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.
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) 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) 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) 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) 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.
