Peer-Reviewed Topical Review: The Importance of Quality in Corrosion-Resistant Alloys in Biopharmaceutical Manufacturing - Pharmaceutical Technology

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Peer-Reviewed Topical Review: The Importance of Quality in Corrosion-Resistant Alloys in Biopharmaceutical Manufacturing
In this topical review, the authors discuss the rationale behind microstructural requirements for biopharmaceutical equipment and problems that may be encountered during the fabrication of high-performance corrosion-resistant equipment.

Pharmaceutical Technology
Volume 3, Issue 32

Fabrication of vessels

The fabrication of vessels made of austenitic stainless steels and the higher corrosion-resistant materials such as the superaustenitic and nickel alloys are the same in many ways, but there are some differences. The obvious difference is the higher value of the raw material being used to fabricate the vessels. There are also the basic welding, forming, and polishing differences. The more important issue, however, is that the vessels made of these higher alloys are generally used in more critical applications with regard to corrosion resistance, some of those being final product or "payload" vessels where the value of the product in these vessels are much higher than any other. The vessels made of these higher alloys are expected to be at a minimum equivalent in surface-finish characteristics to austenitic stainless steels. By meeting this requirement, the cleaning and sterilization will be equivalent to known practices and the higher corrosion resistance will be automatic. Overall, the surface-finish quality and corrosion resistance must be at a premium. If there are any problems encountered during manufacturing of the vessels that do not allow this premium quality to be achieved, it hampers fabrication schedules, which can in turn affect a company's schedule of getting a drug to market. There are also major cost effects of raw materials and shop labor that the vessel manufacturer has put into the product.

The fabrication of these high-alloy vessels begin with the normal purchase of raw materials. The lead times of these raw materials are generally longer than the austenitic stainless steels, and it is important to have the correct material specifications at this stage. Raw materials are received into the fabrication shop normally in the mill hot-rolled, annealed, and descaled condition, but on occasion are in a cold-rolled 2B type finish. The material is thoroughly inspected. Raw-material markings are checked, and material certifications are received and reviewed for compliance. Positive material identification is performed to verify the correct material chemistry. After inspection, components such as vessel heads, liners, heat-transfer coils, and heat-transfer jackets are cold-formed and prepared for welding. Welding is performed in compliance with the applicable code and customer specifications; typically for the American Society of Mechanical Engineers' (ASME) Section VIII, Division I code vessels, ASME Section IX is required (3, 4). Welding of the shells, heads, nozzles, and heat-transfer surfaces are performed. After welding, many forms of mechanical polishing steps are performed to remove any mill hot-rolled, annealed, and pickled finish, weld beads, and weld-heat tint to achieve a uniform, flush, and smooth surface finish. Much of this work is performed at the component and subassembly stage. Many inspections of welds and surface finishes are performed during this process to comply with customer requirements and the ASME bioprocessing equipment standard (5).

The final stage is electropolishing of the vessel components and subassemblies. At this stage, the final surface finish is achieved and inspected. If it does not meet the requirements and if it is found to be a raw-material metallurgical issue, the surface finish generally cannot be changed. This situation becomes a major issue for the vessel manufacturer, raw-material suppliers, and mostly the vessel purchaser. Depending on the vessel, 70–90% of the fabrication costs may be already performed at this point. Normally there are only a few final fabrication steps left such as installation of components and subassemblies into the vessel and finally the insulation and exterior sheathing. In addition, there may be some final factory-acceptance testing to be performed. Figure 1 shows the highly electropolished heat-transfer coils for a vessel. A completed biopharmaceutical vessel is shown in Figure 2.


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