6.7　Fabrication of Large-diameter Coil-welded Steel Pipe

6.7.1　For the fabrication of the large-diameter coil-welded steel pipe，the type setting drawing shall be prepared according to the design drawing and the supply of the steel plate，the pipes shall be segmented and numbered and the connection form shall be determined.

6.7.2　For the large-diameter coil-welded steel pipe，the NC cutting machine tool should be adopted for cutting and mechanical cutting joint should be adopted.For the curve-shaped part，manual cutting joint may be adopted.After processing，the joint shall be subject to de-rustingand pre-coating.

6.7.3　A roller type plate bending machine shall be adopted for bending the large-diameter coil-welded steel pipe.At the plate end，the length of the bent linear segment shall be pre-pressed or reserved.There shall be not linear segment in the joint of the coil-welded pipe.

6.7.4　There should not be more than two longitudinal joints in the same section of the large-diameter coilwelded steel pipe.The distance between two longitudinal joints shall not be less than 200mm.When pairing，the distance between two longitudinal joints shall be larger than 100mm，and the distance between the outer wall of the branch pipe and the weld should not be less than 50mm.

6.7.5　The alignment tolerance of the inner wall of the butt weld of the coil-welded steel pipe should not exceed 10% of the wall thickness and shall not be larger than 2mm.

6.7.6　The arc length of the circular plate of the large-diameter coil-welded steel pipe shall be 1/6-1/4 of the perimeter of the pipe.The gap unfitting to the inner wall of the pipe shall be in accordance with the following requirements:

1　The longitudinal butt weld shall not be larger than 10%+2.0mm of the thickness of the wall and shall not be larger than 3.0mm；

2　The longitudinal butt weld away from 200mm to the pipe end shall not be larger than 2.0mm and that of the other parts shall not be larger than 1.0mm.

6.7.7　The allowable deviation of fabrication of large-diameter coil-welded steel pipe shall meet the provisions of Table 6.7.7.

6.7.8　The allowable deviation of main dimension of welded elbow shall be in accordance with the following requirements:

1　When the nominal diameter is no larger than 1000mm，the allowable deviation of the perimeter is±4.0mm；and when that is larger than 1000mm，the allowable deviation of the perimeter is±6.0mm.

2　The variation in perpendicularity of the end face of the welded elbow and the center line shall not be larger than 1% of the outer diameter of the pipe or 3.0mm（Figure 6.7.8）.

6.7.9　The allowable deviation of circular degree of the welded reducing pipe shall not be larger than 1%of the outer diameter of each end and 5.0mm.The center lines of both ends of the concentric reducing pipe shall be overlapped，and its offset value（a1-a2）/2 shall not be larger than 1% of the outer diameter of the large end or 5.0mm（Figure 6.7.9）.

6.7.10　The allowable deviation of perpendicularity of the branch pipes of the welded three-way pipe shall not be larger than 1% of the length of the branch pipe or 3.0mm.

6.7.11　When leaving plant，the large-diameter coil-welded steel pipe shall be provided with antidistorting support and be in accordance with the following requirements:

1　When the diameter is 600mm-1000mm，a crossed support should be adopted.

2　When the diameter is larger than 1000mm-1500mm，a 60° angular support should be adopted.

3　When the diameter is larger than 1500mm，a support in a shape like a Chinese character mi should be adopted.

4　The support should adopt angle steel and connecting plate，which shall be subject to three-face weld all around.The connecting plate and the pipe wall should be subject to intermittent welding.

6.7.12　The fabrication of the hot air ring pipe shall be in accordance with the following requirements:

1　The computer should be adopted for setting out and the NC cutting machine should be adopted for cutting for the fabrication of the hot air ring pipe.Subsection fabrication shall be performed，multiple ring sections shall be divided for assembly，and overall preassembly shall be performed at last at the plant.Butt welding should be adopted for connection between ring sections.The overall preassembly shall be conducted on the platform，the bearing stool or the molding bed.The platform，the bearing stool or the molding bed shall be arranged in accordance with those specified in Item 3 of Article 6.4.8 of this standard.

2　The preassembly of the hot air ring pipe shall be in accordance with the following requirements:

1）The preassembly of the ring pipe shall be conducted on the platform，the bearing stool or the molding bed，and the height difference of the upper surface shall not be larger than 2mm.

2）The height difference of the upper surface of the ring pipe shall be checked by level gauge，with the allowable deviation of 10.0mm.

3）The difference between the maximum diameter and the minimum diameter of the ring pipe shall be checked by steel ruler.The allowable deviation is 1.5/1000 of the diameter of the ring pipe and no larger than 10.0mm.The allowable deviation of alignment tolerance between the ring sections is 2.0mm，and that of the joint end gap is-1.0mm to+2.0mm.

6.7.13　For the fabrication of the gas uptake duct，downcomer，three-way pipe or five way ball of the blast furnace，the computer should be adopted for setting out and the NC cutting machine shall be used for cutting and baiting.The plant shall fabricate and assemble in subsection as well as meet the requirements of the design document.The preassembly shall be conducted on the platform，the bearing stool or the molding bed.The platform，the bearing stool or the molding bed shall be arranged in accordance with those specified in Item 3 of Article 6.4.8 of this standard.

6.7.14　The steel plate of five-way ball equatorial zone should not be spliced.The butt welds of the equatorial zone and the upper and lower polar zones shall adopt T-shaped intersection with a space of not less than 100mm.