The Polyurethane Direct-Buried Insulated Pipe Industry Enters a New Technological Era

　　Prefabricated polyurethane foam insulated pipes are widely used in pipeline networks for the transportation of liquids and gases, as well as in insulation projects for chemical pipelines, oil and chemical industries, district heating networks, central air-conditioning ventilation ducts, and municipal engineering. High-temperature prefabricated direct-buried insulated pipes are a type of prefabricated, directly buried insulation pipe that offers excellent thermal performance, high safety and reliability, and low construction costs. They effectively address the challenges associated with high-temperature heat transmission—ranging from 130°C to 600°C—in urban district heating systems, including thermal insulation, smooth sliding, and waterproofing of exposed pipe ends. These pipes not only boast advanced technologies and superior service performance that are unmatched by traditional trench-laid or overhead pipelines, but also deliver significant social and economic benefits, serving as an effective measure for energy conservation in heating systems. The adoption of direct-buried heating pipeline technology for high-temperature prefabricated insulated pipes marks a new starting point in the development of China’s heating pipeline technology.

　　Prefabricated polyurethane-foamed insulation pipes: The insulation layer of this type of pipe is composed of two insulating materials bonded together. However, the insulation layer, its protective outer shell, and the steel working pipe through which the heat transfer medium flows are not firmly adhered to one another. The inner layer is made of a new high-temperature-resistant insulation material, such as centrifugal glass wool felt, composite silicate, or glass foam, while the outer layer is formed by laminating rigid polyurethane foam plastic.

　　Structure of prefabricated polyurethane foam insulated pipe:

　　The core tube—the steel pipe that conveys the fluid—features a rigid polyurethane foam insulation layer. This foam is closed-cell, providing excellent thermal insulation, low water absorption, and a low thermal conductivity. The bond strength between the foam and the steel pipe exceeds the foam’s own tensile strength, thereby also offering corrosion protection for the steel pipe. The outer jacket is a plastic pipe extruded from high-density polyethylene material that exhibits high tensile, flexural, and impact resistance.

　　Prefabricated polyurethane foam insulated pipes are primarily used in district heating systems in northern regions to convey various media within the temperature range of –50°C to 150°C. In certain southern areas, they can also be employed in district cooling systems, serving as insulated pipelines for chilled water and hot water, respectively. In short, this type of insulated pipe is suitable for any medium that requires thermal insulation at temperatures below 150°C. The working steel pipe can be selected based on the specific properties of the conveyed medium, and the thickness of the insulation layer can be adjusted according to varying technical requirements.

　　Characteristics of prefabricated polyurethane foam insulated pipes:

　　1. Rigid polyurethane, which has a low thermal conductivity and excellent thermal insulation performance, is used as the insulation material, providing superior thermal insulation.

　　2. After normal operation, when 130°C water is transported over a distance of 40 kilometers, the temperature drop is only about 1°C.

　　3. It boasts excellent waterproofing and corrosion resistance, eliminating the need for trench installation; it can be directly buried underground or submerged in water, with simple and rapid construction and low overall costs.

　　4. It also exhibits excellent corrosion resistance and impact resistance under low-temperature conditions, allowing it to be directly buried in permafrost.

　　5. The prefabricated insulated pipe utilizes a corrosion-resistant (HDPE: high-density polyethylene) outer casing, which is immune to biological and chemical corrosion.

　　6. We can supply piping materials in various diameters ranging from DN20 to DN1200, along with corresponding insulation thicknesses, as well as matching pipe fittings (elbows, tees, reducers, expansion joints, etc.), tailored to customer requirements.

　　7. The product shall comply with the People’s Republic of China urban construction industry standards CJ/T 114-2000 and CJ/T 115-2001.

　　8. To ensure the integrated performance of the prefabricated directly buried insulated pipe, the steel pipe surface shall be blast-cleaned prior to foaming, and the inner and outer surfaces of the plastic pipe shall undergo corona treatment. Specifically, the rust removal grade shall meet the requirements of Sa2.5 as specified in GB/T 8923–1998, and the surface tension of both the inner and outer surfaces of the outer casing pipe shall reach 50 dynes/cm.

　　Characteristics and advantages of prefabricated polyurethane foam insulated pipes:

　　1. Reduce project costs.

　　According to estimates by the relevant authorities, dual-pipe heating pipelines can reduce construction costs by approximately 25% under normal conditions when a glass-fiber-reinforced plastic outer jacket is used, and by about 10% when a high-density polyethylene outer jacket is employed.

　　2. Low heat loss, resulting in energy savings.

　　Prefabricated polyurethane foam insulated pipes have a thermal conductivity of λ = 0.013–0.03 kcal/m·h·°C, which is significantly lower than that of previously commonly used pipe insulation materials, resulting in an insulation performance that is 4 to 9 times better. In addition, their water absorption is very low, at approximately 0.2 kg/m². This low water absorption is attributable to the polyurethane foam’s closed-cell structure, which boasts a cell closure rate of around 92%. Combined with the low thermal conductivity and low water absorption, along with a high-density polyethylene or glass-fiber-reinforced plastic outer protective shell that offers excellent waterproofing, these features have fundamentally changed the traditional practice of laying heating pipelines in trench installations—often described as “wearing a wet cotton jacket.” As a result, overall heat loss from heating pipelines has been greatly reduced, with heat losses in district heating networks now standing at only 2%, well below the international standard requirement of 10%.

　　3. Corrosion resistance, excellent insulation performance, and long service life.

　　Pre-fabricated polyurethane foam insulated pipes feature a rigid polyurethane foam insulation layer that is tightly bonded to the outer surface of the steel pipe, effectively preventing the ingress of air and water and providing excellent corrosion protection. Moreover, the foam cells are predominantly closed, resulting in very low water absorption. The high-density polyethylene and glass-fiber-reinforced plastic outer jackets also exhibit outstanding corrosion resistance, electrical insulation, and mechanical properties. Consequently, the outer surface of the working steel pipe is highly resistant to corrosion by external air and water. Provided that the water quality inside the pipeline is properly treated, according to foreign literature, the service life of high-temperature pre-fabricated directly buried insulated pipes can exceed 50 years—3 to 4 times longer than that of conventional trench-laid or overhead-installed systems.

　　4. Requires minimal land use, enables rapid construction, and promotes environmental protection while minimizing disruption to local residents.

　　Prefabricated polyurethane foam insulated pipes eliminate the need for constructing extensive underground trenches; they can simply be buried directly in the ground, thereby significantly reducing the project’s land footprint, cutting earthwork volume by more than 50%, and reducing civil engineering masonry and concrete quantities by 90%. At the same time, pipe fabrication and on-site trenching can proceed in parallel, with only on-site joint connections required, which substantially shortens the construction schedule. Moreover, by minimizing the transportation of materials such as bricks, cement, sand, gravel, and excess excavated soil, the project reduces vehicle exhaust emissions, dust generation, and noise levels during construction, thus protecting the environment.

　　Prefabricated polyurethane-foamed insulated pipes are composed of an outer steel protective casing with anti-corrosion coating, a polyurethane foam insulation layer, and an inner working steel pipe. Steel-sleeve, steam-composite insulated pipes are suitable for conveying steam or other media at pressures up to 2.5 MPa and temperatures below 350°C. Featuring a steel pipe as the outer protective layer, this product offers high strength, excellent resistance to damage, convenient installation and maintenance, and a long service life.