EHEDG: Gaskets and seals for food equipment

How does a seal work?

If you exclude the subject of dynamic leakage or permeation of gases through seal materials, the answer is simple. A seal works – that is, it seals – when the contact pressure between the seal and the mating surfaces of the parts connected is higher than the system pressure. A seal manufacturer will take this simple fact into consideration by designing a proper interference between the seal and the recommended installation groove. The groove is defined by shape as well as dimensions and allowable tolerances. Figure 1 (page 42) shows the basic principle when a seal is installed by compression and the resulting compressive forces of the seal material create a normal force on the contact surface of the housing. Any added system pressure increases the contact force, because rubber materials act like high-viscous fluids.

Seal materials in food contact

A few sentences about seal materials are required, although this article is focusing on the hygienic design. For seals in product contact, two groups of materials cover the large majority of applications. They are able to create and maintain a proper contact pressure by deformation. The main material group consists of rubber materials, also called elastomers. Rubber materials are highly-viscous polymeric materials which are cross-linked to create the elastomeric properties. The most frequently used materials according to ISO 1629 abbreviation are the synthetic rubbers EPDM, FKM, VMQ, HNBR, NBR and FFKM.

The second material group is the thermoplastic materials which are non cross-linked polymers that often need an elastomeric energiser or metal springs which ensure a long-lasting contact pressure (Figure 2). This is due to the fact that thermoplastic materials suffer from cold-flow and the initial contact forces often diminish too quickly to achieve a good service life of the seal.

For industrial or special applications, other seal or gasket materials like metal or graphite-metal, elastomer-bonded fibre materials or graphite packings are used. For scrapers, TPE materials, ThermoPlastic Elastomers offer good elastic properties combined with often superior wear resistance. Particularly Polyurethane compounds – which are also available as food grade materials – are utilised.

All materials have to meet the regulatory requirements which are often a combination of formulation and testing requirements. In Europe, the framework regulation (EU) No. 1935/2004 is the main regulation that requires all types of food contact materials to be not harmful to consumer health and to not change the organoleptic properties of the food. Also, it requires Good Manufacturing Practice (GMP) to be applied in the production of the materials, the establishing of a tracking system, proper documentation and a proper labelling of the articles made of those materials. For plastic materials, Regulation (EU) No. 10/2011 must be met. For exports into the United States of America, mainly the 21CFR177 applies. Alternatively, materials or substances may have a registered FCS number allocated to the manufacturer by the FDA.

Polymer seals usually die slowly and quietly

It is important for any user to know that polymeric seal materials will lose their initial resilience and consequently, normal contact forces will slowly diminish until finally, leakage occurs. This stress relaxation of the material is expressed as Compression Set (CS) which is calculated by the following equation defined in ISO 815 Standard: CS%= (h0-h2)/(h0-h1)*100% (Figure 3). The standard test piece is a cylindrical button with a diameter of 13mm and 6mm height.

Typical values of food grade rubber materials are compression sets of 20 to 25 per cent after compression by 25 per cent of the original thickness for 22 hours at a temperature close to the limit temperature in air. The lower the CS value, the higher the potential service life. Of course, this process slows down, but never really stops.

In a figurative sense, polymeric seals are shape shifters, meaning that the original cross sectional shape of a new seal is slowly adapting to the housing to which it is installed. It’s only a matter of time, temperature and chemical activity, until this change in shape will lead first to microbial permeability and then to fluid leakage. In aseptic processes, this fact may lead to product contamination that may only be detected later on, because there is no visible fluid leakage but there are already movements of the seal in its groove that could transport matter in either direction.

That’s why it’s of utmost importance for end users to establish preventive maintenance intervals depending on the seal material and the operating conditions in a certain application. Alternatively, the supplier of equipment may establish such intervals for given applications.

Hygienic aspects

First of all it is important to recognise the fact that a seal alone cannot achieve a hygienic joint. The seal and the housing design must match and be designed to achieve a hygienic and easy-to-clean seal when installed. Also, the groove must be cleanable when replacing a worn seal. Cleanability is achieved by good surface quality, sufficient groove radii and groove dimensions that make all areas accessible for cleaning and inspection.

Important hygienic aspects for static seals are:

• Selection of a suitable seal material, providing sufficient chemical resistance in the environment of intended use, meeting regulatory requirements and being non-toxic, non-mutagenic and non-carcinogenic, non-absorbent to microorganisms or spores and non-porous. Pores in rubber could e.g. develop unintentionally during production, when adding too much activator to the formulation in order to accelerate vulcanisation and thus reducing production costs. Also, the material shall not absorb water or cleaning fluids at a level that may compromise hygienic integrity or even destroy the seal. A volume swell of less than five per cent of the original seal volume is usually acceptable, e.g. in a groove design according to DIN 11864 which allows for a certain amount of volume swell and thermal expansion.
• Front-flush seal design shall prevent dead areas where a product can stagnate and spoil during production or cannot be effectively removed during CIP cleaning (of course, for manually cleaned equipment, the situation is different). Front-flush design shall ensure a good accessibility of the entire surface to cleaning fluids. A standard ‘hydraulic’ rectangular O-ring groove design with a short area of metal-to-metal contact on the product side is a hygienic ‘no-go’.
• Proper alignment of all parts is important to ensure drain ability and avoid unsupported gasket areas. If a gasket is not completely supported (compressed) from both sides, a non-cleanable crevice condition will emerge.
• Controlled compression, usually achieved by a metal (or plastic) stop on the non-product side of a polymer seal avoids over-compression or under-compression. Both situations can pose a hygienic hazard. Under-compression can lead to bacterial permeability or even fluid leakage, over-compression can damage or even destroy the seal. Standard ISO 14159 recommends 15 per cent compression of a 70 Sh A hardness elastomer O-ring to achieve bacteria tightness. Required compression depends, for example, on the shape of the seal and hardness of the rubber material.
• Groove-fill and expansion room for polymeric seals. Calculation of a suitable groove for a certain seal would be easy if there wasn’t the effect of thermal expansion and volume swell. Rubber materials can have thermal expansion coefficients of up to 350 x 10^-6/K, which is more than 20 times higher than the coefficient of stainless steel. Volume swell is also often an issue, since e.g. rubber materials are usually of either polar or non-polar structure and they are comparable with a high-viscosity fluid, they are supposed to swell when immersed in fluids of the same character. Swelling is a physical and reversible action. For example, a polar Fluorocarbon material (FKM) takes in a certain amount of water or water-based cleaning fluids. If the amount of thermal expansion and the to be expected volume swell are not considered by the groove design, damage of the seal up to the extrusion of seal material out of the groove can be observed.
• Surface roughness is also a key to a hygienic sealing joint. If the surface of the seal groove is too rough, the valleys of the surface roughness can harbour microorganisms and spores or can even lead to gas or fluid leakage. Also the seal itself should have a good surface quality. A value of Ra 0,8µm for both seal and groove surface usually allows for a good hygienic sealing. For rubber gaskets, it’s also important to have no offset between the mould-halves and all flash removed on the finished part. The best hygienic surface is offered by an unharmed, closed ‘mould skin’.

Hygienic solutions for static seals

The term ‘static seal’ is used when the two parts joined do not move in relation to each other, e.g. on pipe couplings or valve halves joined together. For such applications, the O-ring with a round cross section (ISO 3601-1 and American standard AS 568 define dimensions and tolerances) is the most widely used sealing element. One standardised solution is the DIN 11864 coupling (Figure 4) that was developed by the use of Finite Element Analysis (FEA), a numerical method using a lattice structure of the sealing element that uses mechanical data like the coefficient of thermal expansion or stress-strain curves established on test sheets of the material. By FEA it is possible to simulate the behaviour of a seal during installation, under pressure and swell or thermal cycling. This DIN 11864 design considers the material properties of typically used rubber materials like e.g. FKM, EPDM or VMQ and allows for thermal expansion without destruction of the O-ring. Also, the seal element is safely kept in place to avoid a pumping effect that could transport microorganisms from the environment to the product area. The highest contact pressure is directly at the break-off point on the product side. Similar solutions covering the same principles have also been developed by a number of companies. There are a range of other pipe couplings and process connections. In the US, couplings according to ISO 2852 / DIN 32676 are the most widely used hygienic connections. EHEDG established a position paper of EHEDG-approved couplings and process connections. The EHEDG guideline No. 16 also provides a great deal of insight into static sealing¹.

Piston seals and rod seals

Piston seals and rod seals fall under the category of ‘linear dynamic seals’. They are used to seal, for example, a plunger piston in a homogeniser or a piston in a dosing unit for viscous products in a filling machine. Rod seals are necessary to seal e.g. the rods of dosing pistons or valve stems against the atmospheric drive side. Rod seals are usually mounted in the housing whereas piston seals are mounted on the moving piston.

Seals shall form a barrier between either the product or the hygienic zone against the atmosphere. Such seals are available in hygienic design. See Figures 5 and 6 for examples for rod seals. A piston seal would look similar, just inside out with the dynamic sealing lip on the outside. For aseptic sealing, which requires hygienic design plus being impermeable to microorganisms, a suitable double seal arrangement or a hermetic sealing with bellows or diaphragms is required to avoid microorganisms and spores being drawn from the environment into the product area.

From a hygienic point of view, it’s easier to design a hygienic rod seal than a hygienic piston seal, because for rod seal arrangements, split groove designs and clamping devices can be located outside of the product area. An optimum in hygienic design for a piston can be achieved by e.g. a metal-rubber-plastic bonded part with neither crevices nor hollow spaces and bearing area incorporated (see Figure 7). Figure 8 shows a standard hydraulic piston with countless crevices in comparison.

Shaft seals

Shaft seals are sealing rotary movements. The majority of shaft seals in food equipment are probably mechanical meals, which seal axially by two hard carbon or ceramic rings, one of them being axially spring loaded. Mechanical seals are an own class of sealing systems and are not covered by this article. Centrifugal pumps usually use such sealing systems, because they’ve got a long wear life potential and can cope with high sliding speeds.

Radial oil seals, which are commonly used in combustion engines and gearboxes to seal the rotary shafts usually cannot be used in product contact because the garter spring that keeps the sealing lip in contact with the shaft is difficult to clean and does not meet hygienic design principles.

Shaft sealing with rubber or plastic seals is a difficult task in food equipment where the product is often not lubricating or even abrasive, containing fruit pips, fibers or crystallising sugar. Also, the sealing lip is always running at the same place. This can cause problems with friction heat but also wear marks on the shaft, which is usually made of stainless steel. So, a matching system of seal material and shaft hardening or coating needs to be engineered and its suitability validated by testing.

When it comes to hygienic applications, two different types of seals are used. For low and atmospheric pressures and higher sliding speeds of up to several meters/second, thin rubber or plastic (often PTFE-compounds) without any energising elements are used (Figure 9). Such seals create – when properly designed – low contact forces and low heat generation between the sealing lip and the shaft. However, they cannot cope with higher pressures. For many designs, 0.2 MPa is already the upper limit. To be well cleanable, a front-flush design, as shown in Figure 9, is preferable.

The other type of seal has a sealing lip that is pressed to the shaft surface by means of a preloaded rubber element, e.g. O-ring or moulded part (Figure 10). Metal springs are also common, but they are not suitable for hygienic applications if the spring is inserted into the seal groove on the product side. Special solutions are available on the market, where the springs are inserted from the back side. A metal spring offers the advantage that temperature limits, chemical and physical compatibility with product and cleaning fluids, flavour transfer and the like are only dependent on the plastic material and the rubber material is out of the equation. Preloaded thermoplastic seals can cope with higher pressures up to several dozen MPa, depending on design and speed. The seal manufacturer often gives MPa x m/s (PV) limits. The higher the speed, the lower the acceptable pressure.

There are a lot of other seal challenges in hygienic design like ‘mechanical force seals’ in valve plugs, butterfly valve seals, personal access port gaskets and others. But covering all aspects of hygienic design sealing is simply impossible because of the amount of pages required. For readers who are interested further in this subject, EHEDG guideline number 16 or the upcoming new seal guideline will offer a great deal more information.

Reference

1. http://www.ehedg.org/uploads/EHEDG_pp_connections.pdf

About the author

Ferdinand Schwabe works as a Consultant primarily for equipment manufactuerers food food equipment. His roles include carrying out third party inspections for equipment certification according to 3-A Sanitary Standards and evaluation of the hygienic design of equipment. Ferdinand also runs internal training courses covering preparation for 3-A TPV inspection; hygienic design using EHEDG training material; and sealing technology. Ferdinand has more than 30 years’ experience within the food equipment and seals industry and has various professional qualifications, including Technician in Mechanical Engineering; Certified QMB and Internal Auditor for Quality Management Systems; Certified 3-A Conformance Evaluator; and Authorised EHEDG Trainer for Hygienic Design Courses.