Comparing LSAW and SSAW Steel Pipes

Metallurgical Properties and Production Techniques

LSAW pipes are manufactured from steel plates that are rolled into cylindrical shapes and welded longitudinally. This process allows for the production of pipes with high dimensional accuracy and excellent surface finish. The use of a water cooling system during the hot strip rolling process enhances the low-temperature toughness and weldability of the steel by allowing the use of low alloy compositions to achieve specific strength levels. However, not all steel production plants are equipped with such cooling systems.

SSAW pipes, on the other hand, are produced by spirally welding hot-rolled coil steel. This method results in a pipe with a helical seam that runs the length of the pipe. The spiral angle of the weld seam typically ranges between 50-75 degrees, which affects the pipe's mechanical properties. The coil's alloy content (carbon equivalent) is often lower than that of similar grade plates, improving the spiral welded pipe's weldability.

Welding Technology and Defects

Both LSAW and SSAW pipes use similar welding processes, but the LSAW pipes are more prone to defects due to the presence of longitudinal T-welds. These T-welds can introduce large residual stresses and increase the likelihood of three-dimensional stress cracks. Additionally, submerged arc welding requires each weld to have an arc start and stop point, which can lead to more defects in LSAW pipes as it is challenging to meet these conditions consistently along the longitudinal seam.

Strength and Stress Resistance

When subjected to internal pressure, pipes experience two main stresses: radial and axial. The helical angle of the SSAW pipe's weld seam results in a lower resultant stress compared to the LSAW pipe under the same operating pressure. Consequently, SSAW pipes can have a reduced wall thickness for the same diameter, offering material savings.

Hydrostatic Burst Strength

Comparative tests have shown that the yield pressure and burst pressure of SSAW pipes are generally lower than those of LSAW pipes. However, SSAW pipes exhibit superior plastic deformation during a burst test, with the deformation typically confined within a single pitch, indicating better performance in containing failures.

Toughness and Fatigue Strength

As pipeline diameters and steel grades increase, the importance of ductile fracture resistance and impact toughness grows. Studies have shown that SSAW pipes possess higher impact toughness compared to LSAW pipes. Additionally, the fatigue strength of SSAW pipes is comparable to that of seamless and ERW pipes, with some studies suggesting that SSAW pipes may have a higher average fatigue strength than LSAW pipes.

Field Weldability and Production Consistency

Field weldability is influenced by the steel material and the dimensional tolerances of the pipe ends. Spiral welded pipes benefit from stable and continuous production conditions, which contribute to consistent geometries and high-quality welds. In contrast, the segmented production process of LSAW pipes can lead to variability in shape and geometry, potentially affecting field weldability.

Impact on Fluid Dynamics

The roughness of the pipe's internal surface can significantly impact the flow characteristics of the transmission medium. Studies have found that surface roughness has a more substantial effect on fluid resistance than localized irregularities such as weld seams.

Production and Quality Assurance

SSAW pipe production is highly efficient, with a single production unit capable of replacing multiple LSAW production lines. This efficiency translates to consistent production standards and a robust quality assurance system. In contrast, managing the production and quality of LSAW pipes can be more challenging due to the segmented nature of the process and the variability in operator skill and quality control.

Cost Considerations

The market price of SSAW pipes is generally higher than that of LSAW pipes, primarily due to the higher technical performance requirements of the hot-rolled coil material used in SSAW production. However, when considering the overall project cost, including quality and total cost factors, SSAW pipes may offer an overall advantage.

Development and Standards

The development of spiral welded pipes is geared towards producing larger diameters and thicker walls to increase pressure resistance, developing new steel grades, and improving smelting technologies. Additionally, the application of coatings to the inner walls of pipes can extend their lifespan and reduce maintenance costs.

Common standards for spiral steel pipes include SY/T5037-2000 for general fluid pipes, GB/T9711.1-1997 for the oil and gas industry, API-5L for pipeline specifications, and SY/T5040-92 for pile pipes.

Environmental Considerations

Plasma cutting of SSAW pipes can produce significant amounts of metal vapor and pollutants. Effective dust management systems are essential to prevent air pollution during the cutting process.

Conclusion

In summary, while both LSAW and SSAW pipes have their unique advantages and applications, the choice between them depends on the specific requirements of the project, including strength, toughness, weldability, and cost. Industry professionals must carefully consider these factors to select the most appropriate type of pipe for their needs.

For more detailed information on SSAW pipes, you can visit Hunan Standard Steel Co., Ltd.