Common Failure Modes of O-Rings
After the untimely failure of the space shuttle Challenger, the need for better understanding of vulnerabilities and failure modes of O-rings have become paramount. Over the years, O-rings have become one of the most common mechanical sealing solutions. O-rings are available in various materials, but the most commonly used materials are different synthetic rubbers and elastomers, such as nitrile rubber, FKM, and PTFE. Many of these materials are subjected to numerous external operational factors that can lead to sealing failure. O-ring failures are manifested by leakage of the process fluid through the seal, contamination of the process fluid, or change in the physical appearance of the O-ring. All these manifestations can be due to common causes such as extrusion, thermal degradation, chemical degradation, spiral failure, and installation damage. The failures have causal explanations in any of the following seven parameters:
- Nature of material
- Installation of the O-ring
- Design of gland and size of O-ring
- Static and dynamic properties of the material
- Chemical properties of the material
- Thermal properties of the material
- Impact of pressure or vacuum in the materials
Nature of the material can influence the compression set properties of the O-ring and the interaction between the gland materials with the O-ring material. Improper installation of O-rings, a parameter that is the result of human error in most cases, is completely avoidable. Similarly, the gland design for the O-rings is typically the result of gross human miscalculation or ordering. However, dynamic and static properties of the material are dependent on the static and dynamic nature of the applications. Even with latest innovations in chemical and thermal compatibility software, the impact of prolonged exposure cannot be accurately extrapolated. On the same lines, the impact of pressure and vacuum are also not accurately predictable. All the above parameters result in the following modes of failure in O-rings:
- Compression set
- Extrusion and nibbling
- Spiral failure
- Explosive decompression
- Chemical degradation
- Plasma degradation
- Thermal degradation
When under compression between the gland surfaces, O-rings exhibit a state of increased plasticity and take a long time to retain their original shape and size. This can introduce a space between the gland surfaces and the O-rings during movement, thereby causing failure of the seal.
Extrusion and Nibbling:
As the name suggests, the O-ring material in this failure mode is extruded in between surfaces of glands, small pieces of O-ring material is nibbled between them, or both. This type of failure occurs in dynamic sealing applications, although static sealing applications with high vibration can also result in such failure.
In hydraulic pistons, long strokes can spiral the O-ring between the piston and cylinder. Spiral failure, generally caused by eccentric surfaces or uneven surface finishes, can result in leakage between the spirals.
With exponential or eccentric changes in the pressure of the system, O-rings can entrap gas between their surfaces. The entrapped gas can result in ruptures or small embolisms in the O-rings when compressed between the gland surfaces.
Abrasion can occur in O-rings due to improper gland surfaces in dynamic sealing applications. Furthermore, the abrasion could also be a result of inadequate lubrication and excessive temperatures. This failure mode can manifest itself in both chronic and mild manners.
Incompatible chemicals can erode the surface or form small cracks on the surface causing irreparable damage to the O-rings. This failure mode can be very dangerous since most of these failures occur in reaction with harmful chemicals.
Thermal degradation could occur due to excessive temperature or extended exposure of the O-rings. This failure mode is similar to that of chemical degradation.
This kind of failure mode is intricate in nature and the most difficult to detect. The O-rings undergo reduction in their cross-sectional area due to degassing, thereby reducing their cross-sectional area, causing sealing problems.
This type of degradation is a sub-classification of chemical degradation and the result of erosion of the top surface of the O-rings. This degradation is characterized by powdered surface and discoloration of the O-rings.
Understanding of failure modes of O-rings is essential for the better selection of replacement material and repair of equipment. For assistance in selecting the proper O-ring for your application, please check out a comprehensive list of o ring specialists and manufacturers available on Zycon.com.