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Hygienic design criteria

Dry materials handling must take into account the possibility for material lump formation, creation of dust explosion conditions, high moisture deposit formation in the presence of hot air, and material remaining in the equipment after plant shutdown (even if a degree of selfemptying is achieved).

Product contact surfaces

Product contact surfaces should be smooth and resistant against dry material contact and also against liquid chemicals used in wet cleaning. Product contact surfaces therefore should be free of crevices, pitting, pinholes and any hairline cracking that can cause material penetration and cleaning difficulties. A roughness standard of Ra<0.8 mm is recommended where there is a risk of microbial growth associated with high moisture content in the dry material or wet cleaning. In order to carry out a dry cleaning operation, contact surfaces should be fully accessible for safe manual cleaning and inspection. For a hygienic wet cleaning operation, contact surfaces should not be horizontal, but have a slight slope to facilitate drainage of cleaning solutions. The possibility for product contact on sharp internal corners (r<6 mm) and recesses, etc., where dry material can accumulate, should be avoided. Windows and inspection ports mounted in product contact surfaces should be flush with the surrounding surfaces to minimise dry material build-up. When using nonmetallic materials as contact surfaces, the porosity of the materials should be investigated with regard to ease of cleanability.


Welds must be accomplished in a way that avoids susceptibility to accumulation of dry material and localised corrosion. It is important to ensure that the metallurgical properties of the weld material are as close as possible to the parent metal. General criteria for welding are described in the EHEDG Doc 9 (EHEDG, 1993b) ‘‘Welding stainless steel to meet hygienic requirements’’. Intermittent (spot) welding of dry product contact surfaces is in principle not acceptable. Normally, the surface roughness of welds does not meet the recommended figure of Ra 0.8 mm. The cleanability of these parts in relation to the actual dry material being handled should be validated.

Static seals (gaskets)for duct and flange connections

Static seals should be of an elastic material, have a non-porous surface and be cleanable. Static seals should be clean before assembly and the possibility for penetration of dry material into the gasket or seal during equipment operation should be avoided. PTFE can be used as a static seal in combination with an elastomer (food grade, FDA-approved or national equivalent). The PTFE should be of high-density resilient quality. Metal-to-metal contact duct assemblies and paper-type gaskets between flanges can be applied where a plant operates at atmospheric pressure and requires no wet cleaning. Inflatable seals, e.g. for valves (Fig. 1) or around access doors and operable inspection ports should be used to prevent dry material build-up around the mounting frames.


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