What Kind of Pipe Is Used for Water Supply Systems?

With the development of urban water supply pipelines, the requirements for urban water supply pipeline materials are becoming increasingly stringent. Not only must the pressure-bearing capacity meet the design requirements, but the pipelines must also be corrosion-resistant and safe, and not endanger human health. Currently, water supply pipes are mainly divided into metal and non-metal types. Metal pipes mainly include cast iron pipes and steel pipes; non-metal pipes include concrete pipes, fiberglass reinforced plastic pipes, plastic pipes, and plastic composite pipes.
 
In the water supply pipeline system, gray cast iron pipes, ductile iron pipes, steel pipes and copper pipes account for a significant proportion. These pipes are widely used in various water supply systems due to their durability and adaptability to different applications. Ductile iron and gray cast iron pipes account for about 70%, steel pipes 20%, and copper pipes 10%.
 
(1) Ductile iron pipes
The manufacturing processes of ductile iron pipes include smelting, spheroidizing, casting, cooling, and subsequent processing. Spheroidizing treatment is key. By adding a spheroidizing agent to the molten iron, the graphite forms a spherical structure during the cooling process. Ductile iron refers to cast iron in which spherical graphite (C) precipitates during the solidification of liquid iron (Fe) during production. Ductile iron pipes are flexible pipes that are formed by centrifugal casting. The plasticity and toughness of ductile iron pipes are greatly improved, and the graphite metallographic structure changes, significantly enhancing the properties of ductile iron. The splitting effect of spherical graphite on the metal matrix is smaller than that of flake graphite, making the strength of cast iron reach 70% to 90% of the matrix structure. The tensile strength can reach 120 kgf/mm², and the service life can be up to 50 to 100 years. Its maximum internal pressure resistance is greater than 2.0MPa, and the maximum soil covering thickness is greater than 10 meters, exhibiting good toughness.
 
The stiffness, elongation, and tensile strength of ductile iron pipes are relatively high, and the ground load and soil load capacity they can withstand are generally better than those of pipes made from other materials. Their pipe fittings have complete specifications, can adapt to the continuous water connection of branch pipes when the pipeline is running, can meet various installation needs, and are more convenient to use than non-metallic pipes. They adopt flexible ends, have a better ability to withstand partial settlement, are more convenient to disassemble, and are easy to repair. Ductile iron pipes have been widely used in urban water supply, industrial water supply, urban water supply systems, sewage treatment systems, natural gas transmission, and mining pipeline systems due to their high-pressure resistance, anti-oxidation, anti-corrosion properties, and other advantages. Compared with gray cast iron pipes, the cost of ductile iron pipes is slightly higher. The inner wall of the pipe is prone to corrosion and rust, which can affect the quality of tap water. The wall of the ductile iron pipe is thin and easily perforated. When the pipe diameter is large, it is not economical. In the late 1860s, ductile iron pipes  began to be mass-produced in industry. In 1986, China introduced the centrifugal method to build a ductile iron production line and has developed the hot film, water-cooled metal mold, and resin sand-lined centrifugal ductile cast pipe technology, reaching a high international level. In 2008, China established the national production standards for ductile iron. In recent years, ductile iron pipes have been widely used in pipeline transformation. Ductile iron pipes have replaced gray cast iron pipes and play an increasingly important role in water supply systems. 
 
(2) Gray cast iron pipes
Gray cast iron refers to cast iron obtained by fully progressing the graphitization process during processing. All or most of the graphite exists in the form of flakes. It is the most widely used water supply pipe in China. It is mainly used in municipal water supply and drainage systems, and is especially suitable for for large-diameter underground pipelines. This traditional material is still used, especially in some old urban pipelines. Although the exact proportions have changed, they are still very important. At the end of 1990, gray cast iron accounted for 75% of the water supply pipelines in service in China. According to statistics, as of 2007, 51% of the water supply pipelines in China's urban water supply pipelines were still gray cast iron pipes, and the proportion of gray cast iron pipes in some cities was up to 90%. Gray cast iron pipes have advantages of durability, corrosion resistance, and long service life. However, gray cast iron pipes have been gradually replaced by ductile iron pipes due to their brittle texture, intolerance to vibration, heavy weight, easy breakage, and pipe explosion during operation.
 
(3) Steel pipes
Steel pipes are long and have good adaptability, high-pressure resistance, vibration resistance, and excellent mechanical strength. They are widely used in applications with large pipe diameters and high water pressures, as well as in areas with poor geological and terrain conditions, such as river valleys and seismic zones. However, steel pipes have poor corrosion resistance, high cost and poor economic efficiency. They are often used in industrial water transmission systems, high-pressure water pipelines, and projects requiring higher pressure-bearing capabilities.
 
(4) Copper pipes
Copper pipes have good corrosion resistance, long service life and antibacterial properties, and are easy to install. They are suitable for domestic water supply pipes, but their costs are relatively high. They are mainly used in household water supply pipes and some places requiring high hygiene standards, such as hospitals and food processing plants.
 
Comparison of water supply pipes
Ductile iron pipes, gray cast iron pipes, and steel pipes are commonly used in water supply pipelines. The following table compares their performance. 
 
Table 1-1 Performance comparison of water supply pipes