Simple comparison of technical characteristics between spiral submerged arc welded pipe and straight welded pipe

I. Metallurgical Properties of Materials

Longitudinal submerged arc welded pipe is made of steel plate, while spiral welded pipe is made of hot rolled coil plate. The rolling process of hot-rolled strip mill has a series of advantages. It has the metallurgical ability to produce high quality pipeline steel. For example, a water cooling system is installed on the output bench to accelerate cooling, which allows the use of low alloy components to achieve special strength grades and low temperature toughness, thereby improving the weldability of steel. But this system is basically not available in steel plate factories. The alloy content (carbon equivalent) of coiled sheets is often lower than that of similar grade steel sheets, which also improves the weldability of spiral welded pipes.

More importantly, because the rolling direction of spiral submerged arc welded pipe is not perpendicular to the axis of the steel pipe (its clamping depends on the spiral angle of the steel pipe), while the rolling direction of the steel plate of the straight welded pipe is perpendicular to the axis of the steel pipe, the crack resistance of the spiral welded pipe material is better than that of the straight welded pipe.

II. WELDING TECHNOLOGY

In terms of welding technology, spiral welded pipe and straight welded pipe have the same welding method, but straight welded pipe will inevitably have a lot of T-welds, so the probability of welding defects is greatly increased, and the welding residual stress at T-weld is large, weld metal is often in a three-dimensional stress state, which increases the possibility of cracking.

Moreover, according to the technological regulations of submerged arc welding, each welding seam should have arc initiation and arc extinguishing places, but each straight welded pipe can not reach this condition when welding annular seam, so there may be more welding defects at arc extinguishing places. 3. Toughness and fatigue strength.

The trend of pipeline development is large diameter and high strength. With the increase of diameter and grade of steel pipe, the trend of stable expansion of ductile fracture tip becomes larger. According to the test of relevant research institutes in the United States, although the spiral welded pipe and the straight welded pipe are of the same grade, the spiral welded pipe has high impact toughness.

Because of the change of the transportation capacity, the steel pipe is subjected to random alternating load in the actual operation process. Understanding the low cycle fatigue strength of steel pipes is of great significance for judging the service life of pipelines.

According to the measured results, the fatigue strength of spiral welded pipe is the same as that of seamless pipe and resistance welded pipe, and the test data are distributed in the same area as seamless pipe and resistance pipe, which is higher than that of submerged arc straight welded pipe.

IV. Strength of Static Pressure Blasting

The results of comparative tests show that the yielding pressure of spiral welded pipe and straight welded pipe basically coincides with the measured and theoretical values of blasting pressure, and the deviation is close to each other. However, the spiral welded pipe is lower than the straight welded pipe both in yield pressure and blasting pressure. The blasting test also shows that the circumferential deformation rate of the spiral welded pipe blasting hole is obviously larger than that of the straight welded pipe. It is proved that the plastic deformation ability of spiral welded pipe is better than that of straight welded pipe, and the blasting hole is generally confined to a pitch, which is due to the strong restraint of spiral weld on crack propagation.

V. STRENGTH CHARACTERISTICS

When a pipe is subjected to internal pressure, there are usually two main stresses on the pipe wall, i.e. radial stress and axial stress. The synthetic stress at the weld seam is 1/2 of the synthetic stress delta=delta Y (l/4sin2alpha+cos2alpha), in which alpha is the spiral angle of the weld of spiral welded pipe.

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

According to the above characteristics, we can see that:

When the spiral welded pipe is blasted, because the normal stress and the synthetic stress of the weld are relatively small, the blasting hole generally does not originate from the spiral weld, and its safety is higher than that of the straight welded pipe.

When there is a parallel defect near the spiral weld, the risk of expansion of the spiral weld is not as great as that of the straight weld, because the stress of the spiral weld is less than that of the straight weld.

Because the radial stress is the maximum stress existing in the steel pipe, the weld will bear the maximum load when it is in the direction of vertical stress. That is to say, the vertical seam bears the greatest load, the circumferential seam bears the smallest load, and the spiral seam is between the two.

VI. On-site weldability

The weldability of the site is mainly determined by the material quality of the steel pipe and the tolerance of the matching size of the end.

Considering the requirement of steel pipe installation and construction, it is particularly important for the continuity of steel pipe processing and production and the consistency of geometric dimensions.

The production of spiral welded pipe is a stable continuous process under the same working conditions: while the production process of straight welded pipe is divided into several stages, including whole plate/indenter/pre-winding/spot welding/welding/finishing/group-to-group process. This is an important feature of spiral welded pipe production which is different from that of straight welded pipe production.

Stable production conditions are very convenient for welding quality control and geometric size assurance. Compared with the straight welded pipe, the spiral welded pipe has very good ellipticity of the orifice and perpendicularity of the end surface, which ensures the accuracy of the alignment of the steel pipe welding group on site.

VII. THE INFLUENCE ON THE FLOW CHARACTERISTICS OF TRANSPORTING MEDIUM

The pressure drop in the pipeline is proportional to the length of the pipe, the coefficient of fluid viscosity, the velocity of the fluid and the coefficient of fluid resistance, but inversely proportional to the inner diameter of the pipe. The drag coefficient of fluid is related not only to Reynolds number, but also to the roughness of the inner surface of the tube. It has been determined that the roughness of the inner surface of the pipe has a ten-fold greater effect than the area of the local bulge, such as the spiral or longitudinal weld, or even the inner annular weld.