Black-Jacketed Direct-Buried LSAW Pipe It 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. Lower project cost. According to calculations by relevant authorities, dual-pipe heating systems can generally reduce project cost by 25% (when FRP is used as the protective jacket) and 10% (when HDPE is used as the protective jacket).
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. From the standpoints of energy saving, cost reduction, shorter construction periods, and environmental protection, planning and construction of PU insulated direct-buried heating pipe should note that it not only offers technology and practical performance that traditional trench and overhead laying can hardly match, but also significant social and economic benefits. Outstanding direct-buried heating-pipeline projects, however, also require sound planning, reliable insulated-pipe quality, and careful construction.
Because Black-Jacketed Direct-Buried LSAW Pipe Floor-heating technology started relatively late in China, so the three conditions above still need continuous improvement. Based on quality issues arising in engineering practice, particular attention should be paid to the following in design and construction:
First, in planning and construction, it must be understood that direct laying of heating pipelines falls into two methods: compensated direct burial and uncompensated direct burial. The working methods, characteristics, and applications of both must be thoroughly mastered so that the scheme can be chosen rationally and construction is safe, reliable, and economical. 1. First, master the concept of compensated direct burial. Natural compensation and compensation devices (such as square-loop and bellows compensators) are used to accommodate thermal elongation of the pipeline, thereby minimizing thermal stress. In short, uncompensated buried laying means that when the pipeline is heated it has no compensation devices, but relies on the strength of the pipe itself to absorb thermal stress.
2. Basic principle of the uncompensated laying method: When installing the pipeline, first heat the pipe to a certain temperature, then weld and fix it. When the pipe returns to installation temperature (temperature decreases), the pipe is pre-loaded with a certain tensile stress. When the pipe is heated, as temperature increases, pipe stress becomes zero. As temperature continues to rise, compressive stress in the pipe increases. When temperature rises to operating temperature, the compressive stress (thermal stress) of the pipe is still less than the allowable stress. In this way the pipeline can operate normally without compensation devices. This uncompensated method uses the fourth strength theory. Construction requires preheating of the pipeline and is complex. However, there are many engineering examples at home and abroad; theoretical calculations are reliable and can ensure safety. Another uncompensated method is the calculation method and stress classification selected in recent years by the Beijing Gas and Heating Planning Institute, applying the third strength theory. This method fully utilizes the plastic potential of steel, and construction is convenient without requiring preheating.
3. The two burial depths take different factors into account.
