February 22nd, 2021
Elastomeric seals are essential for a wide range of dynamic and static sealing applications, but even industrial-grade, standard polymers are prone to failure. This typically occurs as the natural result of abrasion, chemical attack, or incremental expansion/contraction due to extreme operating temperatures. Polymer spring-energized seals are a reliable solution for demanding operating conditions of this nature.
Basic Design Elements of Spring-Energized Seals
Spring-energized seals are essentially lip seals, or U-cups, comprised of high-performance elastomers—typically polytetrafluoroethylene (PTFE). This highly-durable polymeric jacket is reinforced with a corrosion-resistant metal spring energizer that supplies a constant load to the lips, creating a reliable gas-tight barrier against all contacting sealing interfaces.
Spring-energized seals are subsequently an effective combination of elastomeric and metallic sealing concepts. Here we will outline some common seal materials suitable for spring-energized configurations:
Although a choice of fluoroplastic compounds and engineered plastics can be used for spring jackets, PTFE is widely considered the gold-standard owing to its exceptional durability and high tolerance to extrusion. Virgin polyetheretherketone (PEEK) is sometimes recommended for back-up rings and high-temperature applications, while formulated ultra-high-molecular-weight polyethylene (UHMW-PE) is valuable for dynamic sealing applications with high wearing forces.
However, the exceptional variability of PTFE-based compounds means there is usually a filled, lubricated, modified, or proprietary solution suitable for all working conditions.
Spring energizers vary depending on the chemical properties of the fluid media they are likely to encounter. Stainless steels offer good all-around performance with near-universal chemical resistance, but diagnosing the right grade for the job will have a direct impact on the lifespan of the spring-energized seal. Type 301 stainless is among the most common alloys for spring energizers, but the likes of Inconel® and Hastelloy® are also applicable.
How Spring-Energized Seals Work
When a spring-energized seal is installed into a cavity it is subjected to radial compression. This spring load energizes the lips, forcing them against the gland walls to create an extremely resilient leak-tight barrier. The inherent radial tension maintains a highly reliable seal from installation onwards, meaning no system pressure is required for decent sealing efficiency. Conversely, when pressure is introduced it further energizes the jacket and increases seal force. This translates to excellent low- and high-pressure sealing capabilities.
Additionally, spring-energizers help to combat seal extrusion at high temperature/pressure. This is a dynamic form of failure observable as nibbling or shaving of seal materials through cycled/sustained periods of stress.
However, it is important to note that additional factors play a role in seal extrusion, notably the size of the extrusion gap and the surface speed. Specialist spring-energized seal configurations such as rectangular and triangular back-up rings offer reliable performance in applications associated with complex extrusion phenomena. If you would like to learn more about the design parameters of spring-energized seals, refer to our OmniSeal® handbook.
Where Spring-Energized Seals Are Valuable
Spring-energized seals are most valuable where leakage is critical and in extreme environments such as cryogenic. This is proven in space launch vehicle applications for fuel tank, valves and thrusters as well as aviation in high temperature jet engines. As the seals are very versatile, they have been used in many different industries, e.g., life science (robotics, liquid chromatography, in vitro diagnostics, and microsurgical tools), energy (onshore/offshore ball valves and LNG loading arms) and industrial (valves and pumps).
If you have any questions about designing and implementing spring-energized seals in your application, why not ask our experts?