Best Materials for Spring Energized Seals | Advanced EMC Technologies
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Seal Gland Design for Spring-Energized PTFE Seals

Seal gland design is an important aspect of effective spring-energized PTFE seals and is also the source of most leaks. Such seals can only perform as well as the seal gland supporting them, and even a small geometry error can lead to issues with extrusion, rapid wear, low contact stress, and spiral leakage.

This blog post reviews what makes PTFE spring-energized seals different from other sealing solutions, and then discusses key aspects of gland design, including the anatomy of a basic spring-energized seal and design aspects of geometry and surface finish that have the greatest impact on seal reliability. 

Anatomy of a Spring-Energized Seal

A short review of spring-energized seals is in order. 

The gland is the machined groove in the metal hardware where the seal sits. In the diagram, you can see the E-gap, or extrusion gap. This is a small clearing between the moving and stationary metal parts of the seal. This gap must be kept tight to prevent the seal from extruding or flowing into it under pressure.

The seal jacket is the U-shaped outer body, which, in PTFE spring-energized seals, is manufactured from PTFE. It provides the main sealing interface against the hardware, and the lips of the jack are the points that actually touch the mating surface.

The spring energizer is contained within the jacket, and it provides constant pressure against the jacket lips and provides pressure outward against the gland walls. This pressure, or sealing force, is maintained by the spring-energizer within the jacket and exists even when there is low to no system pressure. As system pressure increases, the jacket enters the U-cup and further augments the spring’s force and the sealing force.

Practical Seal Gland Design Geometry

PTFE is not flexible like elastomers, and that must be taken into account when specifying the gland geometry. First, proper gland width provides the seal with enough clearance to float, and the gland depth strictly controls the interference, and both are critical for providing sufficient axial and radial freedom instead of just simply squeezing the seal into place. 

When the design creates over-compression, the seal will not be able to flex as it should. This leads to increased friction, accelerated jacket wear, and a potential locked condition where the spring cannot compensate for shaft misalignment. Under-compression brings its own set of problems because it will not be able to fully utilize the spring energy. Leakage at low pressures and a delayed response during pressure spikes severely impact the performance of the seal. 

The goal with the design then becomes the achievement of a uniform load distribution that can conform to surface irregularities and maintain the integrity of the seal.

Seal Gland Design: Corner Radii and Lead-In Geometry

PTFE is softer than steel. If you try to shove this plastic seal into a metal bore that has a sharp 90° corner, that corner acts like a knife. It will slice a layer off your seal before the machine even turns on. This can lead to micro-damage that will later present itself as problematic leaks.

Sharp edges are one of the main causes of damage during PTFE seal installation, and this differs significantly from elastomeric seals. PTFE is stiffer, which makes it more likely to suffer a cut from insertion into a metal bore at a 90° angle. The solution is a shallow lead-in chamfer or on-ramp that is around 15° to 20°, and longer than the seal itself. The length is necessary to ensure the seal is fully compressed before it enters the main bore. In addition, the ramp corners must be rounded, and no sharp edges are allowed.

Seal Gland Design: Surface Finish and Hardware Interaction

Because these seals are constantly rubbing against metal, the surface finish of the metal is of utmost importance. If the surface is too rough, it will abrade the seal and lead directly to leakage. On the other hand, if the surface is too smooth (e.g., mirror polish), it will not be able to hold, and the seal will wipe the sealing surface dry. In addition, because PTFE is self-lubricating and forms a transfer film on the sealing surface, the surface cannot be so smooth that the transfer film does not stick. 2 RMS to 16 RMS, depending on what your application is. 

For gases and liquids at cryogenic temperatures, you want a smoother finish with an RMS between 2 and 4. For gases at non-cryogenic temperatures, the recommended surface finish is between 6 and 12 RMS. For liquids, a surface finish between 8 and 16 RMS is sufficient.

In addition, it is important to specify a plunge-ground finish (0° lead) to prevent spiral tool marks from acting as a screw pump that leaks fluid regardless of seal tightness. 

Seal Gland Design: Squeeze, Clearance, and Extrusion Gap Control

Under extremely high pressures, the behavior of PTFE is significantly different from that of elastomers. At extremely high pressures, PTFE is more likely to flow like a liquid to an area of lower pressure. There will always be a small gap between your piston and cylinder wall to prevent them from grinding against each other. This small gap is known as the extrusion gap, or E-gap. If the gap is too large, the pressure will push the heel of the seal into the gap, shredding the seal. This is where careful specifications come in: when specifying the diameter, include a small enough tolerance to ensure that the gap remains small (e.g., smaller than a human hair) even when the parts move.

Conclusion

Seal gland design for PTFE spring-energized seals is a little different from elastomeric seals and involves more focus on aspects such as geometry, lead-in geometry, surface finish, and extrusion gap control. 

If you need help with a PTFE spring-energized seal, our engineers at Advanced EMC are here and ready to help. Contact us today!

Best Materials for Spring Energized Seals | Advanced EMC Technologies
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Why Spring-Energized PTFE Seals Work in So Many Applications

Spring-energized PTFE seals work incredibly well. Engineers know that when designing equipment for truly hostile operational environments, whether facing cryogenic cold, aggressive media, or extreme pressure cycling, the reliability of the seal is absolutely critical. Traditional seals frequently fail under such harsh conditions, suffering from issues such as chemical attack, thermal degradation, or permanent deformation. The proven industrial solution for conquering these critical limitations lies in the PTFE spring-energized seal.

In this blog post, we discuss exactly why PTFE spring-energized seals work so well, including the use of PTFE and spring-energizers, along with a discussion of where these seals are used.

Where Spring-Energized Seals Conquer Extreme Conditions

Before discussing why these seals work so well, it would be wise to quickly review the many different industries and applications in which they have proven themselves. PTFE spring-energized seals have become indispensable across almost every challenging industrial sector because of their unique performance characteristics. Advanced EMC provides solutions for industries including Oil & Gas/Energy, Medical Devices, Aerospace & Defense, Automotive, and Food, Dairy, and Pharmaceutical.

In Oil & Gas and Energy, seals must handle extremely high pressures, sometimes up to 25 kpsi. They must also maintain integrity when exposed to chemically aggressive media such as sour gas and acid gases. In these applications, the PTFE jacket resists the chemicals while the energized springs help keep the seal in place.

For Aerospace & Defense, especially in cryogenic space applications, seals must survive conditions where temperatures drop below -250°C. Traditional elastomeric seals often shrink and lose sealing force at these cryogenic temperatures. PTFE spring-energized seals compensate for thermal contraction and are suitable for applications such as cryogenic fuel transfer and rocket engines. Virgin PTFE and high-purity filled PTFE variants also exhibit minimal outgassing, meeting NASA/ESA standards for use in a vacuum.

In the Medical Device and Pharmaceutical industries, reliability and purity are paramount. Virgin PTFE is both FDA- and USDA-approved. These seals are crucial in fluid management systems, surgical instruments, and diagnostic equipment. They withstand aggressive sterilization processes, including high-temperature Clean-In-Place (CIP) and Sanitize-In-Place (SIP) procedures, due to PTFE’s high thermal stability and chemical inertness. PTFE is also hydrophobic, repelling water and making it easier to clean complex geometries.

The Material Foundation: Why PTFE?

Polytetrafluoroethylene (PTFE), also know as Teflon®, is a synthetic fluoropolymer of tetrafluoroethylene. This material forms the resilient seal jacket, providing excellent chemical and thermal performance. Pure PTFE is almost completely chemically inert, meaning it resists attack from aggressive media such as acids, solvents, and reactive gases, minimizing the risk of degradation or swelling. It is also hydrophobic and non-wetting.

In addition, PTFE boasts an operating temperature range: it is thermally stable enough for continuous service up to 500°F, and certain specialized compounds can function reliably in cryogenic conditions as low as -450°F. PTFE also has the lowest coefficient of friction of any known solid and is naturally self-lubricating, which means it performs well in dry or non-lubricated applications. All of these are reasons to choose PTFE as the seal jacket in a spring-energized seal.

PTFE Spring-Energized Seals Work So Well

The core principle of a spring-energized seal lies in its architecture: a polymer seal jacket housing an internal metallic spring energizer. The spring’s primary job is to apply a continuous force against the sealing surface. This initial force ensures a consistent seal is maintained, especially at low system pressures.

The energizer’s constant force provides permanent resilience to the seal jacket, compensating for operational issues that would cause conventional seals to fail, including:

  • Jacket wear
  • Hardware misalignment and eccentricity
  • Dimensional changes due to thermal contraction or expansion
  • Misalignment

When system pressure increases, the pressure begins to supplement the spring force. This drives the seal lip against the mating surface, resulting in a tighter, highly efficient barrier. The spring design also ensures that the seal maintains force over time, eliminating the problem of permanent deformation or compression set seen in elastomers.

Engineers can select from various spring types to suit specific needs. The V spring (V ribbon spring energized seal) is an excellent candidate for cryogenic and vacuum applications, where coil springs (spiral pitch springs) work well, where low friction and high pressure are involved, typically in medium-speed applications. The helical flat spring is adapted to a wide range of pressures, from high pressure all the way down to vacuum conditions. An elastomeric O-ring energizer can be used when the use of metal must be avoided, and is adapted well to extreme pressures.

Conclusion

Robust, reliable sealing is absolutely necessary in mission-critical operations, regardless of the industry. Spring-energized PTFE seals provide a superior engineering solution. By combining the chemical inertness, wide temperature range, and low friction of PTFE with the persistent mechanical force of a metallic spring, these seals eliminate issues like compression set and loss of sealing force over time. They flex, adapt, and hold their seal integrity even when conditions shift fast.

For applications that demand unparalleled performance where failure is not an option, PTFE spring-energized seals work extremely well as they provide the durability, adaptability, and precision required. When specifying a PTFE spring-energized seal, never take for granted the finish of the mating surface. Advanced EMC Technologies offers expert polymer sealing solutions, leveraging more than 100 years of combined experience. Contact Advanced EMC today to learn how spring-energized seals can improve the reliability and lifespan of your system.