Reduce Downtime and Costly Seal Replacements: Seal Failure Diagnosis Part 2
As mentioned in part one of our seal failure blog series, O-ring and seal failures are often due to a combination of failure modes, making root cause difficult to uncover. It's important to gather hardware information, how the seal is installed, application conditions, and how long a seal was in service before starting the failure analysis process. In part 1 of our blog series, we discussed compression set, extrusion and nibbling, and spiral failure. In part 2 of our series, we will review four other common failure modes to familiarize yourself with before diagnosing a potential seal failure in your application.
Rapid Gas Decompression
Rapid gas decompression (commonly called RGD, or sometimes explosive decompression (ED)) is a failure mode that is the result of gas that has permeated into a seal that quickly exits the seal cross-section, causing damage.
Detection of this failure mode can be difficult, as the damage does not always show on the exterior. When the damage is visible, it can look like air bubbles on out the outside, or perhaps a fissure that has propagated to the surface. The damage may also be hidden under the surface. If the seal is cut for a cross-section inspection, RGD damage will look like fissures in the seal that may or may not propagate all the way to the surface.
Parker’s guidance as to how to avoid this failure mode is 1) Keep the depressurization rate lower than 200 psi per minute. If this cannot be achieved, we would suggest 2) RGD resistant materials. We offer these RGD resistant options from the HNBR, FKM, EPDM, and FFKM polymer families.
Abrasion
Abrasion damage is the result of the seal rubbing against a bore or shaft, resulting in a reduction of cross-sectional thickness due to wear. As the seal wears, it has the potential to lose compression on the mating surface. This wear is compounded by the fact that dynamic applications already have lower compression recommendations.
To reduce the risk for this failure mode, it requires consideration during design and seal selection. The surface finish and concentricity of the hardware will be very important considerations. Smooth surface results in less friction (suggest 8 to 16 RMS), which in turn results in less wear. Increasing the durometer of the seal material helps resist wear, and there are also internally lubricated materials that could be employed. If the application is a high temperature, one should consider the impacts of thermal expansion on the elastomer being used. The thermal expansion increases contact pressure, which would increase friction/wear.
Installation Damage
Installation damage can occur in both face seals and radial seals but is much more common in radial applications. Installation damage can be the result of excessive installation stretch, cuts or nicks from installation tools, the installation of a seal over a burr or sharp corner or threads, improper size selection, or the insertion of a piston into a bore that does not employ an appropriate lead-in chamfer.
Our guidance for avoiding this failure mode is to:
• Break all sharp corners.
• Provide a 20-degree lead-in chamfer for radial seal applications.
• Cover threads that the seal will travel over during installation.
• Use lubrication during installation (this is the main benefit of an externally applied lubricant).
Fluid incompatibility
Failure due to fluid compatibility issues will usually be evident upon disassembly of hardware after the seal has been in service. Fluid incompatibility can look like many things but will be the result of some sort of chemical attack on the seal itself. The seal could be excessively swollen, from absorbing service fluid(s), softened or gummy which would likely lead to another failure mode such as extrusion or compression set, cracking / embrittlement, or a simple case of rapid compression set.
This failure mode is one that can be completely avoided by selecting an appropriate material for a given application.