PU insulated pipe has become a leading technology in some developed countries. Over the past decade, China's heating-engineering technicians, by absorbing this leading technology, have raised domestic pipe-network laying technology to a higher level. More than ten years of practical performance have fully demonstrated that the PU insulated direct-buried laying method has many advantages over traditional trench and overhead laying. This is also the inherent driving force behind the rapid development of PU insulated direct-buried pipe in China's heating projects.
Advantages of PU insulated direct-buried pipe: 1. Reduce project cost. According to calculations by relevant departments, dual-pipe heating pipelines can generally reduce project cost by 25% (selecting FRP as the protective layer) and 10% (using HDPE as the protective layer).
2. Low heat loss, energy saving. Because the direct-buried pipe is made of PU foam, its thermal conductivity is: λ = 0.013–0.03 kcal/m·h·°C, far lower than other commonly used pipeline insulation materials, improving insulation performance by 4–9 times. In addition, its water absorption is very low, about 0.2 kg/m2. The low water absorption is due to the closed-cell ratio of PU foam being as high as 92%. Low thermal conductivity and low water absorption, together with HDPE or FRP protective jackets that provide a good insulation layer and external waterproofing, have changed the traditional heating-pipe laying practice of “wearing a wet cotton quilt” and greatly reduced overall heat loss of heating pipes. Heat-network heat loss is 2%, below the worldwide specification requirement of 10%.
3. Corrosion resistance, good insulation performance, and long service life. Direct-buried insulated pipe has an excellent anti-corrosion effect because the rigid PU foam insulation layer bonds tightly to the outer surface of the steel pipe and can isolate air and water. At the same time, its foam cells are closed and water absorption is low. The HDPE outer jacket and FRP jacket provide excellent corrosion protection, electrical insulation, and mechanical properties. Therefore the outer surface of the carrier pipe is virtually uncorroded by external air and water, provided that the water quality inside the pipeline is well treated. According to foreign data, the service life of PU insulated pipe can exceed 50 years, which is 3–4 times that of traditional trench installation and overhead laying.
4. Reduce land occupation, speed up construction, and protect the environment. Direct-buried heating pipe does not require construction of large trenches; the insulated pipe only needs to be buried underground, greatly reducing land use, cutting earthwork by about 50%, and reducing civil works and concrete volume by 90%. At the same time, insulated-pipe fabrication and on-site trenching proceed in parallel, and only field joints are required, which can shorten construction time by more than 50%.
5. Prefabricated PU insulated pipes produced abroad are all fitted with leak-detection alarm wires. Once leakage occurs in any part of the pipeline, the transmission wires can be used with a dedicated inspection instrument to display the exact location of water ingress and leakage, as well as the extent of the leak, so that inspectors can promptly locate the leaking section and ensure safe operation of the heating network. Domestic insulated pipes are now also fitted with leak-detection alarm wires, which need to be installed. In short, PU insulated direct-buried pipe not only offers advanced technology and practical performance that traditional trench and overhead piping can hardly match, but also delivers significant social and economic benefits and is an energy-efficient method of district heating. Adoption of direct-buried heating-pipeline technology marks a new starting point in the development of China’s heating-pipeline technology. With further improvement and development of this advanced technology, direct-buried heating pipe must replace trench and overhead laying.
