X-ray Microscopic Examination of Filter Media

X-Ray Microscopic Imaging of Filter Media

Filter media is the key component for all types of particulate retention – removing contaminant substances or selectively extracting one or more components of a mixture – in both air and liquid handling applications. Car engines utilize several filters to protect the sensitive drive components of the vehicle, including those within the oil delivery system. Over time, filters lose efficiency due to selective fouling or saturation that ultimately leads to replacement.

Using the SkyScan 1273 desktop micro-CT, we examined and compared a new off-the-shelf filter with a filter used for about 5,000 miles from a high-mileage personal vehicle. In examining the used filter, analysis from the data located nearly 2,000 individual particles of elevated density trapped within the paper filtration layers. While micro-CT cannot identify the elemental composition of the particles, we observe that the particles are all higher in density than the surrounding paper and oil matrix. If needed, the filter could be destroyed to extract out the particles for downstream analysis as part of a failure investigation using a more traditional elemental analysis technique.

As shown in Figure 2, both filters were well resolved by the SkyScan 1273 at the 35um voxel size and individual components are present. In both cases, the rubber seals at the top and bottom of the filters along with the glue used to adhere the rubber seals and to join the loop of pleated paper filtration media are represented with similar grayscale brightness. Between the two samples, the paper filtration media is barely visible in the new filter images due to the much lower density of paper compared to the rubber and glue components. With the used filter, the paper structure is more clearly visualized because of the wetting by the oil and the presence of the dense particles, which both boost the attenuation of X-rays through the paper.

If you look closely at the top left image of the used filter, a particle is observed as a bright white spot between the paper and glue layer at the top of the filter. The relative brightness of the particle (near pure white) within the image indicates that the density of this particle is higher than all other components within the filter. This relative density information suggests that the particles that were removed and trapped by the filter may be metallic in composition.

In inspecting the new filter, none of the high density particles were present within the paper filtration media, suggesting that the presence of the particles within the used filter was the result of accumulation during filtration of engine oil rather than arising during the manufacturing process of the filter itself.

Using CTVox, we digitally inspected and dissected the sample to better locate the position of the particles within the filter (Figure 3). It appears that the particles are entrapped on the outer surface of the paper filtration media facing the outside of the cylinder. Knowing that engine oil is pumped from the outside of the filter, through the paper, and then back to the engine through the center channel, the presence of the high-density particles on the outside of the filter demonstrates the efficacy of the filter for particles of this size. If the particles were present within the center of the filter channel, this would indicate the filter was only partially restricting the access of these particles from recycling through the engine.

After selectively segmenting the dense particles from the other components of the filter, CTAnalyzer allows us to quantitatively assess the number, size, and location of the particles within the filter (Figure 4). CTVox once again allows us to display either a view of just the analyzed particles like this figure or an overlay of the particles on top of the data from the filter, as seen in Figure 1 in this article.

CTAn provides us a quantitative inspection of the particles removed by the filter, including generating a histogram of particle sizes against the total particle volume (Figure 5). From this analysis, we determined that the average particle size trapped in this filter to be 197.83 ± 146.04 µm with the particles comprising a total volume of about 6 mm³.

Conclusion

The SkyScan 1273 allowed us to inspect both a new engine oil filter and a filter that was heavily used in a high mileage vehicle. Once dense contaminants were observed, suspected to be metallic in nature, CTAn provided us with a quantitative assessment of the particles within the used filter.

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Scan Specifications

These scans were completed on our high-power desktop SkyScan 1273 micro-CT system at the Micro Photonics Imaging Laboratory in Allentown, PA. Reconstructions were completed using NRecon 2.0 while visualization and volumetric inspection of the 2D and 3D results were completed using DataViewer and CTVox.

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