As revealed in the preceding post, the material of construction is a critical factor in the design of “sanitary” plumbing. Although the selection of material is important, there are other considerations that are equally important in the design of a system that will resist contamination.
Surface Finish –
Surface finish can have a significant influence on the ability of a surface to resist contamination. The most acceptable finishes will be free of surface defects including scratches, pitting, and other asperities that might create a rough surface finish. Mill-finished and surfaces finished using a metal removal process such as turning and milling are most resistant to contamination. Surfaces finished using a “smearing” process like buffing, grinding and graining (sanding) tend to harbor contaminants. Electropolishing is often employed to assure that a surface not otherwise qualified is applicable in a sanitary application.
Note – Electropolishing is a process applicable on metallic surfaces which is, in simple terms, the reverse of electroplating. An electric current is used in conjunction with an electrolyte (usually an acid) to remove small amounts of metal from a surface. Because of the nature of the electrolytic process, protruding asperities are selectively attacked as areas of high current density. As a result electro-polishing tends to “level” and polish a surface. A good introductory explanation of the electropolishing process can be found at the following link – The MCP System of Electropolishing – General Process Steps.
Other Features –
Most cleaning systems consist of a collection of tanks along with associated plumbing and other liquid handling components. It is really not that difficult to provide a tank suitable for sanitary use. In general, tank surfaces are relatively accessible allowing fabrication and finishing operations to be carried out with some degree of scrutiny. Naturally, tanks need to be constructed with rounded rather than square corners and all welds must be appropriately finished using sanitary compatible processes. As a final measure, most tanks used in sanitary applications are electropolished as described above as a final assurance. As a result, the process tank in a sanitary system is usually the least likely area for a sanitary system to be deficient in design and construction.
Probably the biggest challenge in the design and construction of a sanitary system is in the inter-connecting plumbing. For starters, the old stand-by threaded pipe fittings are taboo. The roots of threads and the fact that tapered designs usually have only line contact between mating surfaces make threaded fittings a major opportunity for contamination. Similarly, compression and flare fittings have “nooks and crannies” in which contamination can hide. Probably the best answer to sanitary fittings are specially designed devices which have smooth interior surfaces and can be welded in place using orbital welding technology. Orbital welding is a mechanized process which can be controlled to provide a smooth transition on both the interior and exterior of a pipe or tube. Another good solution is the use of flanged fittings with a suitable “O” ring between flat faces that are secured together with a mechanical clamp that captures and squeezes the flanges of components to be connected. This temporary connection offers the ability to disassemble joints to allow periodic internal cleaning and inspection. It is this clamped flange construction that we probably most associate with food processing facilities although it finds use in nearly any high purity application.
The next blog will explore plumbing practices beyond tanks and plumbing connections that are crucial in assuring high purity in a liquid handling system.
– FJF –