The connection mode between pipelines, pipe joints, valves and inlet and outlet nozzles on equipment is determined by factors such as the nature of fluid, pressure and temperature, and the material, size and installation position of pipelines. There are four main methods: threaded connection, flange connection, socket connection and welding.
Threaded connection is mainly suitable for small diameter pipes. When connecting, it is generally necessary to wrap fluoroplastic sealing tape on the threaded connection part, or coat it with thick paint and wrap it with hemp and other sealing materials to prevent leakage. When the pressure is above 1.6 MPa, the end face of the pipeline is generally sealed with gasket. This connection method is simple and can be disassembled, but movable joints should be installed in appropriate places in the pipeline for disassembly.
Flange connection is suitable for all kinds of pipe diameters. When connecting, different flanges and sealing gaskets are selected according to the nature, pressure and temperature of the fluid, and the gasket is clamped by bolts to keep the seal. Flange connection is mostly used in pipe sections that need to be disassembled frequently and where pipelines are connected with equipment.
Socket connection is mainly used to connect cast iron pipes, concrete pipes, clay pipes and their connectors, and is only suitable for water supply, drainage and gas pipes working at low pressure and normal temperature. When connecting, hemp, cotton thread or asbestos rope are usually filled in the groove of the socket first, and then filled with asbestos cement or lead. Rubber sealing rings can also be filled in the socket to make it elastic and allow the pipeline to move slightly.
Welding connection has the best strength and tightness, and is suitable for all kinds of pipelines, saving labor and materials, but the pipe and pipe joint must be cut off when disassembling.
Urban water supply, drainage, heating, gas supply and long-distance oil and gas pipelines are mostly laid underground, while factory process pipelines are mostly laid on the ground, which is convenient for operation and maintenance. The passage, support, slope, drainage and exhaust, compensation, heat preservation and heating, anti-corrosion and cleaning, marking and painting, and safety of pipelines are all important issues, whether they are laid on the ground or underground.
Ground pipelines should try to avoid crossing roads, railways and waterways. When crossing is unavoidable, the crossing height should also enable pedestrians and vehicles to pass safely. Underground pipelines are generally laid along roads, and various pipelines are kept at a proper distance, which is convenient for installation and maintenance. There is thermal insulation layer on the surface of heating pipeline, which should be laid in ditch or protective pipe to avoid being squeezed by soil and making the pipeline expand and move.
The pipeline may be subjected to many external forces, including self-weight, thrust of fluid to the pipe end, snow load, earth pressure, thermal stress caused by thermal expansion and contraction, vibration load, earthquake disaster and so on. In order to ensure the strength and rigidity of the pipeline, it is necessary to set up various supports (hangers), such as movable supports, fixed supports, guide supports and spring supports. The setting of support depends on pipe diameter, material, wall thickness and load. 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 enables the pipeline to only move axially,
In order to remove condensed water, water-containing gas pipelines such as steam should have a certain slope, generally not less than two thousandths. Underground gravity drainage pipeline, 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, and some gas pipelines are also equipped with gas-water separators to discharge water and liquid in time to prevent water hammer from occurring in the pipelines and hinder gas flow. Water supply or other liquid pipelines are equipped with an exhaust device at the highest point to eliminate air or other gases accumulated in the pipeline and prevent air resistance from causing abnormal operation.
If the pipeline can't expand and contract freely, it will produce huge additional stress. Therefore, it is necessary to set expansion joints on pipelines with large temperature changes and normal temperature pipelines that need free displacement to compensate the thermal expansion and cold contraction of pipelines and eliminate the influence of additional stress.
For steam pipes, high-temperature pipes, low-temperature pipes and pipes with anti-scalding and anti-freezing requirements, it is necessary to coat insulation materials on the pipes to prevent heat (cold) loss or freezing in the pipes. For some liquid pipelines with high freezing point, it is necessary to heat and keep warm to prevent the liquid from being too viscous or solidified and affecting the transportation. Commonly used thermal insulation materials are cement perlite, glass wool, rock wool and asbestos diatomite.
In order to prevent soil erosion, the surface of underground metal pipes should be coated with anticorrosive coatings such as antirust paint, tar and asphalt, or coated with glass cloth and linen impregnated with asphalt. Cathodic protection devices shall be provided for pipelines buried in corrosive low-resistance soil to prevent corrosion. In order to prevent atmospheric corrosion, steel pipes on the ground are often coated with various antirust paints.
All kinds of pipes should be cleaned before use, and some pipes should be cleaned regularly. In order to facilitate cleaning, the pipeline is provided with a filter or a purge hole. In long-distance oil and gas pipelines, it is necessary to use sweepers to regularly remove the scale in the pipeline, so a special device for receiving and sending sweepers should be set up.
When there are many kinds of pipes, in order to facilitate operation and maintenance, paint the surface of the pipes with specified colors for identification. For example, the steam pipeline is red and the compressed air pipeline is light blue.
In order to ensure the safe operation of pipelines and prevent accidents from expanding in time, in addition to installing detection and control instruments and safety valves on pipelines, special safety measures have been taken for some important pipelines, such as installing accident safety valves or emergency cut-off valve on gas pipelines and long-distance pipelines transporting oil and natural gas. They can automatically stop transportation in time when catastrophic accidents occur, reducing disaster losses. 1. Characteristics of metal materials for pressure pipelines
Pressure pipeline involves all walks of life, and its basic requirement is "safety and use". For safe use, it must be used safely. The use also involves economic problems, that is, saving investment and long service life, which is of course related to many factors. Material is the foundation of engineering. First of all, we should understand the special requirements of metal materials for pressure pipes. In addition to bearing loads, pressure pipes will be subjected to special tests because they work in different environments, temperatures and media.
Changes of (1) metallic materials at high temperature.
① Creep: When steel is subjected to external force at high temperature, plastic deformation occurs slowly and continuously with the extension of time. This phenomenon is called creep. The creep characteristics of steel are closely related to temperature and stress. Creep speed increases with the increase of temperature or stress. For example, when the working temperature of carbon steel exceeds 300 ~ 350℃, creep will occur when the working temperature of alloy steel exceeds 300 ~ 400℃. The stress required for creep is lower than the yield strength of steel at the test temperature. Therefore, the steel used for boilers, steam pipes and pressure vessels working at high temperature for a long time should have good creep resistance, so as to prevent a lot of deformation caused by creep from causing vicious accidents such as structural rupture and explosion.
② Spheroidization and graphitization: Under the action of high temperature, the cementite in carbon steel gains energy, forms coarse-grained cementite, and mixes with ferrite, and its cementite gradually changes from flaky to spherical, which is called spheroidization. Because the strength of graphite is extremely low, it appears in flake form, which greatly reduces the strength of materials and increases their brittleness, which is called graphitization of materials. Carbon steel will be graphitized if it works in the environment above 425℃ for a long time, especially above 475℃. SH3059 stipulates that the maximum service temperature of carbon steel is 425℃, while GB 150 stipulates that the maximum service temperature of carbon steel is 450℃.
(3) Thermal fatigue performance If steel works alternately in cold and hot for a long time, under the thermal stress caused by temperature difference change, tiny cracks will be generated and will continue to expand, eventually leading to fracture. Therefore, under the working conditions of temperature fluctuation, the thermal fatigue performance of steel should be considered in structures and pipelines.
(4) Material's high-temperature oxidized metal material will be oxidized to form oxide scale in high-temperature oxidized medium environment (such as flue), which is brittle. Carbon steel is easy to produce scale in high temperature gas at 570℃, which makes the metal thin. Therefore, gas, flue and other steel pipes should be limited to work at 560℃.
(2) The performance change of metal materials at low temperature.
When the ambient temperature is lower than the critical temperature of the material, the impact toughness of the material will drop sharply, and this critical temperature is called the ductile-brittle transition temperature of the material. Low temperature impact toughness (impact work) is often used to measure the low temperature toughness of materials, and the pipeline working at low temperature must pay attention to its low temperature impact toughness.
(3) Performance changes of pipelines in corrosive environment
Many pipeline media, such as petrochemicals, ships and offshore oil platforms, are corrosive. Facts have proved that the harm of metal corrosion is very common and serious, and corrosion will cause direct or indirect losses. For example, stress corrosion, fatigue corrosion and intergranular corrosion of metals often cause catastrophic accidents, and metal corrosion will cause a lot of metal consumption and waste a lot of resources. There are mainly the following media that cause corrosion.
(1) The corrosion of chloride on carbon steel is basically uniform, accompanied by hydrogen embrittlement, and the corrosion on stainless steel is pitting corrosion or intergranular corrosion. Appropriate materials can be selected for preventive measures, such as carbon steel-stainless steel composite pipe.
(2) Sulfide crude oil contains more than 250 kinds of sulfides, such as hydrogen sulfide (H2S), mercaptan (R-SH) and sulfide (R-S-R), which are corrosive to metals. The high H2S content in liquefied petroleum gas (LPG) in China led to container rupture, some of which appeared after 87 days of production. After magnetic particle inspection, it was found that there were 465,438+07 cracks on the inner surface of the sphere, but no cracks on the outer surface. Attention should be paid to stress corrosion caused by high H2S content. According to the regulations of Japan Welding Society and High Pressure Gas Safety Association, the H2S content in liquefied petroleum should be controlled below 100× 10-6, while the average H2S content in liquefied petroleum gas in China 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 by crude oil. When the temperature exceeds 220℃, the corrosion begins and reaches the maximum at 270 ~ 280℃. When the temperature exceeds 400℃, naphthenic acid in crude oil has vaporized. 316L (00Cr17Ni14mo2) stainless steel is an effective material for naphthenic acid corrosion resistance, which is often used in high temperature naphthenic acid corrosion environment.
2. Selection of metal materials for pressure pipes
① Meet the requirements of operating conditions. First of all, it is necessary to judge whether the pipeline is under pressure and what kind of pressure pipeline it belongs to according to the use situation. Different types of pressure pipes have different importance, different hazards caused by accidents and different requirements for materials. At the same time, the use environment of the pipeline, the medium transported and the corrosion degree of the medium to the pipe body should be considered. For example, the corrosion rate of steel pipe piles inserted into the seabed in splash zone is 6 times that in seabed soil; The corrosion rate in tidal range area is four times that of seabed soil. Special attention should be paid to material selection and anti-corrosion measures.
② Machinability requirements. Materials should have good machinability and weldability.
(3) The requirements of durable and economical pressure pipes should be safe, durable and economical at first. Before investing in a piece of equipment and a batch of pipeline projects, a feasibility study, that is, an economic and technical analysis, can be conducted when necessary. There are several schemes for the materials selected for economic and technical analysis. Some materials have a slightly higher initial investment, but they are reliable in normal use and save maintenance costs. Some materials seem to save initial investment, but they have poor operation reliability, high maintenance cost and high life cycle cost. As early as 1926, API Gravity (API) issued the API-5L standard, which initially included only A25, A and B grades, and then issued several times, as shown in Table 4. Table 4 pipeline steel grades issued by API
Note: 1972 API issued U80 and U 100 standards, which were later changed to X80 and X 100.
Before 2000, X70 accounted for about 40% in the world, X65 and X60 both accounted for 30%, and X52 steel was used in a considerable number of small-diameter product oil pipelines, and most of them were resistance welded straight pipes (ERW steel pipes).
For more than ten years, China's metallurgical industry has vigorously developed pipeline steel. At present, we are working hard to tackle X70 wide plate. The chemical composition and mechanical properties of X70 and X80 of Shanghai Baoshan Iron and Steel Company and Wuhan Iron and Steel Company are listed in Table 5 ~ Table 9 respectively. Table 5 WISCO X80 Coil Performance Table 6 X70 Steel Pipe Mechanical Properties Table 7 X70 Steel Pipe Bending Performance Test Results Table 8 X70 Steel Pipe Charpy Impact Toughness Table 9 Charpy Impact Toughness of High Strength Conveyor Pipe
The commonly used types of oil pipelines in China are spiral submerged arc welded pipe (SSAW), longitudinal submerged arc welded pipe (LSAW) and resistance welded pipe (ERW). When the diameter is less than 152mm, the seamless steel pipe is selected.
From the end of 1960s to 1970s, the spiral welded pipe factory in China developed rapidly. Almost all crude oil pipelines are spiral welded steel pipes, and spiral welded steel pipes are also used in the first-class area of "West-East Gas Transmission". The disadvantages of spiral welded steel pipe are large internal stress, poor dimensional accuracy and high defect probability. According to expert analysis, the principle of "walking on two legs" should be adopted. First, actively carry out technical transformation on the existing spiral welded pipe factory, which is promising; Second, vigorously develop China's longitudinal submerged arc welding 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 the oil and gas resources in China are distributed in the northeast and northwest regions, while most of the consumption markets are in densely populated areas such as large and medium-sized cities in the southeast coast and central and southern regions. This serious separation of production and marketing markets makes the transportation of oil and gas products the biggest obstacle to the development and utilization of oil and gas resources. Pipeline transportation is the best way to break through this obstacle. Compared with railway transportation, pipeline transportation is a transportation mode of oil and gas products with large capacity, higher safety and more economy. Its construction investment is half that of the railway, and its transportation cost is only one third. Therefore, China government has included the development strategy of "strengthening the construction of oil and gas pipelines and forming a pipeline transportation network" in the Tenth Five-Year Plan. According to the planning of relevant parties, in the next 10 year, China will build 14 oil and gas pipelines, forming a pattern of "two vertical, two horizontal and four hubs and five gas depots", with a total length exceeding 1 10,000 kilometers. This indicates that China is about to usher in the peak period of oil and gas pipeline construction.
The key natural gas pipeline projects under construction and proposed in China include: the West-to-East Gas Transmission Project, with a total length of 4 176 km and a total investment of12 billion yuan, which was officially started in September 2000 and completed in 2004; The gas transmission pipeline project of Saininglan, with a total length of 950km, started construction in May 2000, and is now nearing completion. Natural gas has been sent to Xining. Zhongxian to Wuhan gas transmission pipeline project, with a total length of 760 kilometers, has made great progress in the preliminary work, and four 1 1 tunnels under construction have been run through; Shijiazhuang to Zhuozhou gas transmission pipeline project, with a total length of 202 kilometers, started construction in May 2000 and has now been completed; Shijiazhuang to Handan gas transmission pipeline project, with a total length of about160km; Shaanxi Jingbian-Beijing Gas Transmission Project Double Track: Shaanxi Jingbian-xi 'an gas transmission pipeline project Double Track; The gas transmission project from Shaanxi-Gansu-Ningxia to Hohhot, with a total length of 497 kilometers; Hainan Island natural gas pipeline project, with a total length of about 270 kilometers; Longkou, Shandong to gas transmission pipeline project, Qingdao, with a total length of about 250 kilometers; Sino-Russian gas pipeline project, with a total length of 2,000 kilometers in China; Guangdong LNG project, attracting investment has been completed, and it is planned to be completed in 2005. Oil pipelines under construction and proposed include: Lanzhou-Chengdu-Chongqing product oil pipeline project, with a total length of 1.207 km, and construction started in May 2000; Sino-Russian oil pipeline project, about 700 kilometers long in China; The Sino-Kazakhstan oil pipeline project is 800 kilometers long in China. In addition, the 2000-kilometer refined oil pipeline from Maoming, Guangdong to Guiyang to Kunming and the crude oil pipeline from Zhenhai to Shanghai and Nanjing are about to start construction. In addition to trunk lines, large-scale urban gas pipeline network construction should also be carried out at the same time.
Faced with such a huge market and such a rare development opportunity, it poses new challenges to pipeline construction technology. Under the same throughput, it is more economical to build a high-pressure large-diameter pipeline than to build several low-pressure small-diameter pipelines in parallel. For example, a gas pipeline with a conveying 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, but the former can save 35% investment and 19% steel. Therefore, expanding pipeline diameter has become a symbol of scientific and technological progress in pipeline construction. Increasing the conveying pressure within a certain range can increase economic benefits. Taking the gas pipeline with a diameter of 1 020mm as an example, the operating pressure is increased from 5.5MPa to 7.5MPa, the gas transmission capacity is increased by 4 1%, the material is saved by 7%, and the investment is reduced by 23%. The calculation shows that if the working pressure of gas pipeline can be further increased from 7.5MPa to 10 ~ 12 MPa, the gas transmission capacity will be further increased by 33 ~ 60%. The pressure of the trans-Alaska natural gas pipeline in the United States is as high as 1 1.8MPa, and the oil pipeline is 8.3MPa, which is the highest operating pressure pipeline at present.
The increase of pipe diameter and conveying pressure requires high strength of pipeline. On the premise of ensuring weldability and impact toughness, the strength of the pipe has been greatly improved. Because the pipeline laying depends entirely on the welding process, the welding quality determines the engineering quality to a great extent, and welding is the key link in pipeline construction. Pipe, welding material, welding technology and welding equipment are the key factors affecting welding quality.
China began to build large-diameter long-distance pipelines in the early 1970s. The famous "August 3" pipeline built oil pipelines from Daqing Oilfield to Tieling, Tieling to Dalian and Tieling to Qinhuangdao, which solved the problem of crude oil transportation that plagued Daqing.
The design diameter of the pipeline is φ720mm, the steel material is 16MnR, and the wall thickness of submerged arc welded spiral pipe is 6 ~11mm. The welding process scheme is: manual arc welding method and overhead welding operation process; The welding materials are J506 and J507 covered electrode, which are baked at 400℃ for 65438 0 hours before welding, based on φ3.2 and φ4 for filling and covering; The welding power supply 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 in the "August 3" campaign in Northeast China has been in operation for 30 years and is still in service, which proves that the technical scheme is correct and the construction quality is good.
In the early 1980s, manual downward welding technology was popularized, and cellulose type and low hydrogen type downward covered electrode were developed. Compared with the traditional overhead welding technology, overhead welding has outstanding advantages such as high speed, good quality and saving welding materials, so it has been widely used in pipeline girth welding.
In the early 1990s, the semi-automatic manual welding of self-shielded flux-cored wire was popularized, which effectively overcame the shortcomings of other welding methods in the field operation, and also had the characteristics of high welding efficiency, good quality and good stability. Now it has become the main method of pipeline girth welding.
The application of pipeline all-position automatic welding has been explored for many years, and now there has been a breakthrough. The successful use of the West-to-East Gas Transmission Project, with its efficiency and quality unmatched by other welding processes, indicates that the welding technology of oil and gas pipelines in China has reached a higher level. 2. 1 pipeline steel development history
Ordinary carbon steels of C, Mn and Si types have been used in early pipeline steels, which pay attention to performance in metallurgy and have no strict regulations on chemical composition. Since 1960s, with the increase of pressure and diameter of oil and gas pipelines, low-alloy high-strength steel (HSLA) has been used, which is mainly supplied in hot-rolled and normalized state. Chemical composition of this steel: C≤0.2%, and alloying elements ≤ 3 ~ 5%. With the further development of pipeline steel, by the end of 1960s and the beginning of 1970s, three microalloyed controlled rolling steels, X56, X60 and X65, were put forward by American petroleum organization in API 5LX and API 5LS standards. This kind of steel breaks through the traditional concept of steel, and the carbon content is 0. 1-0. 14%. By adding Nb, V and Ti ≤0.2% alloy elements to the steel, the mechanical properties of the steel are significantly improved by controlling the rolling process. To 1973 and 1985, X70 and X80 steels successively joined API standards, and then X 100 pipeline steel was developed. The carbon content is reduced to 0.0 1-0.04%, and the carbon equivalent is correspondingly reduced to below 0.35, so that modern multi-element microalloyed controlled rolling and controlled cooling steel really appears.
The application and start of pipeline steel in China is relatively late. In the past, Q235 and 16Mn steels were used in most oil and gas pipelines. During the Sixth Five-Year Plan period, China began to develop X60 and X65 pipeline steels according to API standards, and successfully used them together with imported steel pipes for pipeline laying. In the early 1990s, Baosteel and WISCO successively developed X70 pipeline steel with high strength and toughness, and successfully applied it to Saininglan 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 and rolling process of steel. When selecting materials for gas pipelines, steel with high yield strength should be selected to reduce the amount of steel used. But the higher the yield strength, the better. Too high yield strength will reduce the toughness of steel. When selecting steel grade, the proportional relationship between yield strength and tensile strength-yield ratio should also be considered to ensure the forming quality and welding performance of pipes.
After repeated stretching and compression, the mechanical properties of steel will change, and the strength will decrease, with a serious decrease of 15%, that is, Bauschinger effect. This factor must be considered when ordering steel plates for steel pipe manufacturing. The minimum yield strength of this steel can be increased by 40-50MPa.
The fracture toughness of steel is related to chemical composition, alloying elements, heat treatment process, material thickness and orientation. The contents of C, S and P in steel should be reduced as much as possible, and alloying elements such as V, Nb, Ti and Ni should be added appropriately. By adopting controlled rolling and controlled cooling, the purity of steel can be improved, the material can be uniform, the grain can be refined and the toughness of steel can be improved. Most of the methods used are to decrease C and increase Mn.
In the oil and gas environment containing hydrogen sulfide, hydrogen produced by corrosion invades steel, leading to hydrogen-induced cracking. Therefore, the pipeline steel for transporting acid oil and gas should have low sulfur content, effectively control the morphology of non-metallic inclusions and reduce the segregation of micro-components. The hardness of pipeline steel also has an important influence 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
With the decrease of carbon equivalent of pipeline steel, the sensitivity of welding hydrogen-induced cracks decreases, the technological measures needed to avoid cracks decrease, and the performance damage degree of welding heat affected zone decreases. However, pipeline steel has gone through a series of complex non-equilibrium physical and chemical processes during welding, which may cause defects in the welding zone or reduce the joint performance, mainly due to welding cracks and embrittlement of the welding heat affected zone.
Because of the low carbon content, the hardening tendency and cold cracking tendency of pipeline steel decrease. However, with the increase of strength grade and plate thickness, there is still a certain tendency of cold cracking. Welding materials with high hydrogen content, such as cellulose covered electrode and self-shielded flux-cored wire, are often used in field welding, which have low linear energy and fast cooling speed, which will increase the sensitivity of cold cracks, so necessary welding measures, such as preheating before welding, are needed.
Brittleness of welding heat affected zone is often the root cause of pipeline fracture and catastrophic accidents. There are two main areas where local embrittlement occurs, namely, the embrittlement of the coarse-grained area in the heat affected zone, which is caused by the excessive growth of grains and the formation of bad structures in the overheated area; The re-critical embrittlement of coarse grain zone in multi-layer welding, that is, the coarse grain zone in the previous pass is caused by the reheating of the two-phase zone in the subsequent pass. By adding a certain amount of Ti and Nb microalloying elements to steel and controlling the cooling rate after welding to obtain a suitable t8/5, the toughness can be improved.
2.4 Steel pipes for West-East Gas Transmission Project
The steel pipes used in the West-to-East Gas Transmission Project are X70 grade pipeline steel, with the specification of φ10/6 mm×14.6 ~ 26.2 mm, of which 80% are spiral welded pipes, 20% are longitudinal submerged arc welded pipes, and the consumption of pipeline steel is about 6,543,800 tons.
X70 pipeline steel contains not only Nb, V and Ti, but also a small amount of Ni, Cr, Cu and Mo, which delays the formation of ferrite to a lower temperature and is beneficial to the formation of acicular ferrite and lower bainite. Therefore, X70 pipeline steel is essentially a kind of acicular ferrite pipeline steel with high strength and toughness. See table 1 and table 2 for the chemical composition and mechanical properties of steel pipes. Characteristics of field welding
Because the oil and gas fields discovered and exploited are remote, the geographical, climatic and geological conditions are bad, and the social supporting conditions are poor, which brings many difficulties to the construction, especially the low temperature.
When welding on site, the nozzle assembly device is used for nozzle assembly. In order to improve the efficiency, the foundation beam or mound is generally placed under the aligned nozzle, and at the same time, the next docking preparation work is started while welding a docking interface. This will produce great additional stress. At the same time, due to the influence of thermal expansion and cold contraction of steel pipes, problems are most likely to occur due to additional stress when they collide with each other.
On-site welding positions are horizontal fixed or inclined fixed butt joint of pipelines, including flat welding, vertical welding, overhead welding and horizontal welding. Therefore, higher and stricter requirements are put forward for the welder's operation technology.
Nowadays, the pipeline industry requires the pipeline to have higher conveying pressure and larger diameter, and ensure its safe operation. In order to adapt to the high strength and toughness, large diameter and thick wall of pipeline steel, a variety of welding methods, welding materials and welding processes have emerged.
Welding method of pipeline construction
Foreign pipeline welding construction has experienced the development of manual welding and automatic welding. Manual welding mainly includes cellulose covered electrode downward welding and low hydrogen covered electrode downward welding. In the automatic welding of pipelines, there is a pipeline flash butt welding machine developed by the former Soviet Union, which accumulated tens of thousands of kilometers of large-diameter pipelines during the former Soviet Union. Its remarkable characteristics are high efficiency and strong adaptability to the environment. The automatic welding system of CRC multi-head gas shielded pipeline developed by CRC Company in the United States consists of three parts: pipe end beveling machine, internal butt welding machine and internal welding machine combination system, and external welding machine. Up to now, the cumulative length of welded pipelines in the world has exceeded 34,000 kilometers. France, the former Soviet Union and other countries have also studied and applied similar automatic welding technology inside and outside pipelines, which has become the mainstream of automatic welding technology for large-diameter pipelines in the world today.
China steel pipe girth welding technology has undergone several major changes. In 1970s, traditional welding method was adopted, and low hydrogen covered electrode manual arc welding overhead welding technology was adopted. In 1980s, manual arc welding downward welding technology was popularized, and cellulose covered electrode and low hydrogen covered electrode downward welding technology were adopted. Self-shielded flux-cored wire semi-automatic welding technology was applied in 1990s, and today all-position automatic welding technology has been fully promoted.
Manual arc welding includes the application of cellulose covered electrode and low hydrogen covered electrode. Manual arc welding overhead welding technology is the main welding method in pipeline construction in China in the past, which is characterized by large assembly gap of nozzle, arc stopping operation during welding, thick welding layers and low welding efficiency. Manual arc welding downward welding is a welding technology introduced from abroad in 1980s, which is characterized by small gap of nozzle assembly, large current, multi-layer and fast welding, suitable for assembly line operation and high welding efficiency. Due to the thin thickness of each welding layer, the toughness of the ring welding joint can be improved by heat treatment of the front welding layer by the rear welding layer. Manual arc welding method is flexible, simple and adaptable. The organic combination of downward welding and upward welding and the good adaptability of cellulose covered electrode to root welding are still irreplaceable by automatic welding method in many occasions.
Self-shielded flux-cored wire semi-automatic welding technology was applied to pipeline construction in 1990s, mainly for filling and covering. It is characterized by high deposition efficiency, good all-position forming, strong environmental adaptability and easy mastery by welders. It is an important welding process method for pipeline construction.
With the increase of strength grade, pipe diameter and wall thickness of steel pipes for pipeline construction, automatic welding technology is gradually applied to pipeline construction. Pipeline automatic welding technology has the advantages of high welding efficiency, low labor intensity and little influence from human factors in the welding process, and has great application potential in the construction of large-diameter thick-walled pipelines. However, China's pipeline automatic welding technology is in its infancy, the problem of automatic root welding has not been solved, and the supporting facilities such as pipe end groove forming machine are not yet mature, which limits the large-scale application of automatic welding technology. The long-term solidification of sludge and rust scale in the pipeline leads to the decrease of the original pipe diameter;
Long-term precipitation of sludge in the pipeline produces hydrogen sulfide gas, which causes environmental pollution and is easy to cause explosion;
The acid and alkali substances in wastewater are easy to corrode the pipeline wall; Non-standard removal of foreign bodies in the pipeline causes pipeline blockage; 1. chemical cleaning: chemical cleaning refers to the temporary transformation of the pipeline with chemicals, and cyclic chemical cleaning is carried out from both ends of the pipeline by using temporary pipelines and circulating pumping stations. This technology has the characteristics of strong flexibility, no requirement for pipeline shape, high speed and thorough cleaning.
2. High-pressure water cleaning: peel off and clean the dirt on the inner surface of the pipeline with a high-pressure water jet above 50Mpa. This technology is mainly used for short-distance pipelines, and the pipeline diameter must be greater than 50cm. This technology has the characteristics of high speed and low cost.
3. Pigging by pig: The industrial pigging technology of pig relies on the driving force generated by the pump to push the fluid to drive the pig forward in the pipeline and discharge the accumulated dirt in the pipeline, thus achieving the purpose of cleaning. This technology is widely used in the cleaning engineering of various process pipelines and oil and gas pipelines in oil fields. , especially for cleaning long-distance fluid pipelines, has irreplaceable advantages.