What are the special requirements for low-temperature pressure vessels?

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Structural design: The structural design of low-temperature pressure vessels should take into account the need for sufficient flexibility. The main requirements are as follows: ① The structure should be as simple as possible to reduce the constraints between welded components； ② Structural design should avoid excessive temperature gradients ； ③ Sharp changes in cross-section should be avoided as much as possible to reduce local stress concentrations; the inner end of the inserted fitting should be rounded to ensure a smooth transition ； ④ The connecting welds of the attachments shall not use discontinuous welding or spot welding ； ⑤ Gussets, lugs, legs (except for spherical tanks), or skirtings of the container should be equipped with gaskets or connecting plates; welding them directly to the container shell should be avoided as much as possible. The gaskets or connecting plates shall be made from materials suitable for low-temperature conditions ； ⑥ For takeover reinforcement, overall reinforcement or thick-walled tube reinforcement should be preferred; if reinforcement plates are used, the welds should have a smooth transition ； ⑦ For containers that cannot undergo overall heat treatment, if the components welded to them require stress relief, consideration should be given to allowing those components to be heat-treated separately. Connection openings: In low-temperature pressure vessels, connection openings should be placed as far away as possible from the main welds and the areas surrounding them. If it is necessary to make an opening in the weld area, it must comply with the requirements of relevant standards. The nozzles on low-temperature pressure vessels shall meet the following requirements: ① For the pipe sections welded to the vessel shell, the wall thickness shall be not less than 5 mm; for nozzles with a diameter of DN ≤ 50 mm, thick-walled pipes are preferred, while their extended portions shall be made of seamless pipes with normal wall thickness ； ② For turning, elbow joints that have been bent or pressed should be used; straight pipes welded together cannot be used instead ； ③ For plug-in nozzles, the sharp corners at the pipe end inside the shell wall must be turned or ground into rounded corners with a radius of R≥3mm ； ④ For longitudinal welds when using coiled tubes for takeover, as well as circumferential welds at the joints between tube sections, a fully penetrative weld structure shall be employed ； ⑤ For hazardous media that are flammable or extremely/highly toxic, or when the pressure is ≥1.6 MPa, T-shaped fittings shall be either seamless extruded tees or structures formed by drilling holes in thickened pipes and then welding them. Flanges: Welded flanges should be used for flanges that meet the following conditions: ① A design pressure of ≥1.60 MPa, and flanges for containers holding flammable media or media with extremely high or high toxicity, or pipe flanges subjected to significant external loads ； ② Flanges for containers and piping flanges with a design pressure of ≥2.50 MPa. Butt weld flanges should be produced using seamless forging or rolling processes; it is not allowed to use thick steel plates that have been cut to create them ； It is permissible to use steel sections or steel plates that have been bent and welded, but post-weld heat treatment is required. If steel plates are used for bending, they should be cut into strips along the rolling direction; when bending, the surface of the steel plate must be parallel to the center line of the flange, and ultrasonic testing of the steel plate is also necessary. The main requirements for fasteners are as follows: ① For fasteners such as bolts and studs used in flanges of low-temperature pressure vessels, ordinary ferritic fasteners along with their corresponding nuts shall not be used; ordinary commercial nuts may be employed, but the operating temperature must be at least -40 degrees Celsius℃ ； ② It is recommended to use elastic bolts or studs with a shank diameter not exceeding 0.9 times the root diameter of the thread, and without threads in the middle section ； ③ For ferritic steel containers designed for temperatures of -100°C or higher, ferritic steel fasteners (bolts, studs, nuts, washers) should be used; for austenitic steel containers designed for temperatures below -100°C, austenitic steel fasteners should be used ； ④ Commercial fasteners made of A2 grade austenitic steel that meet the requirements of GB 3098.6 \"Mechanical properties of fasteners – Stainless steel bolts, screws and studs\" can be used in low-temperature pressure vessels operating at temperatures of not lower than -196°C ； ⑤ For stress-reducing conditions, when the adjusted impact test temperature is equal to or above -20°C, standard ferritic commercial fasteners can be used. Sealing gaskets: For low-temperature pressure vessels, the commonly used sealing gaskets include those made of metallic materials (including semimetallic gaskets) as well as non-metallic gaskets, with the following conditions and requirements. ① For gasket materials to be used at temperatures below -40°C, metal materials such as austenitic stainless steel, copper, and aluminum should be employed – materials that do not exhibit any significant structural changes at low temperatures. This includes the metal strips used in wound gaskets, the housings of metal-lined gaskets, as well as hollow or solid metal gaskets. ② Non-metallic sealing gaskets should be made of materials that maintain good elasticity at low temperatures, such as asbestos, flexible (expansive) graphite, polytetrafluoroethylene, etc. The applicable conditions are as follows: For flange gaskets in environments where the temperature is not lower than -40°C and the pressure is not higher than 2.5 MPa, high-quality asbestos rubber sheets, asbestos-free rubber sheets, flexible (expanding) graphite sheets, polyethylene sheets, etc., may be used ； Flange gaskets with a temperature not lower than -120°C and a pressure not higher than 1.6 MPa may use high-quality asbestos rubber sheets impregnated with paraffin. The main requirements for welding are as follows. ① Full penetration construction should be adopted for welds of classes A, B, and C. For Class D welds, except for the welding of flanges to the vessel wall, the welding of small-diameter nozzles (DN≤50mm) to thicker heads or covers, and the connection of threaded pipe fittings to the vessel wall, which may follow the relevant provisions of HG 20582, a fully penetrated weld structure shall also be employed in all such cases. ② Before welding low-temperature pressure vessels, a welding procedure qualification must be carried out. This qualification should focus on low-temperature Charpy (V-notch) impact tests on the welds and heat-affected zones, and the acceptance criteria shall be determined in accordance with the requirements of the base material, ensuring that they are not lower than the properties of the base material. ③ During welding, the welding wire energy must be strictly controlled within the range determined through process evaluation; it is advisable to use a lower welding wire energy and carry out multiple passes of welding. ④ The buttweld must be fully penetrated, and the weld bead height should be minimized; it shall not exceed 10% of the thickness of the welded part, nor more than 3 mm. The fillet weld should be smooth, with no protrusions outward. The weld surface must be free from defects such as cracks, pores, and undercut. There should be no abrupt changes in shape; instead, there should be smooth transitions. ⑤ Arcing must not be performed at non-welding areas; arcing should be carried out using an arc starting plate or within the groove. ⑥ Welding accessories, fixtures, tie rods, etc. must use welding materials and welding processes identical to those of the housing material, and must be welded by qualified professional welders; the length of each weld seam must be no less than 50 mm. ⑦ Surface defects on the container caused by machining, welding, or assembly, such as scratches, weld scars, and kerf marks, should all be polished out. The wall thickness after grinding shall not be less than the calculated thickness of the container plus the corrosion allowance; the grinding depth shall not exceed 5% of the nominal thickness of the container, nor more than 2 mm. ⑧ Seams that are discontinuous or joined by spot welding are not allowed.