When the flow rate of the fluid is known, the size of the pipe diameter depends on the allowable flow rate or the allowable friction resistance (pressure drop). When the flow rate is high, the pipe diameter is small, but the pressure drop value increases. Therefore, when the flow rate is high, investment in pipeline infrastructure can be saved, but the operating energy consumption of power equipment such as pumps and compressors increases. In addition, if the flow rate is too high, there may be other adverse factors. Therefore, the pipe diameter should be determined based on comprehensive considerations of construction investment, operating costs and other technical factors.
The connection method between the inlet and outlet pipes on pipes, pipe connectors, valves and equipment is determined by factors such as the nature of the fluid, pressure and temperature, as well as the material, size and installation location of the pipe. The main ones are threaded connections. , flange connection, socket connection and welding.
Threaded connections are mainly suitable for small diameter pipes. When connecting, it is generally necessary to wrap fluoroplastic sealing tape around the threaded connection part, or apply thick paint or wrap around hemp wire and other sealing materials to prevent leakage. When the pressure is above 1.6 MPa, gasket sealing is generally added to the end face of the pipe. This connection method is simple and can be disassembled and reinstalled, but a union must be installed at an appropriate place in the pipeline to facilitate disassembly and assembly.
Flange connections are suitable for a wide range of pipe diameters. When connecting, select different flanges and sealing gaskets according to the nature, pressure and temperature of the fluid, and use bolt clamping gaskets to maintain sealing. Flanges are mostly used in pipe sections that need to be frequently disassembled and installed and where pipes and equipment are connected. join.
Socket connection is mainly used for the connection between cast iron pipes, concrete pipes, clay pipes and their connecting parts. It is only suitable for water supply, drainage and gas pipelines working under low pressure and normal temperature conditions. When connecting, the groove of the socket is usually filled with flax, cotton thread or asbestos rope first, and then filled with asbestos cement or lead and other materials. A rubber sealing ring can also be filled in the socket to make it have better The flexibility allows a small amount of movement of the tube.
Welded connections have the best strength and sealing properties and are suitable for all kinds of pipes, saving labor and materials. However, the pipes and pipe connections must be cut off during disassembly.
Most of the water supply, drainage, heating, and gas supply pipelines and long-distance oil and gas pipelines in cities are laid underground, while process pipelines in factories are mostly laid underground to facilitate operation and maintenance. On the ground. Pipeline passage, support, slope and drainage, compensation, insulation and heating, anti-corrosion and cleaning, identification and painting, and safety are all important issues for both above-ground and underground laying.
Pipelines on the ground should try to avoid crossing roads, railways and waterways. When intersection cannot be avoided, the height spanned by the intersection should also enable pedestrians, vehicles and ships to pass safely. Underground pipes are generally laid along roads, and appropriate distances are maintained between various pipes to facilitate installation and maintenance; heating pipes have an insulation layer on their surface and are laid in trenches or protective pipes to avoid being crushed by the soil and allowing the pipes to expand. move.
Pipes may withstand many external forces, including their own weight, the thrust of fluid on the pipe end, wind and snow loads, soil pressure, thermal stress caused by thermal expansion and contraction, vibration loads and earthquake disasters, etc. . In order to ensure the strength and stiffness of the pipeline, various supports (hangers) must be installed, such as movable brackets, fixed brackets, guide brackets, spring brackets, etc. The setting of the bracket is determined by the diameter, material, pipe wall thickness, load and other conditions of the pipe. The fixed bracket is used to control the thermal elongation of the pipeline in sections to make the expansion joint work evenly; the guide bracket allows the pipe to move only axially.
In order to eliminate condensation water, steam and other water-containing gas pipelines should have A certain slope is generally not less than two thousandths. For underground drainage pipes that utilize gravity flow, the slope shall not be less than five thousandths. Steam or other water-containing gas pipelines are equipped with drainage pipes or traps at the lowest point. Some gas pipelines are also equipped with gas-water separators to drain water in time to prevent water hammer in the pipe and obstruction of gas flow. Water supply or other liquid pipelines are equipped with exhaust devices at the highest points to remove air or other gases accumulated in the pipelines to prevent air locks from causing operational abnormalities.
If the pipe cannot expand and contract freely, it will generate huge additional stress.
Therefore, on pipelines with large temperature changes and normal temperature pipelines that require free displacement, expansion joints need to be installed to compensate for the expansion and contraction of the pipeline and eliminate the influence of additional stress.
For steam pipes, high-temperature pipes, low-temperature pipes and pipes with anti-scalding and anti-freeze requirements, it is necessary to cover the outside of the pipes with insulation materials to prevent the loss of heat (cold) or freezing in the pipes. For some liquid pipelines with high freezing points, heating and insulation are also required to prevent the liquid from being too viscous or solidifying and affecting transportation. Commonly used insulation materials include cement perlite, glass wool, rock wool and asbestos diatomaceous earth.
In order to prevent soil erosion, the surface of underground metal pipes should be coated with anti-rust paint or anti-corrosion coatings such as tar and asphalt, or covered with glass cloth and linen impregnated with asphalt. Pipes buried in highly corrosive low-resistance soil must be equipped with cathodic protection devices to prevent corrosion. In order to prevent atmospheric corrosion, steel pipes on the ground are often coated with various anti-rust paints on their surfaces.
All kinds of pipes should be cleaned before use, and some pipes should also be cleaned regularly inside. For easy cleaning, filters or purging holes are provided on the pipeline. On pipelines transporting oil and natural gas over long distances, cleaners must be used to regularly remove dirt accumulated in the pipes. For this purpose, special devices for sending and receiving cleaners must be set up.
When there are many types of pipelines, in order to facilitate operation and maintenance, paint of a specified color is painted on the surface of the pipeline for identification. For example, use red for steam pipes, light blue for compressed air pipes, etc.
In order to ensure the safe operation of pipelines and promptly prevent the expansion of accidents when accidents occur, in addition to installing detection control instruments and safety valves on pipelines, special safety measures are also taken for some important pipelines, such as gas pipelines and Emergency pressure relief valves or emergency shut-off valves are installed on pipelines transporting oil and natural gas over long distances. They can automatically stop transportation in time when a catastrophic accident occurs to reduce disaster losses. 1. Characteristics of metal materials for pressure pipelines
Pressure pipelines involve all walks of life, and the basic requirements for them are "safety and use". Safety is for use, and use must be safe. Use also involves economic issues, that is, investment Save money and have a long service life, which is of course related to many factors. Materials are the foundation of engineering. We must first understand the special requirements of metal materials for pressure pipes. In addition to bearing loads, pressure pipelines also undergo special tests due to working in different environments, temperatures and media.
(1) Changes in the properties of metal materials at high temperatures
① Creep: When steel is subjected to external forces at high temperatures, it slowly and continuously undergoes plastic deformation over time. The phenomenon is called creep. The creep characteristics of steel are closely related to temperature and stress. As the temperature rises or the stress increases, the creep rate accelerates. For example, if the working temperature of carbon steel exceeds 300~350℃, and the working temperature of alloy steel exceeds 300~400℃, creep will occur. The stress required to produce creep is below the yield strength of the steel at the test temperature. Therefore, the steel materials used in boilers, steam pipes, and pressure vessels that work under high temperatures for a long time should have good creep resistance to prevent massive deformation due to creep, which may lead to structural rupture and explosions.
② Spheroidization and graphitization: Under the action of high temperature, cementite in carbon steel will migrate and aggregate due to energy gain, forming coarse-grained cementite and mixed with ferrite. , the cementite will gradually transform from flaky to spherical, which is called spheroidization. Since graphite has extremely low strength and appears in flakes, the strength of the material is greatly reduced and the brittleness is increased, which is called graphitization of the material. When carbon steel works in an environment above 425℃ for a long time, graphitization will occur, which is more obvious when it is above 475℃. SH3059 stipulates that the maximum use temperature of carbon steel is 425°C, and GB150 stipulates that the maximum use temperature of carbon steel is 450°C.
③ Thermal fatigue performance If steel works alternately between hot and cold for a long time, tiny cracks will occur inside the material under the action of thermal stress caused by changes in temperature, and will continue to expand, eventually leading to rupture. Therefore, the thermal fatigue performance of steel should be considered for structures and pipelines operating under temperature fluctuations.
④ High-temperature oxidation of materials Metal materials will be oxidized in high-temperature oxidizing media environments (such as flues) to produce oxide scales, which are prone to brittleness. Carbon steel is prone to oxide scale when exposed to high-temperature gases of 570°C, causing the metal to become thinner. Therefore, steel pipes such as gas and flue pipes should be limited to operating at 560°C.
(2) Performance changes of metal materials at low temperatures
When the ambient temperature is lower than the critical temperature of the material, the impact toughness of the material will decrease sharply. This critical temperature is called The brittle transition temperature of a material. Low-temperature impact toughness (impact energy) is commonly used to measure the low-temperature toughness of materials. For pipelines working at low temperatures, attention must be paid to their low-temperature impact toughness.
(3) Performance changes of pipelines in corrosive environments
Many pipeline media such as petrochemical industry, ships, and offshore oil platforms are corrosive. Facts have proved that the harmfulness of metal corrosion Very common and very serious, corrosion can cause direct or indirect losses. For example, stress corrosion, fatigue corrosion and intergranular corrosion of metals often cause catastrophic major accidents. Metal corrosion can cause a large amount of metal consumption and waste a lot of resources. The media that cause corrosion mainly include the following.
① Chloride The corrosion of carbon steel by chloride is basically uniform corrosion, accompanied by hydrogen embrittlement. The corrosion of stainless steel is pitting corrosion or intergranular corrosion. Preventive measures can be taken by choosing appropriate materials, such as carbon steel-stainless steel composite pipes.
② There are more than 250 kinds of sulfides in sulfide crude oil. Those that corrode metals include hydrogen sulfide (H2S), mercaptans (R-SH), thioethers (R-S-R), etc. The high content of H2S in my country's liquefied petroleum gas has caused cracks in the containers. Some cracks occurred within 87 days of being put into production. Afterwards, magnetic particle inspection showed that there were a total of 417 cracks in the ring seams on the inner surface, and there were no cracks on the outer surface of the sphere, so the H2S content High stress corrosion caused should be taken seriously. The Japanese Welding Society and the High Pressure Gas Safety Association stipulate that the H2S content in liquefied petroleum should be controlled below 100×10-6, while the average H2S content in my country’s liquefied petroleum gas is 2392×10-6, which is more than 20 times higher than that in Japan.
③ Naphthenic acid Naphthenic acid is an organic matter brought in crude oil. When the temperature exceeds 220℃, corrosion begins to occur, and the corrosion reaches its maximum at 270~280℃; when the temperature exceeds 400℃, corrosion in crude oil The naphthenic acid has completely vaporized. 316L (00Cr17Ni14Mo2) stainless steel material is an effective material resistant to naphthenic acid corrosion and is often used in high-temperature naphthenic acid corrosion environments.
2. Selection of metal materials for pressure pipes
① Meet the requirements of operating conditions. First of all, it should be judged according to the conditions of use whether the pipeline is under pressure and which type of pressure pipeline it belongs to. Different types of pressure pipelines have different importance, different degrees of harm caused by accidents, and different requirements for materials. At the same time, the use environment of the pipeline, the medium transported, and the degree of corrosion of the medium on the pipe body should be considered. For example, for steel pipe piles inserted into the seabed, the corrosion rate of the pipe body in the splash zone is 6 times that of the seabed soil; the corrosion rate of the pipe body in the tidal zone is 4 times that of the seabed soil. Special attention should be paid to material selection and anti-corrosion measures.
② Processability requirements. The material should have good processability and weldability.
③ Durable and economical requirements Pressure pipelines should first be safe, durable and economical. For a piece of equipment or a batch of pipeline projects, before investing in materials selection, conduct a feasibility study, that is, economic and technical analysis, if necessary. Several plans can be made for the materials to be selected, and economic and technical analysis can be conducted. The initial investment for some materials is slightly higher, but the use It is reliable and saves daily maintenance costs; some materials seem to save initial investment, but have poor reliability during operation, high daily maintenance costs and high life cycle costs. As early as 1926, the American Petroleum Institute (API) released the API-5L standard, which initially only included three steel grades: A25, A, and B. It was released several times later, see Table 4. Table 4 Pipeline steel grades released by API
Note: In 1972, API released U80 and U100 standards, which were later changed to X80 and X100.
Before 2000, the world used X70, which was about 40, and X65 and .
my country's metallurgical industry has made great efforts to develop pipeline steel for more than ten years. It is currently working hard to tackle the X70 wide plate. Shanghai Baoshan Iron and Steel Company, Wuhan Iron and Steel Company and other X70 and X80 chemical compositions and mechanics The performances are listed in Tables 5 to 9 respectively. Table 5 WISCO X80 coiled plate properties Table 6 Mechanical properties of X70 grade steel pipes Table 7 Bending performance test results of X70 grade steel pipes Table 8 Charpy impact toughness of X70 grade steel pipes Table 9 Charpy impact toughness of high-strength conveyor pipes
The pipe types commonly used in oil pipelines in my country include spiral submerged arc welded pipe (SSAW), straight seam submerged arc welded pipe (LSAW), and electric resistance welded pipe (ERW). When the diameter is less than 152mm, seamless steel pipes are used.
From the late 1960s to the 1970s, spiral welded pipe factories developed rapidly in my country. Almost all crude oil pipelines used spiral welded steel pipes. Spiral welded steel pipes were also used in the first-class areas of the "West-East Gas Pipeline" pipeline. The disadvantages of spiral welded steel pipes are large internal stress, poor dimensional accuracy, and high probability of defects. According to expert analysis, the policy of "walking on two legs" should be adopted. First, it is promising to actively carry out technological transformation of existing spiral welded pipe factories; second, to vigorously develop my country's longitudinal submerged arc welded pipe industry.
ERW steel pipe has the characteristics of smooth appearance, high dimensional accuracy and low price, and has been widely used at home and abroad. Most of my country's oil and gas resources are distributed in the northeast and northwest regions, while most of the consumer markets are in densely populated areas such as large and medium-sized cities on the southeastern coast and in central and southern China. This severe separation of production and marketing markets makes the transportation of oil and gas products a major issue in the development of oil and gas resources. and biggest barriers to utilization. Pipeline transportation is the best way to break through this obstacle. Compared with railway transportation, pipeline transportation is a large-capacity, safer, and more economical way to transport oil and gas products. Its construction investment is half of that of railways, and the transportation cost is only one third. Therefore, our government has included the development strategy of “strengthening the construction of oil and gas pipelines and forming a pipeline transportation network” into the “Tenth Five-Year Plan” development plan. According to the plans of relevant parties, in the next 10 years, my country will build 14 oil and gas pipelines, forming an oil and gas pipeline transmission pattern of "two vertical, two horizontal, four hubs, and five gas reservoirs" with a total length of more than 10,000 kilometers. This indicates that my country is about to usher in the peak period of oil and gas pipeline construction.
The key natural gas pipeline projects under construction or planned to be constructed in our country include: West-East Gas Transmission Project, with a total length of 4,176 kilometers and a total investment of 120 billion yuan. Construction officially started in September 2000 and was completed in 2004. ; The Seninglan Gas Pipeline Project, with a total length of 950 kilometers, started construction in May 2000 and is nearing completion. The natural gas has been sent to Xining; the Zhongxian to Wuhan Gas Pipeline Project, with a total length of 760 kilometers, preliminary preparations Significant progress has been made, and 4 of the 11 tunnels under construction have been connected; the Shijiazhuang-Zhuozhou gas pipeline project, with a total length of 202 kilometers, started construction in May 2000 and has been completed; the Shijiazhuang-Handan gas pipeline project, The total length is about 160 kilometers; the double line of the Shaanxi Jingbian to Beijing gas transmission project; the double line of the Shaanxi Jingbian to Xi'an gas transmission project; the Shaanxi Gansu Ningxia to Hohhot gas transmission project, the total length is 497 kilometers; the Hainan Island natural gas pipeline project, the total length is about 270 kilometers; the Shandong Longkou to Qingdao gas pipeline project, with a total length of about 250 kilometers; the China-Russia gas pipeline project, with a total length of 2,000 kilometers in China; the Guangdong Liquefied Natural Gas Project, investment promotion work has been completed and is planned to be completed in 2005. The oil pipelines under construction and to be built include: Lancheng-Chongqing refined oil pipeline project, with a total length of 1,207 kilometers, which started construction in May 2000; China-Russia oil pipeline project, approximately 700 kilometers long in China; China-Kazakhstan oil pipeline project , 800 kilometers long in China. In addition, the 2,000-kilometer-long refined oil pipeline from Maoming, Guangdong to Guiyang to Kunming and the crude oil pipeline from Zhenhai to Shanghai and Nanjing are also about to start construction. In addition to the main lines, the construction of large-scale urban gas pipeline networks must also be carried out at the same time.
Faced with such a huge market and such a rare development opportunity, new challenges have been posed to pipeline construction technology. Under the same transmission volume, it is more economical to build one high-pressure large-diameter pipeline than to build several low-pressure small-diameter pipelines in parallel. For example, a gas pipeline with a transmission pressure of 7.5MPa and a diameter of 1,400mm can replace three pipelines with a pressure of 5.5MPa and a diameter of 1,000mm. However, the former can save investment 35 and steel 19. Therefore, expanding the diameter of the pipeline has become a major issue in pipeline construction. symbol of scientific and technological progress. Increasing the delivery pressure within a certain range can increase economic benefits. Taking a gas transmission pipeline with a diameter of 1020mm as an example, the operating pressure is increased from 5.5MPa to 7.5MPa, the gas transmission capacity is increased by 41%, materials are saved by 7%, and investment is reduced by 23%. Calculations show that if the working pressure of the gas pipeline can be further increased from 7.5MPa to 10-12MPa, the gas transmission capacity will be further increased by 33-60%. The pressure of the trans-Alaska gas pipeline in the United States is as high as 11.8MPa, and the oil pipeline reaches 8.3MPa, making them the pipelines with the highest operating pressure at present.
The increase in pipe diameter and delivery pressure require pipes to have higher strength. On the premise of ensuring weldability and impact toughness, the strength of the pipe has been greatly improved. Since pipeline laying is completely completed by welding technology, the quality of welding determines the quality of the project to a large extent. Welding is a key link in pipeline construction. Pipe materials, welding materials, welding processes and welding equipment are key factors that affect welding quality.
my country began to build large-diameter long-distance pipelines in the early 1970s. The famous "August 3rd" pipeline battle built oil pipelines from Daqing Oilfield to Tieling, from Tieling to Dalian, and from Tieling to Qinhuangdao, solving the problem of Daqing’s crude oil export problem is troubling.
The design diameter of the pipeline is φ720mm, the steel material is 16MnR, submerged arc spiral welded pipe, and the wall thickness is 6~11mm. The welding process plan is: manual arc welding method, upward welding operation process; J506 and J507 electrodes are used as welding materials, baked at 400°C for 1 hour before welding, φ3.2 base, φ4 filling, and cover; the welding power source adopts rotating DC Arc welding machine; the groove is 60° V-shaped, and the root is welded on one side and formed on both sides.
The pipeline built during the "August 3rd" Battle in Northeast China has been in operation for 30 years and is still in service, proving that the process plan was correct and the construction quality was good.
In the early 1980s, the manual downward welding process began to be promoted, and cellulose type and low hydrogen type downward welding rods were developed. Compared with the traditional upward welding process, downward welding has outstanding advantages such as fast speed, good quality, and saving of welding materials, so it has been widely used in pipeline circumferential seam welding.
In the early 1990s, semi-automatic manual welding with self-shielded flux-cored wire was promoted, which effectively overcomes the shortcomings of other welding processes that have poor wind resistance in field operations. It also has the advantages of high welding efficiency, good quality and stability. Features, now become the main method of pipe circumferential seam welding.
The application of all-position automatic welding of pipelines has been explored for many years, and now there has been a breakthrough. It has been successfully used in the West-East Gas Pipeline Project. Its efficiency and quality are unmatched by other welding processes. This marks that my country's oil and gas pipeline welding technology has reached a high level. 2.1 The development history of pipeline steel
Early pipeline steel always used ordinary carbon steel of C, Mn, and Si types. It focused on metallurgical performance and did not have strict regulations on chemical composition. Since the 1960s, as the pressure and diameter of oil and gas pipelines have increased, low-alloy high-strength steel (HSLA) has been used, mainly supplied in hot-rolled and normalized states. The chemical composition of this type of steel: C ≤ 0.2, alloying elements ≤ 3 to 5. With the further development of pipeline steel, by the late 1960s and early 1970s, the American Petroleum Organization proposed three microalloy controlled rolling steels, X56, X60 and X65, in the API 5LX and API 5LS standards. This kind of steel breaks through the concept of traditional steel. The carbon content is 0.1-0.14. Nb, V, Ti and other alloying elements ≤0.2 are added to the steel, and the mechanical properties of the steel are significantly improved through the controlled rolling process.
By 1973 and 1985, API standards successively added X70 and Multi-component micro-alloyed controlled rolling and cooling steel.
The application of pipeline steel in my country started relatively late. Most of the oil and gas pipelines that have been laid in the past used Q235 and 16Mn steel. During the "Sixth Five-Year Plan" period, my country began to develop X60 and X65 pipeline steel in accordance with API standards and successfully used them for pipeline laying together with imported steel pipes. In the early 1990s, Baosteel and Wuhan Iron and Steel successively developed high-strength and high-toughness X70 pipeline steel, which was successfully used in the Seninglan Pipeline Project.
2.2 Main mechanical properties of pipeline steel
The main mechanical properties of pipeline steel are strength, toughness and mechanical properties in environmental media.
The tensile strength and yield strength of steel are determined by the chemical composition of the steel and the rolling process. When selecting materials for gas pipelines, steel types with higher yield strength should be selected to reduce the amount of steel used. But the higher the yield strength, the better. Yield strength that is too high will reduce the toughness of the steel. When selecting steel types, the proportional relationship between the yield strength and tensile strength of the steel - the yield-strength ratio - should also be considered to ensure the pipe forming quality and welding performance.
After repeated stretching and compression, the mechanical properties of steel will change, and the strength will decrease, seriously reducing by 15%, which is the Bauschinger effect. This factor must be considered when ordering steel plates for pipe making. It can be increased by 40-50MPa based on the minimum yield strength of this grade of steel.
The fracture toughness of steel is related to chemical composition, alloying elements, heat treatment process, material thickness and directionality. The content of C, S, and P in steel should be reduced as much as possible, alloying elements such as V, Nb, Ti, and Ni should be appropriately added, and processes such as controlled rolling and controlled cooling should be adopted to improve the purity of the steel, make the material uniform, and refine the grains. , can improve steel toughness. Most of the methods adopted are to reduce C and increase Mn.
When pipeline steel is exposed to oil and gas environments containing hydrogen sulfide, hydrogen-induced cracking occurs due to the intrusion of hydrogen produced by corrosion into the steel. Therefore, pipeline steel for transporting acid oil and gas should have low sulfur content to effectively control the morphology of non-metallic inclusions and reduce microscopic component segregation. The hardness value of pipeline steel also has an important impact on HIC. In order to prevent hydrogen-induced cracking in steel, it is generally believed that the hardness should be controlled below HV265.
2.3 Weldability of pipeline steel
As the carbon equivalent of pipeline steel decreases, the susceptibility to welding hydrogen-induced cracking decreases, and the process measures required to avoid cracks decrease, and the welding heat Performance impairment in the affected area is reduced. However, since pipeline steel undergoes a series of complex non-equilibrium physical and chemical processes during welding, it may cause defects in the welding zone or reduce the performance of the joint. The main problems are welding cracks and embrittlement of the welding heat affected zone.
Due to its low carbon content, pipeline steel has a reduced hardening tendency and a reduced cold cracking tendency. However, as the strength level increases and the plate thickness increases, there is still a certain tendency for cold cracking. During on-site welding, welding materials with high hydrogen content such as cellulose electrodes and self-protected flux-cored wires are often used. The line energy is small and the cooling rate is fast, which will increase the sensitivity of cold cracks. Necessary welding measures need to be taken, such as welding Preheating, etc.
The embrittlement of the welding heat-affected zone is often the root cause of pipeline fractures and catastrophic accidents. There are two main areas where local embrittlement occurs, namely, embrittlement in the coarse-grained area in the heat-affected zone, which is caused by the excessive growth of grains in the overheated area and the formation of poor structures. During multi-layer welding, the coarse-grained area becomes critically embrittled. That is, the coarse-grained area of ??the previous weld bead is caused by the reheating of the two-phase area of ??the subsequent weld bead. This can improve the toughness by adding a certain amount of Ti and Nb microalloying elements to the steel and controlling the post-weld cooling rate to obtain a suitable t8/5.
2.4 Steel pipes used in the West-East Gas Pipeline Project
The steel pipes used in the West-East Gas Pipeline Project are X70 grade pipeline steel, with specifications of Φ1 016mm×14.6~26.2mm, among which spiral welded pipes Accounting for about 80%, straight seam submerged arc welded pipes account for about 20%, and the consumption of pipeline steel is about 1.7 million tons.
In addition to Nb, V, and Ti, X70 pipeline steel also adds a small amount of Ni, Cr, Cu, and Mo, which delays the formation of ferrite to a lower temperature and is conducive to the formation of needle-like shapes. ferrite and lower bainite. Therefore, X70 pipeline steel is essentially a needle-shaped ferrite type high-strength and high-toughness pipeline steel. The chemical composition and mechanical properties of steel pipes are shown in Table 1 and Table 2. Characteristics of on-site welding
Because the oil and gas fields discovered and exploited are located in remote areas with harsh geographical, climatic, and geological conditions, as well as poor social support conditions, it brings many difficulties to the construction, especially the low temperature that causes the greatest trouble. .
When welding on site, use a mouthpiece to assemble the pipe mouths. In order to improve efficiency, foundation beams or mounds of soil are generally placed under the aligned pipe openings, and preparations for the next docking are started while the previous docking joint is being welded. This will produce greater additional stress. At the same time, due to the influence of thermal expansion and contraction of steel pipes, problems are most likely to occur due to additional stress when hitting dead ends.
The on-site welding position is horizontal fixed or tilted fixed butt joint of pipes, including flat welding, vertical welding, overhead welding, horizontal welding and other welding positions. Therefore, higher and stricter requirements are put forward for welders’ operating skills.
Today's pipeline industry requires pipelines to have higher delivery pressures and larger pipeline diameters and to ensure their safe operation. In order to adapt to the high strengthening and toughening of pipeline steel, the enlargement of pipe diameters and the thickening of pipe walls, a variety of welding methods, welding materials and welding processes have emerged.
Pipeline construction welding methods
Overseas pipeline welding construction has experienced the development process of manual welding and automatic welding. Manual welding is mainly downward welding with cellulose electrodes and downward welding with low hydrogen electrodes. In terms of automatic pipeline welding, there is a pipeline flash butt welding machine developed by the former Soviet Union, which welded tens of thousands of kilometers of large-diameter pipelines during the former Soviet Union. Its distinctive features are high efficiency and strong adaptability to the environment. The CRC multi-head gas shielded pipeline automatic welding system developed by the American CRC company consists of three parts: a pipe end beveling machine, an internal seamer and internal welding machine combination system, and an external welding machine. So far, the cumulative length of welded pipelines worldwide has exceeded 34,000km. France, the former Soviet Union and other countries have also researched and applied similar automatic welding technology inside and outside pipelines. This technical direction has become the mainstream of automatic welding technology for large-diameter pipelines in the world today.
my country's steel pipeline girth welding technology has experienced several major changes. In the 1970s, traditional welding methods and low-hydrogen electrode manual arc welding upward welding technology were adopted. In the 1980s, manual arc welding downward welding was promoted. The welding technology is downward welding with cellulose electrodes and low-hydrogen electrodes. In the 1990s, self-shielded flux-cored wire semi-automatic welding technology was applied. Today, all-position automatic welding technology has been fully promoted.
Manual arc welding includes the application of cellulose electrodes and low hydrogen electrodes. Manual arc welding upward welding technology was the main welding method used in pipeline construction in my country in the past. It is characterized by a large gap between the nozzles. The arc rest operation method is used during the welding process. The thickness of each weld layer is large and the welding efficiency is low. . Manual arc welding downward welding is a welding technology introduced from abroad in the 1980s. It is characterized by a small gap between the nozzles. The welding process is completed by using a large current, multi-layer, and fast welding operation method. It is suitable for flow operations and welding. Higher efficiency. Since the thickness of each weld layer is thin, the toughness of the girth welded joint can be improved by the heat treatment of the rear weld layer on the front weld layer. The manual arc welding method is flexible, simple and adaptable. The organic combination of downward welding and upward welding methods and the good root welding adaptability of cellulose electrodes cannot be replaced by automatic welding methods in many situations.
Self-protected flux-cored wire semi-automatic welding technology has been applied to pipeline construction since the 1990s, and is mainly used for filling and covering. It is characterized by high deposition efficiency, good forming in all positions, strong environmental adaptability, and easy mastery by welders. It is an important welding process for pipeline construction.
With the improvement of the strength grade of steel pipes used in pipeline construction, and the increase of pipe diameter and wall thickness, automatic welding technology has gradually begun to be applied in pipeline construction. Automatic pipeline welding technology has great potential in the application of large-diameter, thick-walled pipeline construction due to its advantages of high welding efficiency, low labor intensity, and the welding process is less affected by human factors.
However, my country's automatic pipeline welding technology is in its infancy, the problem of automatic root welding has not yet been solved, and supporting facilities such as pipe end bevel shaping machines are not yet mature, which limits the large-scale application of automatic welding technology. The long-term solidification of oil sludge and rust in the pipe causes the original pipe diameter to become smaller;
Long-term sludge precipitation in the pipe produces hydrogen sulfide gas, causing environmental pollution and easily causing explosions;
Hydrogen in waste water Acid and alkali substances are easy to corrode the pipeline wall; irregular removal of foreign matter in the pipeline causes pipeline blockage; 1. Chemical cleaning: Chemical cleaning of pipelines uses chemicals to temporarily transform the pipeline, using temporary pipelines and circulating pump stations. Carry out cyclic chemical cleaning from both ends of the pipeline. This technology has the characteristics of strong flexibility, no requirements on the shape of the pipeline, fast speed, and thorough cleaning.
2. High-pressure water cleaning: Use high-pressure water jets above 50Mpa to peel and clean the dirt on the inner surface of the pipeline. This technology is mainly used for short-distance pipelines, and the diameter of the pipeline must be greater than 50cm. This technology has the characteristics of high speed and low cost.
3. PIG pigging: PIG industrial pigging technology relies on the driving force generated by the pump to push the fluid to drive the PIG (pipe pig) forward in the pipe, and discharge the dirt accumulated in the pipeline out of the pipe. , so as to achieve the purpose of cleaning. This technology is widely used in cleaning projects such as various process pipelines and oil field oil and steam pipelines. Especially for pipeline cleaning that transports fluids over long distances, it has advantages that other technologies cannot replace.