When welding the tank wall, why should the longitudinal seam be welded first and then the circumferential seam? And when welding the bottom plate, why should we weld the short weld first and then the long weld? When welding the tank wall, the circumferential seam cannot be welded first. According to your description, it should be a large tank, assembled vertically, so it is very difficult to weld the circumferential seam first. First of all, it is difficult to assemble the circumferential seam. Second, girth seam welding will cause serious deformation of longitudinal seam, which will affect the assembly and welding of longitudinal seam. Moreover, the longitudinal seam is welded first, and the longitudinal seam can shrink freely without affecting the circumferential seam, so the circumferential seam is welded first.

Welding short welds first is to reduce welding deformation. The long and short welds welded first will be fixed and will not shrink freely, which will lead to a lot of deformation, which will eventually affect the assembly of the floor and the cylinder, and even the whole is unqualified (the deformation of the bottom plate is limited to 50). Similarly, welding the short weld first has no effect on the long weld, and better welding quality can be obtained.

In addition, if the weld is long, it is suggested to retreat from the middle to both sides, so that the welding deformation can be controlled to a minimum.