The international standard of polyethylene pipes for water supply (ISO4427) was not officially released until 1996. In China, the national standard GB/T 13663 was compiled with reference to ISO4427 1999, and was promulgated and implemented in 2000, but the standards and engineering technical specifications of pe pipe fittings are still being improved. This shows that the quality of pe pipes in China is unstable, the matching of pipes and fittings is not perfect, and the quality cannot be guaranteed in production and construction.
Typical uses [1] water supply pipeline, household pipeline, house wallboard, commercial machine shell, electronic product packaging, medical equipment, food packaging, etc. 2. PE water supply pipe 1, scope of application
Since 1990s, China's synthetic resin industry has developed vigorously, and its output and consumption have maintained a good growth momentum. At present, China's synthetic resin production capacity reaches100000 tons, ranking the top five in the world, second only to the United States, Japan, Germany and South Korea. However, there is still a big gap in all kinds of special resins needed for plastic pipes. Although the output of PE and PP resins in China is large, there are not many resins suitable for pipes, especially PE pipes for urban water supply and gas transmission and PP-R special materials for building cold and hot water. It is necessary to import a certain amount of special resin for plastic pipes.
China plastic pipe has developed rapidly and its quality has been continuously improved. At present, a plastic pipe industry mainly consisting of PVC-U pipes, pe pipes and PP-R pipes has been initially formed. Among them, polyethylene (PE) pipes are widely used in building water supply, building drainage, buried drainage pipelines, building heating, gas transmission pipelines, electrical and telecommunications protective sleeves, industrial pipelines, agricultural pipelines and so on because of their unique advantages. Mainly used for urban water supply, urban gas supply and farmland irrigation.
Second, the characteristics and social benefit analysis of PE pipeline
We analyze it by comparing the characteristics of common water supply (cold) pipes in the attached table.
(1) polyethylene has excellent corrosion resistance, good sanitary performance and long service life.
Polyethylene is a non-inert material. Except for a small amount of strong oxidant, it can resist the corrosion of various chemicals and is not easy to breed bacteria. As we all know, the reason why steel pipes and cast iron pipes are replaced by plastic pipes is not only because of their lower energy consumption, lower domestic energy consumption, lighter weight, smaller water flow resistance, simpler and faster installation, lower cost, longer service life and thermal insulation function, but also because plastic pipes have better corrosion resistance than steel pipes and cast iron pipes and are not easy to breed microorganisms.
The service life of polyethylene pipe is more than 50 years, which is not only confirmed by international standards and some advanced foreign standards, but also proved by practice.
Another reason why polyethylene can be popularized and applied is that PVC is under increasing pressure of environmental protection. First of all, the sanitary performance of PVC itself: as we all know, the production of PVC pipes can ensure the sanitary performance under regular production and strict control, and it is allowed to be used in the drinking water field. However, there are still people who worry that there may be problems in places with lax control, such as excessive vinyl chloride monomer in PVC resin and abuse of toxic additives in PVC pipe formula for water supply. Non-toxic PVC pipes and fittings for drainage are misused as water supply pipes and fittings. Secondly, the recycling of PVC pipes: PVC, like polyethylene, is thermoplastic and can be used in theory. However, countries have proved that the proportion of old plastic products that can be recycled is limited, and the main treatment method is incineration to recover energy. Because PVC contains chlorine, it may produce harmful substances if it is not well controlled during incineration, while polyethylene only contains hydrocarbons, which will produce water and carbon dioxide after incineration. Therefore, in Europe and America and other countries, the application of PVC is under increasing pressure from some environmental organizations.
(2) Polyethylene has unique flexibility and excellent scratch resistance.
The flexibility of polyethylene pipeline system has great technical and economic value. The flexibility of polyethylene is an important property, which greatly improves the value of this material in pipeline engineering. Good flexibility enables polyethylene pipes to be coiled and supplied in a long length, avoiding a large number of joints and fittings. For example, Nantong Water Supply Company, a national urban water improvement demonstration unit, made full use of the coiled characteristics of PE small-diameter pipes when selecting materials in the transformation of one household and one meter. From the water meter to the end of the pipeline, there is no need for a pipe joint in the middle, which not only saves the cost, but also improves the work efficiency. This feature of PE small-caliber pipe has been recognized by water supply companies with outdoor water meter projects in cities all over the country and has become their first choice for water improvement. At the same time, flexibility, light weight and excellent scratch resistance make it possible to adopt a variety of economical installation methods that can reduce the impact on the environment and social life, such as excavation-free construction technology. Trenchless construction technology refers to the construction technology of laying, replacing or repairing various underground pipelines without ditching on the surface by using various geotechnical drilling and excavation technologies. A variety of trenchless construction techniques are very suitable for using polyethylene pipes, such as horizontal directional drilling and pilot drilling for laying new pipes, expansion method for replacing old pipes in situ, interspersed renewal lining method for repairing old pipes and various improved lining methods (folding deformation method, hot stretching method and cold rolling method).
The unique flexibility of polyethylene also enables it to effectively resist underground movements and end loads. On the surface, the strength and rigidity of plastic buried pipes are not as good as cement pipes and metal pipes, but in practical application, plastic buried pipes belong to "flexible pipes". Under the correct design and laying construction, the load borne by plastic buried pipes is the same as that of the surrounding soil. Therefore, the plastic buried pipe can meet the requirements of mechanical properties when used underground, without reaching the same strength and stiffness as the "rigid pipe". At the same time, the pressure relaxation characteristics of polyethylene can effectively consume stress through deformation, and its actual axial stress level is far lower than the theoretical calculation value. The elongation at break is generally greater than 500%, and the bending radius can be as small as 20 ~ 25 times of the pipe diameter. It is a kind of high toughness material, which has strong adaptability to uneven settlement of foundation and is the best pipeline to resist earthquake, foundation settlement and temperature difference expansion. For example, in the Kobe earthquake of 1995, PE water supply pipes and gas pipes were the only surviving piping systems.
(3) Polyethylene has outstanding low temperature resistance.
The low temperature embrittlement point of PE pipe is -70℃, which is superior to other pipes. In winter, PVC-U pipes are prone to brittle fracture in field construction. An experience gained from the pilot project of laying PVC-U buried water supply pipe in Beijing is that it is not suitable to lay PVC-U pipe when the temperature is below zero. There is also an obvious evidence that in order to improve the toughness and low-temperature impact resistance of PP, ethylene and propylene monomers can be polymerized into random polypropylene (PP-R). Generally, the mixed gas polymerization of propylene and ethylene is adopted to obtain a polymer with random distribution of propylene and ethylene segments on the main chain (that is, PP-R pipe). However, the low-temperature resistance of PP-R after improvement is still not satisfactory, and it is brittle.
(4) Polyethylene has good fast crack propagation fracture toughness.
When the crack spreads rapidly, it can spread rapidly from several hundred meters to more than ten kilometers at the speed of 100 ~ 45m/s, which leads to the damage of long-distance pipelines and the occurrence of large-scale leakage accidents, followed by combustion explosion (natural gas transportation) or flood (water transportation) accidents. This kind of accident is unlikely to happen, and once it happens, it will be extremely harmful. In recent ten years, this failure mode is considered to be the most dangerous accident in plastic gas pipeline, which needs absolute prevention. For the sustainable development of plastic pressure pipes, the importance of preventing the rapid crack propagation and failure has exceeded the requirements of long-term life strength performance. The reason is that under the same SDR (ratio of pipe diameter to its thickness), the calculated long-term life-long-term strength has nothing to do with the increase of pipe diameter (in fact, large pipe diameter may be safer than small pipe diameter), but with the increase of pipe diameter, the risk of rapid crack propagation increases. In the existing large-scale plastic experimental methods, such as polyethylene, polypropylene, PVC pipes and so on. When a certain pipe diameter is reached, the allowable pressure determined to prevent the rapid expansion and destruction of cracks is always lower than that determined by the long-term strength problem. That is to say, after the allowable pressure is determined according to the requirement of preventing the rapid expansion and destruction of cracks, the requirements of long-term life (such as 20℃, 50 years) can be met by itself; Materials with fast crack growth and poor fracture toughness will be eliminated, regardless of their long-term strength properties. For example, polyvinyl chloride (PVC-U) gas pipes have been basically replaced by polyethylene (PE) gas pipes. The trend that PVC-U water supply pipes in Europe are replaced by pe pipes is obvious.
China has not established a testing device to monitor the rapid crack growth and failure. China's standards for plastic pressure pipes do not involve this problem, which shows that the level of plastic pressure pipes in China is at least one development stage behind the general level in the world.
(5) The installation and connection of polyethylene pipes are convenient and reliable.
The polyethylene pipe can be reliably and firmly connected with the original pipe by convenient hot-melt butt joint and socket (there is a small welding convex ring at the joint), or the polyethylene pipe can be connected by special electric melting fittings. For small diameter pipes, we can also use the card connection produced by our company, which is convenient and reliable (this product is a patented technology introduced by our company and widely used in many domestic water companies). The welded joint of polyethylene pipe can bear axial load without leakage and disconnection.
Today, the connection technology of polyethylene is very mature and reliable. Statistics show that the leakage rate of polyethylene pipes is less than 2/100,000, far lower than 2-3% of ductile iron pipes, which greatly improves the safety and economic benefits of pipelines, which is also a very important reason why polyethylene pipes are widely used in gas pipelines. 3. Cast iron pipe: a pipe cast with cast iron. Cast iron pipes are used for water supply, drainage and gas transmission pipelines, including cast iron straight pipes and pipe fittings. Low labor intensity. According to the different casting methods, it can be divided into continuous cast iron pipe and centrifugal cast iron pipe, in which centrifugal cast iron pipe is divided into sand mold and metal mold. According to different materials, it can be divided into gray cast iron pipes and ductile iron pipes. The flexible interface is sealed by a rubber ring,
The nominal diameter of continuous gray cast iron pipe is 75 ~ 1200mm, and socket or flange joint is adopted. According to functions, it can be divided into flexible interface and rigid interface. The lengths of straight pipes are 4m, 5m and 6m; According to the different wall thickness, it is divided into three grades: LA, A and B. The nominal diameter of sand centrifugal gray cast iron pipe is 200 ~ 1000mm, and the effective length is 5m and 6m. According to the different wall thickness, it is divided into P grade and G grade. High strength, good toughness, thin pipe wall, less metal consumption, able to withstand higher pressure,
Compared with gray cast iron pipe, ductile iron pipe has the advantages of high strength, good toughness, thin wall, less metal consumption and higher pressure resistance, and its effective length is 5 meters and 6 meters. According to the different wall thickness, it is divided into P grade and G grade. It is the development direction of cast iron pipe.
The connection between pipes adopts socket type or flange type interface; According to functions, it can be divided into flexible interface and rigid interface. The flexible interface is sealed by rubber ring, which allows a certain limit of rotation and displacement, so it has good impact resistance and sealing performance, is simpler and faster than the rigid interface installation, and has low labor intensity according to different pouring methods.