Keeping clean
Seal technology for sterile process pumps

The latest mechanical seals meet
the most stringent requirements for
sterile systems. Points to look out for when choosing a sterile seal include suitable materials and surface finishes. Most importantly, elastomer seals should be designed to avoid becoming
contamination traps.

Pumpkin seed disrupts cleaning process.” This is not a joke; it really happened, and it shows the huge range of particle sizes that engineers have to consider when designing mechanical seals for sterile processes. At the other end of the size scale, micro-organisms frequently disrupt processes, too. This article describes the latest developments in mechanical
seals designed to reduce the risk of unintentional product contamination caused by inadequately sterile conditions—and the high costs of lost production.
The main areas of application, and the associated media, for sterile mechanical seals are: pharmaceuticals (highest-purity water); infusion solutions (nutrient solutions, suspensions and alcoholic solutions); cosmetics (highest-purity water, lotions and perfume); biotechnology (cell suspensions and nutrient solutions); and food (beverages, milk products and many more). There is also the special area of WFI (water for injection). This is water of the highest purity, used in the production of pharmaceuticals and semiconductors, and in biotechnology.
It is beyond the scope of this article to describe all the rules and standards that apply to the use of mechanical seals in sterile processes. The key standards that apply to seal design are GMP (Good Manufacturing Practice) and QHD (Qualified Hygienic Design). For testing and inspection in Europe, the most important bodies are the European Hygienic Equipment
Design Group (EHEDG) and TNO, the Dutch research and standards organization. FDA and 3-A standards are also important.
Features in seal design
For seals, the main focus of attention is on the suitability of the materials used, the quality of the surface, and other design features.
Seal materials – both primary seal faces and secondary seals – must be resistant to corrosion by both the product and the cleaning solutions used for SIP/CIP. They must be mechanically stable and wear-resistant, so that fragments do not break off and contaminate the product. For food use, seal materials must also be approved for product contact by the FDA or other
bodies.

Surface quality for sterile use means smoothness (root-mean-square roughness ,0.8 µm), no cracks or striations, and no cavities. Seal surfaces should also resist product adhesion. Suitable surfaces are best achieved by precision machining followed by electropolishing. As well as removing the roughness left by machining, electropolishing passivates the surface for increased resistance to corrosion. Design features that are desirable in a sterile
seal include:
closed surface geometry, without gaps. If gaps are unavoidable, they must be large and easy to clean;
rounded corners and chamfered edges;
no empty spaces;
no screws or threads in contact with the product; clamp-type joints should be used instead; and
no springs in contact with the product. If springs are essential, there must be a minimum gap between the coils.
The mechanical seal is a key component of the sterile process pump, so it is subject to the same stringent performance requirements. For instance, EHEDG certification of sterile process pumps requires that the pump and the mechanical seal be tested as a unit. It follows that a holistic approach, in which the seal is designed in close consultation with the pump manufacturer, is the only way to produce an optimized design which meets the requirements of sterile processes.
Seals for high pressures
Sterile mechanical seals generally fall into two classes: O-ring seals for high pressures (up to 30 bar), and roller bellows seals for medium pressures (up to 16 bar).
The main features of an O-ring sterile mechanical seal, such as the Burgmann Type SHJ97G, are:
the spring is isolated from the product;
molded elastomer seals are in contact with the product;
the seal is installed using a clamp system;
a relief mechanism is built in; and
the maximum operating temperature is 140 °C.
A conventional O-ring seal sits in a groove. Since the O-ring only fills part of the groove, this type of seal can pick up deposits that are impossible to remove by normal cleaning processes.
Molded O-rings developed by Burgmann solve this problem by eliminating the gaps. Design of these molded elastomer rings is a complex and sensitive business, balancing sealing performance against the need for low compression to reduce friction and wear. Dynamic seals are especially hard to design because the need to compensate automatically for shaft run-out limits seal compression even further. Burgmann’s designers have succeeded, however, and the SHJ97G seal complies fully with the stringent requirements of the EHEDG and TNO.
Seals for medium pressures
Roller bellows mechanical seals are suitable for sterile applications at medium pressures. In contrast to O-ring seals, roller bellows seals such as the Burgmann SMF do not use dynamic O-rings or springs. This design, which like the O-ring seals is held in place by clamps, can be used at pressures up to 16 bar and temperatures up to 200°C. In this type of
seal the pumpkin seed referred to at the beginning of this article is a typical problem for designers. To accommodate particles of this size, which are common in food processing, the geometry of the roller bellows is designed with large clearances so that the system is self-cleaning. This type of seal also uses static elastomer seals, to which the same design considerations apply as in the O-ring type of seal. Both these examples show that it is possible to take a proven seal design and develop it to meet the most demanding standards for sterile processes. Ultimately, however, what counts most are the opinions of users who need to optimize the commercial aspects of their processes and avoid the financial losses associated with contaminated production batches.