Preparation Before Installation of Marine Carbon Steel Pipes
There are many hidden dangers to quality in the construction of the ship piping system, and the hidden dangers can only be discovered when the equipment is used in the later stage. At this time, changing will consume a lot of manpower and material resources and waste a lot of time. Therefore, according to the construction, performing construction step by step and reducing hidden dangers can get twice the result with half the effort.
1. Workshop prefabrication of piping systems
1.1 The principle of division of piping systems
There are 4 standard lengths for straight pipes commonly used on ships, that is, 6 m, 4 m, 3 m and 2 m. It is not advisable to take the same length at both ends of the plane bending pipe, the long side is generally 2 to 4 m, and the short side is 0.4 to 1 m. The bending angles should be 15°, 30°, 45°, 60° and 90°. The bending radius is about 3 times the length of the outer diameter of the pipe. After the pipe is bent and formed, it is necessary to check the roundness and angle of bending of the pipe system, turning angle, bending crease height and pipe length. Pipes for special media. For example, liquid pipes for transporting LPG can be connected with shaped elbows with a small bending radius. For piping systems that require a branch pipe, the branch pipe should be provided at a distance of 150 mm from one end of the pipe to facilitate grinding, cleaning and welding of the inner diameter of the branch pipe.
1.2 Cutting and assembly of pipes
1) Marking of the cutting line
According to the corresponding drawings, each bending point and cutting point on the long straight pipe is marked with a horizontal line of different colors to facilitate the operation of the cutter and the pipe bender, and the pipe section number, pipe material, pipe grade, furnace batch number and other information of the pipe are marked with special labels for the pipe system.
2) Cutting and assembling processes of pipelines
Common pipe cutting machines include high-speed cutting machines, grinding machines, flame cutting machines, and sawing machines. After the pipe system is cut, the beveling is processed with a beveling machine and a grinding machine. For pipes with wall thickness less than 2 mm, use "I" bevel; for pipes with wall thickness greater than 2 mm, use "V" bevel. The bevel angle is 65°±5°. For pipes with a pipe diameter of less than 500 mm, the bottom groove gap is 2 to 4 mm, and 3 to 4 welding points are fixed by spot welding during assembly. For pipes with a diameter of not less than 500 mm, the groove gap at the bottom is 3.5 to 5 mm, and 4 to 8 welding points are fixed by spot welding during assembly. When welding a branch pipe on the main pipe, when the wall thickness of the branch pipe is less than 6 mm, it is not necessary to make a bevel. When the wall thickness is not less than 6 mm and the diameter of the main pipe is not less than 2.5 times the diameter of the branch pipe, make a bevel at the end of the branch pipe. When the diameter of the main pipe is less than 2.5 times the diameter of the branch pipe, bevels can be provided at the left and right parts of the end of the branch pipe along the axis of the main pipe, as shown in Figure 1.
Figure 1 Bevels at the end of the branch pipe
In the assembly process, the limit deviation of the inner diameter of the pipe center should be less than 2 mm. When the pipe is not completely centered, the deviation of the inner diameter of the pipe end can be adjusted by means of the hydraulic top or screw top, but harmful stress should not be caused. The adjustment method of the screw top is shown in Figure 2, that is, the cylindricity of the inner diameter of the tube is adjusted by tightening the screws on the tooling, thereby reducing the deviation of the inner diameter of the tube center.
Figure 2 adjusting the screw top
1.3 Welding of the pipe system
For special carbon steel pipes, argon tungsten arc welding (GTAW) is generally used for single-sided welding and double-sided forming. In the welding process, sufficient argon gas protection is required for both the welding torch mouth and the welded pipe to prevent oxidation of the pipeline. Special grinding wheels and stainless steel brushes must be used to remove carburization in the welding process to avoid damage to the welding and the pipe wall due to carburization. For ordinary carbon steel pipes, carbon dioxide gas shielded welding is used; the welding seam has low hydrogen content, good rust resistance, little tendency of cold cracks, good penetration ability, high welding current density, small deformation, and high production efficiency.
In the welding process, when deformation occurs for the welding seam, certain corrections can be made according to the deformation of the welding seam. For pipe fittings with small sizes and thin walls, welding deformation of the weld seam is very likely to occur. During welding, it is necessary to pay attention to the condition of the pipe end in real time, flexibly adjust the welding direction according to the actual situation and keep the temperature of the weldment not too high to reduce deformation.
1. Workshop prefabrication of piping systems
1.1 The principle of division of piping systems
There are 4 standard lengths for straight pipes commonly used on ships, that is, 6 m, 4 m, 3 m and 2 m. It is not advisable to take the same length at both ends of the plane bending pipe, the long side is generally 2 to 4 m, and the short side is 0.4 to 1 m. The bending angles should be 15°, 30°, 45°, 60° and 90°. The bending radius is about 3 times the length of the outer diameter of the pipe. After the pipe is bent and formed, it is necessary to check the roundness and angle of bending of the pipe system, turning angle, bending crease height and pipe length. Pipes for special media. For example, liquid pipes for transporting LPG can be connected with shaped elbows with a small bending radius. For piping systems that require a branch pipe, the branch pipe should be provided at a distance of 150 mm from one end of the pipe to facilitate grinding, cleaning and welding of the inner diameter of the branch pipe.
1.2 Cutting and assembly of pipes
1) Marking of the cutting line
According to the corresponding drawings, each bending point and cutting point on the long straight pipe is marked with a horizontal line of different colors to facilitate the operation of the cutter and the pipe bender, and the pipe section number, pipe material, pipe grade, furnace batch number and other information of the pipe are marked with special labels for the pipe system.
2) Cutting and assembling processes of pipelines
Common pipe cutting machines include high-speed cutting machines, grinding machines, flame cutting machines, and sawing machines. After the pipe system is cut, the beveling is processed with a beveling machine and a grinding machine. For pipes with wall thickness less than 2 mm, use "I" bevel; for pipes with wall thickness greater than 2 mm, use "V" bevel. The bevel angle is 65°±5°. For pipes with a pipe diameter of less than 500 mm, the bottom groove gap is 2 to 4 mm, and 3 to 4 welding points are fixed by spot welding during assembly. For pipes with a diameter of not less than 500 mm, the groove gap at the bottom is 3.5 to 5 mm, and 4 to 8 welding points are fixed by spot welding during assembly. When welding a branch pipe on the main pipe, when the wall thickness of the branch pipe is less than 6 mm, it is not necessary to make a bevel. When the wall thickness is not less than 6 mm and the diameter of the main pipe is not less than 2.5 times the diameter of the branch pipe, make a bevel at the end of the branch pipe. When the diameter of the main pipe is less than 2.5 times the diameter of the branch pipe, bevels can be provided at the left and right parts of the end of the branch pipe along the axis of the main pipe, as shown in Figure 1.
Figure 1 Bevels at the end of the branch pipe
In the assembly process, the limit deviation of the inner diameter of the pipe center should be less than 2 mm. When the pipe is not completely centered, the deviation of the inner diameter of the pipe end can be adjusted by means of the hydraulic top or screw top, but harmful stress should not be caused. The adjustment method of the screw top is shown in Figure 2, that is, the cylindricity of the inner diameter of the tube is adjusted by tightening the screws on the tooling, thereby reducing the deviation of the inner diameter of the tube center.
Figure 2 adjusting the screw top
1.3 Welding of the pipe system
For special carbon steel pipes, argon tungsten arc welding (GTAW) is generally used for single-sided welding and double-sided forming. In the welding process, sufficient argon gas protection is required for both the welding torch mouth and the welded pipe to prevent oxidation of the pipeline. Special grinding wheels and stainless steel brushes must be used to remove carburization in the welding process to avoid damage to the welding and the pipe wall due to carburization. For ordinary carbon steel pipes, carbon dioxide gas shielded welding is used; the welding seam has low hydrogen content, good rust resistance, little tendency of cold cracks, good penetration ability, high welding current density, small deformation, and high production efficiency.
In the welding process, when deformation occurs for the welding seam, certain corrections can be made according to the deformation of the welding seam. For pipe fittings with small sizes and thin walls, welding deformation of the weld seam is very likely to occur. During welding, it is necessary to pay attention to the condition of the pipe end in real time, flexibly adjust the welding direction according to the actual situation and keep the temperature of the weldment not too high to reduce deformation.
