Key points of pressure pipeline welding quality control

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Key points of pressure pipeline welding quality control The welding process is a key and main process in the construction of steel pressure pipeline projects. The quality of pressure pipeline teaming and welding directly affects the flow rate and direction of the pipeline medium, pipeline wear and safe operation. Therefore, there are extremely strict requirements for the welding quality of pressure pipelines. In addition to requiring the welding joint to be a complete penetration weld, there are also specific welding standards for the corrosion resistance of the pressure pipeline and the quality of the weld. The surface of the weld (inside and outside the tank) should be smooth and uniform, and there should be no obvious convex and concave weld beads. Quality control of the welding process plays an important role in ensuring the installation of pressure pipeline projects. For this reason, controlling the welding quality in pressure pipeline engineering is the key to quality control of pipeline installation. 1. Welder preparation before welding. All welders engaged in pressure pipeline welding must take the examination in accordance with the current "Boiler, Pressure Vessel and Pressure Pipe Welder Examination and Management Rules" and "Construction and Acceptance Specifications for Field Equipment Industrial Pipe Welding Projects". Only after passing the examination can they engage in the corresponding welding construction. ； Welding equipment: The hand arc welding machine, argon arc welding machine, electrode drying equipment and weld heat treatment device required for pressure pipeline welding should be complete, intact, stable and reliable in performance, and should be equipped with current, voltmeters and pressure gauges that have passed the weekly inspection (calibration) period. When the on-site conditions for bevel processing and cleaning permit, thermal processing methods such as plasma arc and oxyacetylene should be used as much as possible. After the groove processing is completed, the oxide scale, oil stain, slag and surface layer affecting the quality of the joint must be removed. The removal scope is the groove and the base material on both sides of an area of ​​no less than 20 mm, and the uneven areas should be polished smooth. Positioning/assembly of pipe joints should be done after confirming the quality of bevel processing and cleaning. Alignment and positioning welding of pipe joints is the key to ensuring welding quality and promoting good formation of the back of the pipe joint. If the groove form, assembly gap, and blunt edge size are inappropriate, defects such as concave, welding nodules, and incomplete welding may easily occur. The alignment gap should be even. When positioning, ensure that the inner wall of the nozzle is flush and the offset of the inner wall does not exceed 10% of the pipe wall thickness and should not be greater than 15 mm. If the wall thickness is inconsistent, grinding and transition should be carried out according to regulations. If the positioning plate is welded, it should be in the same direction as the fillet weld of the pipe plate. When assembling pipe fittings, they should be firmly padded, and measures should be taken to prevent deformation during the welding process. When positioning welding, the same welding materials and welding processes as those for the root bead should be used, and the welding should be performed by qualified welders. 2. Welding process control materials and welding consumable construction units should have a complete material management system to ensure that the specifications and models of materials meet the design requirements. On-site materials: The on-site material clerk checks whether the name, specification, model, quantity, quality certificate and other information of the material are consistent with the goods based on the arrival voucher. Materials that have passed the inspection and on-site material personnel are responsible for warehousing and registering them. Sometimes some materials on site are of large specifications and cannot be stored in the warehouse. Therefore, a suitable open space should be selected for storage and protection work should be done. Materials that need to be stored in the warehouse must be properly stored in the warehouse to prevent loss and damage. When distributing materials, be sure to check the project, specification, model, material and quantity of the materials to prevent errors. Welding rods used on site must be dried, and operators must use thermal insulation buckets to prevent moisture return. Only welding rods of the same brand can be used in each bucket to prevent misuse, and the maximum amount cannot exceed 5 kilograms at a time. The storage time in the bucket should not exceed four hours, otherwise it must be re-dried. The quantity of welding wire used at one time should not exceed the minimum package. Before use, check whether the surface is clean of rust, oil and other impurities. The purity of argon gas used for argon arc welding should not be less than 99.9%, and the water content should not be greater than 50ml/m3. Materials between processes: During the pipeline installation process, some materials need to go through several processes before they can be installed. For example, pipes and weldments need to go through processes such as drilling, beveling, rust removal, pickling, passivation, and degreasing depending on the transmission medium. ； Valves, accessories, and instruments need to be cleaned, degreased, inspected, tested, etc. ； Protective measures must be taken between each process, and handover inspections must be done between each process to prevent re-contamination. 3. Welding process assessment and welding process The welding process assessment of various materials and welding joint forms in pipeline welding construction covers 100%. If the unit does not have a welding process assessment suitable for pipeline materials and welding requirements, it should entrust a unit with assessment qualifications to conduct assessment. Welding technicians should develop practical welding process instructions based on design drawings, relevant construction specifications and current standards, based on welding process assessment and combined with the actual conditions of the construction site. Conduct technical briefings for welders and plumbers before construction, including welding materials, process parameters, preheating before welding, interlayer, postheating, heat treatment temperature and time, storage and use of welding materials, non-destructive testing and other requirements. The pipeline number should be marked on the welding process instruction book. Pipelines with different processes are numbered separately. Pipelines with the same welding process can be written in one process instruction book, indicating the pipeline code. The plumber prepares the groove according to the welding instructions, and the on-site quality inspector confirms the groove, size and assembly requirements (signature confirmation). The welder must strictly implement the welding process, and the on-site quality inspector should strengthen supervision and inspection in this area, which is the key to ensuring welding quality. The welder should conduct process handover, confirm the quality of the fracture, and ensure the correct implementation of the welding process. 4. The number of weld repairs for the same part of the weld should not exceed two times. For the first and second repairs, the welding technician will analyze the causes of the defects that need to be repaired and prepare a repair process, which will be reviewed and approved by the welding responsible engineer. For three or more weld repairs, the welding responsible engineer will work with relevant personnel to carefully analyze the reasons and determine the measures to avoid the recurrence of the repair and prepare a repair process, which will be reviewed and approved by the project technical person in charge or the project quality assurance engineer. After weld repair, welding inspection and non-destructive testing should be carried out according to the requirements of the repair process. Pipe welds that require post-weld heat treatment should generally be reworked before heat treatment, such as rework after heat treatment and necessary heat treatment after repair welding. For stainless steel welds that require resistance to intergranular corrosion, the repaired parts must still meet the original requirements. The number, location and status of weld repairs should be recorded in the delivery data, and the repair records, reports and other contents should be imported into the project files. 5. Post-weld inspection 5.1 Weld appearance inspection The inspection of the surface quality of the weld should be carried out before the non-destructive testing and pressure test. Therefore, the surface quality of welds should be 100% visually inspected using standard samples, gauges, hardness testers, etc. and comply with the specific requirements in the GB50236-98 standard, and the results should be truthfully recorded. The surface of a standard weld with a qualified appearance shall not have defects such as cracks, pores, arc craters, and slag inclusions, and shall not have slag and spatter. The undercut and weld reinforcement shall meet relevant requirements. 5.2 Inspection of the internal quality of welds The methods and detection ratios used in non-destructive testing should comply with relevant construction specifications, standards and design requirements. When the RT method is used for random inspection, the welds welded by each welder should be randomly inspected according to the specified proportion. The random inspection of fixed nozzles should not be less than one welding joint. The welding joints for random inspection should be jointly selected by the construction unit and third-party supervisors on site, and the number of the welding joint should be recorded. After the non-destructive testing is completed, the construction unit should accurately indicate the pipeline number, pipe material, specifications, welding joint location, welder code, non-destructive testing location, and non-destructive testing method on the pipeline diagram in a timely manner. Inspection personnel should promptly check the non-destructive testing report and RT film. If rework is necessary, the rework process should be strictly implemented and the rework welding records should be kept. At the same time, the RT film report of the deepening according to the regulations should be reviewed in a timely manner, and the reworked and deepened position should be accurately marked on the single-line diagram of the pressure pipeline. Rework must be performed before heat treatment and pressure test. 5.3 Final inspection and test 5.3.1 Final inspection and test can only be carried out after the inspection and test of the materials used in the pressure pipeline have been completed and qualified, the process inspection and test of the installation process have been completed and qualified, and the information is complete and meets the requirements. (1) After the pressure pipeline is installed, it can only be carried out after passing the joint inspection. ； (2) Inspection and testing procedures and requirements should be strictly followed and records should be kept ； (3) Pipelines transporting highly toxic, poisonous and flammable fluids must determine the leakage amount in accordance with regulations ； After the pressure test of the vacuum system pipeline, a vacuum test must be performed.