When designing a new system, or even specifying replacement parts, engineers often turn to datasheets full of tables and other useful information. While this is a proper starting point, materials rarely behave in the real world as they do in a highly controlled laboratory environment. Elastomers, which are susceptible to temperature and ambient environmental conditions in ways that aren’t always explicitly spelled out, exemplify this phenomenon.

To help understand some of these subtleties, this white paper discusses five myths of elastomer material datasheets, based on this presentation by Chris Allan, oil and gas product manager at Precision Polymer Engineering. The overarching theme is that proper elastomer sealing with O-rings and other seals isn’t simply a matter of specifying physical dimensions. Engineers must also dive deeper to overcome any assumptions, and ensure that the proper materials are specified for an application.

Myth No. 1: Physical properties

Along with physical dimensions, one needs to choose a material with the proper strength and hardness to prevent extrusion. High-pressure applications need a material with high strength and hardness to withstand the forces experienced. This, however, is only part of the story, as the operational temperature and conditions, as well as the material’s long-term reaction to pressure over time — compression set — must be considered as well.

Figure 1. Source: Precision Polymer EngineeringFigure 1. Source: Precision Polymer Engineering

An example elastomer, shown in Figure 1, exhibits a dramatically different tensile strength versus elongation when measured at both 125° C and 22º C. Tensile strength variation over time is also greatly affected by temperature, with higher temperatures equating to faster degradation, illustrated in the middle graph.
The force exerted by this material as it is deformed is also affected by the rate of deformation, with higher stretching speeds corresponding to higher forces in a non-linear curve, as illustrated in the bottom graph of Figure 1.

Figure 2. Source: Precision Polymer EngineeringFigure 2. Source: Precision Polymer EngineeringCompression set is the permanent deformation remaining when a force applied to a material is removed, and will differ depending on the time this force is applied. As illustrated in Figure 2, this too can be non-linear. One example material initially experiences a high compression set but stabilizes over time, while the second is slow at first, but eventually overtakes the first material. Because of these complex interactions as well as material non-linearity, predictive mathematical models for safety and performance factors may not exist, and choices must be based on experimentation and expertise.

Myth No. 2: High temperatures

High temperatures affect tensile strength variation over time, with increased temperatures equating to faster tensile strength breakdown. Seal failure is generally due to chemical breakdown, and for every 10° increase in temperature, the chemical reaction rate doubles, cutting the life of a seal in half. A seal’s maximum temperature rating should be used as a guide, not a hard-and-fast number that guarantees it will survive
forever if it operates just below that point. Conversely, the seal will not immediately fail if the maximum value is exceeded for a short amount of time, though there is no guarantee as to how long it will endure without damage. This concept is analogous to driving a car. A driver could push the engine to the red line, but shouldn’t drive it like that if he or she wants it to run well in the future.

Another consideration when working with seals in high temperatures is their inherent coefficient of thermal expansion. FKM and FFKM elastomers, for example, exhibit vastly different properties in this respect.

Myth No. 3: Low temperature

Like the high-temperature seal limit, the low-temperature ratings for seals should be used as a guide. Seals are not guaranteed to work down to this level in all applications, nor will they immediately fail once they get to a low temperature.

Thermal expansion properties — or contraction in this case — must be taken into account in the opposite way that they are in high-temperature applications. As elastomer seals contract, the force they exhibit on what they are sealing also decreases.

At a certain low temperature, elastomeric materials will experience a glass transformation where they become brittle. While this may be detrimental to sealing, the transition doesn’t happen all at once, similar to how a frozen body of water can be a mixture of liquid with ice scattered throughout the surface.

Also like water, this transition is reversible, and seals may be used below the glass transition point within reason. Often, the suitability of a material for low-temperature applications will depend on the judgment and experience of the engineers specifying certain seals.

Myth No. 4: Industry approvals

Industry approvals could be interpreted as ensuring that a certain material will 100% meet specifications, but like high and low-temperature limits, approvals should be treated as a guide, or perhaps as a minimum criteria for use consideration in an application. There are several different testing standards, and an engineer must decide which one best indicates that a material would perform well in an application.

Figure 3. Source: Precision Polymer EngineeringFigure 3. Source: Precision Polymer Engineering

Choosing the proper industry approvals needed for an application includes understanding how each test is performed, along with any variations that a supplier might use to represent a material as meeting a certain specification. For example, the NORSOK M710 Rev. 2 Annex B standard, summarized in Figure 3, is often used for rapid gas decompression (RGD) in the oil and gas industry. The standard specifies that tests may be done at different temperatures and pressures, and the provider would likely use the lowest temperature (100° C) along with the lowest acceptable pressure (15 Mpa). While understandable, engineers should ensure that the actual test conditions are applicable to their use case. Other conditions used here can include the type of gas mixture and compression percentage.

Figure 4. Source: Precision Polymer EngineeringFigure 4. Source: Precision Polymer Engineering

Heat aging, also known as sour aging or sour gas testing, is another industry approval that must be carefully considered. Gas, oil and water phase composition, as well as the elements that make up each phase, can vary greatly depending on the standards used, as well as pressure and test duration. Temperature is also a huge variable in this type of testing, and the common NORSOK M710 Rev. 2 Annex A standard leaves it up to the supplier to specify three temperature values for the test. It is therefore important not only to understand the test itself, but understand what to look for to produce an applicable comparison when specifying seals.

Myth No. 5: Chemical resistance

Datasheets provide little-to-no information about whether a seal will withstand a particular medium. If they do, it is generally based on an ASTM automotive requirements that will likely not be applicable. Even the concept of chemical resistance can have multiple meanings. For instance, a seal may swell in response to exposure — which can even enhance sealing in some cases — or it may instead start to break down and lose its elastomeric properties in response to chemicals attacking the material.

Figure 5. Source: Precision Polymer EngineeringFigure 5. Source: Precision Polymer Engineering

As an example of what properties are necessary to fully specify a seal, consider one used in a subsea Christmas tree. The seal might be an S-seal with a design temperature of -18° C to 177° C, maximum pressure of 689 bar, be compatible within produced fluids, control fluids, corrosion inhibitors and sea water, with 14% compression and a life of 25 years. This seal must then meet the NORSOK M710 Rev. 2 Annex A and Annex B, API 6A – F.1.13.5.2 Immersion Fluid FF/HH, and API 6A – F.1.11.5.2 PR2 testing requirements.

While each material chosen — including NBR, HNBR, FEPM, FKM and FFKM — will meet one or more approval standards, only FKM will pass all the required standards, as shown in Figure 5.

Summary

In this example, and in many real-life situations, knowing which standards must be applied to a particular seal setup is a matter of experience and careful process evaluation. Use datasheets as a tool to aid in narrowing down material options and select potential candidates, and ask questions of the supplier about what would be appropriate for your situation.

Finally, be sure to understand the methods used when testing seal materials to help choose the perfect material for an application. And of course, consult a knowledgeable supplier invested in customer success, such as Precision Polymer Engineering.