Greene Tweed Discusses New SAF Testing with Fluorine-Containing Elastomeric Seals
Experts at Greene Tweed discussed their newest research on sustainable aviation fuel (SAF) in a webinar, exploring the potential for creating a more sustainable option with lower carbon emissions.
According to Andy Clark of Greene Tweed, “SAF is currently required to be blended in a 50/50 ratio with a control fluid, which is normally conventional jet fuel. And this is because that control fuel, the jet fuel, contains aromatics and therefore provides the target aromatic fuel content for drop in.”
Clark noted that Greene Tweed tested methods for achieving a more sustainable SAF that requires less conventional jet fuel to be mixed in, further reducing greenhouse gas emissions.
Why is SAF research important?
Clark explained the importance of pursuing new SAF applications that improve the sustainability of the solution.
He said, “A number of 2050, decarbonization goals exist, including Fly Net Zero and the Destination 2050 meeting these different 2050 goals will require fuels that have a lower carbon footprint.”
Elaborating on the benefits of SAF and its potential to accelerate progress toward these goals, Clark shared, “SAF and SAC (synthetic aromatic kerosene) are capable of delivering greenhouse gas reductions of over 65% compared to conventional aviation fuel, and some organizations would consider this figure to be conservative. I've seen values of over 80% quoted elsewhere in terms of the amount of greenhouse gas reduction that's possible as a result of these fluids.”
He continued, “There's a much shorter lead time to adoption for this type of fluid and the potential for a rapid reduction in greenhouse gas emissions.”
“And this is because there's no potential need for new aircraft platforms or technologies as opposed to other energy systems, such as battery and fuel cell. There are no anticipated changes to distribution infrastructure or to the airport fueling infrastructure either,” Clark added.
Clark then showcased how this concept informs Greene Tweed’s research, saying, “As part of this roll out, Greene Tweed is seeking to validate that no changes to aircraft equipment or operation are required, namely, that our traditional FKM seal materials and FBM Q seal materials remain suitable for use in the sustainable aviation fuel and SAC lens.”
How can synthetic aromatic kerosene (SAK) improve SAF sustainability?
Clark offered more detail on the need to mix aromatics into SAF, noting, “Aromatics are necessary only to ensure the swelling of nitrile O-rings and seals that have previously been exposed to fuel that had a high aromatic content. Seals that have not previously been exposed to fuel do not require aromatics for acceptable performance.”
Clark continued, “And this, in turn, is driving the need to mix with conventional jet fuels and is potentially limiting the amount of greenhouse gas reduction. But rather than blending with a conventional petroleum-based hydrocarbon, there is the potential to instead blend with a synthetic aromatic kerosene instead.”
Introducing the solution explored in Greene Tweed’s research, Clark said, “Virent is producing this synthetic aromatic kerosene via a bio forming process where they convert plant feed stocks into renewable jet range hydrocarbons. The resulting product is a mixture of clean burning aromatics that would otherwise be missing from other alternative jet fuels. The resulting SAK can be combined with SPK types of sustainable aviation fuel to enable a 100% jet fuel that meets all of the ASTM.D7.566 requirements.”
Clark added, “In addition to providing the aromatic content to meet the performance for legacy nitrile seals…this also helps to provide an option that meets the minimum density specification for ASTM D7566 while still providing a good overall energy content.”
What tests did Greene Tweed perform?
On the Greene Tweed materials used in this testing, Clark said, “Greene Tweed has tested our portfolio of aerospace FKM materials…731 compound, 772 compound and 665 compound.”
Clark outlined the testing procedures, stating, “Five sets of data were gathered, the first two of which are the baseline material properties and heat, aging and air, our control data, which was used for comparative purposes for later testing. The third set of tests was fluid immersion testing. For this, material specimens were immersed for 168 hours at 125 degrees Celsius. The materials were immersed in 100% blends and also 50/50 ratio blends.”
On the fluids used in immersion testing, Clark said, “The first is an FT-SPK per Annex A one of ASTM D7566, it's a synthetic paraffinic kerosene provided by Sasol. The second fluid is a HEFA-SPK per annex A2, a sustainable aviation fuel supplied by World Energy paramount. And the third is an ATJ-SPK per annex S5 this is the Gevo jet blend stock.”
“For the 100% concentration blends, in addition to the three sustainable aviation fuels…we were also testing with a control fuel which, in this instance, is a Jet A-1 reference fuel consisting of 15% toluene and 85% isooctane by volume. This control jet fuel was then used in the 50/50 blends, where it was blended in a one-to-one ratio with the three different sustainable aviation fuels,” Clark added.
Becky Petry of Greene Tweed then explained that one of the variables measured in testing was material change in each substance.
When exploring the material changes in properties observed during testing, Clark clarified that they measured several material properties, including compression set, tensile strength and elongation of break modulus at 100% elongation.
On why they tested hardness change, Petry said, “Hardness is a good indicator of what a fluid does to a elastomeric seal. If it gets very, very soft, swells a lot, etc. you can lose the resistance to pressure on a sealing application.”
Petry also explained the testing phase for the blends containing Virent’s SAK instead of conventional jet fuel.
Petry shared, “For phase one, we tested in 100% of the control fluid, 100% of the different SPK fluids and then the recommended 50/50 blend for each of those. For phase two, we still have the same control data from the first round of testing with the jet fuel. And then we also tested samples in 100% SAK immersion, as well as the recommended 80/20 blend.”
What results did Greene Tweed find?
Petry shared the overall conclusions of the experiments, stating, “Based on the data presented, we feel that the three FKMs—the 731, 772 and 665—had very good and consistent properties with the SPKs and blends and the SAK and blends. And we would recommend that, based on the properties presented, these could be used with those three SAF fuels that are listed and with the SAK.”
Petry continued, “In the straight fuel, we're seeing a change in hardness that is to a lesser degree in the SPK fluids than it was in the control jet fuel. So, there is a lower degree of hardness change there. When we look to the right at the 50/50 blends…we still see a reduction in hardness, although it is not as significant as the change was in the straight fuel.”
In terms of volume change, Petry said, “With the straight fuels, therefore the three FKM types…the volume change was lower in the SPKs than it was in the control fluid. They are fairly consistent across the different fluids, and they are all lower. When we move to the 50/50 blends on the right, we see similar behavior in a reduction of the change in volume.”
She also discussed the fluid-switching tests, adding, “This is to simulate filling the aircraft with traditional jet fuel, and then it is switched over to one of the SPK blends without changing the seals out, what would happen…The takeaway from this is that there is variability in the 40409 compound, but the FKMs do look fairly consistent.”
When comparing the results of the conventional SAF tests to those where SAK was added, Petry explained, “There is just a very slightly lower hardness change in the pure SAK and the blended SAKs, than there is in the jet fuel. So, there is a minor effect in the hardness change.”
Petry noted, “This was the first set of tests that Greene Tweed performed with our products in SAF fluids. It was meant to be the beginning of a longer series of tests, just to kind of see where we stand with our materials and these fluids as a starting point for ongoing evaluation.”
Petry also shared a few additional reasons why they deemed future testing necessary, including batch-to-batch variation in materials, fluid lot-to-lot variation and the need for larger sample sizes.
Andy Clark supported this, saying, “We are recommending that certainly further testing is needed in order to build up a larger picture. I am also mindful that the content of these sustainable aviation fuels can change significantly depending on the manufacturing approach and the original source.”
