Comparison of technical characteristics of spiral welded pipe and straight seam welded pipe

Comparison of technical characteristics of spiral welded pipe (SSAW) and straight seam welded pipe (LSAW/ERW Pipe):

1. Metallurgical properties of materials

Longitudinal submerged arc welded pipes (LSAW) are produced from steel plates, while spiral welded pipes are produced from hot rolled coils. The rolling process of hot strip mill has a series of advantages and has the metallurgical process capability to produce high-quality pipeline steel. For example, a water cooling system is installed on the output stage to accelerate cooling, which allows the use of low alloy compositions to achieve special strength grades and low-temperature toughness, thereby improving the weldability of the steel. However, this system is basically not available in steel plate production plants. The alloy content (carbon content) of coiled plates is often lower than similar grades of steel plates, which also improves the weldability of spiral welded pipes.

What needs more explanation is that since the rolling direction of the spiral welded pipe is not perpendicular to the axis of the steel pipe (its clamping depends on the spiral angle of the steel pipe), and the rolling direction of the steel plate of the straight seam steel pipe is perpendicular to the axis of the steel pipe, therefore, the spiral The crack resistance of welded pipe materials is better than that of straight seam steel pipes.

2. Welding process

In terms of welding process, the welding methods of spiral welded pipes and straight seam steel pipes are the same, but straight seam welded pipes will inevitably have many T-shaped welds, so the probability of welding defects is also greatly increased, and the welding residues at the T-shaped welds are The stress is large, and the weld metal is often in a three-dimensional stress state, increasing the possibility of cracks.

Moreover, according to the process regulations of submerged arc welding, each weld should have an arc starting point and an arc extinguishing point. However, each straight seam welded pipe cannot meet this condition when welding the circumferential seam, so there may be an arc extinguishing point. More welding defects.

3. Strength characteristics

When a pipe is subjected to internal pressure, it usually produces two main stresses on the pipe wall, namely radial stress δY and axial stress δX. The resultant stress at the weld is δ=δY(l/4sin2α+cos2α)1/2, where α is the helix angle of the spiral welded pipe weld.

The helix angle of the spiral welded pipe weld is generally 50-75 degrees, so the synthetic stress at the spiral weld is 60-85% of the principal stress of the straight seam welded pipe. Under the same working pressure, the wall thickness of spiral welded pipes of the same diameter can be reduced compared with straight seam welded pipes.

According to the above characteristics, we can know:
A. When a spiral welded pipe explodes, since the normal stress and synthetic stress on the weld are relatively small, the blast hole generally does not originate from the spiral weld, and its safety is higher than that of straight seam welded pipes.
B. When there are defects parallel to the spiral weld near the spiral weld, the risk of expansion of the spiral weld is not as great as that of the straight weld because the force on the spiral weld is smaller.
C. Since radial stress is the maximum stress existing on the steel pipe, the weld will bear the maximum load when it is in the direction of vertical stress. That is, the straight seam bears the greatest load, the circumferential weld bears the smallest load, and the spiral seam is between the two.

4. Static pressure bursting strength

Through relevant comparative tests, it has been verified that the measured values of yield pressure and burst pressure of spiral welded pipes and straight seam welded pipes are basically consistent with the theoretical values, and the deviations are close. However, whether it is yield pressure or burst pressure, spiral welded pipes are lower than straight seam welded pipes. The blasting test also shows that the circumferential deformation rate of the blast hole of the spiral welded pipe is significantly greater than that of the straight seam welded pipe. This confirms that the plastic deformation ability of spiral welded pipes is better than that of straight seam welded pipes, and the blast opening is generally limited to one pitch. This is because the spiral welds play a strong role in constraining the expansion of the crack.

5. Toughness and fatigue strength

The development trend of pipelines is large diameter and high strength. As the diameter of the steel pipe increases and the grade of steel used increases, the tendency of the ductile fracture tip to expand steadily increases. According to tests conducted by relevant research institutions in the United States, although spiral welded pipes and straight seam welded pipes are of the same level, spiral welded pipes have higher impact toughness.
Due to changes in the transmission volume of transportation pipelines, steel pipes are subject to random alternating loads during actual operation. Understanding the low cyclic fatigue strength of steel pipes is of great significance to judging the service life of pipelines.

According to the measurement results, the fatigue strength of spiral welded pipes is the same as that of seamless pipes and resistance welded pipes. The test data are distributed in the same area as seamless pipes and resistance pipes, but are higher than ordinary submerged arc straight seam welded pipes.

6. On-site weldability
On-site weldability is mainly determined by the material of the steel pipe and the dimensional tolerance of the port fit.

Considering the requirements for steel pipe installation and construction, the continuity of steel pipe processing and production and the consistency of the shape and geometric dimensions are particularly important.

The production of spiral welded pipes is basically a stable and continuous process under the same working conditions: while the production process of straight seam welded pipes is segmented, including the whole plate/pressure head/pre-rolling/spot welding/welding/finishing/assembly, etc. Multi-step process. This is an important feature that distinguishes spiral welded pipe production from straight seam welded pipe production.

Stable production conditions make it very convenient to control welding quality and ensure geometric dimensions. Due to the regular shape of the spiral welded pipe and the even distribution of welds, compared with the straight seam welded pipe, the spiral steel pipe has very good pipe mouth ovality and end face verticality, ensuring the assembly accuracy of the on-site steel pipe welding assembly.

7. Influence on the flow characteristics of the conveyed medium

The pressure drop in the transmission pipeline is directly proportional to the length of the pipe, fluid viscosity coefficient, fluid velocity, and fluid resistance coefficient, and inversely proportional to the inner diameter of the pipe. The fluid resistance coefficient is related to both the Reynolds number and the roughness of the inner wall surface of the pipe. It has been determined that the impact of the roughness of the inner wall surface of the pipe is ten times greater than the impact of the local raised area (such as a spiral weld or a longitudinal weld, or even an inner annular weld).

Read more: Classification and application of welded steel pipe