At present, China adopts a composite decoupling structure for steam pipelines, that is, the insulation layer is divided into two layers: the inner layer is made of high temperature resistant inorganic insulation material, and the outer layer is made of polyurethane foam. The outer protective layer of the directly buried insulation pipe uses three materials, namely high-density polyethylene plastic pipe, glass steel pipe, and steel pipe. Directly buried steam pipelines must have sufficient mechanical strength. There are three different requirements in addition to the reliable resistance to groundwater immersion and the corrosion resistance of the housing and metal parts:

(1) There must be sufficient thermal displacement of the pipeline, and the thermal displacement will occur when the pipeline is working. These displacements must be guaranteed. This requires that when designing the insulation structure, the straight pipe part should allow the working steel pipe to move between the shafts in the insulation structure. The workload at the bend can be moved axially and laterally.

(2) Control of soil temperature inside and around the thermal insulation structure. At present, some of the heat preservation, anti-corrosion and shell materials used in the heat preservation structure cannot work under the high temperature of the steam pipeline. The concrete of the same pipeline structure, surrounding water pipes, cables, etc. may be damaged beyond the temperature range. Slight negligence can cause large losses. Therefore, it is necessary to correctly grasp the temperature values ​​that can be reached in each part and location of the pipeline after it is buried, and there must be reliable guarantee measures.

(3) Must have the function of removing moisture. Insulation pipes may contain some moisture and absorb some moisture during production, transportation, and installation. The vapor pressure penetrates the insulation layer to increase the temperature and pressure between the layers, thereby damaging the insulation structure.

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