Selection of water supply pipeline and its connection method
Foreword In the water supply project, the modern water treatment plant takes water from the water source, settles and filters it into the clean pool, and then pumps it to the urban pipe network to reach the user directly. The main pipelines and urban pipelines are under high pressure, and they are generally laid in the ground. Therefore, the pipeline is required to have high pressure bearing capacity, convenient installation and long service life. With the advancement of technology, it is also required to treat the inner wall of the water pipeline so that it does not breed bacteria, reduces friction, and does not pollute water quality. The above requirements have brought opportunities and challenges to the pipe-making industry. Zhejiang Jinzhou Group, a large domestic pipe-making company, has invested a lot of manpower and financial resources to cooperate with scientific research institutions to conduct basic research, scientific and technological research, and equipment transformation. , Pipe wall coating, pipe connection and other aspects have achieved gratifying results. 2 Steel pipes The use of steel pipes was started by a Scottish inventor in 1815 when the tubes were connected to convey gas for lights. In 1836, the United Kingdom had a patent for the production of seamless steel pipes by the extrusion method, but it was not until 1885 that the Manmesmann brothers invented the process of directly producing seamless pipes from steel bars. In terms of welded pipes, a patent was obtained in 1923. After low-frequency resistance welding and direct current welding, it has now entered the era of high-frequency induction welding (Samak-Julu method) mainly used for the production of medium and small diameter steel pipes. In addition, for large-diameter steel pipes, the UO forming submerged arc automatic welding process is used to produce straight seam pipes, and the spiral forming submerged arc automatic welding process is used to produce spiral steel pipes. 2.1 Seamless steel tube The production method of seamless steel tube is roughly divided into cross-rolling method (Mennesmann method) and extrusion method. The first method is to first pierce the broken tube with a slanting roller, and then extend it with a rolling mill. This method has a fast production speed, but has high requirements on the machinability of the tube blank, and is mainly suitable for the production of carbon steel and low-alloy steel pipes. The latter rule is to use a piercing machine to pierce the tube blank or steel ingot, and then use an extruder to extrude into a steel pipe. This method is less efficient than the cross-rolling method and is suitable for the production of high-strength alloy steel pipes. Both the cross-rolling method and the extrusion method must first heat the tube billet or steel ingot, and the produced steel tube is called a hot-rolled tube. Steel pipes produced by hot working can sometimes be cold worked as needed. There are two methods of cold working: one is the cold drawing method, which is to draw the steel pipe through a pipe drawing die to make the steel tube gradually thinner and elongated; the other method is the cold rolling method, which is invented by the Mennesmann brothers The hot rolling mill is used in cold working methods. Cold processing of seamless steel pipes can improve the dimensional accuracy and processing finish of the steel pipes, and improve the mechanical properties of the material. 2. 2 Electric Resistance Welded steel pipe, referred to as ERW steel pipe or high frequency welded pipe, has the following advantages: (1) High production efficiency and low cost, and its price is about 85 of UOE straight seam submerged arc welded steel pipe %; (2) The dimensional accuracy is high, and its roundness (Roundenss) is better than that of spiral welded pipes, which can save a lot of man-hours in the field; In the past, due to the poor reliability of ERW steel pipe welds, a large number of accidents occurred in the pipeline, which restricted the use of ERW steel pipes. Recently, Zhejiang Jinzhou Group has invested huge sums of money. Through the joint efforts of researchers and pipe manufacturers, this situation has changed a lot. The weld quality of ERW steel pipes is comparable to that of LSAW steel pipes. The ERW pipe production process of Jinzhou Group has adopted the following technical measures: (1) The control of the heat input in the ERW steel pipe production process has many factors affecting the heat input, the first is the influence of the Convergency Angle. The test results show that under the condition that other factors remain unchanged, too large a convergence angle will cause cold welding, and too small a convergence angle will cause overburning. Another factor that affects the heat input is the advancing speed of the steel pipe on the forming line. The test results show that if other factors remain the same, too fast speed will cause cold welding, and too slow speed will easily cause over-burning; (2) Strengthen on-line automatic detection, in order to ensure the quality of ERW steel pipes. , Added multi-channel flaw detection in the pipe making process. After uncoiling, ultrasonic testing is performed on both sides of the steel strip, and eddy current testing is performed after the welding is completed. In addition, several visual inspections are required, and samples are taken for flattening tests and other mechanical performance tests. 2. 3 Spiral seam submerged arc welded steel pipe is formed by spirally bending the strip steel, and the inner seam and outer seam are welded by automatic submerged arc welding to make spiral seam steel pipe. It can be widely used in the production of large-diameter steel pipes due to the following reasons: (1) As long as the forming angle is changed, steel pipes of various diameters can be produced with the same width of strip steel; (2) Because it is continuously bent and formed, the steel pipe The cut-to-length length is not limited; (3) The welding seam is evenly distributed on the circumference of the steel pipe, so the steel pipe has high dimensional accuracy and high strength; (4) The equipment is cheap, easy to change the size, and suitable for small Mass production of multiple varieties of steel pipes. The weld seam of spiral steel pipe is longer than that of straight seam pipe. If the pipe length is L, the weld seam length is L/cosα (α is the forming angle). The vast majority of steel pipe defects are concentrated in the weld seam and heat-affected zone. The long weld seam means that the probability of defects is high. This is the main reason that has restricted the wider application of spiral welded pipe for a long time. Straight seam pipe, especially the question of who is superior compared with UOE steel pipe. With the development of spiral steel pipe manufacturing technology today, we should comprehensively and correctly evaluate and compare, and re-understand the problem of long spiral steel pipe welds. First of all, because the defect is parallel to the weld, so for the spiral steel pipe, the defect of the weld is “oblique defect”. In the process of use, the main stress direction of the steel pipe, that is, the equivalent defect length in the axial direction of the steel pipe is smaller than that of the straight seam pipe; secondly, because the pipeline steel is a rolled steel plate, the impact toughness has a large anisotropy, and it is along the rolling direction. The CVN value can be 3 times higher than the CVN value perpendicular to the rolling direction. The principal stress of the straight seam pipe is exactly perpendicular to the direction of the lowest impact resistance of the pipe, while the spiral steel pipe is staggered by the α angle. Furthermore, in the production process of spiral steel pipes, due to continuous technological and technological improvements, continuous advancement and improvement of inspection equipment and technology, the probability of weld defects has been quite low, and 100% non-destructive testing of welded joints has allowed the steel pipe to leave the factory. The qualification rate has truly reached 100%, especially the computer-controlled X-ray real-time continuous inspection of the full weld after the hydraulic test, which further improves the reliability of the spiral steel pipe and turns the disadvantage of the spiral steel pipe’s long weld into an advantage. 2. 4 Galvanized tube zinc has a more negative electrode potential than iron. When zinc and iron form a microbattery, zinc is the anode and iron is the cathode. When corroded, zinc dissolves and iron is not damaged, so zinc is plated on iron. Play the role of sacrificial anode. (1) Hot-dip galvanized pipe-hot-dip galvanized pipe is to make the molten metal react with the iron matrix to produce an alloy layer, so that the matrix and the coating are combined. Hot-dip galvanizing is to pickle the steel pipe first. In order to remove the iron oxide on the surface of the steel pipe, after the pickling, it is cleaned in a tank with ammonium chloride or zinc chloride aqueous solution or a mixed aqueous solution of ammonium chloride and zinc chloride, and then sent to In the hot dip plating tank. Hot-dip galvanizing has the advantages of uniform coating, strong adhesion and long service life. (2) Cold galvanized pipe-cold galvanizing is electro-galvanizing, and the amount of galvanizing is very small, only 10-50g/m2, and its corrosion resistance is much different than that of hot-dip galvanized pipe. Most of the regular galvanized pipe manufacturers do not use electro-galvanization (cold plating) in order to ensure the quality. Only those small enterprises with small scale and outdated equipment use electro-galvanization, and of course their prices are relatively cheaper. At present, the Ministry of Construction has officially announced that cold-galvanized pipes with backward technology should be eliminated, and cold-galvanized pipes are not allowed to be used as water and gas pipes in the future.
3 Pipe wall coating water engineering trunk pipelines and urban pipe networks are laid in the ground. In order to prolong the service life of pipelines and avoid metal corrosion, people have buried the corrosion mechanism and anti-corrosion methods of metal pipelines since the beginning of water supply projects in the world. , Coating structure, etc. have been unremittingly explored and studied, and a variety of anti-corrosion systems have been formed. The outer wall of buried metal pipelines is coated, and the life of the pipeline is greatly improved. Drinking water is the most basic element for human survival. Modern people have higher and higher requirements for water quality. In order to meet high water quality requirements, the most important link is to prevent bacteria from growing or polluting water during the water delivery process. 3. 1 The outer (coating) anti-corrosion corrosion of the pipeline is a kind of damage caused by the chemical and electrochemical action of the surrounding medium. Metal corrosion can be divided into chemical corrosion and electrochemical corrosion according to its nature. Chemical corrosion is the process of dissolving metal ions caused by direct contact between metal and the medium to cause chemical action. Electrochemical corrosion is the electrolysis process that occurs when metal and electrolyte form a primary battery. Metals generally undergo electrochemical corrosion in electrolyte solutions such as soil, water or moisture. The reasons for external corrosion of buried pipelines are more complex, generally summarized into the following three reasons: (1) Electrochemical corrosion-because the soil is a complex mixture of solid, liquid, and gaseous substances, the soil particles are filled with air and water With various salts, the soil has the characteristics of an electrolyte solution. The exposed metal of the buried pipeline (such as mechanical damage, microbial damage, etc.) forms a galvanic cell with the soil electrolyte, leading to electrochemical corrosion of the metal. (2) Corrosion of pipeline by stray current-due to the influence of external leakage, stray current in the soil passes through the pipeline, and electrolysis occurs, causing the pipeline to corrode. (3) Corrosion caused by bacterial action—According to research, different types of bacteria have different corrosion behaviors and conditions in the process of microbes participating in the corrosion process. For example, there are anaerobic sulfate-reducing bacteria in anoxic soil, which can The soluble sulfate is converted to increase the concentration of H+ in the soil and accelerate the corrosion of buried pipelines. The investigation and research on the corrosion of buried pipelines indicated that the damage of the outer wall insulation of buried pipelines is the main reason that the pipeline suffers from soil corrosion. In order to ensure the long-term safe production of pipelines, in view of the characteristics of soil corrosion, the following ways can be used to prevent corrosion. The occurrence and reduction of corrosion: (1) Use corrosion-resistant pipes; (2) Increase the transition resistance between the pipe and the soil and reduce the corrosion current, such as using petroleum asphalt, enamel paint, adhesive tape, epoxy powder, three Layer insulation layer such as PE, make the insulation layer resistance larger, the insulation layer is very dense (no pinholes), thereby reducing the corrosion current to a minimum; (3) Adopt electrical protection (cathodic protection and sacrificial anode protection), general electrical protection should be combined with insulation protection to reduce the consumption of protection current. In the past, petroleum asphalt anti-corrosion coatings were mostly used in China. In recent years, China has successively researched and developed anti-corrosion technologies such as polyethylene jacket (two-layer structure), fused epoxy powder coating, polyethylene adhesive tape and enamel paint, and achieved good results. Anti-corrosion effect. However, in practical applications, the shortcomings of these anti-corrosion technologies have also been exposed, such as poor adhesion of the two-layer polyethylene jacket, low mechanical properties of the fused epoxy powder layer, environmental pollution problems of enamel paint, and poly The construction quality of vinyl adhesive tape is difficult to guarantee. Therefore, some European countries have researched, developed and successfully applied the anticorrosive coating technology of fusion bonded epoxy/extruded polyethylene three-layer structure since the 1980s. This is currently considered internationally. The most advanced external anti-corrosion technology for pipelines. Extruded polyethylene coating, with its excellent physical and mechanical properties, good chemical resistance, extremely low water and oxygen permeability and low environmental pollution, it is recognized as one of the most promising anti-corrosion materials. . However, the non-polar polyethylene coating has poor adhesion to the surface of the polar steel pipe. In order to solve this problem, many methods have been adopted. The traditional method is to apply a layer of “soft base” mastic or rubber adhesive or “hard base” polyethylene copolymer primer between the polyethylene coating and the steel pipe to form a two-layer coating structure. In some countries, high-temperature sintered polyethylene powder is used. However, whether it is a two-layer structure coating or a sintered polyethylene powder coating, there are two shortcomings: one is the weak cathodic peeling strength, and the other is the resistance to steel. The bonding performance is low and unstable, especially at higher temperatures. In order to better solve the problem of adhesion between polyethylene coating and steel pipe surface, European countries represented by Germany and Italy widely adopted three-layer polyolefin pipeline anticorrosive coating (hereinafter referred to as three-layer PE) in the 1980s. Polyethylene copolymer primer, forming a two-layer coating structure. In some countries, high-temperature sintering of polyethylene powder is used. However, whether it is a two-layer structure coating or a sintered polyethylene powder coating, there are two shortcomings: one is the weak cathodic peel strength, and the other is that it has a strong resistance to steel. The bonding performance is low and unstable, especially at higher temperatures. In order to better solve the problem of adhesion between polyethylene coating and steel pipe surface, European countries represented by Germany and Italy widely adopted three-layer polyolefin pipeline anticorrosive coating (hereinafter referred to as three-layer PE) in the 1980s. Polyethylene copolymer primer, forming a two-layer coating structure. In some countries, high-temperature sintered polyethylene powder is used. However, whether it is a two-layer structure coating or a sintered polyethylene powder coating, there are two shortcomings: one is the weak cathodic peeling strength, and the other is the resistance to steel. The bonding performance is low and unstable, especially at higher temperatures. In order to better solve the problem of adhesion between polyethylene coating and steel pipe surface, European countries represented by Germany and Italy widely adopted three-layer polyolefin pipeline anticorrosive coating (hereinafter referred to as three-layer PE) in the 1980s.
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