High density polyethylene (HDPE) pipe |
| Publisher:Jiangsu LvDao Pipe Valve Co.,Ltd Release time:2020-10-15 10:36:18 Hits:544 Close |
| Introduction to materials High density polyethylene (HDPE) is called "high density polyethylene" in English. HDPE is a kind of thermoplastic resin with high crystallinity and non polarity. The appearance of the original HDPE is milky white and translucent to a certain extent in the thin section. PE has excellent resistance to most chemicals used in life and industry. Certain types of chemicals produce chemical corrosion, such as corrosive oxidants (concentrated nitric acid), aromatic hydrocarbons (xylene) and halogenated hydrocarbons (carbon tetrachloride). The polymer is non hygroscopic and has good water vapor resistance, which can be used for packaging. HDPE has good electrical properties, especially high dielectric strength of insulation, which makes it very suitable for wire and cable. Medium to high molecular weight grades have excellent impact resistance at room temperature and even at - 40F low temperature. Although HDPE was introduced in 1956, the plastic has not yet reached the mature level. This general material is still developing its new uses and markets. PE can be made by a wide range of different processing methods. These include sheet extrusion, film extrusion, tube or profile extrusion, blow molding, injection molding and rotational molding. nature chemical property Polyethylene is resistant to a variety of organic solvents and acid-base corrosion, but not antioxidant acids, such as nitric acid. Polyethylene is oxidized in an oxidizing environment. physical property Polyethylene can be regarded as transparent in the film state, but in the presence of blocks, due to the existence of a large number of crystals, there will be strong light scattering and opaque. The degree of crystallization of polyethylene is affected by the number of branch chains. The more branches, the more difficult it is to crystallize. The melting temperature of polyethylene is also affected by the number of branch chains, which is distributed from 90 ℃ to 130 ℃. The more the chain, the lower the melting temperature. Polyethylene single crystals are usually prepared by dissolving HDPE in xylene at temperatures above 130 ℃. Pipe properties HDPE is a good pipe, not only should be good economic, but also has the advantages of reliable interface, simultaneous interpreting of materials, impact resistance, cracking resistance, aging resistance and corrosion resistance. Compared with traditional pipes, HDPE pipeline system has the following advantages: (1) reliable connection: the polyethylene pipe systems are connected by electrothermal melting, and the strength of the joint is higher than that of the pipe body. (2) good low temperature impact resistance: the low temperature embrittlement temperature of polyethylene is very low, and it can be used safely in the temperature range of - 60-60 ℃. In winter construction, because of the good impact resistance of the material, the pipe will not be brittle. (3) good stress cracking resistance: HDPE has low notch sensitivity, high shear strength and excellent scratch resistance, and environmental stress cracking resistance is also very outstanding. (4) good chemical corrosion resistance: HDPE pipeline can resist the corrosion of various chemical media, and the chemical substances in the soil will not cause any degradation to the pipeline. Polyethylene is an electrical insulator, so it will not rot, rust or electrochemical corrosion; in addition, it will not promote the growth of algae, bacteria or fungi. (5) aging resistance and long service life: polyethylene pipes containing 2-2.5% evenly distributed carbon black can be stored outdoors or used for 50 years without being damaged by ultraviolet radiation. (6) good wear resistance: the wear resistance comparison test of HDPE pipe and steel pipe shows that the wear resistance of HDPE pipe is 4 times of that of steel pipe. In the field of mud transportation, compared with steel pipe, HDPE pipeline has better wear resistance, which means that HDPE pipeline has longer service life and better economy. (7) good flexibility: the flexibility of HDPE pipe makes it easy to bend. In engineering, it can bypass obstacles by changing the pipeline direction. In many cases, the flexibility of pipeline can reduce the amount of pipe fittings and installation costs. (8) low flow resistance: HDPE pipe has smooth inner surface, and its Manning coefficient is 0.009. The smooth performance and non adhesion characteristics ensure that HDPE pipeline has higher conveying capacity than traditional pipe, and also reduces the pipeline pressure loss and water transmission energy consumption. (9) convenient handling: HDPE pipe is lighter than concrete pipe, galvanized pipe and steel pipe, and it is easy to handle and install. Lower manpower and equipment requirements mean that the installation cost of the project is greatly reduced. (10) a variety of new construction methods: HDPE pipeline has a variety of construction technologies. In addition to the traditional excavation method, it can also adopt a variety of new trenchless technologies, such as pipe jacking, directional drilling, lining pipe, pipe cracking, etc., which is a good choice for some places where excavation is not allowed. Other instructions Plastic pipe has made great development and improvement in design theory and construction technology, and accumulated rich practical experience, which makes plastic pipe occupy a very important position in building water supply and drainage pipeline engineering, and forms an irresistible development trend. In China, PVC-U water supply pipe, PP-R pipe, aluminum plastic composite pipe (Pap), steel plastic composite pipe (SP), HDPE pipe are mainly used in water supply pipeline. HDPE pipe is formed by hot extrusion with advanced production process and technology. It has the characteristics of corrosion resistance, smooth inner wall, low flow resistance, high strength, good toughness and light weight. After PVC-U drainage pipe, HDPE pipe has become the second largest consumption of plastic pipe in the world. 1、 Development of HDPE water supply pipe In the water supply and drainage pipeline system, plastic pipe gradually replaced the traditional pipe materials such as cast iron pipe and galvanized steel pipe, and became the mainstream pipe. Compared with traditional pipe, plastic pipe has the advantages of light weight, corrosion resistance, small water flow resistance, energy saving, simple and rapid installation, low cost and so on. At the same time, with the rapid development of petrochemical industry and the continuous progress of plastic manufacturing technology, the output of plastic pipes is increasing rapidly, and the types of products are more diversified. After PVC-U drainage pipe, HDPE pipe has become the second largest consumption of plastic pipe in the world. The medium or high density polyethylene pipes of grade PE80 and PE100 should be used for gas transmission; the medium or high density polyethylene pipes of grade PE80 and PE100 are usually used for water supply pipes, and pe63 has been gradually phased out. In terms of water delivery, the fastest growth is PE100 pipeline system, and the growth rate is expected to reach more than 10% in the next five years. In Europe, HDPE pipe has been more rapid development and application, HDPE pipe has gradually replaced the position of PVC-U pipe, become the main consumer pipeline. At the same time, HDPE pipes are gradually used outside Europe. 2、 Specification and connection mode of HDPE pipe The diameter of HDPE pipe ranges from DN32 to dn 1000, which is divided into 26 grades. The pressure level is between 0.6MPa and 1.6Mpa, with 5 grades in total. HDPE pipe will be melted between 190 ℃ and N240 ℃. By using this characteristic, the two melted parts of pipe (or pipe fitting) can be fully contacted and maintained with proper pressure. After cooling, the two parts can be firmly integrated. Therefore, PE pipe is different from U-PVC pipe in connection mode, which usually adopts electrothermal fusion connection and hot-melt butt joint. According to the pipe diameter, PE pipe can be divided into: when DN ≤ 63, injection hot-melt socket connection is used; when DN ≥ 75, hot-melt butt connection or electric fusion socket connection is used; when connecting with different materials, flange or screw thread connection is used. In the pipeline repair method, there are expansion pipe construction method and lining HDPE method. 3、 Application fields of HDPE HDPE pipeline is mainly used in municipal engineering water supply system, building indoor water supply system, outdoor buried water supply system and residential area, plant buried water supply system, old pipeline repair, water treatment engineering pipeline system, garden, irrigation and other fields of industrial water pipe. However, it should be noted that HDPE pipes cannot be used in hot water pipelines. 4、 Advantages of HDPE water supply pipeline in application HDPE water supply pipeline is widely used because of its incomparable advantages compared with other pipelines 1. It is easy to form a closed anti-seepage system by butt welding and electrofusion welding. When laying along the trench, the earthwork volume of trench excavation and the amount of accessories can be reduced. 2. Light weight and easy to install and handle; 3. Strong wear resistance and excellent hydraulic performance, the buried pipeline can not need outer protection. It can be applied to earthquake and subsidence area of mining area. It can also be laid in river bottom by sinking method. 4. It is resistant to chemical corrosion, internal, external and microbial corrosion, strong corrosion resistance and health. It is suitable for transporting acidic and alkaline substances, sewage, natural gas, gas and other substances; 5. Good environmental adaptability and frost resistance. It can be used for water supply and outdoor piping. 6. It has a long service life of more than 50 years; 7. Easy to recycle. 5、 Matters needing attention in application of HDPE water supply pipeline 1. Outdoor laying, where there is sunlight, it is recommended to take shielding measures. 2. For buried HDPE water supply pipeline, the pipeline with DN ≤ 110 can be installed in summer, and the pipeline with DN ≥ 110 can resist thermal stress due to sufficient soil resistance, so there is no need to reserve pipe length; in winter, there is no need to reserve pipe length. 3. When HDPE pipe is installed, if the operation space is too small (such as: pipe well, construction in ceiling, etc.), electric fusion connection method should be adopted. 4. During hot-melt socket connection, the heating temperature should not be too high or too long, and the temperature should be controlled at 210 ± 10 ℃, otherwise, too much molten slurry will be extruded from the fittings, and the inner diameter of water supply will be reduced; during the socket connection, the pipe fittings or pipe joints should be clean, otherwise the socket will be disconnected and leaking; at the same time, the angle and direction of pipe fittings should be controlled to avoid rework. 5. During hot-melt butt connection, the voltage is required to be between 200 and 220 v. if the voltage is too high, the temperature of the heating plate will be too high, and the voltage will be too low, then the butt welding machine can not work normally; during the butt joint, it should keep the alignment of the interface, otherwise it will cause insufficient butt joint area, insufficient weld strength, and improper crimping; when heating the heating plate, the pipe joint is not cleaned or heated If there are impurities such as oil stain and sediment on the plate, it will cause water leakage to the interface; if the heating time should be well controlled, the heating time should be short, and the heat absorption time of the pipe material is not enough, which will result in too small crimping and too long heating time, which may lead to the formation of false welding. Common phenomena Table: Causes and treatment of common phenomena in HDPE pipe production Common phenomena Causes Recommended solutions The surface is dull 1. Raw material moisture 1. Raw material pretreatment 2. The melt temperature is not suitable 2. Adjust the temperature 3. The molten material extruded by extruder is not uniform 3. Increase back pressure, use fine filter screen and design suitable screw structure 4. The sizing sleeve is too short 4. Lengthening sizing sleeve 5. The molding section of die is too short 5. Lengthen the molding section of die. Surface spots 1. Moisture in raw materials 1. Drying raw materials 2. There are bubbles on the pipes in the sink 2. Eliminate bubbles. Adjust process temperature The outer surface is bright and transparent 1. The head temperature is too high 1. Reduce the head temperature 2. The cooling water is too small or insufficient or uneven 2. Open the cooling water or clean the sizing sleeve The smooth outer surface of the pipe has regular stripes Tube tends to adhere to sizing sleeve Increase the cooling water flow, clean the water way or reduce the speed Deep ripple on the outer surface of pipe No alignment of sizing sleeve die Alignment, keep sizing box and die in the same axis Rough inner surface 1. The raw material is wet 1. Drying or pretreatment of raw materials 2. The core mold temperature is low 2. Increase the temperature or prolong the holding time 3. The gap between die and die is too large 3. Core changing die 4. Die setting section is too short 4. Change the die with longer setting section The inner wall of the pipe is corrugated 1. The output of extruder changes and the feeding is unstable 1. Reduce the screw feeding zone temperature 2 2. Adjust the traction air pressure 3. Uneven cooling of pipes 3. Adjust the waterway There are pits in the inner wall of the pipe 1. The moisture content of raw materials is large 1. Preheating and drying of raw materials 2. Poor dispersion of filler, unplasticized and impurities 2. Refuel, adjust temperature and clean raw materials There are coke particles on the inner wall of the tube 1. The extruder head and die inner wall are not clean 1. Mould cleaning 2. Local temperature is too high 2. Check whether the thermocouple is normal 3. Die accumulation is serious 3. Clean the die and reduce the die temperature properly The outer diameter or wall thickness changes at any time 1. Change of extrusion speed 1. Check the tractor 2. Traction speed changes or slips 2. Increase the pressure appropriately 3. Unstable cutting (uneven particle size of returned material) 3. Screening or granulation of raw materials 4. Instability of melt 4. Increase the material temperature, reduce the linear speed and increase the die gap 5. Uneven cooling 5. Clear the waterway Uneven wall thickness of pipe 1. Die not aligned 1. Adjust die concentricity 2. Uneven die temperature 2. Adjust the temperature 3. The tractor, sizing sleeve and die are not aligned 3. Keep on the same axis 4. The distance between sizing sleeve and die is too far 4. Close the distance Poor fusion joint 1. The molding section of die is too short 1. Use a longer die forming section 2. Low melting temperature 2. Increase the material temperature 3. The plastic in the die head is dispersed 3. Clean the die head 4. The head machine structure is unreasonable 4. Replacement or modification Premature failure and perforation of pipes 1. Blister 1. Drying raw materials 2. Bubbles 2. Dehumidification or temperature reduction 3. Impurities 3. Clean raw materials or use filter screen 4. Poor dispersion of pigments or fillers 4. Adjust the temperature or change the raw materials Premature failure and brittle failure of pipes 1. The material temperature is low 1. Increase the material temperature 2. If the temperature is too high, the raw materials will decompose 2. Clean the mold and reduce the temperature Pipe cracking 1. Low die temperature, high extrusion speed 1. Increase temperature and reduce speed 2. Too much cooling water 2. Reduce the cooling water flow Pipe roundness is not good, bending 1. The center position of die and mandrel is not correct 1. Adjust the concentricity 2. The head temperature is uneven around 2. Adjust the temperature 3. The cooling water is too close to the die 3. Adjust the position of cooling water 4. Too much cooling water spray 4. Adjust the nozzle angle 5. The cooling water spray is too small 5. Clear the waterway 6. The water level is too high 6. Drainage 7. The tractor pressure is too high 7. Adjust the air pressure Note: the thickened part in the table is the analysis and solution of various problems easily occurred in the production of PE pipes. Economic evaluation Price comparison of HDPE pipe with PVC-U pipe, PP-R pipe and aluminum plastic composite pipe: Outdoor water supply pipeline mainly includes ductile iron pipe, galvanized steel pipe, PVC-U water supply pipe, HDPE pipe, etc. the reason why HDPE water supply pipeline can achieve such rapid development and application lies in the economic and practical characteristics of HDPE pipe. The following is an engineering example to illustrate. For example, the total length of the water supply pipe from the outside of the project is 800 meters, which is about 500 meters from the outdoor water supply pipe. The original design is ductile iron pipe. According to the company's regulations, it is necessary to compare the economy of the pipeline system. We analyze the cost of ductile iron pipe and HDPE pipe respectively and estimate the project cost. After comparison and analysis, we found that: 1 (1) The price of ductile iron pipe is dn400 yuan / m, DN300 yuan / M; the price of HDPE pipe is dn400 yuan / m, DN300 yuan / m. from the perspective of pipe unit price, the price of ductile iron pipe is better than that of HDPE pipe. (2) Because HDPE pipe is connected by hot melt, the construction is difficult, and the installation cost is about 40% higher than that of ductile iron pipe. (3) Due to its good flexibility, HDPE pipe can be directly laid without pipe fittings in case of obstacles or unsmooth excavation of pipe trench, but ductile iron pipe can not. Sometimes, elbow or zigzag pipe must be used for transitional connection. In this way, HDPE pipe is better than ductile iron pipe in pipe fitting part. (4) HDPE pipes are connected by hot-melt butt joint, and the strength of the interface is very good. Nodular cast iron pipe is connected by socket and socket, and concrete reinforcement pier is required at tee joint, elbow and other parts. In this respect, the cost of using ductile iron pipe is higher than that of HDPE pipe. (5) HDPE pipe does not need anti-corrosion. If ductile iron pipe is selected for water supply pipeline, it is required to carry out anti-corrosion treatment for inner and outer walls of ductile iron pipe. (6) The normal service life of ductile iron pipe is 20-25 years after anticorrosion treatment of inner and outer wall. However, HDPE pipe has no corrosion and its normal service life is 50 years. 7、 Several problems in popularizing HDPE pipe 1. The popularization and application of HDPE pipe should be promoted by strengthening the popular science education of HDPE pipe. 2. As soon as possible to formulate the national unified pipeline design specification and construction acceptance specification, as of 2012, there is no national unified pipeline design specification and construction acceptance specification for HDPE pipe. 3. Strengthen the quality training for pipeline designers and construction and installation personnel, because HDPE pipe has high requirements for installation technology, so as to avoid design problems and rework due to poor sense of responsibility of design and construction personnel. construction technology Construction technology: survey → setting out → trench excavation → foundation treatment → connection, pipe laying and pipe calibration → pipe trench backfilling → inspection well masonry and backfilling → acceptance. In addition to the construction specification, the following points should be paid attention to in the construction of HDPE pipe (1) HDPE pipe has serious thermal expansion and cold contraction, so HDPE pipe should be constructed and used in the environment with little temperature difference. (2) When the buried depth of HDPE pipe is lower than the bottom surface of the building foundation, the pipe shall not be set within the range of foundation diffusion angle compression under the building foundation. (3) In the area where the groundwater level is higher than the elevation of the trench bottom, the groundwater level drops to 0.3 m-0.5 m below the nearest point of the trench bottom. (4) During the excavation of pipe trench, the excavation depth shall be strictly controlled, and the undisturbed soil at the bottom of the foundation shall not be disturbed. In case of over excavation and disturbance, the natural graded sand and gravel with particle size of 0.1 m-0.5 m shall be replaced and leveled and compacted. (5) HDPE pipe foundation adopts gravel foundation instead of concrete foundation, with thickness of 0.05 m-20 m, which is determined according to geological conditions. (6) In the process of pipe laying, pipe laying shall be carried out manually or by crane. When the crane runs the pipe, non-metallic rope buckle shall be used to tie the pipe ends 1 / 4 away from the pipe orifice. It is strictly forbidden to lift through the center. (7) There are strict requirements for backfill in the backfill, some of which are medium (coarse) sand (locally called asbestos sand) with certain specifications. The hard edges and corners of gravel (soil) are easy to damage PE pipes and the backfill is not easy to compact. Asbestos sand has the function of automatic compaction under the optimal moisture content, and the compactness requirements can be met by adding small tamping machines. (8) The part above 0.7 m above the pipe top shall be backfilled with machinery. The backfill below the top of the pipe must be backfilled from both sides of the pipe at the same time and tamped, otherwise the HDPE pipe will be subject to uneven pressure, resulting in deformation and displacement of HDPE pipe. Other backfill shall be carried out according to the specification. (9) The pipeline is connected to the inspection well. In the original construction scheme, the pipe mouth was retracted into the inner wall of the inspection well by 0.3 m ~ 0.5 m, and the mortar was used to paint it into a "trumpet shape". After that, the bricks were built, but the closed water test could not meet the requirements. The following problems were found after inspection: ① the heat expansion and cold contraction of HDPE pipe will crack the support of the inspection well, resulting in water leakage of the inspection well. ② The inspection well was not cracked, but the sealing of the connection between the pipeline and the inspection well could not meet the requirements, resulting in water leakage. In view of the above two problems, the following solutions are put forward: ① increase the buried depth of HDPE pipe, so that the HDPE pipe is in the environment with little change in relative temperature. At the same time, the HDPE pipe shall be backfilled immediately after laying and completed in a short time. The construction length should be controlled within a certain range. ② After the HDPE pipeline is laid, the inspection well shall be laid. ③ In view of the second problem, the connection mode as shown in the figure is adopted again, and the hot-melt fiber seal is used at the joint of inspection well and HDPE pipe to replace the original mortar sealing. |
