Adaptability analysis of long distance pipeline welding process under different environments

Construction quality is very important for long-term safe operation and service life of long-distance pipeline, and welding technology is the key technology in long-distance pipeline construction. In recent years, there have been mature manual downward welding technology and semi-automatic gas shielded welding technology in the welding processing technology of long-distance pipeline. The application of full-automatic gas shielded welding technology is starting. Due to its high welding quality and good reproducibility, this automatic welding method will be more and more widely used in the construction of long-distance pipeline.

20211103073951 56046 - Adaptability analysis of long distance pipeline welding process under different environments

High pressure and high density transportation technology is the development trend of large flow pipeline transportation technology, and long-distance pipeline is also developing in the direction of large diameter and high pressure transportation. The welding method of long-distance pipeline is very important to the manufacturing quality of long-distance pipeline, which is related to its use reliability and operation safety. Reasonable selection of welding methods and application of corresponding welding technology are the key to ensure the welding quality of long-distance pipeline.

Welding requirements for site construction conditions of long-distance pipeline

  • (1) Influence of fluidity construction on welding quality. The operation points in the construction of long-distance pipeline are constantly changing with the construction progress, so it is relatively difficult to ensure the welding quality.
  • (2) The site construction topography and landform of long-distance pipeline have an impact on the welding quality. Long distance pipelines will encounter a variety of topography and landforms, and their differences lead to the diversification of nozzle weld positions, poor interface group conditions, difficult control of weld gap, and large constraint stress caused by forced group, which have a direct impact on the welding quality. Therefore, different welding methods should be selected according to local conditions to meet the needs of the project.
  • (3) Influence of climate environment on welding quality. Wind, rain, temperature, humidity and other natural environments also have a certain impact on welding quality.
  • (4) There are many working conditions for all position welding, the construction site is narrow, and there are many obstacles hindering the welding operation, which often brings inconvenience to the welding.
  • (5) Mechanized welding methods have poor applicability in the field, and there are many restrictive factors for the application of advanced welding technology in the field.

Requirements for field welding of long-distance pipeline

Requirements of long-distance pipeline materials on welding methods

At present, the steel used for long-distance pipeline is mainly carbon steel and low-alloy steel. For the welding of carbon steel pipeline, there are many welding methods, but for the welding method of pipeline low alloy steel, attention should be paid to the input of welding heat energy. For X70 and below pipeline steel, manual welding, semi-automatic welding or full-automatic welding can be selected, while for X70 and above pipeline steel, manual welding or some special automatic welding methods are more selected, because at present, self-protection welding wire with good process performance and comprehensive mechanical properties cannot be produced. When duplex stainless steel and incony alloy are used to transport oil and gas with high sulfur and halogen ions, accurate control of heat input during welding is very important to welding quality.

Requirements of pipe specification on welding technology

For large-diameter thin-walled pipeline (≤ 8mm), four layers of all position downward welding can be used to complete the welding, which can fully reflect its rapid advantages and obtain high welding qualification rate. For large diameter thick wall pipeline (> 8mm), when the downward welding needs to increase the welding layer greatly, the composite welding process of downward and upward welding can be considered. For small-diameter thin-walled pipe, argon arc welding backing, manual welding filling and capping can be considered. When the conditions are limited (such as field operation, equipment level, construction cost, etc.), the electrode arc welding method can be used. Stainless steel pipe shall consider its corrosion resistance requirements, and TIG welding or electrode arc welding is preferred.

Welding technology and application of long-distance pipeline

Manual downward welding method and Application

Manual welding method overcomes the shortcomings of complex equipment and inconvenient operation under poor natural conditions in the field. It is widely used in the field welding of long-distance pipeline, especially the welding of large-diameter thin-walled long-distance pipeline. At present, the commonly used manual welding methods are manual electrode downward welding and manual tungsten argon arc welding.
Downward welding method and application of electrode arc welding:

At present, the downward welding method of electrode arc welding has been mature and widely used in pipeline engineering construction. There are mainly three types of downward welding methods of electrode arc welding: all cellulose downward welding, mixed downward welding and composite downward welding.

  • (1) All cellulose down welding. All cellulose down welding is a widely used welding method at home and abroad. It is mainly used for the welding of thin-walled large-diameter pipes with steel below X70. It has fast welding speed, good root welding performance and high one-time qualification rate of weld radiographic inspection. There are some areas in pipeline construction, such as water network environment and areas that cannot be accessed by automatic and semi-automatic welding equipment. The key to guarantee the welding quality of this method is that one side welding and two sides forming are required during backing welding, and the back depression and hot metal adhesion of molten metal under the action of gravity are prevented in the overhead welding position. All cellulose electrodes were used in the early downward welding in China.
  • (2) Hybrid downward welding. Hybrid downward welding refers to the manual downward welding method of cellulose electrode backing welding, thermal welding, low hydrogen electrode filling and cover surface during on-site assembly welding of long-distance pipeline. This welding method is mainly used for the connection of high-grade steel pipe materials and sour oil and gas medium pipelines with harsh climate and environment and high sour gas transmission. It is required to weld pipelines or thick wall pipelines with good low-temperature impact toughness.
  • (3) Compound downward welding. Compound downward welding means that the root welding layer and thermal welding layer adopt downward welding method, and the filling and cover welding adopt upward welding method. It is mainly used for welding pipes with large wall thickness. Compared with traditional upward welding, downward welding has low heat input, shallow penetration and thin weld bead. With the increase of steel pipe wall thickness, the number of weld bead layers will increase rapidly, and the welding time and labor intensity will increase. Simple downward welding is difficult to give full play to its characteristics of fast welding speed and high efficiency. Before the gas shielded semi-automatic downward welding technology was applied to pipeline construction, the compound downward welding technology was mostly used for large wall thickness pipelines, which can greatly improve the production efficiency.

Semi automatic downward welding method and Application

Semi automatic downward welding methods are mainly divided into flux cored wire self-protection semi-automatic downward welding and active gas protection semi-automatic downward welding.

  • (1) Flux cored wire self-protection semi-automatic downward welding. Flux cored wire self-protection semi-automatic welding is especially suitable for outdoor windy occasions. It is an efficient and high-quality welding method to protect the arc and molten pool by a large amount of gas released by high-temperature decomposition of flux core, and a small amount of slag to protect the molten pool and solidified weld metal. The continuous operation mode of flux cored wire can reduce the number of welded joints, improve weld quality and labor productivity. Flux cored wire has excellent process performance, stable arc and beautiful shape. It is suitable for all position pipeline welding, and the mechanical impact toughness can be greatly improved. It is suitable for field operation, but the weld root is prone to incomplete fusion during backing welding. Therefore, the welding shall be carried out in strict accordance with the process specification parameters.
  • (2) CO2 active gas shielded semi-automatic downward welding. CO2 gas shielded welding is a cheap, efficient and high-quality welding method. The traditional short-circuit transfer CO2 welding can not fundamentally solve the contradiction between large welding spatter, controlling penetration and forming. Based on the high-speed controllability of the inverter welding machine, the waveform control technology CO2 semi-automatic welding machine is adopted. The droplet transition is completed by the liquid surface tension, and the current and voltage waveform is controllable. It can be used for backing welding of all position single-sided welding and double-sided forming.

Author: Tan Yinhua

Source: China Welded Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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Reference:

  • [1] Liu Jun. discussion on welding quality management in long-distance natural gas pipeline installation [J]. Modern commerce and industry, 2017 (14)

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