Polyethylene (PE) pipes for water supply

(1) polyethylene has excellent corrosion resistance, better hygienic properties and longer service life.

Polyethylene is a kind of inert material, which is resistant to many chemicals and not easy to breed bacteria except a small amount of strong oxidant. It is well known that the reason why steel pipes and cast iron pipes are replaced by plastic pipes is not only because the plastic pipes have lower energy consumption, lower living energy consumption, lighter weight, less water resistance, simple and rapid installation, low cost, long life span and heat preservation function than steel, but also because the plastic pipes are better than steel in corrosion resistance and microorganism resistance. Tube and cast iron pipe.

The service life of polyethylene pipe is more than 50 years, which has been confirmed not only by international standards and some advanced foreign standards, but also by practice.

Another reason why polyethylene can be popularized is that PVC is under increasing environmental pressure. The first is the hygienic properties of PVC itself: it is well known that the production of PVC pipes under regular production and strict control can ensure hygienic properties, allowing applications in the field of drinking water. But there are worries that problems may arise in places where control is lax: for example, vinyl chloride monomers in PVC resins exceed standards and poisonous additives are misused in the formulation of PVC pipes for water supply. Misuse of PVC pipes and fittings that are not guaranteed to be harmless is misused to water pipes and fittings. Secondly, the recycling of PVC pipes: PVC and polyethylene are thermoplastic plastics, theoretically available, but countries have proved that the proportion of recycling of old plastic products is limited, the main way is to incinerate recycled energy, PVC because of chlorine, in the incineration control is not controlled. Good can produce harmful substances, while polyethylene contains only carbon and hydrogen, and water and carbon dioxide are generated after burning.

flexibility

Polyethylene has unique flexibility and excellent scratch resistance.

The flexibility of PE piping system is of great technical and economic value. Flexibility of polyethylene is an important property, which greatly improves the value of the material for pipeline engineering. Good flexibility allows polyethylene pipes to be coiled and supplied in longer lengths, avoiding a large number of joints and fittings. At the same time, the flexible, light weight and excellent scratch resistance make it possible to adopt a variety of installation methods which can reduce the impact on the environment and social life and cost-effective, such as excavation-free construction technology. Excavation-free construction technology refers to the use of various geotechnical drilling techniques, in the surface without ditches (grooves) under the conditions of laying, replacing or repairing a variety of underground pipelines construction technology. Various excavation-free construction techniques are suitable for polyethylene pipes, such as horizontal directional drilling and directional drilling for laying new pipelines, in-situ replacement of old pipelines by expanding pipelines, interpolation and renewal of old pipelines by lining, and various improved lining methods (folding deformation, hot drawing and cold rolling).

PE's unique flexibility also enables it to effectively resist underground movement and end load. On the surface, the strength and rigidity of plastic buried pipes are inferior to cement pipes and metal pipes, but in practical application, plastic buried pipes belong to "flexible pipes". Plastic buried pipes are loaded together with the surrounding soil under the correct design and laying facilities. Therefore, the plastic buried pipe does not need to meet the "steel pipe" the same strength and rigidity can meet the requirements of mechanical properties in the use of buried. At the same time, the pressure relaxation property of polyethylene can effectively dissipate stress through deformation. Its actual axial stress level is much lower than the theoretical value, and its elongation at break is generally greater than 500%, and the bending radius can be reduced to 20-25 times of the pipe diameter. It is a kind of high toughness material, and its adaptability to uneven settlement of foundation is not good. Changqiang, these characteristics make it the best pipeline to withstand earthquakes, ground settlement and temperature difference expansion. For example, PE water and gas pipes were the only survivors of the Kobe earthquake in 1995.

Low temperature resistance

Polyethylene has very prominent low temperature resistance.

The low temperature embrittlement point of PE pipe is -70 C, which is superior to other pipes. Polyvinyl chloride (PVC-U) pipes are easy to be brittle and cracked during field construction in winter. One of the experiences summarized in the pilot project of laying PVC-U buried water supply pipes in Beijing is that it is not suitable to lay PVC-U pipes below zero temperature. There is also an obvious evidence that in order to improve the toughness and low temperature impact resistance of PP, ethylene and propylene monomer can be copolymerized into random copolymer polypropylene (PP-R), which generally uses the iPP process route and method to copolymerize the mixture of propylene and ethylene, resulting in irregular distribution of propylene and ethylene in the main chain. The content of ethylene in the PP-R tube material is about 3%, which is the copolymer of PP-R. However, the low temperature resistance of the improved PP-R is still unsatisfactory. The brittleness point of the modified PP-R is about - 15 C, which is much higher than that of the polyethylene pipe - 70 C.

fracture toughness

Polyethylene has good fast crack growth and fracture toughness.

When rapid crack growth failure occurs, the crack can spread rapidly for several hundred to ten kilometers at a speed of 100-45 m/s, resulting in long-distance pipeline damage, large-scale leakage accidents, and subsequent combustion explosion (natural gas transmission) or flood (water transmission) accidents. The probability of such an accident is not great. Once it happens, it will cause great harm. For the continuous development of plastic pressure pipes, the importance of preventing rapid crack growth has exceeded the long-term strength requirements. The reason is that at the same SDR (pipe diameter to thickness ratio), the calculated long-term life-long strength has nothing to do with the increase of pipe diameter (in fact, the large diameter pipe may be safer than the small diameter pipe), but the risk of rapid crack growth increases with the increase of pipe diameter. The allowable pressure determined by preventing rapid crack growth is always lower than the allowable pressure determined by long-term strength problem when the pipe diameter reaches a certain value. That is to say, long-term life (e.g. 20 C, 50 years) can be satisfied by itself after the allowable pressure is determined according to the requirement of preventing rapid crack growth damage; materials with poor fracture toughness due to rapid crack growth will be eliminated regardless of their long-term strength performance. For example, polyvinyl chloride (PVC-U) gas pipes are basically replaced by polyethylene (PE) gas pipes. The trend of European polyvinyl chloride (PVC-U) water supply pipe replaced by polyethylene (PE) pipe is clear.

Our country has not yet set up a test device to monitor the rapid crack growth failure. China's standards for plastic pressure pipes do not deal with this problem, which shows that the level of China's plastic pressure pipes is at least one development stage behind the world's general level.