Abstract: The liquefaction of gases in industrial installations, as well as the production, storage, and application of liquefied gases, are becoming increasingly widespread. The development of low-temperature technology has promoted the application of various low-temperature pressure vessels.
The design of low-temperature pressure vessels is more complex than that of normal temperature vessels. The author categorizes the usage characteristics and existing failure modes of low-temperature pressure vessels, as well as the considerations for material selection, structural design, and manufacturing process inspection when designing low-temperature pressure vessels
Analyze and provide more references for the design of low-temperature pressure vessels in work.
Keywords: low-temperature pressure; Container; Design points; matters needing attention
Low temperature technology has developed in the liquid air industry, and with the progress of science and technology, it has been rapidly developed and widely applied.
And low-temperature pressure vessels are key equipment in low-temperature industrial processes, greatly promoting the development of low-temperature pressure vessel construction. Low temperature pressure vessels are used at low temperatures. When steel is used at low temperatures, its toughness and plasticity will decrease to varying degrees compared to room temperature, and its brittleness will increase.
When the operating temperature of a low-temperature pressure vessel is below a certain temperature, there may be sufficiently sharp notches or defects, which may lead to brittle fracture under low stress, and this fracture will suddenly occur.
Many pressure vessels, chemical equipment, pipelines, etc. in production facilities have experienced multiple brittle fractures, resulting in huge losses.
In order to avoid accidents, it is necessary to make reasonable designs from the aspects of determining the design temperature of low-temperature vessels, reasonable selection of materials, structural design, welding material selection, manufacturing inspection, post weld stress relief heat treatment, etc. during design.
1. Determination of design temperature
The design temperature below -20 ℃ is the key to determining whether carbon steel, low alloy steel, duplex stainless steel, and ferritic stainless steel containers are low-temperature containers; The design temperature below -196 ℃ is the key to determining whether austenitic stainless steel vessels are low-temperature vessels.
When designing, it is necessary to have a comprehensive understanding and analysis of the relevant factors that affect the temperature of the container, including the location of use, whether the installation location is indoor or outdoor, the impact of normal working environment temperature on the metal temperature of the container shell, and the impact of medium temperature inside the container on the metal.
2. Key points for material selection
Due to the main failure mode of low-temperature pressure vessels being brittle fracture, the brittleness of steel increases and its toughness decreases at low temperatures. The selection of materials poses higher requirements compared to the selection of non low-temperature container materials.
Low temperature steel has strict regulations and requirements in terms of smelting method, chemical composition, heat treatment state, etc., and low-temperature impact requirements are put forward for the steel used.
P ≤ 0.025% and S ≤ 0.012% for steel with a design temperature below -20 ℃ and a lower limit of tensile strength less than 540MPa; For steel with a design temperature below -20 ℃ and a lower limit of tensile strength greater than 540MPa, P ≤ 0.02% and S ≤ 0.01%.
The material for non pressure components directly welded between and between pressure components of low-temperature pressure vessels should be steel with good welding performance.
At present, low-temperature steel in China is mainly divided into the following three categories according to the service temperature.
1) The design temperature should not be lower than -20 ℃ but not lower than -40 ℃. This type of container is represented by 16MnDR steel and is mainly used for outdoor installation of air
Storage Tanks, nitrogen storage tanks, etc. affected by environmental temperature.
2) If the design temperature is lower than -40 ℃ and not lower than -196 ℃, Ni series low-temperature steel can be selected for manufacturing low-temperature containers; 09MnNiDR used for design temperature -70 ℃ can be used for the manufacturing of liquid ammonia equipment; 08Ni3DR can be used to manufacture low-temperature containers with a usage temperature of -100 ℃; 06Ni9DR
It can be used to manufacture low-temperature containers with a usage temperature of -196 ℃.
3) The design temperature is below -196 ℃ and not below -273 ℃, and austenitic stainless steel is usually used for container manufacturing. Austenitic stainless steel such as S30408.
3. Key points of structural design
3.1 Key points to be followed in container structure design
1) The structure should be as simple as possible, reducing constraints, avoiding the generation of excess additional stress, avoiding excessive temperature changes, and selecting materials consistently.
2) Sudden changes in structural shape should be avoided as much as possible to reduce local stress, as higher local stress is the key factor causing cracks.
3) The fillet weld at the connection between the connecting pipe and the shell should have a concave and smooth transition, and the inner wall of the connecting pipe end should be rounded. 4) The reinforcement of the connecting pipe should be as much as possible using integral reinforcement or thick walled pipe reinforcement. If a reinforcement ring structure is used, it should be a fully welded structure with a smooth transition of the weld seam. 5) The welding between the support and the shell should be equipped with a backing plate.
3.2 Design of sealing fasteners
Stud fasteners for flanges of low-temperature pressure vessels shall not use ordinary ferrite commodity fasteners. Stud materials with design temperatures below -40 ℃ to -70 ℃ shall be 35CrMoA; The design temperature is below -70 ℃ to -100 ℃, and the material of the stud is 30CrMoA.
Propose limiting the chemical composition of sulfur and phosphorus, and conduct low-temperature impact tests. Austenitic steel studs should be used when the design temperature is lower than the above temperature.
The stud should be an elastic stud with a core rod diameter of no more than 0.95 times the root diameter of the thread or full thread in the central unthreaded part.
3.3 Design of sealing gaskets
The non-metallic materials used for sealing gaskets should be asbestos rubber sheets, flexible graphite, and other materials with good elastic-plastic properties at low temperatures. Sealing gaskets made of metal materials should be used when the temperature is below -40 ℃ (such as the metal shell of metal clad gaskets, the metal strip of spiral wound gaskets, and solid metal gaskets),
Austenitic stainless steel, copper, aluminum, and other metal materials with no obvious transformation characteristics at low temperatures should be used.
4. Design of welding joints and selection of welding materials
1) When working in low-temperature pressure vessels, various welding joints bear different forces and properties, and each welding joint has different design requirements.
The welded joints of pressure components in low-temperature pressure vessels are classified into four categories based on their location: A, B, C, D, and so on. The classification principle is the same as that specified in Pressure Vessels GB150-2011.
In addition, the connection joints between the base, support ears, brackets, base plates, and other non pressurized accessories and the inner and outer shell walls of the container are designated as Class E welded joints. Class A welding joints should be welded using double-sided butt welding, or using single sided butt welding that ensures penetration and double-sided forming and has the same quality as double-sided welding.
If single sided welding with a backing plate is used, the backing plate must be removed after welding. Although B-type welded joints mainly bear axial stress, they should also adopt the same structural design as A-type welded joints.
Regardless of the form used, Class C welded joints are of a welded through structure. If the flange, cylinder, and connecting pipe adopt a flat welding structure and are full section welded through structure, they are suitable for working conditions with a design temperature not less than -40 ℃ and a design pressure not exceeding 4.0MPa.
The corner joint between the D-type welding joint nozzle and the container wall should adopt a fully welded structure.
2) Low hydrogen alkaline electrode shall be used for Shielded metal arc welding of pressure components of low temperature pressure vessels or welding of pressure components and non pressure components, and alkaline or neutral flux shall be used for submerged arc automatic welding. 3) Performance requirements for dissimilar steel welded joints in low-temperature pressure vessels.
Direct welding of ferrite steel and austenitic steel can cause thermal stress at low temperatures due to the significant difference in linear expansion coefficient between ferrite steel and austenitic steel. Additionally, during fusion welding, carbon transfer from the ferrite side to the weld metal can lead to a decrease in strength on the ferrite side and a decrease in ductility of the weld metal.
The following requirements should be followed during welding.
(1) Generally, Cr23Ni13 or Cr26Ni21 high chromium nickel or nickel based welding materials should be selected, and stress relief heat treatment should not be carried out after welding in principle;
(2) The welding process qualification and product welding test plate of this type of dissimilar steel should meet the following requirements.
① The tensile strength of the welded joint shall not be lower than the smaller value of the lowest tensile strength in the base metal on both sides.
② The impact energy of the fusion line and heat affected zone on the ferrite side should be determined based on the tensile strength of the ferrite steel, with the corresponding V-notch impact energy value KV2.
③ The joint should undergo a side bending test, with a bending center diameter equal to 4 times the thickness of the sample and 180 °. After the bending test, there should be no cracking defects exceeding 1.5mm measured in any direction on the tensile surface, and no cracking defects exceeding 3mm at the fusion line.
5. Manufacturing inspection
The welding of low-temperature vessels should strictly control the line energy, usually using thinner welding rods and small welding line energy for multi-pass welding.
When local flaw detection is allowed for butt welds of low-temperature pressure vessels, the detection length shall not be less than 50% of the length of each welded joint. This is considered from a safety perspective based on the characteristics of low-temperature pressure vessels, and the requirements for the detection length are different from those for normal temperature vessels.
It is incorrect to measure the length of each welded joint by no less than 20% according to the normal temperature container.
For low temperature vessels with Class A longitudinal welded joints, product welding specimens should be prepared one by one. During the product production process, the production welder must use the same materials, welding processes, and welding conditions as the product during production
The welded joint of the product shall be welded simultaneously and shall not be made by other welders or repaired after the product is completed.
6. Post weld stress relief heat treatment
If the material and welding joint thickness of low-temperature vessels meet the heat treatment requirements of design and manufacturing standards, stress relief heat treatment must be carried out on the vessels.
Post weld heat treatment can eliminate residual welding stresses generated during the welding process, improve the mechanical properties of welded joints, and reduce the tendency of steel to brittle fracture at low temperatures.
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July 15, 2023
