Polyethylene pipe is a new type of pipe material. Western developed countries began to use polyethylene pipes to transport city gas on a large scale since the 1960s. China has been doing research work in this area since the early 1980s, and promulgated and implemented the national standard for polyethylene pipes in gas. Harbin is one of the few cities in the country that uses medium-density polyethylene pipes to transport gas in large areas. This article intends to briefly introduce the characteristics of medium-density polyethylene pipes and their application in the gas pipeline network in Harbin for peers to choose and use. Density polyethylene pipe (hereinafter referred to as plastic pipe) 2 Features of plastic pipe 2.1 Main properties of plastic pipe Polyethylene is a kind of molecular polymer, and its molecular expression is + h. For different polyethylenes, the number of molecules n Not the same. The main properties of medium density polyethylene pipes are listed in the following table: Project Performance Data Melt Flow Rate Softening Point (V1CAT) Brittleness Temperature (° c) Yield Tensile Stress (MPa) Fracture Elongation Drop Weight Impact Test Heating Internal Pressure Creep Test (h) Environmental stress cracking test (FOi) The main specifications of plastic pipes SDR; The specifications and dimensions of 11 series plastic pipes are listed in the following table: Unit: mm Nominal outer diameter Du wall thickness e (working pressure plastic pipe has a higher resistance Corrosion and durability. Because plastic tubes will not produce electrochemical corrosion and have acid and alkali resistance, their service life can reach more than 50 years.
Convenient transportation and simple construction. The plastic pipe has a light specific gravity of only 1/8 of the steel pipe, and has good flexibility. Low cost, can save engineering investment. The sub-high pressure building pressure regulation method is used to replace the traditional three-stage transmission, distribution system of the middle, low pressure, which can reduce the pipe diameter and avoid the repeated laying of pipelines, thus greatly reducing the cost of pipeline engineering.
The inner wall of the plastic pipe is smooth, which can reduce the resistance loss along the pipeline transportation process.
3 Plastic gland connection methods The plastic tube connection methods include electrothermal fusion connection, thermal fusion butt connection, thermal fusion socket connection, flange connection, continuous spinning wheel, screw connection and thermal welding connection. At present, the connection method of the plastic pipe mainly adopts electrothermal fusion connection and thermal fusion butt joint. These two connection methods have the advantages of easy operation and stable welding performance. The cost of electrothermal fusion connection is higher than that of thermal fusion but the influence of human factors is smaller than that of thermal fusion butt. Electrothermal fusion connection is to insert the two pipe ends of the required connection into the connector where the heating wire is buried. The electric heating wire heats the outer wall of the pipe and the inner wall of the pipe to be melted with a certain current, and forms a firm joint after cooling. The welding schematic diagram is shown in the figure.
Hot-melt butt-joint hot-melt butt-joint is to contact the end face of the connecting pipe with the heating plate, after reaching the melting temperature, withdraw the heating plate, align the two pipe end faces with a certain pressure, and form a firm joint after cooling. The strength of a good butt joint is higher than that of the base material. The welding schematic diagram is as shown in the right picture. 4) Application of plastic pipes in Harbin Gas Project The Harbin Municipal Construction Committee and the Planning Commission presided over the Harbin City Plastic Gas Pipe Community Test Expert Demonstration Meeting, which combined the community test situation and integrated foreign plastic gas pipes. The technical situation and the actual situation of domestic gas business construction suggest that it is feasible to adopt the gas plastic pipe sub-pressure gas supply process and technology in Harbin.
In 1990, the urban area of ​​Harbin began to formally lay plastic pipes. The raw materials used for the pipes and fittings are all imported resins. The pipes and fittings are produced by Harbin Dongguang Factory and Daqing Plastic Products Factory. Their production equipment is imported equipment (made in Germany and Italy) . The wall thickness of the pipe adopts the SDR11 series, and the specifications are before 1994. The connection of the plastic pipe adopts the hot-melt butt joint method. After the welding of the joint has occurred several times, in April 1994, the electric hot-melt socket joint method has basically been adopted. Imported parts are used as connecting parts. The plastic pipe is laid with the tracer line at the same time. When backfilling, first fill 330mm (from the top of the pipe) with fine soil or sand, then lay protective tiles or bricks directly above the pipe, and then lay the yellow plastic warning tape, and then backfill and tamping.
At present, the total length of plastic pipes laid in Harbin is 762KM. The operating pressure in the plastic pipes is 245.2KM of medium pressure (0.2MPa). The typical plastic pipe leakage accidents caused by the construction quality and improper operation of the typical plastic pipe leakage accidents have occurred since 1993. Since then, there have been several gas leakage accidents caused by improper construction quality and improper operation in Harbin, and most of the accidents occurred at the connection ports. In October 1993, the pl25 plastic pipeline in Situ Street, Xiangfang District was connected to the three-way swing head. When the welding port was fixed on the welding side, the inner wall weld of the pipe circumference of 3-4 cm was not sharp due to the uneven cutting of the pipe mouth, that is, It is said that within the range of 3-4cm in circumference, only 1/2 wall thickness is welded, but 1/2 wall thickness fails to be effectively welded. Due to uneven piping, the welding port cracks and gas leaks. In this accident, the welding port used hot-melt docking. The operating procedures met the specifications. After the fusion, the appearance inspection passed. It seems that the accident should not happen, but it is a gas operation, and it touches the fixed port (close to the dead port). Because the pipeline flattening device cannot effectively stop the airflow, the operator works on the anti-virus area, and the vision is affected by resistance and the mood is irritated, which causes the quality of the interface to deviate. Through this accident, we realized that the welding of plastic pipe joints is not as simple as imagined. The proficiency, mental state, weather environment, operating tools and other factors of the operator directly affect the quality of the welded joint.
Leaks caused by the quality defects of the base material itself During the production process of the plastic pipe, the raw material is not melted well and the inner tube wall is shaped like a slub. Such defects are generally exposed in the strength test and air tightness test of the pipeline. In the early stage of construction of gas pipelines in Harbin, steel-plastic joints, that is, the connections between plastic pipes and steel pipes, leaked more. With the continuous improvement of the production process and technology of steel-plastic joints, few air leaks have occurred recently. .
Leakage caused by man-made damage Harbin ’s municipal and telecommunications facilities are still being improved, and water pipes, telecommunications, and building construction will inevitably encounter gas pipelines. If the construction unit handles the countersign in accordance with the normal procedures, we will send someone to monitor on site. However, some construction units plan to save time and grooving construction without going through the countersign procedures with our company. Due to the low cultural level of some migrant workers during manual excavation, we do not understand our warnings and the role of protective bricks. Excavation caused damage and leakage of plastic pipes. In January, 1998, a construction company broke ground above my second plastic pipe, and did not contact the company before construction. Due to night construction, our pipeline inspectors were no longer in the post and failed to effectively stop the gas leakage, causing gas Large area leakage. Nearly 20 similar accidents occurred in 98 and 99. Most of our pipeline inspection personnel discovered in time or related personnel reported in time without serious consequences. In response to these accidents, the relevant departments of the municipal government issued documents requesting that all construction units must go through the signing procedures before construction. In addition, our company has increased the patrol of the pipeline to ensure that the pipeline is not damaged.
6 Several issues worthy of discussion Yan Yu Steel-plastic joints Steel-plastic joints are important components for connecting plastic pipes and steel pipes. Welding methods are used to connect with steel pipes. It is still unknown whether their temperature affects plastic pipes when welding. In the past, we used water or wet cloth to cool the steel-plastic joint steel pipe and plastic pipe contact section during construction. At present, we are increasing the length of the steel-plastic joint steel pipe section to avoid excessive temperature at the contact surface of the steel pipe and plastic pipe.
About the connection method of plastic pipes At present, most of the plastic gas pipelines in Harbin use electrothermal fusion sockets. The contact surface of the sockets is larger than that of the butt joints. The tightness and strength of the socket joints should be better than the butt joints. However, the electric fusion sleeve has too strict requirements on the diameter and roundness of the pipe, and it is not easy for the workers to control, and it is easy to produce incomplete bonding of the joint. In addition, the price of electrofusion sleeve fittings is also relatively low.
An expert said that Japan's plastic pipes basically use hot-melt butt joints. We asked Japan whether to consider socket joints instead of butt joints. The answer is no, because they think that hot-melt butt joints are more economical and reliable.
We believe that after training a group of experienced operators, it is desirable and economical to use plastic butt welding for the plastic pipes.
Non-destructive testing of plastic pipe joints None of our plastic pipe joints have undergone non-destructive testing. After the fusion splice is welded, the strength and air tightness of the joint are generally checked by means of appearance inspection, strength test and air tightness test. After the gas exchange tube, the strength test and the air tightness test cannot be carried out. The quality and air tightness of the interface can only be checked by the appearance and soapy water. The fusion and strength of the interface cannot be checked. Perform non-destructive testing on the interface to check the quality of the interface. It is understood that there is no non-destructive flaw detection instrument with a plastic pipe interface in China. I only heard that Canada, the United States and other developed countries are using this product.
Regarding winter construction Harbin City is relatively cold in winter and is also a frequent occurrence of gas leakage accidents. It is inevitable to have emergency work and repair work. Plastic pipes are very brittle at low temperatures, and flattening plastic pipes can easily cause brittleness when throttling. Cracking; temperature drops too fast during pipeline welding in winter will affect the strength of the interface. At present, we solve this problem by locally heating and prolonging the welding time. We believe there will be a better way.
About the tracer line. The tracer line is a metal wire laid in the same trench as the plastic pipe in order to detect the position and depth of the buried plastic pipe. Current underground pipeline detectors can easily and accurately determine the position and depth of metal pipelines, but plastic pipes are insulators and cannot be detected by conventional electromagnetic methods. Ground penetrating radar is technically feasible, but the cost of detection is relatively high. Therefore, the plastic gas pipelines in Harbin are laid with tracers along the same ditch. At present, there is no product standard for trace lines in China. Our company uses polyethylene-coated metal wires (that is, because the wires are not detected and positioned in the pipeline, the effect and function of the trace line has not yet been reflected.
Harbin has a large number of plastic gas pipelines and a large area. After several years of operation, we feel that it is feasible and economical to use plastic pipelines as gas transmission pipelines. In the construction and operation management of plastic gas pipelines, we have accumulated some experience and also done a lot of exploration work. At present, there are still many problems to be solved. To this end, we are willing to work together with domestic experts and peers to make due contributions to the promotion and application of domestic plastic pipes and technological development.
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