Food Processing and PTFE Spring-Energized Seals
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

PTFE + Spring-Energized Seals: A Reliable Solution for Food and Pharmaceutical Processing

The primary challenge in specifying a seal is finding a solution that achieves consistent seal integrity for the operating conditions involved. However, when food or pharmaceuticals are involved, additional challenges arise — and can be met using PTFE spring-energized seals.

Design Concerns for Food and Pharmaceutical Seals

There are a number of critical design considerations involved with any type of sealing application, such as operating temperature, pressure, velocity, wear rate, friction, and chemical compatibility.

When food, dairy, or pharmaceuticals are involved, however, there are additional criteria. The first of these is finding a material that is compliant with relevant standards. In the United States, the main standard is the Food and Drug Administration standard FDA 21 CFR Part 177. This standard covers indirect food additives and thus applies to seals. For a material to be considered FDA compliant, it must be safe for human consumption and chemically inert.

Another potential challenge related to food and pharmaceuticals is MRO (Maintenance, Repair, and Operations): 3-A (Dairy and Milk) sanitary standards 18-03 for rubber materials and 20-27 for polymers, as well as NSF (National Sanitation Foundation) sanitary practice standards such as NSF/ANSI 2-2021. Such standards and practices deal with CIP (Clean-In-Place) and SIP (Sanitize-In-Place).  CIP/SIP processes often involve …

  • Extremely high temperatures, which can affect seal integrity and dimensional stability if the right material is not selected
  • Exposure to hot water and steam, which can prove problematic for materials that have a significant water absorption rate
  • Aggressive media, which can permanently compromise seal integrity if the jacket material is not compatible with the cleaning media

Wear resistance is also a critical factor: as the seal begins to wear, its particles become a potential source of contamination. This is particularly problematic when food, dairy, beverage, or pharmaceuticals are involved because those wear particles will likely be ingested. The seal material must, therefore, have a low rate of wear and be safe for occasional ingestion.

Extreme temperatures are often involved and can range from cryogenic (where elastomers and polymers may develop brittle behavior) to extreme heat (where the strength and stiffness of the seal material may be significantly reduced). Seal materials for food and pharmaceutical applications may experience both temperature extremes during regular operation, which may involve the CIP/SIP procedures discussed earlier.

Lubrication is also a major design choice for food and pharmaceutical sealing solutions. But, again, contamination must be considered and the chances are not good when it comes to finding a food-safe lubricant that is compatible with the sealing material and provides the necessary reduction in friction. A better solution would be a material with an extremely low coefficient of friction that is also self-lubricating.

Finally, any application involving food, beverages, or pharmaceuticals must have highly reliable seals. A seal failure can result in ruined products, dangerous contamination, and the potential for lawsuits.

PTFE Spring-Energized Seals

One highly reliable solution for food, dairy, and pharmaceutical applications is spring-energized PTFE seals. Spring-energized seals include an energizer that maintains seal integrity during …

  • Extreme temperatures and temperature variation, including temperatures related to CIP/SIP processes
  • Changes in pressure as well as reliable performance over a range of pressures (including vacuum conditions)
  • Seal or shaft wear
  • Shaft misalignment, eccentricity, or dimensional changes

In addition, spring-energizers add permanent resilience to the seal jacket — and an excellent option for the seal jacket is PTFE.

Virgin PTFE (also referred to as unfilled PTFE) is both FDA and USDA approved. PTFE also provides excellent high-temperature performance, experiences no water absorption, and is extremely chemically inert, all of which combine to give it the ability to maintain seal integrity during the most aggressive CIP/SIP procedures. In addition, PTFE is hydrophobic, thus repelling water and making it even easier to keep clean.

PTFE also exhibits good wear resistance, and what wear the seal jacket does experience will be compensated for by the spring energizer. And virgin PTFE provides excellent performance over a range of temperatures, from cryogenic -450°F to high temperatures up to 450°F. PTFE also has the lowest coefficient of friction of any solid at 0.1. Furthermore, it does not require lubrication because it is self-lubricating.

Conclusion

PTFE spring-energized seals are an excellent solution to the sealing challenges of food, dairy, beverage, and pharmaceutical processing. Combining the outstanding properties of FDA-approved virgin PTFE with the reliability and integrity of spring-energizers leads to high integrity and consistent sealing even in aggressive or extreme operating conditions.

If you are looking for seals related to food or drug processing, contact Advanced EMC today. Our sealing solution experts will work with you to find the right type of spring-energized seal for your application, including everything from the seal jacket geometry to the spring material and configuration.

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Seals for Space Vehicle Propulsion Systems

In addition to the growing market for commercial applications, Statista estimates that 80 countries will have their own space program by 2025. And a critical factor in the success of any space venture is the type of seals used, including those for space vehicle propulsion systems.

However, finding the optimum sealing solution that provides highly dependable performance can be formidable for several reasons, including extreme pressures and temperatures, chemical compatibility, SWaP, and outgassing, not to mention vendor requirements involving seal size manufacturing.

Extreme Pressures

Depending on the applications, the pressures faced by seals can involve not only high pressures in the range of 100 bar but ultra-high vacuum conditions as well. For example, consider a typical hypergolic rocket propulsion system where pressures can reach between 1,500 and 16,000 psi during Stage 2. To further complicate matters, pressure fluctuations make it much more challenging to retain a reliable seal. 

Extreme Temperatures

One of the challenges involved in designing seals for propulsion systems often involves wide temperature ranges from cryogenic to extreme heat. For example, the fuel tanks on many modern rockets must provide sealing at cryogenic level temperatures at which many polymeric and elastomeric materials become brittle and unreliable. And when temperatures fluctuate, traditional seal solutions can experience significant dimensional changes, leading to leaks.

Material Compatibility

Seals for vehicle propulsion systems must be compatible with the fuels involved. For example, cryogenic hyperbolic bipropellants often require seals resistant to oxidizers. Also, keep in mind that, according to a NASA Lesson Learned on Static Cryogenic Seals for Launch Vehicle Applications, seal materials must also be compatible with any anticipated purge or cleaning material that they may come into contact with.

SWaP

SWaP (Size, Weight, and Power) is a significant factor in both space vehicle engineering and aerospace in general. Designs must fall within tight constraints for weight to minimize fuel requirements and size to reduce the overall volume of the space vehicle. In addition, seals, including those for the propulsion systems, must also be optimized to meet SWaP requirements.

Outgassing

Vacuum conditions make outgassing, the release of gasses from solid materials, a serious problem. This phenomenon not only introduces new chemicals to what may already be a volatile environment but can also lead to the deposition of chemicals on critical surfaces. Therefore, NASA makes available spacecraft material outgassing data to aid in selecting safe materials.

Size

Large fuel tanks necessitate large seals. For example, some of the seals for NASA’s SLS (Space Launch System) are over 6’ in diameter, and other fuel tanks have similar requirements. Such seals are challenging to both manufacture and test. Such large seals must usually be custom manufactured with small production runs, making it difficult to balance cost and performance. In addition, not all seal vendors have the manufacturing capabilities required for creating seals of this size at the proper levels of accuracy and precision.

Spring-Energized Seals

Image from NASA

Spring-energized seals, widely used in aerospace, are a practical solution for space vehicle propulsion systems. They can retain seal integrity even when subject to extreme pressure and temperatures due to the energizer. In addition, these seals can account for misalignment, eccentricity, thermal expansion and contraction, wear issues, and other dimensional changes. And they provide a consistent force over a wide deformation range.

The most commonly used materials for spring-energized seals in space environments include PTFE, FFKM, and Hytrel. However, when outgassing is a concern, materials such as PEEK, PAI, PI, Acetal, PCTFE, and antistatic PTFE can be considered. And many of these materials are also available with fillers to enhance their properties.

There are also a variety of choices regarding the spring energizer used. For example, cryogenic and vacuum pressure environments most often implement V ribbon springs (also known as V-springs),  helical springs work exceptionally well for static cryogenic applications, and high-pressure conditions usually employ coil seals. And remember that canted coil springs can be highly customized to produce consistent forces.

Advanced EMC: Your Source for Space Vehicle Seals

When it comes to vendor concerns, Advanced EMC has the equipment and experience required to manufacture large seals accurately. Our sealing solutions team has the knowledge and understanding needed to help you at every step of selecting propulsion system seals for everything from small LEO satellites to passenger-laden space vehicles. Contact us today!