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Basics of Hygienic Design


In view of the increasing requirements on safety and cleanability and regardless if for sanitary or sterile production,the quality requirements for valves and measuring instruments are also increasing. Thus, when choosing the correct valve or measuring instrument, the choice of material and the required surface finish quality is a decisive factor.
Austenitic stainless steel is used as a standard material for wetted areas. In the food and beverage industry and also in the pharmaceutical industry, 1.4404 and 1.4435 grades are preferred. Stainless steels are inert to the majority of foods and pharmaceuticals, while also offering good corrosion resistance to disinfectants and cleaning agents. The 1.4435 material will have much lower Sulfur and Ferrite content and thus a higher resistance to corrosion. In this respect, 1.4435 displays a better corrosion resistance than 1.4404. For specific applications, special alloys are used, such as the fully austenitic stainless steel, 1.4539 (904L), or Hastelloy C, and also plastic coatings such as PFA (perfluoroalkoxy) or PTFE (poly-tetra-fluoro-ethylene). As a standard material for all those metallic surfaces that will come into contact with the process medium, PharmAseptic specifies stainless steel 1.4435.
An important aspect in the cleaning of a plant using CIP/SIP processes is the quality of the surfaces in contact with the process medium. To enable the easy cleaning of the measuring instruments, and also to prevent the creation of biofilms, all potential product contact surfaces must be passive and free from microscopic flaws. In addition to the surface topography, the surface roughness is an important criterion for cleanability. In the standards, such as EHEDG Doc. No. 8, “Hygienic equipment design criteria”, a roughness of Ra < 0.8 μm is considered as sufficient for normal cleaning processes. For sensitive biotechnology processes, surfaces with a lower
roughness are needed, e.g. Ra < 0.38 μm per ASME BPE.
By using electrolytic polishing, the cleanability of the surfaces can be improved. In this way, essentially, the topographical structure of the surfaces is smoothed and therefore the roughness is decreased. A further advantage is that electropolishing increases the passive layer of stainless steel, and thus the corrosion resistance is improved, especially with reducing media.
Sealing materials
In the selection of a suitable sealing material, various process parameters and also the process media are important. Sealing materials must be not only toxicologically harmless and sufficiently resistant to abrasion, but also resistant against aggressive cleaning and disinfecting media as well as stable in super-heated steam at high sterilization temperatures. Special compounds are used for O-rings and sanitary clamp seals, for example fluororubber-based (FKM) such as VITON®, ethylene-propylene-diene material (EPDM) or poly-tetra-fluoro-ethylene (PTFE). It is important to note that not all polymers are the same. Not only the quality and formulation of materials used for the sealing elements but also the manufacturing processes are critical. The final commercial product must also conform to the rules of the regulatory authorities and organisations.