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The last edit of this post was made by Wang Wei2 on 2023-10-19 at 10:19. A brief discussion on the advantages and disadvantages of fiberglass-reinforced plastic and steel outer protective pipes: 1. Advantages of steel protective pipes: The technology for burying steel protective pipes (steel inside steel) has been continuously improved over nearly 20 years, especially through practices and advancements in the past 5 years; it has successfully addressed the key issues related to waterproofing, impermeability, and compressive strength in the burial of steam pipelines. Currently, in the design process, there is a complete set of methods and strict management measures regarding aspects such as the system’s embedding structure, stress analysis, insulation, corrosion prevention, drainage, thermal bridges, and construction. Practice has shown that the steel casing (steel-in-steel) burial technology is a new burial method that provides waterproofing, leak prevention, impermeability, compressive strength, and full sealing; it represents a significant advancement in the use of direct burial techniques in areas with high groundwater levels. The primary concern regarding protective pipes for directly buried pipelines is the reliability of their waterproofing properties. In addition, they need to possess good mechanical strength. Steel sleeves, thanks to their high strength, are connected by welding, which ensures a very high level of reliability in terms of waterproof sealing. Moreover, their resistance to high temperatures is unmatched by other types of external protective pipes. In areas with high groundwater levels, to ensure that groundwater does not affect the proper operation of steam direct-buried pipelines, it is advisable to use a sturdy, airtight steel pipe casing as the outer protective layer. “The main advantages of the \"steel-in-steel\" directly-buried insulation structure are: (1) the highest reliability for long-term safe operation (it features good strength, stiffness, temperature resistance, and advanced technology) ； ⑵ Best sealing and waterproof performance (new technologies and new structural designs are easy to implement) ； ⑶ Pipeline anti-corrosion is ensured to provide a long service life (anti-corrosion coatings typically use HD epoxy coal tar pitch paint for effective protection) ； ⑷ Potential risks are low (the steel tube used as the outer layer has high fatigue strength; it operates over the long term in an environment of thermal expansion and contraction, and its joints are not as prone to separation as those of fiberglass-reinforced plastic outer tubes) ； ⑸ Significant energy savings (good sealing performance, no leaks, improved insulation) ； ⑹ Easy to install; the joints are well-sealed. Repair work and prefabricated insulated \"steel-in-steel\" pipes are all more convenient than \"fiberglass-in-steel\" pipes ； ⑺ The welding techniques for constructing steel pipes are more mature, and the leak detection methods are advanced (backed by national standards). 2. Disadvantages of fiberglass-reinforced plastic: Although using fiberglass-reinforced plastic as the outer protective layer in the composite insulation structure for steam pipes offers certain advantages—such as a lower cost compared to the \"steel-in-steel\" design—there are still some disadvantages ； Compared to \"steel in steel\", there is no need to consider corrosion protection issues. However, due to the numerous accidents that have occurred in engineering practices in recent years, it has been found that fiberglass protective layers have many defects; it can be said that they currently fail to meet China’s engineering requirements, both in terms of material properties and construction techniques. ⑴ Temperature resistance. Fiberglass reinforced plastic, also known as glass fiber reinforced resin, has temperature resistance properties that depend on the resin used. In China, unsaturated polyester fiberglass pipes are widely used; their maximum allowable operating temperature is generally not more than 70°C. Although epoxy resin fiberglass pipes can operate at temperatures around 100°C, their cost is higher. This defect is a critical issue that cannot be ignored when transporting high-temperature steam through direct burial. As the outer protective tube for the insulation structure of high-temperature steam, especially in cases involving steam at temperatures above 300°C, it is impossible to ensure that the surface temperature of this outer protective tube remains below 70°C for various reasons. As a result, it operates under high temperatures for extended periods, which leads to a decline in its mechanical properties, causing the pipeline to become unusable in a short time ； ⑵ Construction quality. Another key issue associated with the use of fiberglass-reinforced plastic outer protective pipes is the handling of on-site joints, especially in areas with high water levels where construction conditions are poor. Inadequate manual handling, poor bonding of thermoplastic shrinkage strips that can lead to tearing, and incomplete curing of the fiberglass repairs result in low strength, which in turn causes poor sealing at those joints and leads to leaks and damage. Such quality problems related to joints are quite common in construction projects in China at present. For long-distance pipelines, there are many joints, and the quality of joint treatment directly affects the overall quality of the project ； ⑶ Thermal bridge treatment. To secure the core tube, fixing brackets are generally used between the core tube and the outer protective tube. Since these fixing brackets are usually made of steel, and the temperature of the outer protective tube is very high in this area, a heat bridge effect occurs. If thermal bridging is not properly addressed, high temperatures will cause the pipes to creep, leading to cracks in them and thus resulting in leaks. In other words, a small defect can lead to widespread problems. ⑷ Voltage withstanding capacity. Compared with steel protective pipes, fiberglass reinforced plastic protective pipes have a lower yield strength; when external pressures are high and the load is excessive, compression failure of the pipe wall, bending failure of the pipe wall, and deformation of the pipe diameter can easily occur ； ⑸ Installation and transportation. During installation and transportation, improper use of construction equipment and inadequate operations can easily lead to damage to the pipes, thereby compromising the quality of the entire piping system and creating potential risks for safe operation in the future ； ⑹ Tube mass. The quality of the pipe bodies and their enclosures made of fiberglass-reinforced plastic in China is also a cause for concern; some products have only two or three layers of glass fabric, uneven resin coatings, insufficient thickness, and a variety of different brands. These indirect quality issues also make it difficult to ensure the quality of the construction work.