Sealing Shielding

New Fluorocarbon Seal Material Extends Safe Life of Jet Fuel O-Rings

For decades, choosing seals for low-temperature (-65°F) aircraft fuel applications in the aerospace industry has meant selecting a material with the most tolerable failure points. Low-temperature nitrile, fluorosilicone and fluorocarbon materials all have obvious weaknesses:

New VX065-75 Fluorosilicone Seal Material Extends Safe Life of Jet Fuel O-Rings - Common jet fuel sealing materials and their weaknesses | Parker Oring Division

Sealing low-temperature fuels over the long-term shouldn’t have to mean choosing the lesser of several evils, though. Why should extended seal life come at the cost of appropriate temperature tolerance? What if there was a material with good resistance to aircraft fuels down to -65°F, durable enough to last the entire five years of a typical maintenance cycle?

There is now. Parker Engineered Materials Group’s O-Ring Division has developed VX065-75, a new extreme-low-temperature fluorocarbon (FKM) compound that solves the lifecycle/performance conundrum.

Low-temperature performance

Reliable sealing at a specified temperature is a function of mechanical design (the seal and the mating gland) and material behavior. Standard O-ring and groove—the most practical design in most applications—relies on material temperature properties to achieve acceptable performance, usually defined as the TR-10 point (temperature at which the material maintains 10% of its elasticity).

Functional sealing in static applications usually can be attained at 15°F below the TR-10 point, as shown in Table 2. The new VX065-75 compound has an extremely low TR-10 point without the wear and tear problems that are inherent to fluorosilicone as noted Table 1.

Table 2 Comparison low-temp performance of common jet fuel sealing materials

Fuel resistance

The amount of swell an elastomer exhibits in a particular fluid is usually a good indicator of chemical degradation caused by the fluid upon a seal material. (Note: elevated temperatures are often used during testing to accelerate the process, but these methods tend to skew results for low-temperature nitrile materials.)

Tables 3 and 4 compare relative fuel resistance of various elastomers at room temperature and elevated temperature, demonstrating significant advantages offered by Parker’s VX065-75 compound. In terms of fuel compatibility, VX065-75 stacks up well with the best-in-class FKM materials, even those that do not offer improved low temperature performance.

Comparison fuel resistance at room temperature

Comparison fuel resistance at 302 degress fahrenheit

 

Compression set

An O-ring in a properly designed gland fails primarily from long-term loss of resiliency: the seal permanently flattens out and the joint begins to seep. Compression set is the most common way to test this material property change (over time, the extent to which a seal assumes the shape of a groove into which it is compressed).

Table 5 shows comparable compression set values for the different high-temperature elastomers. Testing was performed at elevated temperatures to heighten the effect. VX065-75 has the best performance and has a -65°F functional sealing temperature, unlike AMS-7287 (traditional low-temperature fluorocarbon) which is only rated to

-40°F (refer to Table 1).

Table 5: Compression set resistance of high-temperature fuel-resistant materials.  Note: AMS 7276 material exhibits greater than 100% set due to thermal expansion.

Other benefits

Three other benefits of using Parker VX065-75 compound instead of fluorosilicone:

  1. Less prone to installation damage because it recovers faster from stretch; see Figure 1.
  2. Much better pressure resistance, eliminating need for backup rings to prevent extrusion
  3. Wears out much more slowly, so it can be used for intentionally dynamic applications

Various high-temperature O-rings were stretched 20%, released and photographed after 3 seconds | Parker Hannifin O-Ring division .

 

Figure 1: Various high-temperature O-rings were stretched 20%, released and photographed after 3 seconds against a shadowgraph of the original O-ring dimensions.

Conclusion

Parker fluorocarbon compound VX065-75 represents an opportunity to dramatically improve seal performance in aircraft fuel applications. It provides the low-temperature performance, compression resistance and overall durability required to dramatically extend service life in real-world applications, particularly where fluorosilicone is currently used. It also offers meaningful improvements over both low-temperature nitrile (e.g., AMS-P-5315) and traditional low-temperature fluorocarbon elastomers (e.g., AMS-R-83485,) so it’s an excellent candidate for use as a universal seal material throughout an aircraft fuel system.

If you are planning to attend the 2016 Farnborough International Air Show, please visit the Parker Hannifin Corporation booth - Hall 4, Stand E50, click here for more information about the show.

Dan Ewing, Senior Chemical Engineer, Parker Hannifin O-Ring Division

This article contributed by Dan Ewing, Senior Chemical Engineer, Parker Hannifin O-Ring Division

 

 

 

Related content:

O-Ring Squeeze - More is Not Always Better

Aerospace Technologies and Key Markets

 

 

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