Comparison of Straight Seam Steel Pipe and Spiral Steel Pipe

The production process of straight seam steel pipes is relatively simple, and the main content of the production process is high-frequency welded straight seam steel pipes (ERW) and submerged arc welded straight seam steel pipes (LSAW). The production efficiency of straight seam steel pipes is high, the cost is low, and the economy develops rapidly. The strength design of spiral steel pipes (SSAW) is usually higher than that of straight seam steel pipes.
 

The main content of steel pipe production technology is submerged arc welding. Spiral steel pipes can be used to produce welded pipes with the same width of billets and different diameters, or narrow billets can be used to produce welded pipes with larger diameters. However, compared with the straight seam steel pipe of the same length, the welding length is increased by 30% and 100% respectively, and the production speed is relatively low. Therefore, most large-diameter steel pipes are welded by spirals, while most small-diameter steel pipes are welded by straight seams. In the industrial production of large-diameter straight-seam steel pipes, T-welding construction technology is adopted, that is, two small straight-seam steel pipes are butt-connected to meet the length required for enterprise projects. The defects of T-shaped straight seam steel pipes are more obvious. The residual stress in the T-shaped weld welding process is very large. The welded metal structure material is usually in a triaxial stress state, which increases the possibility of cracking.

In the welding process and process design, the welding method of spiral steel pipe and direct welded steel pipe is the same, but it will inevitably lead to the appearance of a large number of T-shaped welds, and the welding residual stress of T-shaped welds has a great influence. Therefore, the technical defects of welding also need to be improved. The improved weld metal structure reduces the possibility of steel pipe cracking.

In addition, according to the technical specifications of submerged arc welding, each weld must be treated in time for arc ignition and arc extinguishing. However, each steel pipe cannot meet this condition when welding the circumferential seam. Therefore, there may be a problem in the arc suppression process. There are more and more welding structural defects.

When the pipeline is under internal working pressure, two main stresses can usually be generated on the pipeline wall, radial stress and axial stress. Comprehensive analysis of the stress at the weld, where α is the helix angle of the weld of the spiral structure steel pipe.

The comprehensive stress at the weld of the spiral structure is different from that of the straight seam steel pipe. Under the same working pressure, the wall thickness of the spiral steel pipe with the same diameter is smaller than that of the straight seam steel pipe.

When parallel defects appear near spiral welds, the expansion risk of spiral welds is less than that of straight welds because the force of spiral welds is very small. Since the different radial stress is the maximum stress on the steel pipe, the weld must be able to withstand the maximum load in the vertical stress direction. The load on the weld has the greatest influence, the load on the circumferential seam is the smallest, and the load factor on the spiral weld is between the two.
 

The development trend of pipelines is large diameter and high strength. With the increase of steel pipe diameter and steel type in my country, although spiral steel pipe and straight seam steel pipe have the same grade, spiral steel pipe has relatively high impact toughness, which is the difference between different steel pipes.

Tips: ASTM A53 Grade B is more popular than other grades. These pipes can be bare pipes without any coating, or it may be Hot-Dipped or Zinc-Coated and manufactured by Welding or by a Seamless manufacturing process. In Oil and Gas, A53 grade pipes are used in the structural and non-critical applications. 

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