature changes on the vessel to avoid structural damage due to temperature differences. Key points in the structural design include:

Simplified structure: The structure should be as simple as possible to reduce unnecessary constraints and additional stresses. Avoid overly complex designs, especially abrupt structural shapes, to minimize local stress concentrations.

Welding joint design: The connection between nozzles and the shell should use concave, smooth transition fillet welds to avoid sharp angles and stress concentrations. Ensure smooth transitions in welding joints to prevent excessive stress.

Reinforcement design: Nozzle reinforcement should preferably use integral reinforcement or thick-walled pipe reinforcement structures. If reinforcement rings are used, ensure full penetration welds and smooth transitions to enhance the vessel's overall strength and stability.

Support and shell welding: Welding joints between supports and the shell should include backing plates to prevent stress concentrations in the welding area, ensuring vessel stability.

Fasteners and Sealing Design

The design of fasteners and seals in cryogenic pressure vessels directly affects the vessel's sealing performance and safety. The performance of materials at low temperatures must be carefully considered when selecting bolts and gaskets.

Bolt selection: Bolts for flange connections in cryogenic pressure vessels should be selected based on design temperature. For temperatures below -40°C, 35CrMoA is recommended; for temperatures below -70°C, 30CrMoA should be used. For even lower temperatures, austenitic stainless steel bolts are preferred.

Gasket materials: In low-temperature environments, gaskets should be made of non-metallic materials with good elasticity and plasticity, such as asbestos rubber sheets or flexible graphite. For lower temperatures, metal gaskets made of austenitic stainless steel, copper, or aluminum are often used due to their stable performance at low temperatures.

Welding Processes and Heat Treatment Requirements

Welding processes for cryogenic pressure vessels are stringent, especially in low-temperature environments where the performance requirements for welding joints are more demanding. Welding joints are categorized into A, B, C, D, and E classes based on the stress and location they endure, each with different design standards. Common welding requirements include:

Class A welding joints: Double-sided butt welding is typically required for Class A joints to ensure full penetration and double-sided formation. If single-sided welding is used, the backing plate must be removed after welding to ensure quality.

Dissimilar steel welding: When welding ferritic steel to austenitic steel, the difference in thermal expansion coefficients can cause thermal stress at low temperatures. Welding materials with high chromium and nickel content, such as Cr23Ni13 or Cr26Ni21, should be used, and post-weld stress relief heat treatment should be avoided.

Post-weld heat treatment: Vessels usually require stress relief heat treatment after welding to eliminate residual stresses, improve the mechanical properties of welding joints, and reduce the risk of brittle fracture at low temperatures.

Quality Inspection After Welding

The welding quality of cryogenic pressure vessels must be strictly inspected to ensure the strength and stability of welding joints. For cryogenic pressure vessels, the inspection length of welding joints is more stringent than for pressure vessels under normal temperature, typically requiring inspection of at least 50% of the joint length. This requirement is based on the special performance considerations of cryogenic pressure vessels, ensuring welding quality meets safety standards.

Conclusion

The design and manufacturing of cryogenic pressure vessels must fully consider factors such as temperature, pressure, materials, and welding. By rationally selecting materials, optimizing structural design, strictly controlling welding quality, and applying appropriate heat treatment processes, the risk of low-temperature brittle fracture and other failure modes can be effectively mitigated, ensuring the long-term safe operation of vessels in cryogenic environments. With the continuous development of cryogenic technology, the design standards and manufacturing processes of cryogenic pressure vessels are constantly updated and improved to meet increasingly stringent industrial demands.