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Welded Steel Pipes for High Pressure

Posted: 12/05/2024 13:18:45 Hits: 2

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Technical Standards for Pipeline Steel

There are two primary types of straight-seam welded pipes used in high-pressure oil and gas pipelines worldwide: straight-seam submerged arc welded (SAW) pipes (with pipe diameters of 400 to 1400 mm, wall thicknesses of 8 to 40 mm, and the highest grade being X80) and high-frequency straight-seam welded (ERW) pipes (with pipe diameters of 250 to 710 mm, wall thicknesses of 8 to 16 mm, and the highest grade being X70). The former is produced from medium-thick steel plates, while the latter is made from hot-rolled steel strips. Therefore, pipeline steel is primarily governed by two distinct international technical standards: steel plates for straight-seam submerged arc welded pipes, designed for high welding power (50 kJ/cm) and delivered according to SAW (Submerged Arc Welding) standards, and steel strips for high-frequency straight-seam welded pipes, delivered according to ERW (Electric Resistance Welding) standards. This is because the production processes for steel plates and steel strips differ, which leads to different welding methods and, consequently, the steels used for each must also differ. For example, Japan’s SAW and ERW series pipeline steels, ranging from X42 to X70, were fully developed and standardized by 1985.

China’s development of steel and welded pipes for high-pressure transmission pipelines has lagged behind that of other countries. Currently, the spiral-welded pipes used in China’s high-pressure transmission pipelines are based on methods that have been phased out globally since the 1980s. However, countries like China, Pakistan, Saudi Arabia, and Algeria continued to use spiral-welded pipes after 1990 due to less advanced welding equipment. Based on Japanese industry experience, high-frequency straight-seam welded pipes and spiral-welded pipes are generally suitable for X42–X52 grade steel strips, while X60–X70 strength-grade welded pipes require specialized standards. As a result, Japan began developing spiral-welded pipe steel strips specifically for international markets after 1980, procuring the necessary steel strips from domestic steel mills. The SPW (Spiral Welding) standard was created specifically for these products to meet the needs of international markets and is not used in China. For example, the SPW series X65 was developed in 1990, followed by the SPW series X70 in 1991.

Japanese steel companies export spiral-welded pipe steel plates to international markets according to customer specifications, complying with two key standards for oil and gas transmission pipelines. Between 1987 and 1992, Japan supplied 8,800 tons of X60 oil pipeline spiral-welded pipe steel strips (wall thickness of 8–18 mm, pipe diameter of 1100–1498 mm), 18,663 tons of X60 gas pipeline spiral-welded pipe steel strips (wall thickness of 12.7 mm, pipe diameter of 1100–13076 mm), and 3,850 tons of X65 gas pipeline spiral-welded pipe steel strips (wall thickness of 12.7 mm, pipe diameter of 1100–1346 mm) to the Saudi Arabian National Petroleum Corporation (NPC).

Japanese steel companies must develop steel strips for spiral-welded pipes because spiral-welded pipes use a submerged arc welding method with medium heat input (10 kJ/cm), which cannot be used with ERW steels designed for high-frequency straight-seam pipes. The latter require lower heat input and high-frequency normalizing of the weld seam and heat-affected zone after welding.

The pipeline steel strips currently produced in China are designed for spiral-welded pipes with medium heat input (10 kJ/cm), making them suitable for spiral-welded pipe applications. However, they are not suitable for high-frequency straight-seam welded pipes, which require lower heat input and high-frequency normalizing of the weld seam and heat-affected zone after welding. These two welding methods should not follow the same standard. In the future, separate standards for pipeline steels specifically for high-frequency straight-seam welded pipes (ERW) should be established.

Straight-seam submerged arc-welded pipes are produced using the U-O-E process and are used to manufacture pipeline sections with diameters ranging from 16 to 56 inches (406 to 1420 mm) and wall thicknesses up to 40 mm. The forming process requires a hydraulic press to shape a single steel plate into a pipe, followed by pre-welding with oxygen protection, groove preparation, and multi-wire submerged arc welding on a precision welding workbench. Finally, the pipe is formed using mechanical or hydraulic expansion devices, which also aid in the elimination of welding stress.

The advantages of straight-seam submerged arc-welded pipes include high-quality pipe formation, excellent weld quality, and effective elimination of welding stress, making them particularly suitable for gas pipeline applications. Additionally, the non-destructive testing accuracy for straight welds is high, with a low missed detection rate, and the pipe end size is precise, which facilitates on-site construction, especially for underwater pipeline projects. Currently, foreign markets emphasize the use of UOE welded pipes for critical pipeline projects, such as underwater pipelines and high-pressure gas pipelines passing through industrial and traffic-intensive areas. In international high-pressure pipeline projects, it is clearly specified that straight-seam submerged arc-welded pipes are used to construct high-pressure natural gas and underwater pipelines.

Spiral-welded pipes were phased out from high-pressure transmission pipeline projects due to their relatively thin wall thickness, typically ranging from 6 to 17 mm. These pipes are formed using two single-wire submerged arc welding machines to weld the inner and outer surfaces while the pipe body is simultaneously curled and formed. During the welding process, the pipe body moves while the welding machine remains fixed, with welding speeds typically below 1.3 m/min. The welding energy input is approximately 10 kJ/cm, and no groove preparation is performed prior to welding. The dilution of the parent material at the weld site is about 50%, referring to the proportion of the steel plate material in the weld.

The wall thickness of straight-seam submerged arc-welded pipes typically ranges from 6 to 25 mm, with a maximum thickness of up to 45 mm. Groove preparation ensures strong welds, and a substantial amount of filler metal is deposited into the groove during the process. Higher welding speeds, typically around 3 m/min, are employed for greater productivity, qualifying as high-speed welding. The multi-wire submerged arc welding process is used, with dual-wire or triple-wire configurations. Carbon dioxide gas-shielded welding is also used in conjunction with dual-wire submerged arc welding. This method uses two sets of units to weld both the inner and outer surfaces of the pipe. The welding energy input is typically 50 kJ/cm, ensuring high weld quality.

Straight-seam submerged arc-welded pipes can produce thick-walled pipes. The maximum wall thickness of an X80-strength grade welded pipe can reach 25 mm, with the potential to increase to 40 mm if required. In comparison, spiral-welded pipes have a maximum thickness of just 12 mm.

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Landee Pipe is a professional industrial pipe manufacturer based in China, we have been producing pipe for a variety of applications, and covering areas of pipe manufacturing, exporting and trading. welcome to access our website: https://www.landeepipe.com.

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Teresa

Teresa is a skilled author specializing in industrial technical articles with over eight years of experience. She has a deep understanding of manufacturing processes, material science, and technological advancements. Her work includes detailed analyses, process optimization techniques, and quality control methods that aim to enhance production efficiency and product quality across various industries. Teresa's articles are well-researched, clear, and informative, making complex industrial concepts accessible to professionals and stakeholders.