Preventive measures for common manufacturing deformations in pressure vessels

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1 Overview Deformation in pressure vessel manufacturing refers to the situation where the geometric dimensions of a pressure vessel or one of its components do not conform to the requirements specified in the drawings and standard specifications, with the errors exceeding the limits set by those drawings and specifications. 　　Based on the causes of deformation, pressure vessel deformation can be divided into two categories: one is deformation caused by stress, including flame cutting deformation, processing instability deformation, welding deformation, and heat treatment deformation, etc ； Another category consists of deformations caused by processing errors, including deformation due to cutting errors, deformation due to shaping errors, and deformation due to assembly errors. 　　Some of these deformities can be corrected through complex orthopedic treatments, while others cannot be altered and must be discarded as defective products, resulting in waste. Therefore, great attention must be paid to deformation during the manufacturing of pressure vessels; manufacturing processes must be carefully developed and strictly followed in order to prevent deformation and ensure that the quality of these vessels meets the requirements specified in the drawings and standard specifications. 2 Stress Deformation and Prevention 2.1 Flame Cutting Deformation (1) Shell Segments: When cutting short shell segments of large-diameter shells (which are long and narrow), the edges that are processed by flame cutting at their ends are prone to deformation. After cooling down from the high temperature of cutting, the machined edges contract, causing straight edges to turn into curved ones. Once the tube section rolls into a circular shape, its ends are no longer on the same horizontal level; when the error is significant, this makes it impossible to meet the requirements for alignment and welding. Symmetrical cutting or machining methods should be employed to avoid deformation. 　　(2) Head: After flame cleaning and cutting of the molded head, contraction occurs around its opening, resulting in a smaller diameter for the head. In severe cases, the diameter of the closed end after contraction fails to meet the size requirements. For the machining of the ports on the integrally formed head, if flame cutting is used, the shrinkage amount after cutting must be taken into account when designing the molding die ； For the machining of the ports on lobe-type combined end caps, if flame cutting is used, the diameter of the end caps must be increased appropriately during assembly to compensate for the shrinkage resulting from cutting. Mechanical processing can also be used to avoid deformation. 　　(3) Machined parts blanks (mainly steel plate blanks): These blanks are often used for large flanges or seals on pressure vessels. After flame cutting, uneven expansion and contraction of the steel plate result in an uneven surface of the blank; in severe cases, this leads to insufficient machining of the blank’s surface. Straightening and shaping should be carried out after the blank sheet is cut; for blank sheets that are difficult to shape, the machining allowance can be increased appropriately. 2.2 Deformation due to processing instability Deformation due to processing instability often occurs when large holes are made in already formed heads or shell sections (such as loading and unloading holes on containers); the reduced stability in the area around these holes leads to deformation of certain parts of the shell. Try to avoid making large holes directly in individual tube sections or end caps; depending on the circumstances, the shell can be assembled into larger sections or as a whole before such holes are made ； Before making the openings, the area where the openings will be created is reinforced using struts that are attached closely to the shell; once the shell is in a stable state after the welded tubes are in place, the reinforcing plates are removed. 2.3 Welding Deformation The welding process refers to the technical requirements and operational specifications for welding containers, including: the welding method used, the weld groove design, the types and diameters of electrodes, welding process parameters, the sequence of welding, the number of weld layers, pre- and post-welding treatments, requirements regarding the welding environment, as well as measures to prevent and counteract deformation. 　　The welding process must undergo process qualification to be deemed acceptable, and the process requirements must be strictly followed during welding operations. 　　Based on the welding conditions and volume of work for pressure vessels and large components, the extent and pattern of deformation that will occur during welding are analyzed in advance, allowing for targeted control measures to be implemented: (1) For large pressure vessels with multiple weld passes, such as spherical vessels, they should first be assembled into a single unit before welding; the welding process must be carried out symmetrically, and the prescribed welding sequence must be followed. 　　(2) For large components with multiple weld passes, such as segmental combined heads and shell transition sections made up of segments, in addition to meeting the above requirements, mouth-shaped fixing fixtures shall also be installed at the welding site. 　　(3) For pressure vessels that are long and composed of multiple sections, the dimensions of the tube sections should be adjusted to account for the welding contraction, in order to prevent shortening of the vessel after welding. 　　(4) For pressure vessels, especially those with complex structures, a reasonable assembly sequence and welding measures to prevent deformation must be adopted to ensure that they do not deform during manufacturing. 　　(5) Counter-deformation measures: Based on practical experience or calculations, a deformation is applied in advance to the welded component in the direction opposite to that of the welding deformation; after welding, this pre-deformation amount precisely cancels out the welding-induced deformation. The specific method involves leaving a counter-deformation amount in the direction opposite to that of the welding deformation at both ends of the longitudinal seam joints of the pressure vessel shell sections when applying pressure for welding ； The mold dimensions for the modular flap head and transition section are designed to account for the counter-deformation amount that offsets welding deformation. 2.4 Preventive measures against deformation during heat treatment (1) The heat treatment furnace must meet regulatory requirements; the temperature inside the furnace should be uniform and accurate. A flame shield should be installed at the flame nozzles on the furnace walls to prevent the flame from coming into direct contact with or approaching the parts being heat-treated. 　　(2) When pressure vessels of larger length are introduced into the furnace, temporary supports must be used for padding, and the number of such supports depends on the specific dimensions of the vessel. 　　(3) Shells with a larger diameter and thinner thickness generally should be internally reinforced. 　　(4) For pressure vessels prefabricated in sections, reinforced supports should be provided at the section joints. 　　(5) Pressure vessel components that are prone to losing stability at high temperatures should also be reinforced according to specific circumstances. 3 Processing error deformation 3.1 Cutting error deformation Due to inaccurate cutting dimensions, the shape of the finished component deviates from the standard specifications. Inaccurate cutting dimensions are mainly due to errors in calculations or drafting. In addition to improving the technical skills of the cutting staff, a system for checking cutting dimensions should be implemented, and computer software for managing cutting dimensions should be used whenever possible. 3.2 Forming error deformation During the processing and shaping of pressure vessel components, deformation can occur due to improper operations or non-standard molds. There are specific requirements regarding the demolding temperature for thermally formed heads; if demolding occurs at too high a temperature, the head will contract significantly, and in severe cases, its geometric dimensions may exceed the specified limits ； Container components made by mechanical rolling or pressing, deformed due to improper handling ； Inadequate or incorrect mold design results in the geometric dimensions of the pressure vessel components after molding not meeting the required standards. The main preventive measures are: (1) The molding process must be carried out strictly in accordance with the technical requirements. 　　(2) Use inspection templates to strictly control the shape of the workpieces. 　　(3) Mold design should be based on the theoretical dimensions and shape of the component to be processed, taking full account of the changes that occur during and after the manufacturing of pressure vessel components. For cold-forming molds, the amount of springback of the formed parts must be considered, while for hot-forming molds, the shrinkage of the parts after cooling needs to be taken into account. 3.3 Assembly error deformation The deformation that occurs during the assembly of pressure vessel shells due to errors such as misalignment or lack of straightness exceeding acceptable limits is referred to as assembly deformation. Preventive measures: (1) Positioning fixtures should be used during the assembly of the shell. For shells with a large diameter and thin thickness, support should also be provided for the tube sections during assembly, in order to strictly control any misalignment at the joints between the shell parts. 　　(2) The horizontal assembly of the housing should be carried out on idlers, and its straightness should be checked with a straightedge. 　　(3) For pressure vessels prefabricated in sections, positioning fixtures should be used during installation, and a theodolite should be employed to check for any deviation in straightness.