Process control in pressure vessel manufacturing

Thread Content

Process control: In the manufacturing process of pressure vessels, the first step is to prepare a complete set of process documents and inspection plans for each vessel. These documents serve to guide production, ensure quality, and improve efficiency. Of course, preparing these documents is often not enough; the key is to strictly enforce them during the construction process. During product manufacturing, no subsequent stage shall proceed until the previous stage has been inspected, verified, and signed off on, and the products or components must not move on to the next stage. This relates to the key points that need to be controlled during the manufacturing process of pressure vessels. Cutting plans and layout diagrams: The cutting plans and layout diagrams are key elements that need to be carefully controlled. The cutting of end caps and cylinders, as well as the assembly of cylinders and the creation of openings for pipes, all rely on these cutting plans and layout diagrams. Before cutting the head and cylinder parts, technicians prepare cutting diagrams based on the dimensions of the supplied sheets and the design drawings; they then create layout diagrams that specify the location and size of the holes according to these cutting diagrams and design drawings. Since the location and diameter of the openings in the workshop are determined based on the layout diagrams prepared by the technical staff, it is essential for them to carefully verify these diagrams; in particular, they need to recheck the location and dimensions of the openings indicated in the diagrams, in order to avoid irreversible losses resulting from errors in the size and position of those openings. Welding Procedure Specification: The quality of pressure vessel manufacturing is directly related to its safety, reliability, and service life during operation. Welding quality control during the manufacturing process is key to ensuring its quality. To ensure welding quality and improve its control, it is necessary to develop reasonable welding procedure specifications based on welding procedure qualification and taking into account the actual conditions of the manufacturing plant. When creating such specifications, welding technicians must not ignore practical considerations; they need to work together with welding operators to determine the appropriate welding method, the type of groove to be used, the angle of the groove, and the size of the root margin. Only through collaboration between welding technicians and operators can procedures that are in line with the qualification results, capable of ensuring quality, and easy for welders to follow be developed. Technical instructions: To better transform the design shown in the drawings into a physical product, to avoid unnecessary detours and mistakes during the manufacturing process that could lead to waste, it is necessary for technicians to provide technical instructions based on the manufacturing challenges identified during the review of the drawings, as well as the technical requirements and points that require special attention during production. It is necessary to convey the construction drawings, as well as the requirements of standards and specifications, to the operators in a timely, accurate, and clear manner. Only then can they carry out the construction in accordance with those drawings, standards, and specifications, thereby producing products that meet the required quality standards. Inspection checkpoints: For pressure vessel manufacturing, what are the inspection checkpoints we need to consider, and how can we better control them? These include cutting of heads and cylinders, marking out lines for fabrication, welding of joint plates, inspection of welds, straightening of cylinder sections, assembly of the cylinder body, making openings in the cylinder body, and welding of connections. To ensure error-free production of pressure vessels, it is essential to strictly control these inspection checkpoints; no inspections, confirmations, or sign-offs should take place at these points, and products or components must not proceed to the next stage of production until such checks have been completed. For example, one of the steps in this process is takeover, the assembly and prefabricated welding of manhole sections, forgings, and flanges. During this process, it is necessary to conduct inspections at appropriate stages, and to ensure that construction is carried out strictly in accordance with the drawings. It is important to closely monitor and verify whether the shape and specifications of the flanges, as well as the type of sealing surface, pressure rating, forging grade, and flange standard numbers, match those specified in the drawings. Only after confirming that everything is correct can the components be assembled and welded, thereby avoiding rework due to non-compliance with the design specifications. At the same time, it is also essential to closely monitor and control the use of welding materials to prevent the use of the wrong type due to differences in material quality. The connection between the pipe fitting and the shell is a challenging aspect of welding, primarily due to the variety of connection methods between them, the complexity of the forces involved, as well as the difficulties associated with groove preparation and assembly quality. The connection between the take over and the shell is available in flush, protruding, and mounted types. The construction drawings specify the requirements for the connection method between the pipes and the shell, and there is more than one such connection method for a single container. To ensure that there are no errors in the pipe connections, it is necessary to prepare welding procedure sheets based on the requirements outlined in the construction drawings during the manufacturing process of pressure vessels. The pipe fitters must prepare the grooves and arrange the pipes according to the connection methods specified in the drawings, while welders must carry out the welding work strictly in accordance with the welding methods specified in the drawings. The stress conditions at the junction between the nozzle and the vessel shell are quite complex. The discontinuity in the structure at the corner where the nozzle meets the vessel shell leads to high stress concentrations. Combined with any residual stresses present, this results in very high stress levels in that area, which often becomes the most stressed part of the entire vessel. Furthermore, since the plasticity of the weld metal is usually lower than that of the base material, the root of the weld and the heat-affected zone are prone to becoming sources of cracks under fatigue loads. Therefore, strict control is required during this process; forced alignment is not allowed, and the welds must have smooth transitions to avoid defects such as lack of fusion, incomplete welding, and undercutting. The quality of groove preparation and assembly is difficult to manage and control. From the perspective of the welder’s work, such welds represent the most challenging to control in terms of manufacturing processes among all the welds on a container, and they are also the areas where problems are most likely to occur. This is because manual methods such as gas cutting and gas gouging are still widely used to create the saddle-shaped openings in the casing, making it difficult to ensure the bevel angle and the size of the root edge ； The oxide scale on the groove surface is difficult to remove, and the welding position for welded pipes often makes it hard for welders to control the shape of the weld; as a result, defects such as cracks, incomplete penetration, lack of fusion, and slag inclusions are very likely to occur. At the same time, due to the special structure of the assembly, RT and UT inspections are difficult to carry out, and the drawings do not provide clear specifications for the connection between the pipes and the container; this reduces the welder’s sense of responsibility, making it hard to ensure the quality of the welds. Therefore, during this process, operators need to strengthen the control over the quality of groove machining and assembly, improve the accuracy of groove dimensions, and ensure proper groove cleanliness. At the same time, when aligning the joints, it is necessary to strictly adhere to the requirements specified in the drawings regarding the height of the joints and the inclination of the flange surfaces, ensuring that these parameters remain within the standard limits. After alignment, welding can only proceed after technical quality inspection personnel confirm that everything is correct, thereby effectively preventing errors that could lead to unnecessary rework and waste.