Hygienic Conveying How to Convey Hygienic Sensitive Bulk materials

Author / Editor: Martin Stephan / Dipl.-Ing. (FH) Tobias Hüser

CIP–compliant rotary valves are especially required for pneumatic conveying in the food industry. Although the guidelines of EHEDG dictate how the systems should look like, at the end the requirements depend on the selected cleaning method. Read this article, how the implementation of these specifications can function.

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Fig. 1: Easily dismantled discharge valve type ZRD (hygienic version)
Fig. 1: Easily dismantled discharge valve type ZRD (hygienic version)
(Picture: Coperion)

When foods are processed, the individual products are subjected to a number of processes before the finished product is ready for distribution. When products are made by artisans, for example in a bakery, these processes are carried out in a small space and the ingredients are transported by hand from one stage to another. With the increase in industrial food production, the distances between the various process stages are becoming longer and the processes themselves are run continuously. This requires automated transport between the process steps.

Without Hygiene, No Enjoyment

There are a number of mechanical methods for handling this task such as conveying screws, transport belts, bucket conveyors and pneumatic conveying systems. Mechanical systems offer a high level of product conservation for the transport of fragile products, but require complex configurations and there is always the risk of product contamination caused by attrition generated by component wear. Hermetic sealing from the environment is also difficult.

In contrast, a pneumatic conveying plant is a closed–circuit system with lower investment costs which only has mechanical components such as rotary valves at the supply or discharge points. The only disadvantage of pneumatic conveying is the often lower efficiency in comparison to a mechanical system. The advantages outweigh the disadvantages when non–fragile products are transported, which is why a pneumatic conveying system is often selected.

Cleaning Decides on Plant Design

However, several points must be observed for hygienic plant design when planning a pneumatic conveying system and these key components. First, the cleaning process that is to be used for this system section must be specified, as the selected cleaning method influences the design and construction. This aspect can best be described by using the flange connections as an example.

A centred flange connection with an O-ring seal is very suitable for dry cleaning, but not for wet cleaning. A flange connection that is suitable for wet cleaning must be equipped with a seal that is flush at the front to prevent penetration of capillary water into the metal to metal connection which occurs with an O–ring seal. This and other basic requirements for hygienic design are described in the guidelines of the EHEDG and illustrated with examples.

When the applicable cleaning process has been defined, the necessary design can be determined with the aid of the EHEDG guidelines. The design can be officially tested by a construction appraisal or test (for automatic wet cleaning) by the testing and certification institutes of the EHEDG and its suitability confirmed with a certificate. Certificates are currently available for closed and open-circuit processes, for dry or wet cleaning in manual or automatic cleaning modes.

There are currently three conventional cleaning methods for dry food processing systems:

  • Dry cleaning: Normally manual cleaning by vacuuming, dry brushing or wiping, sometimes also blowing off or out. Components are opened or dismantled for cleaning. During dry cleaning it is not possible to achieve completely residue–free cleaning results — slight layers of dust remain. A great advantage of dry cleaning is that the entire system is kept dry and that microbial contamination cannot occur. Obviously the exterior must also be kept dry, especially when the components are opened. As cleaning is carried out manually, it is always necessary to check the cleaning results and correct any oversights.
  • Manual wet cleaning (COP) by washing, spraying or wiping with cleaning solutions (acidic, alkaline), rinsing with water and subsequent drying. Some stages can be automated. Residue-free cleaning can be achieved without any problems with this method. Special care must also be taken to ensure that no contaminants are brought into the open components from the outside. As this is a manual cleaning method, inspection of the cleaning results is essential after cleaning and drying.
  • Automatic wet cleaning (CIP) by purging with cleaning solutions (acidic, alkaline), as well as rinsing water and subsequent drying. Components are not dismantled or opened. Clearly defined separating points (supply and discharge points) are essential. Automatic wet cleaning must be determined separately for each product and system. This cleaning procedure must then be validated. In addition to the low time and personnel requirements for automatic cleaning, the consistency of the cleaning results is ensured, as the human factor can be almost completely excluded.

Consider Constructive Principles

Several principles apply for all cleaning methods for the hygienic design of the rotary valve and pipe diverter components that are in contact with the product:

  • Smooth design in areas in contact with the product
  • Surfaces polished to a maximum Ra of 0,8 µm
  • Corners and edges with wide radii
  • Simple dismantling and easy visibility for inspection during manual cleaning
  • Gas purged shaft seals on continuously rotating shafts
  • Inert materials (no interaction between the product and component)

Fig. 1 shows an dismantled rotary valve with polished surfaces, suitable for manual wet cleaning or dry cleaning. For automatic wet cleaning (CIP), the metal to metal connections must be equipped with a front-flush seal line as described for the flange connections, as the components are not dismantled. A gap is created in conventional connections with an O–ring seal into which water can penetrate during cleaning due to capillary effects. During subsequent cleaning it cannot be fully ensured that this water is reliably removed. This is prevented with special, front-flush seals with a front–flush sealing line.

Coperion CIP–compliant rotary valves and diverter valves are always equipped with these patented seals. These components were launched on the market several years ago and were therefore the first rotary valves and diverter valves for bulk material systems that were successfully tested and certified by the EHEDG. Such CIP-compliant components are essential for the design and construction of CIP–compliant conveying systems. The company also supplies rotary valves and diverter valves for dry cleaning that have been certified by EHEDG. The components are designed so that they can be dismantled quickly and the connection points are sealed with O–rings.

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As the cleaning method determines the design of the key components, it must always be decided at the planning stage which cleaning method is to be used for the individual system sections. Special attention must be paid to the separation points between the areas that are cleaned with wet cleaning methods and those that are cleaned with dry methods. It is essential that moisture and water are kept away from dry cleaning areas. During manual cleaning (dry or wet), attention must be paid to ensure easy accessibility, simple dismantling and cleaning and easy inspection of the components. During automatic wet cleaning the component version must ensure that there are never any product or moisture residues.

The Design of the plant in accordance with EHEDG guidelines and the use of EHEDG–certified components is the prerequisite for a successful and reproducible cleaning quality. The most frequent error made during plant design is the use of unsuitable components next to the dry–wet separation points. For example, CIP–compliant components are often installed together with unsuitable flexible connections, or the connections for measuring instruments are incorrectly planned. Frequently the platforms that ensure easy access to the components for manual cleaning are often missing. During manual cleaning itself, it is often observed that cleaning personnel do not handle the components correctly. Fully automatic CIP is also advantageous in this respect, as the components are only opened occasionally.

* The author is Product Manager Components of Coperion, Weingarten, Germany. Contact: Phone +49-751-408398