At present, most PU direct-buried insulated-pipe heating enterprises nationwide are concentrated in the Three-North regions. Although some newly built large district-heating systems are relatively advanced, the technical level of most small and medium-sized heating systems is still low. Many heating enterprises have a situation of “three highs and one low”—high construction investment, high operating cost, high accident rate, and low fee-collection rate. The technical status of these enterprises is mainly shown in the following respects:

　　Backward heating systems still exist in large numbers

　　In China today the main production areas are in North China; as demand has grown, production has also developed in the central plains. The structure of insulated pipe generally consists of four parts: the carrier pipe, the insulation layer, the protective jacket, and the leak-detection alarm wire. These four parts are responsible respectively for conveying the medium, thermal insulation, protecting the insulation layer, and preventing leakage. None of the four can be omitted. 3. Factors to consider when determining burial depth for the two laying methods. This method fully utilizes the plastic potential of steel; construction is easy and requires no preheating—there is no such requirement for die-casting production. Steel-casing burial is a new waterproof, leak-proof, seepage-resistant, pressure-resistant, and fully enclosed burial technology, and represents a major breakthrough in applying direct-buried laying in areas with high groundwater tables. In addition, the technical indicators of insulated pipe: its density is generally around 45–60 kg/m3. China’s specified thermal conductivity is generally 0.016–0.024 W/m·K. This is set so that insulated pipes can convey higher-temperature media. The applicable temperature range is −90°C to 121°C. Each pipe section between two fixed supports is provided with a moisture vent so that every section can effectively vent moisture. In practice we have found that:

　　Prefabricated direct-buried PU insulated pipe is a steel pipe with anti-corrosion properties and good thermal insulation performance. It is widely used and promoted in construction and is continually being improved and popularized. Anti-corrosion insulated steel pipes have different processing types and methods; different processing methods determine different performance and functions of the steel pipe, and the scope of use also changes accordingly. The thicker the pipeline insulation layer, the smaller the heat-dissipation loss of the pipeline, saving fuel; but as thickness increases, investment in the insulation structure increases. “Economic insulation thickness” comprehensively considers both the investment in the pipeline insulation structure and the annual operating cost of pipeline heat-dissipation losses, converting them to the insulation-layer thickness at which the “annual calculated cost” is minimized over a given number of years. One curve shows that the cost of insulation increases with insulation-layer thickness; another shows that the cost of heat loss decreases with insulation-layer thickness; the third curve is the result of superimposing curves A and B, and the thickness corresponding to the minimum point of curve C (i.e., minimum cost) is the economic thickness of the insulation material.

　　Over the past decade or so, China’s heating engineers have been digesting and absorbing this advanced technology and are pushing domestic pipe-network laying technology to a higher level. Practical results over more than ten years have fully demonstrated that, compared with traditional trench and overhead laying, the PU insulated direct-buried pipe laying method has many advantages. Direct-buried insulated pipe is formed by tightly combining a steel pipe conveying the medium, an HDPE outer casing, and a rigid PU foam insulation layer between the steel pipe and the outer casing. As an ideal, commonly used, and favored pipe insulation profile for energy-saving pipeline conveyance and application worldwide, PU insulated pipe differs somewhat in application according to material, specification, and composition.