Requirements for the manufacturing and inspection of low-temperature pressure vessels

Thread Content

1 Raw materials for manufacturing 1.1 Materials used for manufacturing low-temperature pressure vessels shall meet the requirements of GBT 150. 1.2 The steel plates used for manufacturing pressure-bearing components of low-temperature pressure vessels shall be re-tested by the vessel manufacturing unit for low-temperature impact toughness in accordance with the requirements of GBT 150. If the steel quality certificate lacks data on low-temperature Charpy (V-notch) impact testing, the low-temperature impact toughness test must be repeated twice as required. 1.3 For the steel used to manufacture the pressure-bearing components of low-temperature pressure vessels, when the steel is delivered without undergoing non-destructive testing, the vessel manufacturer shall carry out such non-destructive testing as required. 2 Forming of compressed components 2.1 Markings such as material identifiers and welder stamps must not be engraved or punched on compressed components. A small number of datum line countersinks are allowed (the tips of the punches should be rounded), but the depth must not exceed 0.5 mm. Material and part number markings are applied with paint. The welder’s record diagram is indicated and shipped out along with the quality certificate. 2.2 For vessels that are not subjected to post-weld stress-relief heat treatment, they shall not be shaped or assembled by forceful methods such as hammering. Steel plates and steel pipes shall not be shaped or straightened by hammering in a cold state. If shaping or sizing is required at room temperature, it is necessary to use molds to cause slow deformation or to tap gently with a wooden hammer or rubber mallet, while also controlling the degree of deformation (fiber elongation). The allowable degrees of cold working for various materials are as follows: (1) For low-alloy steels and carbon steels with a nickel content of less than 1.5%, the degree of cold working should be less than or equal to 2% for steel plates and 5% for steel pipes. (2) For ferritic nickel alloy steels with a nickel content of 1.5% or more, the degree of cold working deformation shall be 5% or less ； When the deformation degree is greater than 2%, an aging impact test (with a deformation degree of 5%) must be conducted; if the impact toughness is below the specified value, appropriate action shall be taken as per regulations. (3) For chromium-nickel austenitic stainless steels, the degree of cold working deformation should be less than or equal to 15%. If it can be demonstrated that the elongation rate of the material after cold working remains greater than or equal to 15%, the allowable degree of cold working deformation can be increased. 2.3 The processing deformation degree of the material shall be calculated in accordance with the relevant provisions of HG 20584. 2.4 When the processing deformation degree of the material exceeds the allowable value. Heat forming or cold forming must be followed by stress-relief heat treatment. The final pressing temperature for hot forming must not be lower than the recrystallization temperature of the material. 2.5 The heating of the billet prior to hot forming must be carried out in a soaking furnace; direct heating with coke flames is not allowed. If, during heating, the material suffers from loss of alloying elements, damage to its microstructure that cannot be restored through heat treatment, or surface cracking, it should be discarded. 2.6 For materials specified for use in the normalized state, they shall be hot-formed under controlled temperature using the normalization process, or undergo re-normalization after hot forming. For processes using controlled-temperature hot forming, in addition to controlling the heating and forming temperature, heat treatment simulation test plates should also be used for evaluation (including the base material and welded joints). Materials specified for use in the quenched and tempered condition must be re-quenched and tempered after hot forming. Chromium-nickel austenitic stainless steel must be quenched (solution-treated) after hot forming. 3 Manufacturing of butt weld flanges: 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, but it is permissible to use shaped steel or steel plates that have been bent and welded together. If steel plates are to be bent, they should be cut into strips along the rolling direction. During bending, the surface of the steel plate must be kept parallel to the centerline of the flange. Additionally, ultrasonic testing must be conducted on the steel plate to ensure that there are no delamination defects. 4 Welding 4.1 The selection of welding materials for low-temperature pressure vessels and the evaluation of welding procedures shall comply with relevant regulations. 4.2 For low-temperature pressure vessels whose pressure-bearing components are made of ferritic steel, and which fall under one of the situations in (1) or (2) below, the welding grooves must be subjected to magnetic particle or penetrant testing prior to welding. (1) The total content of alloying elements is greater than 3%. (2) For low-alloy steels specified in steel standards with a minimum tensile strength of over 540 MPa, whose welding grooves are prepared by flame cutting or carbon arc gas gouging. (3) At locations where welding is performed on the surface of steel plates (rather than on their ends) in structures connecting attached nozzles, flat heads, tube sheets, and shells (such as J7, R4–R9, R11, G34–G41 as specified in HG 20583), the surface of the steel plate within a range equal to three times the weld width around the groove shall be subjected to ultrasonic testing, with no delamination defects present. 4.3 The welding of the pressure-bearing components of low-temperature pressure vessels must meet the following requirements: (1) Arc starting shall be carried out using an arc starting plate or within the groove; arc starting is not permitted at non-welding areas. (2) Welding accessories, fixtures, tie rods, etc. must be welded using the same welding materials and procedures as those used for the shell. The welding must be performed by qualified welders, and the length of each weld bead must not be less than 50 mm. (3) The welds connecting the nozzle and manhole roots to the vessel shell, as well as the butt welds of nozzles and flanges, must be fully penetrated. (4) Butt welds must be fully penetrated; the reinforcement height should be minimized as much as possible. The reinforcement height of butt welds shall not exceed 10% of the thickness of the welded plates, and shall not be greater than 3 mm. Any excess reinforcement must be ground down ； The fillet welds should be smooth; any outward bulging is not permitted. Welds with poor smoothness or poor formation must be ground down ； The weld surface must be free of defects such as cracks, pores, slag inclusions, and undercuts; there should be no sudden changes in shape on the surface, which must have a smooth transition. 4.4 The welding procedure qualification for pressure-bearing components of low-temperature pressure vessels shall be carried out in accordance with the provisions of JB 4708 “Welding Procedure Qualification for Steel Pressure Vessels”. The number of samples for impact toughness testing (excluding retests) shall be no less than two sets (three samples per set). The center lines of the V-notches are located at the center of the weld and in the heat-affected zone (the phase transformation shadow zone about 2 mm outside the fusion line), respectively. If necessary, a K-type groove can be used to evaluate the impact toughness of the weld seam. The dimensions of the low-temperature impact specimens, the testing methods, the testing temperature, and the acceptance criteria shall meet the corresponding requirements. 4.5 Welding procedure qualification shall be carried out in accordance with JB 4708; when welding joints are formed from base materials of different group numbers, their low-temperature impact tests must be requalified. 4.6 During welding, the welding line energy must be strictly controlled within the range specified in the process qualification; special care should be taken to ensure that it does not exceed the upper limit of the process-qualified line energy. 5 Post-weld stress relief heat treatment
5.1 When the thickness of the welded joint of pressurized components exceeds 16 mm, after all welding work on low-temperature pressure vessels or components has been completed, stress relief heat treatment shall be carried out. The heat treatment process shall be consistent with the heat treatment schedule (temperature profile) specified in the welding process evaluation. Heat treatment should include the welds connecting the stressed and unstressed components. Welding must not be performed after heat treatment. 5.2 The calculation principles for the thickness of welded joints are as follows: (1) For butt joints, the thickness of the thinner member shall be used. (2) Welded joints such as those between the cylinder body and the tube sheet or flat head are calculated based on the thickness of the cylinder body. (3) The welding joints between the take over and the shell are calculated based on the thickness of the shell. (4) The weld joint between the pipe nipple or shell and flange is calculated based on the thickness of the pipe nipple or joint. (5) The weld joints between the attachments and the compressed elements are calculated based on the thickness of the fillet welds (for fillet welds, it is calculated based on the height of the weld). 6 Product welding test plates and their inspection 6.1 At least one product welding test plate shall be prepared for each cryogenic pressure vessel. When the main pressure-bearing components of a vessel (the cylinder and the convex head) are welded using several evaluated welding processes, the number of product welding test plates should be increased accordingly. 6.2 The product welding test plates must be welded simultaneously with the product welds during the product manufacturing process, using the same materials (including base metal and welding materials), as well as the same welding procedures and conditions; they cannot be fabricated afterwards once the product has been completed. 6.3 The preparation, inspection items, and testing methods for the welding test plates of products shall comply with the provisions of JB 4744 \"Inspection of Mechanical Properties of Welding Test Plates for Steel Pressure Vessel Products\" ； For the low-temperature impact test, at least two sets of specimens (three specimens per set) are taken for each test plate; one set has the notch centerline located at the center of the weld, and the other set has it located in the heat-affected zone. The specimens are taken from the side of the final weld seam, with the notch centerline perpendicular to the plate surface. 6.4 When the impact test on the product test plate fails, take appropriate action as required, or heat-treat the test plate along with the container or the component it represents and then recheck it. 7 Welded joints and surface non-destructive testing 7.1 Butt welds of low-temperature pressure vessels (type A and B joints) that meet one of the following conditions shall be subject to 100% radiographic or ultrasonic testing. (1) Containers holding flammable media with a design pressure greater than 0.6 MPa. (2) Vessels with a design pressure of 1.60 MPa or greater. (3) Vessels with a shell plate thickness greater than 25 mm. (4) Containers whose minimum tensile strength as specified by the steel standards is >540 MPa, or whose alloy element content is greater than 3%. (5) Vessels designed for temperatures below -40°C. (6) Vessels undergoing a pneumatic test. (7) The diagram indicates containers used to hold media with extreme or high toxicity. 7.2 Except as specified in 7.1, butt welds may be subject to local inspection; the inspection length shall be not less than 50% of the total length of the corresponding weld, and not less than 250 mm. 7.3 For low-temperature pressure vessels, the following parts shall be subject to surface magnetic particle or penetrant testing in accordance with JB/T 4730. (1) Butt welds that meet the requirements of 7.1 but for which radiographic or ultrasonic inspection is not possible. (2) The accessible surfaces of Class C and Class D welds on the container shell that meet the requirements of 7.1, as well as the fillet welds and corner welds used for attaching accessories. (3) All welds and the surfaces of the heat-affected zones on the high-strength steel vessel shell, for which the minimum tensile strength specified in the steel standards is >540 MPa. (4) The surfaces of weld seams resulting from the removal of temporary attachments such as tooling fixtures and tie plates on the compressed shell, the grooves prior to welding repairs, the surfaces after welding repairs, and areas damaged by arc erosion. 7.4 When the fastener material for equipment flanges with a design pressure of 1.60 MPa or higher and a design temperature below -40°C is ferritic steel, magnetic particle testing shall be performed on each individual part. 7.5 The non-destructive testing methods and evaluation criteria shall meet the following requirements: (1) Radiographic testing of butt welds shall be carried out in accordance with JB/T 4730.2. The quality of radiography should be at least grade AB, and the quality of welds should be at least grade II to be considered acceptable (100% inspection and partial inspection). (2) Ultrasonic testing of welds shall be carried out in accordance with JB/T 4730.3; both 100% testing and partial testing must meet the requirements of Grade I or higher. (3) Magnetic particle testing shall be carried out in accordance with JB/T 4730.4 and meet Class I requirements. (4) Penetrant testing shall be carried out in accordance with JB/T 4730.5 and must meet Class I requirements. (5) Magnetic particle testing for fasteners shall be carried out in accordance with the provisions of JB/T 4730.4, and shall meet Class I requirements. 7.6 Surface defects on low-temperature pressure vessels and their pressure-bearing components that arise during the material production or processing shall be inspected visually. If any harmful defects such as cracks, folds, embedded scale, scars, tears, splatter, arc scratches, undercuts, cratering, sharp mechanical scratches, impact dents, or weld scars caused by fixtures and tools are found, they should be removed by grinding. The ground depression should blend smoothly into the base material. The thickness of the material after grinding shall be no less than the calculated thickness at that location (including the corrosion allowance), and the depth shall not exceed the following values. Steel plate: 7% of the nominal thickness, and less than 2 mm. Steel pipe: 12.5% of the nominal wall thickness. If the grinding depth or the thickness of the material after grinding does not meet the above requirements, weld repair, replacement, stress analysis, or other safety measures should be considered. 7.7 Retesting (1) For welded joints inspected by radiography or ultrasonic testing, if any unacceptable defects are found, repair welding shall be carried out after the defects have been completely removed. The affected area must then be re-inspected using the original inspection method until it meets the requirements. (2) For welded joints that are subject to local inspection, if any unacceptable defects are found, 100% inspection of that welded joint shall be carried out. (3) Defects that are detected by magnetic particle testing and penetrant testing and are not allowed to exist must be ground and repaired as necessary, and that area must be retested using the original testing methods until it meets the requirements. 7.8 For areas requiring weld repair, magnetic particle or penetrant testing shall be conducted prior to welding. The reinforcement height at the repair weld should be greater than 1.6 mm, and then it should be ground down to the surface of the base metal. After rewelding, all rewelded areas shall be subjected to magnetic particle or penetrant testing. After patch welding the heat-treated areas, they should be heat-treated again using the original heat treatment process. The number of repair welds should not exceed 2. The requirements for patch welding shall comply with the provisions of GBT 150. 8 Pressure testing and tightness testing 8.1 The pressure testing methods and pressure values shall comply with GBT 150 or the provisions of the design documents. 8.2 During pressure testing, the wall temperature of the vessel must be at least 20°C higher than the impact test temperature of the shell material and welding joints, whichever is higher. 8.3 After the hydrostatic test, no welding or other processing that may cause welding stress and notch stress concentration shall be carried out on the pressure-bearing components; otherwise, the test must be repeated. 8.4 Density test: When the test pressure is equal to the design pressure, the test temperature shall be no lower than the design temperature. When the test pressure is 10% or less higher than the design pressure, the test temperature must be 20°C or more higher than the design temperature. 9 Certificates of Quality, Marking, Packaging, and Transportation 9.1 The nameplate of the container shall not be nailed directly to the pressure vessel; if it is necessary to install a nameplate on the container shell, a nameplate holder must be welded in advance, with the nameplate being mounted on that holder. 9.2 The factory certification, quality certificate, nameplate, packaging, transportation, etc. of the container shall comply with the relevant provisions of GBT 150 and HG 20584.