African Fusion June 2018

gap, weldingeffects andspringback. • Possible effects on pipe ends from cutting to length operations. • Possible transport effects related to welding stress relief and packaging. • Spring back associated with the heat-affect zone around the seam welding. The net result is pipes and tubes are nei- ther circular nor of even wall thickness. In the case of thin wall pipes related to hygienic fabrication, the tolerance impacts can be significant in terms of increased CIP requirements. Manufacturing effects may be over- come by using clamping or pipe end forming methods. Also, in terms of product development, the potential move from 316L stainless steel towards stronger duplex grades, e.g. 2304, could lead to even thinner pipe walls. All inputs affect the fay matching and alignment of pipes ends that are otherwise correctly cut. Speaking to welding craftsmen, one of the realities noted was the effect on productivity on the need to crimp or flare pipe ends to achieve a good fit up. In their experience, because of the fit up complications, typi- cally about one good 50 mm diameter pipe joint per hour was achieved. In a production environment where time is of a premium and thousands of welds are required, compromise seems likely. However, whilstwelders cannot con- trol the input material, it seems feasible either to modify adjacent pipe ends to achieve better fit up prior to welding or to provide clamped alignment to achieve good welds. This implies that the pre-weld check is more important than the post-weld check as weld replacement implies two welds in the place of one. In addition, fabricator centroid misalignment can be important both in terms of pipe geometry and for the welding process. What has all this to do with NDT? The question that arises from this analysis is whether the role of the NDT should be reconsidered. It is possible to examine pipework after assembly using various tools, including borescopes to check structural integrity and a healthy geometry. Hygiene requirements, potentially more demanding to achieve than struc- tural integrity, are an addition to engi- neering fabrication/operational require- ments. Welding is a source of surface

increased surface roughness, in- creased temperatures, and lower flow speeds, which are associated withoccludedanddeadarea effects. Also as a result of the formation of biofilm, local galvanic activity and MIC affects the inert surface and Fe(OH) 2 tubercules form, as shown in Figure 3. Welded joints support biofilm growth where there exists: • Inadequate penetration – innerweld profile leaves crevices (dead areas). • Over penetration, cauliflowering – inner weld profile proud (dead areas). • Porosity. • Cracks. • Misalignment (manufacture effect or fabricator) – occluded areas. • Welding process effects on thewidth of HAZ zone – laser

Figure 4: Left, an ideal TIG orbital weld: pipe faying edges are identical and pipe fit up is correct. Source: Orbital welding of stainless steel tuning for biopharmaceutical, food and dairy use; JD Cluett; www.arcmachines.com.

Figure 5: Plant realities of joint problems. Source JD Cluett; www.arcmachines.com. roughness. The welded area is a small part of the total area of installation. Whilst good design standards exist for hygienic fabrication, major gaps are found in the lack of practical guidelines. Realistically, hygienicwelded fabrication, particularlywithon-sitewelding, ismore challenging, more costly. The NDT ques- tion is whether we are looking deeply enough intonon-structural performance issues such as health and corrosion. In summary: • Operating circumstances have changed. • Materials have changed; standards have changed. • Health legislation has introduced a new challenge. • Life extension of older plants intro- duced new challenges. NDT approaches and techniques require constant review and adaptation. New NDT approaches and tools may well be required to meet client expectations. On the health side, more use can be made of borescopes linked by optical fibre to a camera and PC. But this may be too late. More im- portant is the need to increase levels of communication beyond simply pass/fail to assess what works from the point of view of assisting clients, manufacturers and fabricators. Then an informed cost/ benefit analysis may be completed.

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June 2018

AFRICAN FUSION