When the pipe is subjected to internal pressure, two main stresses are usually generated 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 weld of the spiral welded pipe.
The helix angle of the spiral welded pipe weld is generally 50-75 degrees, so the synthetic stress at the spiral welded seam is 60-85% of the main stress of the straight seam welded pipe. Under the same working pressure, the wall thickness of the spiral welded pipe of the same diameter can be reduced compared with that of the straight seam welded pipe.
According to the above characteristics:
A. When the spiral welded pipe is blasted, due to the relatively small normal stress and composite stress on the weld, the blasting hole generally does not originate at the spiral welded seam, and its safety is higher than that of the straight seam welded pipe.
B. When there is a parallel defect near the spiral weld, because the force of the spiral weld is small, the risk of its expansion is not as great as that of the straight weld.
C. Since the radial stress is the maximum stress existing on the steel pipe, the weld bears the maximum load when it is in the direction of the vertical stress. That is, the straight seam bears the largest load, the circumferential weld bears the smallest load, and the spiral seam is between the two.
