Basic guidelines for hygienically designed machines

Process equipment must not only keep functioning properly, it must also be easy to clean and protect products from contamination. The hygienic design guideline for process equipment provides excellent guidance for achieving that. In 1989, EHEDG (European Hygienic Engineering & Design Group) was created, in which food manufacturers as well as machine builders and suppliers are represented. Together, they build food safety by establishing guidelines that encourage hygienic design. This article summarizes the guidelines regarding the hygienic design of process equipment. Several basic criteria must be taken into account.

Surfaces and geometry

Direct and indirect product contact surfaces must be readily cleanable, non-absorbent and pose no toxicological hazard with respect to leaching of substances into food. Well cleanable means smooth and free of imperfections (cracks or tears). That way, microorganisms or other residues do not have a chance to remain there. Therefore, avoid:

Surface textures that are difficult to clean. Since the surface finish affects cleaning time, an Ra value of equal to or less than 0.8 µm is recommended for stainless steel surfaces. Higher Ra values are justified in special types of devices or special functional requirements;

• Direct metal-to-metal connections (except welding), as they can corrode and retain dirt and liquids;
• Step transitions due to misalignment of equipment and pipe connections
• Use of O-rings unless in static joints or otherwise stated in EHEDG documents for special applications;
• Visible threads, metal-to-metal connections of fasteners, gaps, playetes in hinges ...;
• Sharp angles. All internal angles of 135° or less must have a radius of at least 3 mm;

Welding

The weld seam of permanent metal-metal joints in contact with the product must be unbroken and without defects. This applies equally to welds on the non-product contact side. In addition, all welds must be sufficiently smooth and well placed so that they can be easily cleaned.

Drainability

The interior and exterior of all equipment and piping should be self-draining or drainable. Avoid horizontal surfaces, but always slope to one side at a minimum angle of 3°. For external surfaces, direct all liquid away from the main product area.

Insulation

Two hygienic options exist for insulation of equipment and piping: sealed enclosure or vacuum. In the first case, insulation materials must be made of corrosion-resistant materials and completely sealed to prevent air or moisture from entering and microbial growth. Avoid excessive moisture in the production environment when applying the sheathing. This is because trapped moisture increases the risk of corrosion. Pipes can additionally be insulated by evacuating air from the space between the walls of double-walled pipes. This is an extremely effective way to avoid all common problems. 

Installation, supports and layout

In factory buildings, there is always the risk of condensation from equipment, piping and the building's internal surfaces falling onto product contact surfaces. This requires adequate attention during design, layout and installation to ensure that condensate is routed away. Equipment and support structures should be installed so that there are no voids or openings. Try to minimize floor contact points in number and area and provide adequate distance between equipment for cleaning and inspection purposes. Supports for pipes or equipment should be built or installed so that no water or dirt can remain on or in them. Also consider possible adverse galvanic reactions between dissimilar materials. In addition, supports should be sealed to prevent accumulation of residue and moisture. When fixtures, brackets ... are attached to hollow supports, the support must not be pierced, which could allow moisture to enter. Regarding layout, ensure that the risk of cross-contamination between process lines is minimized.