Key design points and precautions for low-temperature pressure vessels

Thread Content

Summary of key design points and precautions for low-temperature pressure vessels: In industrial facilities, the liquefaction of gases, the production of liquefied gases, as well as their storage, transportation, and application are becoming increasingly widespread. The development of low-temperature technology has facilitated the use of various low-temperature pressure vessels. The design of low-temperature pressure vessels is more complex than that of normal-temperature vessels. The author provides a categorized analysis of the usage characteristics and failure modes of low-temperature pressure vessels, as well as the considerations regarding material selection, structural design, and manufacturing process inspection during their design, in order to offer more guidance for the design of such vessels in practical applications. Keywords: low temperature and pressure; containers; design considerations; precautions. Low-temperature technology was developed in the liquid air industry, and with advances in science and technology, it has seen rapid development and wide application. Low-temperature pressure vessels are key equipment in low-temperature industrial processes, and they have greatly promoted the development of their construction. Low-temperature pressure vessels operate at low temperatures; when steel is used at such temperatures, its toughness and ductility decrease to varying degrees compared to normal temperatures, while its brittleness increases. When the operating temperature of a low-temperature pressure vessel is below a certain threshold, a sufficiently sharp notch or defect can lead to brittle fracture under low stress, and such fracture occurs suddenly. In production facilities, many pressure vessels, chemical equipment, pipelines, etc. have experienced brittle fractures on multiple occasions, resulting in significant losses. To avoid accidents, it is necessary to carry out proper design during the planning phase, covering aspects such as determining the design temperature of the low-temperature vessel, selecting appropriate materials, designing the structure, choosing welding materials, conducting manufacturing inspections, and performing stress-relief heat treatment after welding. 1. Determination of the design temperature: A design temperature below –20°C is key to determining whether carbon steel, low-alloy steel, duplex stainless steel, and ferritic stainless steel containers are low-temperature containers; a design temperature below –196°C is key to determining whether austenitic stainless steel containers are low-temperature containers. During the design process, it is necessary to conduct a comprehensive understanding and analysis of the factors that affect the temperature of the container, including the location where the container will be used, whether it is installed indoors or outdoors, the impact of the normal operating environment temperature on the metal temperature of the container’s shell, and the effect of the temperature of the medium inside the container on the metal. 2. Key points for material selection: Since the main mode of failure in low-temperature pressure vessels is brittle fracture, steel becomes more brittle and less ductile at low temperatures. The selection of materials imposes higher requirements compared to the selection of materials for non-low-temperature containers. Low-temperature steel is subject to strict regulations and requirements regarding its manufacturing methods, chemical composition, and heat treatment conditions, and it also has specified requirements for low-temperature impact resistance. For steel designed for temperatures below –20°C with a minimum tensile strength of less than 540 MPa, P ≤ 0.025% and S ≤ 0.012%; for steel designed for temperatures below –20°C with a minimum tensile strength of more than 540 MPa, P ≤ 0.02% and S ≤ 0.01%. The materials of the pressure-bearing components in low-temperature pressure vessels, as well as the non-pressure-bearing components that are directly welded to these pressure-bearing components, should be steels with good weldability. At present, low-temperature steels in China are mainly classified into the following three categories according to their operating temperature. 1) The design temperature is below –20°C and not lower than –40°C; such containers are represented by 16MnDR steel and are mainly used for air storage tanks, nitrogen storage tanks, etc. that are installed outdoors and subject to environmental temperature influences. 2) For design temperatures below –40°C and not lower than –196°C, Ni-series low-temperature steels can be used to manufacture low-temperature vessels. 09MnNiDR, suitable for design temperatures of –70°C, can be used in the manufacture of liquid ammonia equipment; 08Ni3DR can be used to produce low-temperature vessels intended for use at –100°C; 06Ni9DR can be utilized for manufacturing low-temperature vessels designed for operation at –196°C. 3) When the design temperature is below –196°C but not lower than –273°C, austenitic stainless steel is usually selected for manufacturing the containers. Austenitic stainless steels such as S30408. 3. Key points of structural design 3.1 Key points to be followed in the design of container structures: 1) The structure should be as simple as possible, with fewer restraints, in order to avoid unnecessary additional stresses and excessive temperature changes; the materials used should be consistent. 2) Sudden changes in the structural shape should be avoided as much as possible to reduce local stresses, as high local stresses are a key factor in the formation of cracks. 3) The fillet weld at the connection between the pipe and the shell should have a concave, smooth transition, and the inner wall at the end of the pipe should be rounded. 4) For takeover reinforcement, integral reinforcement or thick-walled tube reinforcement should be used as much as possible; if a reinforcement ring structure is employed, it must be of fully weld-through type with smooth weld transitions. 5) Shims should be used for the welding of the support to the shell. 3.2 Design of sealing fasteners: For stud fasteners used in the flanges of low-temperature pressure vessels, ordinary ferritic commercial fasteners shall not be used. When the design temperature is below –40°C to –70°C, the material for the studs should be 35CrMoA; when the design temperature is below –70°C to –100°C, the material for the studs should be 30CrMoA. It is proposed to set limits on the chemical contents of sulfur and phosphorus, and to conduct low-temperature impact tests. Austenitic steel studs should be used when the design temperature is lower than the above temperature. The studs should be elastic studs with a shank diameter in the unthreaded middle section that is not greater than 0.95 times the root diameter of the thread, or studs with fully threaded sections. 3.3 Design of gaskets: For gaskets made of non-metallic materials, substances such as asbestos rubber sheets and flexible graphite should be used, as they maintain good elastoplastic properties at low temperatures. When the operating temperature is below –40°C, metallic gaskets should be employed (such as the metal casing of metal-clad gaskets, the metal strips in wound gaskets, and solid metal gaskets); metallic materials such as austenitic stainless steel, copper, and aluminum should be used, as they do not exhibit significant structural changes at low temperatures. 4. Design of welding joints and selection of welding materials 1) When low-temperature pressure vessels are in operation, the forces acting on various welding joints vary in magnitude and nature, and each type of welding joint has distinct design requirements. The welding joints of the pressure-bearing components in low-temperature pressure vessels are classified into four categories: A, B, C, and D, according to their location; the classification principles are in line with those specified in the standard GB150‑2011 for Pressure Vessels. Furthermore, the joints connecting the base, lugs, brackets, shims, and other non-compressive accessories to the inner and outer shell walls of the container are specified as Class E weld joints. Class A welded joints shall employ double-sided butt welding, or single-sided butt welding that ensures full penetration and double-sided formation, with quality equivalent to that of double-sided welding. If single-sided welding with shims is used, the shims must be removed after welding. Although Class B welded joints are primarily subjected to axial stress, they should also adopt the same structural design as Class A welded joints. Weld joints of category C are fully penetrated structures regardless of the form used. When flanges, cylinders, and nozzles adopt a flat welding structure with full-thickness penetration, they are suitable for applications where the design temperature is not lower than –40°C and the design pressure is not greater than 4.0 MPa. The fillet joints between the nozzle of a Class D welded joint and the vessel wall shall adopt a fully penetrated design. 2) For welding the pressure-bearing components of low-temperature pressure vessels, or when welding pressure-bearing components to non-pressure-bearing components, low-hydrogen alkaline electrodes should be used for manual arc welding; alkaline or neutral fluxes should be used for submerged arc automatic welding. 3) Performance requirements for welded joints of dissimilar steels in low-temperature pressure vessels. When ferritic steel and austenitic steel are welded directly, the significant difference in their linear expansion coefficients leads to thermal stress at low temperatures. During welding, carbon from the ferritic side migrates into the weld metal, resulting in a decrease in the strength of the ferritic side as well as a reduction in the plasticity of the weld metal. The following requirements should be followed during welding. (1) Generally, high-chromium-nickel or nickel-based welding materials of the Cr23Ni13 or Cr26Ni21 type should be used, and stress-relief heat treatment is not required after welding in principle; (2) The welding procedure qualification for such dissimilar steel joints as well as the product welding test plates must meet the following requirements. ①The tensile strength of the welded joint shall be not less than the lower of the minimum tensile strengths of the base materials on both sides. ②The impact energy at the ferrite side of the weld line and heat-affected zone shall determine the corresponding V-notch impact energy value KV2 based on the tensile strength of the ferritic steel. ③The joint shall be subjected to a bending test, with a bend radius equal to 4 times the thickness of the specimen, at 180°. After the bending test, no cracking defects exceeding 1.5 mm shall be present in any direction on the tensile surface, and no cracking defects exceeding 3 mm shall be present at the weld line. 5. For the welding of low-temperature vessels in manufacturing inspections, the wire energy must be strictly controlled; typically, thinner electrodes and a lower welding wire energy are used, with multiple passes of welding applied. When partial testing is permitted for the butt welds of low-temperature pressure vessels, the inspection length must be no less than 50% of the length of each welded joint. This requirement is based on the characteristics of low-temperature pressure vessels and is intended to ensure safety; it differs from the inspection length requirements for vessels operating at normal temperatures. It is incorrect to state that, according to the standards for containers at normal temperature, the inspection length should be no less than 20% of the length of each weld joint. For low-temperature vessels with Class A longitudinal weld joints, product welding specimens must be prepared for each vessel individually. These specimens shall be welded by the welder who is responsible for manufacturing the vessel, using the same materials, welding procedures, and conditions as those employed for the actual weld joints in the vessel; they cannot be fabricated by another welder or created after the vessel has been completed. 6. Post-weld stress relief heat treatment: If the material and weld joint thickness of low-temperature vessels meet the requirements specified in the design and manufacturing standards for heat treatment, it is necessary to perform stress relief heat treatment on the vessels. Post-weld heat treatment can eliminate the welding residual stresses generated during welding, improve the mechanical properties of the welded joint, and reduce the tendency of brittle fracture in steel at low temperatures. References: [1] Meng Yawei, Ge Zhiqiang. Analysis of Some Issues Related to the Design of Low-temperature Pressure Vessels [J]. Science, Technology Innovation and Application, 2012(32):114. [2] Gao Jun. Steels for Low-temperature Pressure Vessels, as well as Their Welding and Inspection Techniques. Research and Application in Machinery, 2014. Author: Wang Jiancheng; Institution: Jilin Xialin Chemical Separation Machinery Industry Co., Ltd