This concept has materialized on account of the high requirements made on the pigging technique. Welding, on one hand, higher wall thicknesses, while also placing high requirements on the weld seam quality. This applies particularly to the root sagging and the root suck back. Normally used pipe diameters in chemistry building construction will be at DN 150 and a material thickness of 2.6 mm.

Taking the comparable nominal pipe width, in pipeline construction, to which pigging technique may be applied, a material thickness of 4.5 mm will in fact apply. At normal orbital welding, these material thicknesses may no longer be handled without weld filler. This is because all welding positions will be run through during the welding process. Particularly critical in this context is the fell seam position for which the preceding melting charge will have a negative effect on the arc tension, and the slightest displacement of the edges will cause root defect(s). Also, the fell seam position has poor degassing conditions, a fact which may well cause the occurrence of pores in this connection.

A further drawback of the traditional orbital welding system is that starting in 9.00 hours position will generate a root suck back of the preceding heat. The limit for normal orbital welding is thus at appr. 3.2 mm.

Application of the part seam welding patented by Westfalen AG will divide the orbital weld seam normally welded round 360°, in two part seams. At this procedure, the first part seam will start in 6.00 hours position and will be welded until the 13.00 hours position on one side. The second part seam will then be welded, after being subjected to a certain amount of heating in 6.00 hours position, and a certain overlapping in 12.00 hours position (picture 1).

The critical fell seam area is thus avoided. Applying this process technique enables the welding of material thicknesses of up to 5 mm without filler. Also an edge displacement or gap which may always occur on a building site may be worked safely.