Prefabricated Direct-Buried Polyurethane Insulated Steel Pipes: Production and Processing Range φDN20—ΦDN2400

A good thermal insulation material should have a low thermal conductivity; remain stable when exposed to moisture; exhibit excellent heat resistance; not corrode metals; be lightweight with a high porosity; possess sufficient mechanical strength to avoid damage under external forces; and be easy to process and inexpensive. Commonly used thermal insulation materials include: expanded perlite and its products, glass wool and its products, rock wool products, microporous calcium silicate, aluminum silicate fiber products, foam plastics, and foam asbestos.

The high-temperature pre-fabricated directly buried thermal insulation pipe consists of a steel pipe, an inner protective sheath made of glass fiber reinforced plastic, and an outer shell also made of glass fiber reinforced plastic. Its distinguishing feature is that it further includes a high-temperature-resistant thermal insulation layer, a lubrication layer, and elastic sealing elements. This utility model effectively addresses the challenges of thermal insulation, sliding lubrication, and waterproofing of exposed pipe ends in pre-fabricated directly buried thermal insulation pipes used for high-temperature heat transmission—from 130℃ to 600℃—in centralized heating systems. The high-temperature pre-fabricated directly buried thermal insulation pipe primarily comprises four main components.

(1) Working steel pipe: Depending on the technical requirements of the conveyed medium, either welded steel pipes with seams, seamless steel pipes, or double-sided submerged arc spiral welded steel pipes may be used.
(2) Thermal insulation layer: Made of rigid polyurethane foam plastic.
(3) Protective casing: Made of high-density polyethylene or fiberglass.
(4) Leakage Alarm Wire: During the manufacture of high-temperature pre-fabricated directly buried thermal insulation pipes, an alarm wire is embedded in the insulation layer close to the steel pipe. Once a leak occurs at any point along the pipeline, the alarm wire will transmit the signal, triggering an alarm on dedicated detection instruments and precisely indicating both the exact location of the leak and the severity of the leakage. This allows maintenance personnel to promptly address the leaking section of the pipe and ensure the safe operation of the heating network.

Advantageous features

1. Reduced project costs. According to estimates by relevant authorities, double-pipe heating pipelines can typically reduce project costs by approximately 25% (when using fiberglass reinforced plastic as the protective layer) and 10% (when using high-density polyethylene as the protective layer).
2. Low thermal loss and energy savings. The high-temperature pre-fabricated directly buried insulation pipe has a thermal conductivity coefficient of λ = 0.013–0.03 kcal/m·h·°C, which is significantly lower than that of other commonly used pipeline insulation materials in the past, resulting in an insulation performance improvement of 4 to 9 times. Moreover, its water absorption rate is extremely low, at only about 0.2 kg/m². The low water absorption rate is attributable to the closed-cell structure of the polyurethane foam, which boasts a cell closure rate of around 92%. With its low thermal conductivity and low water absorption, coupled with a high-density polyethylene or fiberglass reinforced plastic protective shell featuring excellent waterproofing properties, this pipe system has fundamentally changed the traditional practice of laying heating pipelines in underground trenches—where pipes were effectively “wearing wet cotton jackets.” As a result, overall heat loss from the heating pipelines has been greatly reduced, with heat network losses dropping to just 2%, well below the international standard requirement of 10%.
3. Excellent corrosion resistance, insulation performance, and long service life. The high-temperature pre-fabricated directly buried thermal insulation pipe features a rigid polyurethane foam insulation layer that is tightly bonded to the outer surface of the steel pipe, effectively preventing the penetration of air and water and thus providing outstanding corrosion protection. Moreover, its foam cells are all closed-cell, resulting in very low water absorption. Both the high-density polyethylene outer shell and the fiberglass reinforced plastic outer shell offer excellent corrosion resistance, insulation properties, and mechanical strength. Consequently, the outer surface of the working steel pipe is highly resistant to erosion from external air and water. Provided that the water quality inside the pipeline is properly treated, according to foreign data, the service life of high-temperature pre-fabricated directly buried thermal insulation pipes can exceed 50 years—four times longer than that of traditional trench-laid or overhead pipelines.
4. Requires little land, allows for rapid construction, and is beneficial to environmental protection. Directly buried heating pipelines do not require the construction of large underground trenches—instead, insulated pipes are simply buried underground, significantly reducing the project’s land footprint. This reduces earthwork excavation by more than 50% and cuts down on masonry and concrete work by 90%. Meanwhile, insulation pipe fabrication can proceed in parallel with on-site trench excavation, and since only on-site connections are needed, the construction period can be shortened by more than 50%. High-temperature pre-fabricated directly buried insulated pipes.
5. Safety: All high-temperature pre-insulated direct-buried thermal insulation pipes manufactured outside China are equipped with leak detection wires. Once a leak occurs at any point along the pipeline, the leakage signal transmitted through these wires will immediately display on dedicated monitoring instruments the precise location and severity of the water or heat leakage in the insulated pipe. This allows leak-detection personnel to promptly address the leaking section, ensuring the safe operation of the heating network. In summary, high-temperature pre-insulated direct-buried thermal insulation pipes not only possess advanced technologies and practical performance that are unmatched by conventional trench- and overhead-installed pipelines, but also deliver significant social and economic benefits, making them a powerful measure for energy conservation in heating systems. The adoption of direct-burial heating pipeline technology marks a new milestone in the development of China’s heating pipeline technology. As this advanced technology continues to be refined and developed, it is inevitable that direct-burial heating pipelines will gradually replace trench- and overhead-installed systems.