City gas pipelines include gas pipelines, distribution pipelines, user entrance pipelines and indoor gas pipelines, each with different uses. According to the different gas transmission pressure, it can be divided into low pressure (not higher than 0.01MPa), medium pressure (0.01~0.4MPa), secondary high pressure (0.4~1.6MPa) and high pressure (1.6~4.0MPa). ). Generally, the higher the pipeline pressure, the greater the possibility of pipeline leakage and the higher the quality of the pipeline; however, higher pressure pipelines can reduce the initial investment of the entire pipeline. Pipes with different uses, different pressures and different diameters must have different materials and different processes.
In urban gas pipeline networks, high-pressure, ultra-high-pressure pipelines and gas pipelines are usually selected from steel pipes, but there are also problems in the selection of steel pipe types. In recent years, straight seam steel pipes have shown a tendency to gradually replace spiral steel pipes in pipelines with higher pressures. At present, high-frequency resistance welded (ERW) steel pipes are lower than spiral steel pipes, and their quality and reliability are higher than spiral steel pipes. It has good cost performance and is widely used. One method is to use straight seam resistance ERW steel pipes with an outer diameter of ≤406 mm and straight seam double-sided submerged arc welded steel pipes with an outer diameter of 406>406 mm.
In terms of medium and low pressure pipelines, PE pipes have good flexibility, corrosion resistance and smoothness of the inner wall of the pipe, making PE80PE pipes the main pipeline in small diameter medium and low pressure gas pipeline networks. Abroad, due to the unparalleled advantages of high-density PE100 PE pipes in terms of price and maintenance costs, it has become the first choice for buried low-pressure gas pipelines. However, when the pipe diameter is larger than the economic limit diameter of PE pipe, steel pipe should be selected. For example, medium-pressure pipelines above DN200 mainly consist of straight seam resistance welded ERW steel pipes.
In cities, there are concentrated populations, concentrated vehicles, large building areas, complex underground facilities, and gas pipelines in special environments. Therefore, higher requirements are placed on the selection of pipelines. Straight seam pipes should be used in critical areas to reduce the length of pipe welds, improve welding quality and reduce the possibility of accidents. In addition, when determining the steel type of the pipeline, the grade of the steel type should be selected according to the economic ratio to increase the wall thickness of the pipeline and increase the linear stiffness of the pipeline.
