Reduce Downtime and Costly Seal Replacements: Seal Failure Diagnosis Part 1
There are many situations where an O-Ring may not last as long as one thinks that it should. When the expectation is realistic and yet the seal fails earlier than expected, the Applications Engineering team is often asked to help discover the failure mode(s).
Seal failures are often due to a combination of failure modes, making root cause difficult to uncover. When we begin a failure analysis, we will ask for: hardware information, how the seal is installed, application conditions (temp, fluids, and pressure exposure), and how long into the service that the seal failed. These details help bring the overall application into focus and enable us to quickly diagnose and resolve seal failures. In part one of our seal failure blog series, we will discuss the compression set, extrusion, and spiral failure.
Compression set
- Compression set is likely the most common failure mode for elastomer seals. Compression can be defined, or rather quantified, by the seals' ability to return to its original shape after compression is removed. The zero-percent compression set indicates that no relaxation (permanent deformation) has occurred, while a 100% compression set indicates that total relaxation (seal no longer applies a force on the mating surface). When investigating material options, note that the lower the % compression set for a given compound, the more resilient the material is. However, it is extremely important to ensure you are making an equal comparison in terms of time and temperature for the test conditions.
- There are many potential causes for the compression set. Poor material properties, improper glands, fluid incompatibility, or temperature exposures above the recommended range for the material.
- For more information on the compression set including animation to further demonstrate, visit our O-Ring eHandbook online.
Extrusion and nibbling
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The driving force (pun intended) for this failure mode is the pressure load that the seal is exposed to. Extrusion most often occurs when a seal material deforms into the space between the bore and the outside of the tube (commonly referred to as the extrusion gap or “E-gap”). An approximation for the pressure rating for a seal can be determined by evaluating figure 3-2 of our O-Ring handbook. The X-axis shows the size of the clearance gap (total gap, or diametral gap), and the Y-axis is the pressure load. The curves on the chart correspond to the hardness of the rubber. Extrusion can also occur due to gland overfill when the deformation from compression of the seal fills the entire groove and lips over into the extrusion gap.
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Face seals do not usually have an extrusion gap, so this orientation can achieve much higher pressure loads than a radial seal. Without a gap for the seal to extrude into, the risk of significant extrusion is highly diminished.
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Extrusion in radial seals can be combated by reducing the clearance gap or by adding a backup ring.
Spiral failure
- Spiral failure can be more simply described as the O-Ring rolling in the groove. This failure more is most common in dynamic reciprocating O-Ring applications. However, spiral failure can also occur during installation. An image of spiral failure is unique, and relatively easy to diagnose, but the root cause of spiral failure can sometimes be difficult to pinpoint. Uneven surface finish, poor lubrication, side loading, eccentricity, or perhaps stroke speed can all contribute to spiral failure.