The water supply network has three characteristics that make the cycle long (30 years or longer), difficult to overhaul and renovate, and the accident is not easy to detect (buried underground). The safety and reliability of the water supply network has two meanings: first, the uninterrupted water supply of the pipe network, avoiding pipe rupture, leakage, and even blasting; the second is the quality of the tap water delivered by the pipe network, not due to the pipeline itself (such as The pipe is rusty, which affects the water quality of the pipe network. In the process of pipe network transmission and distribution, the water quality of the pipe network can not be changed too much; from the water plant to the user, it can arrive smoothly, and at the same time meet the pressure of water quantity and water supply service. In pipeline engineering, there are various pipes with different materials, and the application range is wide. Due to technological advances, the materials used in the manufacture of water supply pipes are constantly evolving, and the performance of new water supply pipes has been greatly improved. From the requirements of two aspects of safe and reliable water supply network, one is uninterrupted water supply, and the other is that the water quality of the pipe network is safe and reliable. This requirement is also the requirement of practical application to discuss the application and development of new water supply pipe.
First, the metal pipe
In the early construction of the distribution pipe network, the lead pipe was used, which is a metal pipe with better corrosion resistance. The oxide film formed on the surface of the lead pipe has been considered to be non-toxic. With the advancement of analytical instruments, trace elements can be determined and it is finally clear that lead will be separated from the lead pipe. In the presence of oxygen, when the pH is less than 7, a trace amount of lead is precipitated. Subsequently, a small-diameter galvanized steel pipe replaced the lead pipe. Now analyzing the water quality changes in the pipe network water distribution process, it can be found that the main reason for the water quality reduction of the pipe network is small-diameter galvanized steel pipes, especially the internal pipes of users. On the one hand, when using small-diameter galvanized steel pipes, protective measures such as lining coating are rarely used, and the pipe quality of the internal pipes of the user is poor; on the other hand, the small-diameter pipes are mostly at the end of the pipe network, and the flow rate in the pipes is slow. The detention time is long and the secondary water supply facilities are poorly managed and maintained. Therefore, small-diameter galvanized steel pipes have been replaced by plastic pipes and composite pipes.
1, cast iron pipe
It is the most familiar and widely used pipe. Cast iron is an alloy of iron (Fe) and carbon. The mass fraction of carbon in cast iron is higher than 2.11%, and the carbon content is lower than this ratio, which is low carbon steel. Steel pipes made of low carbon steel are easy to process and install. The mechanical properties of steel pipes are superior to those of other pipes, but their corrosion resistance is very poor. The service life of steel pipes is 20-30 years, and the life of cast iron pipes can reach 80-100 years. Since the surface anti-corrosion treatment of steel pipes is troublesome and the anti-corrosion cost is high, steel pipes are generally used only in special sections (now widely used).
2, ductile iron pipe
It is the overall heat treatment method of heating and cooling to change the performance of the pipe. For example, the ductile iron pipe is placed in a heating furnace, heated to 830 ° C, and then the pipe and the furnace are simultaneously cooled to room temperature. The overall heat treatment method for cooling such pipes and furnaces with furnaces is called "annealing". This method improves the impact toughness of ductile iron pipes. Comparing the metallographic analysis of ductile iron pipes and gray cast iron pipes, we can see spheroidized graphite (carbon) and filamentous graphite (unspheroidized carbon): gray cast iron pipes contain raw materials and production processes, A large number of cementite and spheroids, white mouth tissue (commonly known as gray mouth) can be seen in the pipe fracture, and the pipe performance is not as good as the ductile iron pipe.
3. Comparison of ordinary cast iron pipe and ductile iron
With the development of cast iron pipe production technology and technological progress, today, centrifugal ductile iron pipe has become the world's most important cast iron pipe production process. Gray cast iron pipes used in China have gradually shifted to centrifugal ductile iron pipes. Ductile iron pipes should not only have high tensile strength, but also have high elongation. According to the requirements of international standard IS02531, the tensile strength of ductile iron pipe is ≥420MPa, the pipe of DN40-DNl00mm, the elongation of ≥10%, the pipe of DNl200-2600mm, the elongation is ≥7%. Gray cast iron pipe has a wide control range for harmful components of molten iron raw materials because it does not require the elongation of the pipe. As shown in the following table:
Comparison of chemical composition between ductile iron pipe and gray cast iron pipe (%) Table 1
Carbon (C) silicon (Si) manganese (Mn) phosphorus (P) sulfur (S)
Ductile iron pipe 3.5～3.61.6～1.80.2～0.4≤0.04≤0.02
Gray cast iron pipe 3.3～3.81.4～2.60.6～～0.9≤0.3≤0.10
Second, plastic pipes
The plastic pipes commonly used in water supply networks can be divided into two categories: one is polyolefin materials: polyethylene (PE), polypropylene (PP), polybutene (PB), and these plastic pipes are suitable for fusion (heating). )connection.
The other type is vinyl chloride material: polyvinyl chloride (PVC), rigid polyvinyl chloride (U-PVC), perchlorinated polyvinyl chloride (C-PVC), suitable for bonding (glue) bonding. It is now described as follows:
1, PVC plastic pipe
PVC and PE pipes were used in water supply pipeline projects in the 1960s. In the initial stage of application, the connection method of PVC pipe is relatively simple. The small-diameter PVC pipe adopts glue and adhesive connection. The medium-diameter PVC pipe adopts the socket interface and is sealed by rubber ring, which is an ideal flexible interface. Therefore, before the 1990s, the application of PVC pipes was extensive. For the promotion of the initial PE pipe, the method of heating connection is adopted, and the construction is inconvenient. Early PE pipes were not good in compressive strength and crack resistance due to material properties. In order to withstand external pressure and internal pressure, the larger the diameter of the PE pipe, the thicker the pipe wall. Because the thicker wall can ensure that the pipe has sufficient rigidity. Large-diameter PE pipes have relatively thick pipe walls and increased material consumption, which increases costs. Therefore, the promotion of the initial PE pipe, generally not more than DN300mm, is mainly used for indoor pipes and water distribution networks.
Polyvinyl chloride (PVC) is usually a multi-component plastic, and the structure of PVC changes under the action of heat, oxygen and light. Therefore, stabilizers must be added to PVC processing. In order to improve the surface properties of PVC pipes, reduce the shrinkage performance and coloring of pipes, other processing aids, fillers and pigments are required. The U-PVC commonly used in pipes is to improve the brittle and hard performance of PVC, and to add plasticizer to the production. PVC pipes with a plasticizer content ranging from 0 to 12% are called U-PVC pipes.
C-PVC is a product obtained by further chlorination of PVC. The difference between C-PVC and PVC is the difference in chlorine content, and the degree of chlorination of C-PVC is higher. Generally, the chlorine content of PVC is 56.7%; the C-PVC chlorine content is 63-74%. This is caused by the different chlorination methods and production technologies of various manufacturers. Due to the increase of C-PVC chlorine content, the anti-aging performance and pressure resistance of C-PVC pipes are greatly improved. It can be seen that the C-PVC pipe is an improvement and improvement of the traditional U-PVC pipe.
Due to its simple construction and good hydraulic conditions, PVC pipes have been widely used in water supply networks. As environmental protection requirements become stricter and water quality indicators increase, considering the health and safety factors, PVC pipes are questioned in the following two aspects: 1. The additives used in PVC pipes contain heavy metals and a stabilizer such as decyl acid. It contains metal lead, which may be deposited in water; 2. The PVC pipeline is too high in chlorine and may have a negative impact. The above two problems have led to the expansion of other plastic pipes such as PE (polyethylene) and PP (polypropylene).
2, PE plastic pipe
PE (polyethylene) is rich in raw materials. From the molecular structure (C2H4), the molecular structure of PE is the simplest one. PE pipelines are developing and constantly overcome their own shortcomings. The heat welding of the PE pipe requires a certain operating space and skilled operation techniques. Later, electric heating joints were invented: two pipe sections or pipe fittings were connected by an electric heating welded sleeve. This connection method
Connection of PE pipes in the trench, or where the construction space is small. The material properties of PE pipes are also improving. From PE63, PE80, to PEl00, PEll5 will be developed in the future. PEl00 means that at 20 ° C, this grade of PE material can maintain a minimum strength (MRS) of 10 MPa after 50 years of use; PE80 is 8.0 MPa.
As the grade of PE material increases, the same pipe, the wall thickness of PEll5 is lower than the pipe wall thickness of PEl00. Under the premise of meeting the requirements, the wall thickness of the PE pipe is thinned, the amount of PE material is reduced, the cost is reduced, and the application of the PE pipe in the large-diameter pipe is facilitated. The connection of the PE pipe needs to be heated, and a certain cooling time is required. It takes about 20 minutes for each welding. The larger the diameter of the pipe, the longer the heating and cooling time. Therefore, the installation period of the PE pipe is relatively long.
PE pipes have a high market share in pipeline excavation and old pipe renovation. Therefore, PE pipes are often used in pipeline excavation projects; due to the comprehensive performance of PE (polyethylene), it is the production of composite pipes ( The main raw material of aluminum-plastic composite pipe and steel-plastic composite pipe). The composite pipe combines the metal material and the plastic reasonably, so that the good mechanical properties of the metal and the anti-corrosion performance of the plastic can be fully utilized, thereby ensuring the safety and reliability of the water supply pipe network.
Third, the development of different types of pipe
In order to meet the requirements of the operation of the water supply network, the development of the pipe has two aspects: First, the selection of new pipes, restrictions and phasing out old pipes that do not meet the development requirements. Develop new pipe with excellent performance and good corrosion resistance, choose suitable metal or non-metal pipe according to actual requirements; secondly, take protective measures on the pipe surface, external anti-corrosion and internal lining to ensure the safety of pipe network water quality. It is now described as follows:
l, cast iron pipe
In pipeline engineering, it still occupies an important position, especially steel production will not decline in the next few decades (currently the world's output is 700-800 million tons). The performance of cast iron pipes has been greatly improved. In the international standard IS02531, the tensile strength of the centrifugal ductile iron pipe is ≥ 420 MPa, which is similar to the strength of the low alloy steel, but the corrosion resistance of the centrifugal ductile iron pipe is superior to that of the steel pipe. In order to resist corrosion and ensure the quality of the pipe network, the lining and anti-corrosion materials inside and outside the cast iron pipe are being improved and improved.
2, plastic tube
There will be greater development, and the material properties of PVC pipes will be improved to avoid the use of stabilizers containing heavy metals. Various new technologies for new plastics are constantly being promoted. However, the stability and aging of plastic pipes is still a concern.
3, advanced composite tube
It has been greatly developed in the 20th century, such as prestressed steel and reinforced concrete pipes, PCCP pipes (steel tube concrete pipes) and resin-based composite materials (glass pipe sand pipes GRP) have been applied in large-diameter water pipeline projects. Steel-plastic composite pipes and aluminum-plastic composite pipes have great demand in the small-diameter user pipe market. The development of new water supply pipes is not a mere technological advancement, but also takes into account market competition conditions and market orientation. New types of pipes that meet development and environmental requirements are still being developed.
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