The benefits of direct burial method are obvious, but this method puts forward higher requirements for thermal insulation pipelines, mainly its maintenance and service life, which is particularly harsh on the anti-corrosion of thermal insulation pipelines. High density polyethylene casing outside polyurethane insulation layer is its first maintenance. Most plastics are difficult to be degraded in natural environment. Especially high density polyethylene is more resistant to corrosion under environmental conditions after being buried in the open. It is relatively stable even in wet, microbial and acid-alkali-salt media. In addition, its water-gas permeability is very low, which can effectively maintain polycondensation. Urethane insulating pipe.

由于封锁的泡孔构造，也使得聚氨酯对工作钢管的外壁起到一定维护作用，使之免受来自外部环境对钢管外壁的腐蚀。这种复合保温管道处理了防腐保温两大问题，在公开的运用寿命可在20年以上，并且在运转年度里不用维护。当然这只是一种理想状态下的状况，在实践应用中也曾呈现诸多问题而形成运用寿命降落。其呈现的缘由主要有以下几方面。

Because of the blocked cell structure, polyurethane also plays a certain role in maintaining the outer wall of the working steel pipe, so that it can avoid the corrosion of the outer wall of the steel pipe from the external environment. This kind of compound insulation pipeline has solved two major problems of anti-corrosion and heat preservation. Its open service life can be more than 20 years, and it is not maintained during the operation year. Of course, this is only an ideal state of the situation, in practice, there have been many problems and the formation of service life decline. The main reasons for its presentation are as follows.

①保温层偏心，即聚氨酯保温层与钢管不同心，构成保温厚薄不均，严重者可使外层塑料软化而被损坏。

(1) The eccentricity of the insulation layer, i.e. the polyurethane insulation layer is not concentric with the steel pipe, constitutes the uneven thickness of the insulation layer, which can cause the outer plastic to soften and be damaged seriously.

②运用不当而遭受毁坏，在运输及装置过程中易受损伤，在埋地后距空中深度不够或上部土壤及道路较软，形成载重车辆碾压后被损坏。

(2) It is easy to be damaged in the process of transportation and installation due to improper use. It is not deep enough in the air after burial or the upper soil and road are soft, which results in the damage of heavy vehicles after rolling.

③接口处置不当，在管道敷设装置过程中，相接二根管焊接不紧密形成渗水，或在“补口”过程中操作不认真而形成外部污水渗入保温层等形成的毁坏。

(3) The improper disposal of the interface, in the process of pipeline laying device, the two jointed pipes are not welded tightly to form water seepage, or in the process of "repairing" the operation is not careful to form external sewage infiltration into the insulation layer and other damage.

④运用的塑料和聚氨酯保温管不合理，如经常发现运用分拣杂乱的回收塑料，经过挤出成形后的塑料管不能充沛被塑化而致使外管劣化。聚氨酯保温管密度低形成强度低而在搬运过程中就已损坏，另外体系中含有游离酸太高，形成对钢管外壁的腐蚀等。

(4) The plastic and polyurethane insulating pipes used are unreasonable. For example, it is often found that the recycled plastic pipes with disorderly sorting can not be fully plasticized after extrusion, resulting in deterioration of the outer pipes. Polyurethane insulating pipes have low density, low forming strength and are damaged during handling. In addition, the system contains too much free acid, which results in corrosion to the outer wall of steel pipes.