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.
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.
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.
Extrusion in radial seals can be combated by reducing the clearance gap or by adding a backup ring.
Parker provides numerous resources to support the diagnosis of seal failures and the best sealing solutions. Check out our latest tech webinar on seal failures modes or utilize our leak troubleshooting app. Also, be sure to keep an eye out for part 2 of our seal failure mode blog series, where I will discuss the rapid gas decompression, abrasion, installation damage, and fluid incompatibility failure modes!
This article was contributed by William Pomeroy, applications engineer, Parker O-Ring & Engineered Seals Division.