Performance Comparison of Steel-in-Steel Direct-Buried Insulated Pipe
1. Light weight, high strength, convenient transportation and installation. The specific gravity of filament-wound fiber-reinforced FRP pipe is 1.65–2.0, only 1/4 that of steel, but the hoop tensile strength of steel-in-steel direct-buried insulated pipe is 180–300 MPa and the axial tensile strength is 60–150 MPa, similar to alloy steel. Therefore its specific strength (strength/specific gravity) is 2 to 3 times that of alloy steel. In this way it can meet user requirements and be designed as piping that satisfies various internal and external pressure requirements. For the same pipe diameter, the unit weight of FRP pipe is only about 1/2.5 that of carbon steel pipe (spiral-welded steel pipe), 1/3.5 that of cast-iron pipe, and about 1/8 that of prestressed reinforced-concrete pipe, making transportation and installation very convenient. Each section of steel-in-steel direct-buried insulated pipe is 12 meters long, which can reduce the joints of concrete pipe by two-thirds. Its socket connection method makes installation fast and simple, while reducing lifting costs and increasing installation speed.
2. Anti-fouling and resistance to marine growth. The surface of unsaturated polyester resin is clean and smooth and is not fouled by microorganisms such as mussels and fungi in seawater or sewage, which would otherwise increase roughness, reduce the flow cross-section, and raise maintenance costs. Steel-in-steel direct-buried insulated pipe has no such fouling and remains as clean after long service as when new. At the same time, because of its smooth inner wall and excellent corrosion resistance, it does not scale or support microbial growth, effectively ensuring water quality and stable hydraulic resistance. Conventional pipes, by contrast, tend to show increasing hydraulic resistance and surface scaling over time.
3. Good heat resistance and freeze resistance. At -30°C, it still has good toughness and high strength. It can be used for a long time in the range of -50°C to 80°C. Using a specially formulated resin, it can operate at temperatures above 110°C.
4. Good corrosion resistance, with no effect on water quality. Steel-in-steel direct-buried insulated pipe can resist corrosion by acids, alkalis, salts, seawater, untreated sewage, corrosive soil or groundwater, and many chemical fluids. Its service life is longer than that of conventional pipe, with a design life typically exceeding 50 years. Sand-filled FRP pipe is used more in municipal and urban water transmission and distribution pipelines. Because it is non-toxic, rust-free, and odorless, causes no secondary pollution of water quality, requires no additional anti-corrosion treatment, and has a greatly extended service life, and because it is easy to install, it is welcomed in the water-supply and drainage industry.
