Overview
The strength of the steel pipe used in the West-East Gas Pipeline Project is API5LX70, the outer diameter is 1016 mm, the wall thickness ranges from 14.6 to 26.2 mm, the pipeline design transmission volume is 12 billion cubic meters, and the design pressure is 10 MPa. To complete the main construction task of such a large-diameter, high-pressure gas pipeline, welding technology is one of the important links that restrict the quality and efficiency of construction.
Weldability of X70 pipeline steel
The coiled steel supplier of API5LX70 pipeline steel for the West-East Gas Pipeline Project has both domestic manufacturers and foreign manufacturers. The cold crack sensitivity coefficient (PCM) of the supplied steel pipes does not exceed 0.21%, and the carbon equivalent (CEQ) does not exceed 0.42%. Considering that the carbon content of the base metal is low and the carbon equivalent is greater than the level of the general low alloy steel, the weldability of the base metal can be preliminarily determined, but the necessary preheating measures should be taken before welding.
Referring to the theoretically calculated preheating temperature value, an oblique Y-groove weld crack test was performed. The test results show that the surface crack rate is qualified when preheated to 80 degrees Celsius by solid wire welding; the surface crack rate is preheated to 100 degrees Celsius by welding with cellulose electrode. Accordingly, the preheating temperature of the welding during the construction is determined to be not less than 100 degrees Celsius, and the temperature between the welding layers is determined to be not less than 80 degrees Celsius. In addition, considering the heat resistance of the outer coating of the steel pipe and the influence of the heating process of the base material itself, it is recommended that the upper limit of the heating temperature of the preheating temperature and the interlayer temperature does not exceed 200 degrees Celsius.
Groove design of welded joints
The groove type usually used for pipe welding construction is API standard 30 degree groove. However, for large-diameter, thick-walled steel pipes, such welded joints have a large heat-affected zone, welded joint deformation and stress, and a large amount of filler metal, high labor intensity and low welding efficiency. After investigation and process test verification, it is determined that the West-East Gas Transmission Project adopts a 23-degree V-groove type.
The groove type adopted in the West-East Gas Pipeline Project greatly reduces the consumption of welding materials and the labor intensity of welding. Taking the semi-automatic welding method used in the project to weld a 14.6 mm wall-thickness steel pipe as an example, using a 23-degree V-shaped groove and a 30-degree V-shaped groove, the 3,200-kilometer-long pipeline can reduce the consumption of welding materials by 460 tons, reducing The welding workload is 226%, the material cost is saved by about 25 million yuan, and the cost of welding construction is about 36.4 million yuan.
Application characteristics of various welding methods
Semi-automatic welding method
The semi-automatic welding methods currently used are divided into two types. One is a method of down-welding using a self-shielded flux cored wire. This welding method is flexible in operation, strong in environmental adaptability, high in welding efficiency, good in welding quality, and has considerable construction experience, and there are no more difficulties in use. The other is a method of using a solid wire for carbon dioxide gas shielded arc welding down welding, such as the application of an STT type carbon dioxide inverter welder. This welding method has high welding efficiency and flexible operation, but the welding process is greatly affected by the ambient wind speed.
Automatic welding method
Although China began research on automatic pipe welding technology in the 1970s, it was not until the 1990s that automatic welding technology was applied in pipeline construction. For example, in 1998, the PAW2000 automatic welding machine of the Pipeline Bureau conducted the welding test on the Henan Yima gas pipeline. In 1999, the automatic welding machine imported from the British NOREAST company conducted the welding test in the Gangqing gas pipeline project, and in 2000 in Suining. In the Lan gas pipeline project, the PAW2000 automatic welding machine of the Pipeline Bureau and the automatic welding machine imported from NOREAST of the United Kingdom jointly carried out the automatic welding process test.
The automatic welding technology has poor adaptability to various changes during the construction process, so maintaining the consistency of the nozzle ovality and the groove parameters plays a key role in the quality stability of the automatic welding technology. In the West-East Gas Transmission Project, the automatic welding technology is mainly applied to the flat terrain in the western terrain. A total of 11 construction units have set up 15 automatic welding construction units, and the pipeline is welded by the full-position automatic welding method to complete the pipeline of about 670 kilometers. , accounting for about 17% of the overall welding task.
The automatic welding technology is suitable for pipeline welding construction in flat terrain, especially in areas with harsh natural environment, such as Gobi, desert, no man's land, etc. Automatic welding technology has irreplaceable application space and advantages. Combining different engineering characteristics and construction environment factors, and properly distributing the welding tasks of automatic welding, semi-automatic welding and manual welding, the relationship between welding efficiency, welding quality, labor intensity and construction cost will be the most reasonable.
Manual welding method
In the West-East Gas Pipeline Project, a large number of down-weld methods for cellulose-type electrodes have been used. Most of the repair welding, continuous welding and special welding of special sections and special welds use cellulose-type electrode root welding and low-hydrogen welding rods. The method of filling and welding on the cover surface.
Electrode arc welding has the characteristics of flexibility, flexibility and adaptability. The organic combination of the two methods of down-welding and up-welding and the good root-welding adaptability of cellulose electrodes are still not replaceable by other welding methods in many occasions. of.
Welding station
In the water network and mountainous areas, a special welding station was established near the construction site. The submerged arc welding method or the semi-automatic welding method was used to complete the welding of the double pipe or the triple pipe, and the number of welds on site construction was reduced by 40%-50%. , greatly shortening the cycle of laying operations.
Welding construction measures
The welding method of automatic welding is relatively strict for the groove type and the nozzle group. If the groove type cannot meet the requirements, or the nozzle group is not accurate enough, it will cause welding defects such as burn-through and incomplete penetration. Therefore, the ellipticality of the nozzle is required, and the pipe end beveling machine must be equipped at the welding construction site to ensure the accuracy of the groove type and the nozzle group.
The effects of external stresses on high-strength, thick-walled steel tubes must be considered. Therefore, during the construction process, attention should be paid to the reliability of the steel pipe support and the sufficient weld bead thickness when the mouthpiece is evacuated. It is forbidden to strongly assemble the counterpart to avoid the occurrence of welding cracks.
in conclusion
The construction of the West-East Gas Pipeline Project shows that China's welding technicians use a large number of domestic and foreign leading high-tech technologies to solve the problems in welding construction efficiently and quickly, comprehensively improve the construction level of long-distance pipelines in China, and realize the pipeline industry. A major breakthrough in localization. Through the construction practice of the West-East Gas Pipeline Project, the capacity and level of domestic pipeline construction have been improved as a whole, and the strength of China's pipeline construction units to participate in international competition has been enhanced.
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