The manufacture of prefabricated insulated pipes and direct-burial technology were first applied and developed in Northern Europe in the late 1960s. Design departments introduced them to China in the early 1980s and also adopted the Nordic elastic-deformation analysis method for the engineering design of direct-buried hot-water pipelines. Elastic-deformation analysis is intended to ensure that the hot-water pipe always remains within the elastic-deformation range and in an elastic state. Straight pipe sections generally employ thermal-compensation devices, using preheating and installation of one-time compensators. After more than twenty years of development, this technology has made considerable progress. In the 1990s, after years of design, installation, and operation of direct-buried hot-water pipeline systems, Nordic heating engineers found through engineering practice that using elastic analysis for temperature-stress calculation of direct-buried hot-water pipelines was overly conservative. A stress-classification method was proposed for calculating the strength of direct-buried hot-water pipes. Years of design, installation, and operation experience with direct-buried hot-water pipeline systems were also summarized. In 1994 a district heating handbook was issued with explanatory standards on the European standards and uncompensated direct-burial technology, clearly specifying the use of stress classification for the design of uncompensated direct-buried hot-water pipeline systems.
The theoretical basis of uncompensated direct burial is the third strength theory, under which stress is divided into primary stress: stress produced by working pressure in the straight pipe, i.e. hoop stress from internal pressure; secondary stress (temperature stress): thermal expansion and contraction cannot be freely released, producing stress in the straight pipe such as axial stress due to temperature rise; and tertiary stress (peak stress): the main pipe releases deformation to fittings while carrying primary and secondary stresses, producing stress on the fittings.
In China’s power systems, the stress-classification method is used to calculate strength, whereas heating-network systems have always used elastic stress analysis to calculate pipe strength. In the 1980s and 1990s, China introduced advanced prefabricated insulated-pipe production lines from Europe. They were put into production in Harbin, Tianjin, Beijing, and Dalian. Prefabricated insulated pipe then began to be widely used in domestic direct-buried heating-network projects. With reference to European standards, China compiled an industry standard for the production of prefabricated insulated pipe, namely “High-density polyethylene outer-pipe polyurethane foam prefabricated direct-buried insulated pipe” (GJ/T3002-92); it also compiled the Technical Specification for Direct-Buried Heating Pipelines in Urban Areas (CJJ/T81-98) to promote the application of direct-buried and uncompensated laying technology in China’s urban heating systems, and formulated industry codes for design, construction, and acceptance. Drawing on the European standards, this procedure uses the stress-classification method to calculate the strength of direct-buried hot-water pipes, bringing China’s calculation theory for direct-buried hot-water pipelines in line with the advanced level.
1.2 Overview of Direct-Burial No-Compensation Technology Application
In the early 1980s, direct-buried hot-water pipelines began to be used in China. After more than 20 years of application and development, the design, manufacture, installation, and acceptance of direct-laid hot-water pipelines have made considerable progress and have begun to be widely used. Heating designers generally regard this as having become the principal laying method for urban hot-water network construction in China. Although as early as 1994 the European standards that fully embody the new uncompensated direct-burial technology were issued—insulated pipes EN253, insulated pipe fittings EN448, insulated valves EN488, insulated joints EN489, and so on—and China’s regulations based on the European standards, namely the Technical Specification for Direct-Buried Heating Pipelines in Urban Areas (CJJ/T81-98), were also implemented on June 1, 1999, to this day uncompensated direct-buried laying of hot-water pipelines in China’s urban heating projects (pipe diameter greater than DN500; insulated pipes, insulated fittings, insulated valves, and insulated joints all conforming to European standards; essentially no inspection chambers or compensators) still has few widely applied engineering examples. The proportion of uncompensated direct-buried hot-water pipelines is too small. Hot-water pipelines are either direct-buried without compensation or laid with compensation.
Only a few design institutes in China have fully mastered and applied this technology. Most design institutes, heating designers, and construction units lack understanding of uncompensated direct-burial technology. The Technical Specification for Direct-Buried Heating Pipelines in Urban Areas (CJJ/T81-98) applies only to hot-water pipelines of DN500 or smaller. Trunk networks in northern cities are generally larger than DN500. There are few manuals and specifications for uncompensated direct-buried hot-water pipelines. These reasons have hindered the application and promotion of uncompensated direct-buried laying technology for hot-water pipelines. Compiling relevant manuals and formulating relevant technical standards (such as standards and codes) is urgently needed, and there is still a long way to go in the research, application, and promotion of uncompensated direct-burial technology.
