Steel pipes are the most widely used pipes in urban gas projects. Its main advantages are: high strength, good toughness, large load bearing stress, good impact resistance and tightness, good plasticity, easy welding and hot processing, thin wall thickness and metal saving. However, its corrosion resistance is poor and proper anti-corrosion measures are required. The steel pipes used in gas engineering are generally made of high quality low carbon steel
(generally referred to as Q235B) or low alloy steel (generally referred to as Q345B).
1. Material selection of steel pipes
The steel pipes used for urban gas pipelines mainly include two types of seamless steel pipes
and welded steel pipes. The strength of seamless steel tubes is very high, but limited by the production process and cost, it is generally small diameter steel pipes below DN200. There are many types of welded steel pipes, which can be divided into straight seam welded steel pipes (LSAW
) and spiral seam welded steel pipes (SSAW
) according to the welding method. Among them, straight seam welded steel pipe (hereinafter referred to as straight seam steel pipe) includes straight seam double-sided submerged arc welding (LSAW) steel pipe and high frequency electric resistance welding (ERW
) steel pipe.
Spiral seam welded steel pipe (hereinafter referred to as spiral steel pipe) is a spiral seam double-sided submerged arc welding (HSAW) steel pipe. The overall quality of the process quality of straight seam steel pipes is a concern for spiral steel pipes. At present, the production situation of domestic straight seam steel pipe is: high-frequency resistance welded steel pipe with nominal diameter DN400 and below, and straight-slot double-sided submerged arc welded steel pipe with nominal diameter above DN400. For straight seam double-sided submerged arc welded steel pipes, there are five kinds of pipe forming methods such as UOE, JCOE, HME, RBE and PFP according to different molding processes. The former two are the most common. Because the forming equipment of UOE forming welded steel pipe production line has high technical content, high production efficiency, simple molding process, good forming, and low welding stress after forming; and due to the overall expansion of water pressure in production, the internal stress of steel pipe can be effectively eliminated and improved. The strength and dimensional accuracy of the pipe, so the quality is better than JCOE welded welded steel pipe. However, the spiral steel pipe is made of a coiled steel pipe, and the cost is lower than that of a straight seam steel pipe rolled by a steel plate. Moreover, the weld seam of the spiral tube forms a line on the pipe that is more uniform than the straight seam steel pipe, and the weld seam avoids the direction of the principal stress. Therefore, the spiral pipe is still used in the city gas transmission and distribution network.
2. Selection of steel pipes
City gas pipelines include gas pipelines, distribution pipelines, user inlet pipes and indoor gas pipelines, each of which has different uses. According to the difference of 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). In general, the higher the pipeline pressure, the greater the possibility of pipeline leakage and the higher the quality of the pipeline; however, the higher pressure pipeline can reduce the initial investment of the entire pipeline. Pipes of different purposes, different pressures and different diameters must have different materials and different processes.
In the city gas pipeline network, high-pressure, sub-high-pressure pipelines and gas pipelines are generally 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 pipes with higher pressures. At present, high-frequency resistance welding (ERW) steel pipe is lower than spiral steel pipe, and the quality reliability is higher than that of spiral steel pipe. It has good cost performance and is widely used. One method is to use a straight seam electric resistance ERW steel pipe with a pipe outer diameter ¢≤406 mm, and a straight seam double-sided submerged arc welded steel pipe with a 406>406 mm.
In respect of medium and low pressure pipelines, PE pipe has the advantages of good flexibility, corrosion resistance and smooth inner wall of the pipe, which makes PE80 PE pipe become the main pipe of small diameter medium and low pressure gas pipe network. In foreign countries, high-density PE100 PE pipe has become the first choice for buried medium and low pressure gas pipelines because of its incomparable advantages in terms of price and maintenance cost. However, when the pipe diameter is larger and larger than the economic limit pipe diameter of the PE pipe, the steel pipe should be selected. For example, the medium-pressure pipeline above DN200 is mainly composed of straight seam electric resistance welding ERW steel pipe.
In the city, the population is accumulated, the vehicles are concentrated, the construction area is numerous, the underground facilities are complex, and the gas pipeline is in a special environment. Therefore, higher requirements are imposed on the selection of pipelines. Straight seam pipes should be used in key areas to reduce the length of pipe welds, improve weld quality and reduce the possibility of accidents. In addition, when determining the steel grade of the pipeline, the grade should be lowered on the steel grade selected according to the economic ratio to increase the wall thickness of the pipeline and increase the linear stiffness of the pipeline.