Application of ERW pipes in transporting gas

1. Types of pipeline steel pipes for natural gas transmission

At present, the pipes used in domestic natural gas pipeline projects can be divided into three categories: seamless steel pipes, high-frequency electric resistance welded steel pipes (HFW or ERW) and submerged arc welded steel pipes. Arc welded pipe (SSAW), straight seam submerged arc welded pipe (LSAW), high frequency straight seam resistance welded pipe (HFW) (ie ERW) and other 4 types.

1.1 Seamless steel pipe (SML)
Seamless steel pipes for natural gas transmission are formed by hot-rolled killed steel. Since there are no welds, there is no groove corrosion problem on the pipe wall, but the size deviation is large. When the pipe diameter is too large, the wall thickness deviation is large, and the consumption of pipe materials is large. It is 8%~10% lower than the straight seam electric resistance welded steel pipe, and the longitudinal impact toughness of the seamless steel pipe is lower than the transverse impact toughness of the straight seam electric resistance welded steel pipe, especially when the impact toughness drops significantly below 0 °C. Domestic hot-rolled seamless steel pipe diameter ≤ DN600, mostly below DN400. Seamless steel pipes are the most expensive due to their excellent performance. They are generally used for small-diameter pipeline projects or difficult control projects, and small and medium-sized pipes in stations.

1.2 Spiral submerged arc welded steel pipe (SSAW)
The spiral submerged arc welded steel pipe is rolled by killed steel hot-rolled (controlled rolling) steel plate (strip), and welded by automatic submerged arc welding. The weld has deposited filler metal, and the weld strength is generally not lower than that of the base metal; Uniformity, more specifications, less weld force, better quality control, and lower price than seamless steel pipes. The DN300~DN1000 spiral welded steel pipes produced in my country are widely used in domestic long-distance oil and gas pipelines.

1.3 Directly welded submerged arc welded seam steel pipe (LSAW)
The straight seam submerged arc welded steel pipe is made of killed steel hot-rolled (controlled rolling) steel plate (strip), formed by UOEJCOE or HME method, and welded by automatic submerged arc welding. The weld has deposited filler metal, and the weld strength is generally not less than - . Base material; it is characterized by good weld quality, high port size accuracy, and easy butt welding; the disadvantage is that the investment in pipe making equipment is large, the cost of pipe making is high, and the price of pipes is high. Domestic LSAW steel pipes have a diameter of DN300~DN1000 and are widely used in important projects.

1.4 High frequency straight seam electric resistance welded steel pipe (ERW)
High-frequency straight seam welded pipe (ERW) is formed by forming a hot-rolled coil through a forming machine, using the skin effect and proximity effect of high-frequency current to heat and melt the edge of the tube blank, and pressure-welding it under the action of the extrusion roller. to achieve production. The high-frequency resistance welding method began to be applied to the production of welded pipes in the 1950s. In the past ten years, its production technology has been perfected day by day, and the product quality has been continuously improved. At the end of the 20th century, the production technology of ERW steel pipes in the world has been relatively mature. Foreign companies do not shy away from using ERW steel pipes in pipeline design. ERW steel pipes have been widely used in both land and sea oil well pipes and oil and gas transmission pipes. In addition to being used for conventional oil and gas transportation, the newly developed ERW steel pipe has been used in polar cold regions and acidic medium conditions, and Russia even uses it to transport natural gas under extremely low temperatures in Siberia. At the beginning of the 21st century, domestic experts in the industry publicly advocated the selection of ERW steel pipes in long-distance oil and gas pipelines, but because domestic manufacturers have different understandings and mastery of the technology, and product positioning is different. Major quality accidents, as well as the specification adaptability of spiral seam submerged arc welded steel pipes and domestic production capacity have greatly restricted the use of ERW steel pipes in long-distance pipelines. The use of steel pipes in China is mostly used for oil pipelines with low pressure and urban natural gas/coal gas pipeline network projects.

The shortcomings of carbon segregation, slag inclusion and unstable pipe quality in ERW pipe welds have always been the reasons for the limited use of ERW pipes in some gas fields in the past few years. With the advancement of various technologies and the accumulation of experience, the above-mentioned problems have basically been solved or found a suitable solution. First of all, regarding the problem of carbon segregation, when our company purchases steel plates, it pays great attention to the use of products from steel mills with high qualifications, controls the quality of the base metal from the source, and strictly requires the use of low-carbon, low-phosphorus, low-sulfur, micro-alloy The manganese content is also controlled in an appropriate range. In recent years, the development of metallurgical technology in this field can fully meet the requirements of steel pipe production for plates. New technologies such as converter smelting, ladle refining, vacuum degassing, and continuous casting in the smelting process The adoption of technology; the use of controlled rolling and controlled cooling technology in the rolling process has greatly improved the appearance size, appearance quality, and physical and chemical properties of hot-rolled steel strips for large and medium-diameter ERW steel pipes.

In addition, the heat treatment process of steel pipe production is also strictly controlled, and the fluctuation range of heat treatment temperature is controlled to the minimum as much as possible. To solve the problem of weld slag inclusion, the most important thing is to control the manganese-silicon ratio in the base metal of the steel plate above 3.5; manufacturers with strong technical strength can control the shape of the edge of the plate (under the premise that the technical conditions are met, the opening angle As large as possible), welding speed (as fast as possible), welding temperature (moderate temperature, base metals of various materials and specifications adapt to different temperatures) and other means to avoid the occurrence of weld slag inclusions. With the emergence of high-purity high-quality steel, mature welding technology, the adoption of many new technologies, strict quality control and a complete testing system, the quality of domestic ERW pipes has been greatly improved, and the pressure rating and use occasions have continued to expand. Used in gas pipelines. For example, the products of major domestic ERW manufacturers such as Baoji Zhujin and Baosteel have a one-time pass rate of 92%~93%, the problems of carbon segregation and slag inclusion are basically solved, and the reliability and corrosion resistance of welds are greatly improved. , It meets the requirements of use under various conditions, and the quality stability has reached a high level. Internationally high-quality ERW steel pipes are not only indistinguishable from the weld seam, but also have a weld seam coefficient of up to 100%, realizing the matching of the weld seam area structure and the base metal.

At present, the output of ERW pipe in my country ranks first in the world, and the scope of application continues to expand. It is not only used in long-distance pipeline projects for transporting commercial gas, but also in important station projects (such as: pipes for station stations of the second line of West-East Gas Pipeline ERW steel pipe is used). The relevant departments of China National Petroleum Corporation are also formulating relevant regulations on the use of HFW. In this case, it is worth discussing and trying to use ERW steel pipes in pipeline projects with certain water content or acidity.

2.ERW vs seamless pipe

When selecting pipes, there are several pipes mentioned above for comparison. The big difference between ERW steel pipe billet and seamless steel pipe (SML) pipe billet is that the former is manufactured by cold rolling of hot-rolled steel strip, while the latter is formed by piercing round steel in a hot-rolled state. Obviously, the grain size and weaving density of the SML tube blank formed in the hot rolling state have a large gap compared with the ERW tube blank, and the performance that can be caused by this must also be very different. In addition, the dimensional accuracy of ERW steel pipe is higher than that of SML. This is because the seamless pipe is formed by continuous piercing of the round steel in the hot rolling mill, while the ERW steel pipe is formed by the steel strip in the cold rolling state. In contrast, ERW is easier to control in terms of appearance and size than SML. Because of this, the appearance quality defects of ERW steel pipes are less, and the appearance quality is better than that of SML steel pipes. Due to advanced production technology and high production efficiency, the production cost of ERW steel pipes is much lower than that of seamless steel pipes.

Comparison of ERW steel pipe and SSAW

There is little difference in price. The length of the SSAW weld is about 2 to 3 times that of the ERW weld, and there is no process capability to remove the internal weld bead, so the friction of the inner wall of the pipeline is inevitably increased when the medium is transported. However, the length of ERW weld is short, and the internal welding tumor can be removed. When the medium flows in the pipeline, the friction resistance generated by the inner wall of the pipeline is relatively small compared with the spiral seam submerged arc welded steel pipe. According to the measurement results of residual stress, it is shown that the circumferential forming internal stress of ERW welded pipe is small, which is beneficial to the pipe. For the SSAW welded pipe, due to its large weld area, the internal stress of the weld is both tensile and compressive, and the stress state is more complicated than that of the ERW pipe, which is related to the forming process of the spiral welded pipe. The large-scale tensile stress of the SSAW welded pipe in the weld seam and the inner wall of the heat-affected zone will have an adverse effect on the crack initiation and stress corrosion of inner wall defects [5]. Since the length of SSAW weld seam is about 2~3 times of that of high-frequency straight seam pipe, the anti-corrosion treatment of finished pipe weld seam is more difficult; ERW steel pipe has short weld seam (compared with spiral seam steel pipe), and the wall thickness of steel pipe is uniform , Stable length, high precision of geometric dimension, anti-corrosion and easy to guarantee the quality of assembly and welding. At the same time, because the gas of this project contains a small amount of water and carbon dioxide and has weak acidity, it is not suitable to use SSAW welded pipes. Technically speaking, ERW steel pipes and LSAW steel pipes have similar advantages and characteristics in comparison with SML steel pipes and SSAW steel pipes.

Previously, the low-temperature toughness and corrosion resistance of straight welded seams of LSAW steel pipes were relatively high, and they were recommended to be used in gas-gathering branch lines and trunk lines for wet gas transportation in gas fields above 200mm. On the other hand, the price of ISAW steel pipe is relatively high, which is close to the price of seamless steel pipe. Under the premise that the weld seam quality of ERW steel pipe is guaranteed, it is feasible to use ERW pipe for wet gas transportation in western Sichuan and save a lot of investment. In view of the above analysis of advantages and disadvantages, ERW steel pipes are used in the Lianhuashan structure test mining main line project. During the procurement, special emphasis was placed on the requirements for the impact performance of the pipe welds and the non-destructive testing of the steel pipe welds, which further ensured the quality of the steel pipes. Shortly after the pipeline project was put into production, on May 12, 2008, the Wenchuan earthquake occurred in the Longmen Mountains area of Sichuan. The Lianhuashan structure, also located in the Longmen Mountains, tried to mine the main pipeline project, and it survived the severe test. In the three years since the earthquake, the pipeline has been running safely and smoothly, during which it passed the official acceptance in May 2010.

3. Conclusion

ERW steel pipe technology has tended to be mature and perfect. Theory and practice have proved that ERW steel pipe has the characteristics of reliability, safety and economy. After being used for the first time in the field of wet gas transportation in Lianhuashan structure trial production main line project, it has also been applied in the wet gas transportation of Jiulongshan gas field trial production main line project (Guangyuan, Sichuan). Of course, its reliability and corrosion resistance of the weld still need to be monitored and verified in long-term use. But all of them undoubtedly provide a valuable reference for the wider popularization and use of ERW steel pipes in future natural gas pipeline projects.