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.
1. First of all, we should control the concepts: there is a compensated direct burial laying method, which is to deal with the thermal elongation of pipeline by natural compensation and compensator (such as square and bellows compensator), so that the thermal stress is * small; non-compensated direct burial laying simply means that the pipeline does not have any compensation measures when heated, but relies on the pipe itself. Body strength absorbs thermal stress.
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 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.
(1) When compensated direct burial is definitely 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.
(2) When using non-compensated direct burial method, the burial depth should consider the stability requirements of pipelines. The stability is mainly related to the thickness of overlying soil, which is generally deeper than that with compensation. When using non-preheated direct buried pipelines without compensation, * the depth of small overlying soil should be in accordance with Article 7.2.15 of the Urban Heat Network Design Standard (CJJ34-90). The thickness of the overlying soil should be proportional to the diameter of the pipe.
4. In the design, the non-compensated laying or compensated laying should be adopted eventually. The criterion is that the straight pipeline is longer, the middle branches are fewer, and the non-compensated laying should be preferred when the heating medium does not exceed 100 C. Otherwise, the compensated 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.
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