African Fusion November 2015

plants subject to health regulation

Figure 2: An unacceptable manual weld with defects including lack-of-penetration, misalignment, a crevice, and a poor ID purge.

Figure 1. An example of an ideal weld, a TIG orbital weld on 316L electro- polished stainless steel pipe.

axis) andminimum(minor axis) external diameters and three equally spacedwall thicknesses were tabled. Of these 90 pipes, 27 related to 50 mm ID, 1.5 mm wall thickness pipes were analysed. These naturally fell into two distinct groups, one with tight manufactur- ing tolerances (ovality 0.04-0.07 mm) and one with wider tolerances (ovality 0.17‑0.51 mm). A Monte Carlo-based mathematical algorithm was developed to assess in- terconnecting pipes against the criteria of a point-by-point minimum overlap of 80%around the pipe circumference. (Or- bital weldingmanufacturers prefer 95%). The algorithm included three parts: an external ellipse; an internal shape; and a randommisalignment. The external el- lipsewas definedby themeasuredmajor andminor axes, and the internal surface by the external ellipse less the local wall thickness. This was modelled as a ran- domly orientatedquartic functionbased on the three wall thicknesses taken for each specific pipe and themisalignment by allowing for a random6%variation of wall thickness (0.1 mm). Twelve pipe alignment situations were measured. Each involved 1 000 iterations. The minimum overlap for each iteration case was recorded onto a histogram. The cases included: fitting one straight, high-tolerance pipe to another with no orientation control; and fitting one randomly selected, high-tolerance pipe to another with major axis align- ment. The fit-up models were repeated using low tolerance pipes, and connect-

effect of changes in 316 composi- tion limits). • Tungsten geometry is diamond ground and correct. Figure 2 shows a manual weld taken from an operating pharmaceutical plant. This unacceptable weld has defects including lack-of-penetration, misalignment, a huge crevice, and a poor ID purge. Impact of pipemanufacturing and fabrication Several factors lead to inadequate welded joints and pipe fabrication. One of these is pipe geometry. Pipes designed to be circular are often oval due to manufacturer inputs related to the rolling, slitting, forming, seamweld- ing and cutting to length operations. Pipes are manufactured to diameter, wall thickness and ovality tolerances. These tolerances affect the matching and alignment of pipe-ends, which are otherwise correctly prepared. Whilst this is a matter beyond welder control, the fabricator can influence choices. In ad- dition, fabricator centroidmisalignment is important. In the case of thin walled pipes, the impacts of tolerances on hygienic fabrication can be significant in terms of the increasedCIPoperations required to maintain an acceptable product. To test the impacts of pipe manu- facture on weld integrity, 90 × 316L stainless steel pipes and bends from the then current Okahandja brewery project in Namibia were measured by the fabricator in India. Maximum (major

• Heat induced oxidation in the HAZ (inert purging gas needs to be used). • Geometric effects from misalign- ment and/or uncontrolled welding of root beads leading to stagnant regions. • Cooling of heterogeneous filler metal leading to micro cracking. • Pickling and passivating chemical treatments affecting surface rough- ness. • Fit-up effects from poorly cut and/ or aligned pipe-ends leading to local under or overfill. Pipe welding For pipework, the preferred fabrication method is automatic orbital welding as this is capable of producing consis- tently high qualitywelds. Process plants require a significant level of on-site assembly welding of small-bore thin- walled pipes, in particular. Welds on the product-contact side must be con- tinuous; they must be smooth enough to allow proper cleaning. Figure 1 shows an ideal welded joint, a TIG orbital weld on 316L electro- polished stainless steel pipe. Theweld is fully penetrated with a uniform crevice- free inner weld bead showing good pipe-to-pipe alignment. The internal diameter (ID) was purged with argon. To achieve this: • Pipes must be dimensionally matched (within 20% by weight). • Alignment, fit-up, orientation and cleanliness must be controlled. • Weld procedure used is alignedwith the sulphur % (Note changes and

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November 2015

AFRICAN FUSION