African Fusion June 2018

New approaches to NDT

complywith increasingly onerous health legislation introduced over the past two decades or so. The legislation limits ac- ceptable bacteria and spore counts in the final product. A major constituent of most products is water. As private sources of water have declined, more and more use is made of municipal water. This is of inconsistent quality over time. Product input materials are shipped fromsources often far removed for the process plants. These generally differ per batch. Bacteria are directly associatedwith the build-up of bio films on product con- tact surfaces. Welded surfaces include geometric and surface effects, which may encourage biofilm development. Clearly some of the existing process plants often designed prior to new leg- islation will be in early to mid-life. Biofilms formon the exposed surfac- es of process plant (See Figure 2 below). A thin layer of microorganisms adheres to the surface to form the film. This may be organic or inorganic, together with the polymers that they secrete – and biofilms include bacteria. Biofilm depth increases with in- creased surface roughness, increased temperatures, and lower flow speeds (note occluded and dead area effects). One source of surface roughness and local occlusions are welded joints. Resulting from biofilm formation, bacterial growth develops and is re- leased into the product. Biofilm forma- tion on inside surfaces encourages bac- teriagrowthanddepresses theminimum inhibitory concentration (MIC). Weld imperfections, therefore, have a direct impact onhealth–andbiofilmformation is not limited to health related sectors. The depth ( δ ) of the film also in- creases with increased surface rough- ness, increased temperatures, and lower flow speeds. Sequential biofilm formation can be summarised as follows: (1) Attachment of small organic mol- ecules to the surface. (2) Microbial cells deposited on and adsorbed into the forming layer. (3) Cells produce hair-like exopolymers (fimbriae) , which feed on organic matter. (4) Biofilm layer grows and includes bacteria. (5) Product flow degrades some of the formation. (6) An equilibrium layer of depth δ is formed. This depth increases with

Older assets operated for decades are of older materials and older design. Consequently themaintenance ormod- ernisation needs to take different design standards, different material standards and different human skill levels into account. The practice of life extension, part of ‘sweating the assets’, may well mix older manufactured materials with newer. Also the reduction in manpower has often affected corporate memory. Corporatememory, the ‘knowledge and information from the companies past that can be accessed and used for pres- ent company activities’ (Railway Inves- tigation Unit, 2010), is not always avail- able and/or accessible. With changed personnel and/or poor record keeping or access, corporate memory loss can prove important when considering life extension. The aspect of corporate memory loss may become significant in assessing NDT needs. What is the weakest link? when will it break? how can life be extended? Cur- rently one problem that is emerging is delamination on pressure and thermal vessels and pipes. Others have been noted in welds. Roll refurbishment that seeks to add wear-resistant layers risk delamination if not properly assessed. Whilst clients want to extend life, indus- trial research using NDT, appropriate tools are increasingly required. The triple bottom line – planet, people and profit – health issues Chemical engineers manufacture prod- uct on a commercial scale. Technology has made life simpler, safer and more convenient. With increasing urbanisa- tion, the C20 th introducedprocess plants tomass-producepharmaceuticals, food, beverages, wine and beer. Concentra- tion was on optimising cost, time to market and reliability. Solutions were financial and technical. But the C21 st has introduced other demands fromwelded fabrication. Triple bottom line reporting places emphasis on the suite of people, planet and prosperity. In particular health is- sues have become more relevant. Inter- nationally and locally, health legislation has been progressively tightened in the pharmaceutical, dairy, food, beverage and brewery sectors. So whilst criteria used to focus on structural integrity, the need for hygienic fabrication adds a new dimension to NDT needs. Process plants are typicallymade up

Figure 3: Scanning electron micrograph of a biofilm on the inner surface of a dispensing line. A: protruding fimbrae; B: Yeast cells; C: Bacteria. Source: Erna Storgårds; Process hygiene control in beer production and dispensing; VVT publication 410 2000. of tanks, heat exchangers, distillation columns, pumps and interconnecting thinwall pipes. In addition, a second set of piping is required to facilitate Clean- ing in Place (CIP) without disassembly. Typically, many components arewelded together. Whilewelding an enabling technolo- gy, process plants are often now subject to health legislation with demanding hygienic requirements that are often difficult to achieve, particularly onsite. Production hygiene is required to guarantee medicine and food safety. Complete sterility of plant, input mate- rials and water is the ideal, whilst not a realistic expectation. In the absence of complete sterility, other methods need tobe considered for plant establishment or refurbishment practices to eliminate or mitigate against bacterial load. For hygienic fabrication, whilst good design standards exist, major gaps are found in the lack of practical guidelines, educationand, pertinent toSouthAfrica, skills traininganddevelopment together with appropriate supervision. Tanks, distillation columns and heat exchangers are factory-built from stan- dard flats and shapes welded together into the required form. The focus on product quality is high. However,most of the small bore thin wall pipe inter-con- nections are made on site. These pipes are inaccessible from the inside. Site welding is inherently farmore difficult to control partly because of the operating circumstances and because of the focus on the production schedule. Inadequate welded joints can compromise product quality in an otherwise hygienically de- signed plant. Poor joints and/or welding can trap biofilm, the source of bacteria. Theymay also lead tomicrobial induced corrosion and/or stress corrosion crack- ing, both of which would potentially affect hygienic performance. Both existing and new food, bever- age and brewing process plants need to

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June 2018

AFRICAN FUSION