From the aspects of energy saving, cost reduction, construction period shortening and environment maintenance, the high temperature prefabricated directly buried heat preservation pipe not only has the technical and applicability performance that is difficult to compare between traditional trench and overhead pipeline laying, but also has remarkable social and economic benefits. However, a high quality directly buried heat supply pipeline project must also have reasonable design and reliable quality of heat preservation pipeline. Three construction conditions. Due to the late start of direct buried heating technology in China, the above three conditions need to be improved from time to time. From the point of view of quality problems in engineering theory, the following problems should be paid special attention to in design and construction:
High Temperature Prefabricated Direct Buried Insulation Pipe
First, in the design and construction, we must really understand that the direct burial laying of heating pipelines can be divided into compensated direct burial laying and non-compensated direct burial laying. We must control the working principles, characteristics and application sites of the two methods, so as to make rational selection in design and safe, reliable and economical construction.
First of all, we should control the concepts: there is a compensated direct burial laying method, which is to deal with the heat elongation of pipeline by natural compensation and compensator (such as square and bellows compensator), so as to minimize the thermal stress; non-compensated direct burial laying simply means that the pipeline does not have any compensation measures when heated, but absorbs the thermal stress by the strength of the pipeline itself.
2. The basic principle of non-compensated laying method: when installing pipeline, heating pipeline to a certain temperature first, then welding and fixing pipeline, when the pipeline restores to the installation temperature (temperature decreases), the pipeline accepts a certain tensile stress in advance. When the pipeline is heated, with the increase of temperature, the stress of the pipeline is zero. When the temperature continues to rise, the pressure stress of the pipeline increases. When the temperature rises to the working temperature, the pressure stress (thermal stress) of the pipeline is still less than the allowable stress. In this way, the pipeline can work normally without compensation installation. This non-compensatory method applies the fourth strength theory, which needs to preheat the pipeline during construction. It is difficult to compare the construction. However, there are a lot of engineering theories at home and abroad. The theoretical calculation is reliable and can ensure safety. Another non-compensatory method is the stability analysis and the third strength theory for the calculation method and stress classification proposed by Beijing Gas Thermal Design Institute of China in recent years. In this way, the plastic potential of steel can be fully developed, and the construction is convenient without preheating.
3. Consider the different elements of two laying depths.
High Temperature Prefabricated Direct Buried Insulation Pipe
Firstly, when compensated direct burial is adopted, the burial depth only considers that the stability of pipeline will not be destroyed due to the action of air load, and considers from the aspects of economy and construction convenience. When compensated direct burial is adopted, shallow burial should be used as far as possible. The thickness of common overlying soil is more than 0.6 meters, and it has nothing to do with the diameter.
Secondly, when using non-compensated direct burial method, the burial depth should consider the stability requirements of pipelines. Stability when using non-preheated non-compensated pre-made high-temperature pre-buried insulating pipes directly buried pipelines, mainly related to the thickness of overlying soil. Generally, the depth of burying with compensation is deeper, and the thickness of overlying soil should be proportional to the diameter of pipelines.
4. In the design, the non-compensatory laying or compensatory laying should be adopted. The criterion is that the straight pipeline is longer, the middle branches are fewer, and when the heating medium does not exceed 100 C, the non-compensatory laying method should be preferred. Otherwise, the compensatory laying method should be considered. Detailed heat network trunk lines should be laid without compensation, while branch courtyard pipeline networks should be laid with compensation. However, the current designers prefer to lay with compensation, and should advocate optimal design.
