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

Thread Content

Requirements for the manufacture and inspection of cryogenic pressure vessels 1 Raw materials for manufacture 1.1 Materials used in the manufacture of cryogenic pressure vessels shall comply with the requirements of GBT 150. 1.2 The steel plates used to manufacture the pressure-bearing components of low-temperature pressure vessels shall have their low-temperature impact toughness retested by the vessel manufacturer 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 counterbore holes 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 aligned 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 steel, the degree of cold working deformation should be less than or equal to 15%. If it can be proven that the elongation rate of the material after cold working remains greater than or equal to 15%, then the allowable degree of cold working deformation can be increased. 2.3 The processing deformation degree of the material is calculated in accordance with the relevant provisions of HG 20584. 2.4 When the processing deformation degree of the material exceeds the allowable value. Thermal 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 to be used in the normalized state, they shall be heat-formed under controlled temperature using the normalization process, or normalized again after heat-forming. For processes using temperature-controlled 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 Welding neck flange manufacturing: Welding neck flanges shall be produced using seamless forging or rolling processes. It is not permitted to manufacture them by cutting from thick steel plates; however, it is permissible to fabricate them by bending and welding section steel or steel plates. If steel plates are to be bent, they should be cut into strips along the rolling direction. When bending, the surface of the steel plate should be parallel to the center line of the flange; furthermore, ultrasonic testing of the steel plate must be carried out 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 either of the following situations (1) or (2), 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 more than 540 MPa, whose welding grooves are prepared by flame cutting or carbon arc gas gouging. (3) At the welded connections of socketed nozzles, flat heads, and tube sheets to the shell on the surface of the steel plate (rather than on its end face), such as J7, R4–R9, R11, G34–G41 in HG 20583, ultrasonic testing shall be carried out on the surface of the steel plate within a range of 3 times the weld width at the groove area, and there shall be no delamination defects. 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 use the same welding material and welding process as those used for the housing, and must be welded by qualified professional welders; the length of each weld seam shall not be less than 50 mm. (3) The welds at the root of the nozzles and manholes that connect to the vessel shell, as well as the butt welds between the nozzles and flanges, must be fully penetrated. (4) The butt weld must be fully penetrated, and the weld bead height should be minimized as much as possible (the height of the butt weld bead shall not exceed 10% of the thickness of the welded plate, nor more than 3 mm; any excess height must be ground off) ； The fillet welds should be smooth; any protrusion outward is not allowed. Welds with poor smoothness or inadequate shape must be polished ； 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 JB 4708 “Welding Procedure Qualification for Steel Pressure Vessels”. The number of samples for the impact toughness test (excluding retests) shall be no less than two sets (three specimens per set). The centerlines of the V-notches are located at the center of the weld and in the heat-affected zone (a transformation shadow zone about 2 mm outside the fusion line), respectively. If necessary, a K-type groove may be used to evaluate the impact toughness of the fusion line. The dimensions of low-temperature impact test specimens, test methods, test temperature, and acceptance criteria shall meet the respective requirements. 4.5 When conducting a welding procedure qualification in accordance with JB 4708, if the welded joint is composed of base metals with different group numbers, the low-temperature impact test for such welded joint must be re-evaluated. 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 welded joints in pressure-bearing components exceeds 16 mm, stress relief heat treatment shall be carried out after all welding work on low-temperature pressure vessels or components has been completed. The heat treatment process shall be consistent with the heat treatment schedule (temperature profile) specified in the welding process evaluation. Heat treatment shall include the weld joints connecting pressurized and non-pressurized 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) The weld joints between the shell and tube sheets, flat heads, etc., are calculated based on the shell thickness. (3) The weld joint between the nozzle and the shell is determined based on the shell thickness. (4) The weld joint of the nozzle or shell and flange is calculated based on the thickness of the nozzle or joint. (5) The welding joints between attachments and pressure-bearing components shall be measured based on the thickness of the fillet weld (for fillet welds, measurement is based on the leg length). 6 Product weld test plates and their inspection 6.1 For each cryogenic pressure vessel, at least one product weld test plate shall be made. When the main pressure-bearing components of a vessel (shell, convex head) are welded using several qualified welding procedures, the number of product weld test plates shall 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 as those used for the product (including base metal and welding materials), as well as the same welding procedures and conditions. They shall not be welded after the product is completed. 6.3 The preparation of product weld test plates, inspection items, and test methods shall comply with the provisions of JB 4744 “Mechanical property testing of product weld test plates for steel pressure vessels” ； For the low-temperature impact test, at least two sets of specimens (three specimens per set) must be taken from each test plate: one set with the notch centerline located in the middle of the weld and another set in the heat-affected zone. The specimens are taken from the side of the final weld pass, with the notch centerline perpendicular to the surface of the plate. 6.4 When the impact test on the product test plate fails, it shall be processed as required, or the test plate shall be re-inspected after heat treatment together with the container or the component it represents. 7 Non-destructive testing of welded joints and surfaces 7.1 Butt welds in low-temperature pressure vessels (Type A and Type B joints) that meet one of the following conditions shall be subjected to 100% radiographic or ultrasonic testing. (1) Containers intended for flammable media, and containers with a design pressure greater than 0.6 MPa. (2) Vessels with a design pressure of 1.60 MPa or higher. (3) Vessels with a shell plate thickness greater than 25 mm. (4) Vessels made of steel whose minimum tensile strength specified in the standards is >540 MPa, or those with an alloy element content greater than 3%. (5) Vessels designed for temperatures below -40°C. (6) Vessels undergoing a pneumatic test. (7) The drawing indicates containers intended for containing media with extremely hazardous or highly hazardous toxicity. 7.2 Except as provided in Clause 7.1, partial inspection is permitted for butt welds; the inspected length shall be no less than 50% of the total length of the respective weld, and shall not be less than 250 mm. 7.3 For low-temperature pressure vessels, surface magnetic particle or penetrant testing shall be conducted on the following parts in accordance with JB/T 4730. (1) Butt welds that meet the requirements of 7.1, but cannot be inspected by radiography or ultrasonic testing. (2) Accessible surfaces of Class C and Class D welds on the container shell that comply with 7.1, as well as fillet welds and corner welds for attachments. (3) The surfaces of all welds and heat-affected zones on the shells of high-strength steel vessels whose minimum tensile strength specified in the steel standards is >540 MPa. (4) Surfaces with weld marks resulting from the removal of temporary attachments such as fixtures, stiffening plates, etc. on pressurized shells; grooves prior to welding repair; surfaces after welding repair; and areas with arc burns. 7.4 When the material of fasteners 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 conducted on each individual piece. 7.5 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 shall be no lower than Class AB, and the weld quality shall be no lower than Class 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 at a minimum. (3) Magnetic particle testing shall be carried out in accordance with JB/T 4730.4 and must 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 must meet Class I requirements. 7.6 Surface defects on the surfaces of low-temperature pressure vessels and their pressure-bearing components, caused during the material production or processing, shall be inspected visually. If any harmful defects are found, such as cracks, folds, embedded scale, scabs, tears, spatter, arc burns, undercut, crater marks, sharp mechanical scratches and impact dents, or weld beads caused by removal of fixtures, etc., they must be ground off. The ground depressions 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 pipes: 12.5% of the nominal wall thickness. If the grinding depth or the thickness of the material after grinding does not meet the aforementioned criteria, welding repair, replacement, stress calculation, 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 subjected to partial inspection, if any unacceptable defects are found, a 100% inspection of that welded joint shall be conducted. (3) Unacceptable defects detected by magnetic particle and penetrant testing shall be ground down and appropriately repaired by welding; the area in question must then 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 repair welding, all repaired areas shall be subjected to magnetic particle or penetrant testing. After repair welding of heat-treated areas, heat treatment shall be carried out according to the original heat treatment procedure. The number of re-welding attempts should not exceed 2. The requirements for repair welding shall be in accordance with GBT 150. 8 Pressure testing and leak tightness testing 8.1 The pressure testing method and pressure values shall be in accordance with the provisions of GBT 150 or the design documents. 8.2 During the pressure test, the wall temperature of the vessel must be at least 20°C higher than the impact test temperature of the shell material and weld joints (whichever is higher). 8.3 After the hydraulic test, no operations such as welding should be performed on pressurized components, as these may cause welding stress and notch stress concentration; otherwise, the pressure test must be repeated. 8.4 Leakage test: When the test pressure is equal to the design pressure, the test temperature shall not be lower than the design temperature. When the test pressure is less than 10% above the design pressure, the test temperature must be at least 20°C higher than the design temperature. 9 Quality certificates, markings, packaging, and transportation. 9.1 The nameplate of the container shall not be directly nailed onto the pressurized shell. Should it be necessary to attach a nameplate to the container shell, a nameplate holder must be welded in advance, and the nameplate should then be mounted on this holder. 9.2 The factory certificates of conformity, quality certificates, nameplates, packaging, transportation, etc., for containers shall comply with the relevant provisions of GBT 150 and HG 20584.