Construction methods
(1)Inspection: Before the use of steel pipes, inspection must be carried out first. It is required that the material, specification, pressure rating and processing quality of the pipe sections shall comply with the design regulations. Visual inspection shall be conducted under sufficient lighting conditions. There shall be no defects such as cracks, laps, rolled folds, delaminations, hair cracks, and scabs on the inner and outer surfaces.
(2)Cutting: Before cutting the pipeline, different cutting methods should be selected according to different materials and diameters, such as manual cutting, mechanical cutting, flame cutting, etc., but electric welding shall not be used instead of cutting. When flame cutting is adopted, the oxides on the cutting surface must be polished with an angle grinder to ensure correct dimensions and a flat surface. The cut ends should be smooth, and the burrs on the inner wall should be removed in a timely manner. The quality of the cut should meet the following requirements: The cut surface should be smooth, free from cracks, double - skins, burrs, bumps, shrinkage openings, slag, iron oxide and iron filings. The deviation of the inclination of the cut end face should not be greater than 1% of the outer diameter of the pipe, and should not be greater than 3mm.
(3)Grooving: Grooving should be carried out according to the requirements of the design documents and welding process before pipeline welding. The processing of pipeline grooves is preferably done by mechanical methods, and heating methods such as plasma arc and oxy - acetylene can also be used. After processing the groove by thermal processing methods, the oxide scale, slag and surface layer affecting the joint quality on the groove surface should be removed, and the uneven parts should be ground smooth. After grooving the pipeline, visual inspection should be carried out. There shall be no defects such as cracks and delamination on the groove surface.
(4)Assembly: Before the assembly of welded joints, clean the inner and outer surfaces by manual or mechanical methods. Within a range of 20mm from the groove, there should be no paint, rust spots, oxide scale, burrs and other substances harmful to welding. Special assembly fixtures should be used for pipeline assembly to ensure the straightness of the pipes. Measure with a spirit level or straightedge at a position 200mm away from the center of the interface, and the allowable deviation is 1mm, with the total length deviation not exceeding 10mm. For the butt - welded joints of pipelines, the inner walls should be flush. The misalignment of the inner walls of steel pipes should preferably not exceed 10% of the wall thickness and should not be greater than 2mm. When the misalignment of the inner wall exceeds the specified value or the misalignment of the outer wall is greater than 3mm, correction should be carried out. The non - circular pipe ends should be made round or trimmed before assembly. The butt joint clearance should meet the requirements, and no forced alignment should be used to avoid causing additional stress. When assembling, place the welded parts firmly to avoid generating additional stress or deformation during the welding process.
(5) Tack welding: The tack - welding for pipeline assembly and positioning should be carried out by the welder who will weld the same pipe. The electrodes or welding wires used for tack - welding should be the same as those for formal welding. The welding process conditions for tack - welding are the same as those for formal welding, and full penetration must be ensured. The length of each tack - weld is 10 - 15mm, the height is 2 - 4mm, and it should not exceed two - thirds of the pipe wall thickness. If defects such as cracks are found in the weld bead of the tack - weld, they should be dealt with in a timely manner.
(6)Welding: The construction process for pipeline welding should be determined according to the pipeline material, specifications, process requirements, design pressure and operating temperature selected in the design. All welders involved in the welding must hold welder certificates. Welding should be carried out in strict accordance with the requirements of the specifications. Electrodes and fluxes should be dried as specified in the factory instruction manual before use, and should also be kept dry during the use process. The position of pipeline welds should meet the following requirements:
a.For the distance between the center planes of two butt - welded joints on a straight pipe section, when the nominal diameter is greater than or equal to 150mm, it should not be less than 150mm; when the nominal diameter is less than 150mm, it should not be less than the outer diameter of the pipe.
b.The distance between the weld and the starting point of a bend (excluding pressed bends) shall not be less than 100mm, and shall also not be less than the outer diameter of the pipe.
c.The longitudinal weld of the coiled pipe should be placed in a position that is easy to repair, and it is not advisable to be at the bottom.
d.The net distance between the circumferential weld and the support or hanger should not be less than 50mm.
e.It is not advisable to make holes on pipeline welds and their edges.
(7)Inspection: After pipeline welding, visual inspection of the welds must be carried out. Before weld inspection, spatter and slag should be removed thoroughly. The appearance of the weld should be well - formed, and the width is preferably 2mm beyond the edge of the groove on each side. There should be no defects such as cracks, pores, slag inclusions, and lack of fusion on the surface of the weld and heat - affected zone. The depth of undercut on the pipeline should not be greater than 0.5mm. The continuous undercut length should not be greater than 100mm, and the total length of undercut on both sides of the weld should not be greater than 10% of the total length of the weld. The weld surface should not be lower than the pipeline surface. The weld reinforcement △h ≤ 1 + 0.2b1, and should not be greater than 3mm. Here, b1 is the maximum width of the groove after the assembly of the welded joint (mm). The misalignment of the welded joint should not be greater than 10% of the wall thickness, and should not be greater than 2mm.
Precautions
Before the use of steel pipes, inspection must be carried out first. The materials, specifications, pressure ratings and processing qualities of the pipe sections are required to comply with the design requirements.
The setting of pipeline welds should facilitate welding and inspection, and must meet the following requirements at the same time.
Except for the shaped elbows without an intuitive section, the distance between the center of the pipeline weld and the starting point of the elbow bend shall not be less than the outer diameter of the pipe, and also not less than 100mm.
The net distance between the weld and the edge of the support or hanger should not be less than 50mm.
The distance between the centers of two adjacent welds on the pipeline should be no less than 100mm and also no less than the outer diameter of the pipe.
For carbon steel pipes, the allowable number of repair times for the same weld shall not exceed three times.
