There are a number of reasons why O-rings can fail. Understanding what risks are posed to your sealing components and how to identify and mitigate these is essential in quickly understanding possible failures and reducing downtime.
Elastomer sealing experts, Precision Polymer Engineering (PPE) explore five of the most common failure modes that their engineers come across
Abrasion
In cases of abrasion the O-ring may appear to have a grazed surface. With excessive wear there could be deep lacerations and breaking.
The cause is common in dynamic application where abrasion can results from excessive repeat rubbing and friction between the component surface and the housing.
The risk can be increased by improper lubrication, the surface finish of the metalwork surrounding the component and in some cases, when abrasive contaminants have compromised the seal.
In order to better avoid abrasion, the correct lubrication for the sealing system is important and materials with improved abrasion resistance.
Wiper and scraper rings can reduce the abrasive contaminants from damaging the seal.
Chemical Attack
Chemical attack can show as a number of signs including blisters, cracking, increased hardness, discolouration or change in consistency.
Unless the sealant has experienced a severe case of chemical attack, it may not be always be visibly identifiable and could require physical measurement.
If an elastomer material isn't compatible with its environment, the chemical media can attack the rubber and change it’s cross-linking properties.
Additional cross links will cause it to become hard and brittle and less cross links will create a tacky and soft elastomer.
Correct elastomer material selection is vital to ensure your sealing components are compatible with application media. Elevated temperatures, excessive stretch and squeezing can speed up the process of chemical attack and can be best prevented with the use of elastomers with high temperature tolerance such as Perlast perfluorolastomers (FFKM).
Compression set
Compression set is unique to identify as the O-ring becomes less circular with flattened surfaces. In these cases, the O-ring takes a permanent ‘set’ to the shape of the groove/gland and is unable to recover its original shape.
The result of this distortion means that the component will lose its elasticity and ability to return to its original shape. This increased the risk of leakage, particularly in systems where thermal/ pressure cycling may occur.
Improper gland design, component swell or incomplete curing of the seal are also contributing causes to this common defect.
Elastomer materials with low compression set and high temperature capability will extend the seal life. Glad design will also need to be checked to ensure that the O-ring isn’t compressed.
Extrusion and nibbling
Extrusion O-ring failure
Extrusion and nibbling are easily identifiable failure modes as the edges of the O-ring become chipped or ‘filly’ in appearance. In severe cases, the surface of the O-ring can appear shaved or peeled off.
High pressures and the force or materials are the most common cause of this type of failure. Pulses of pressure cause clearance gaps to open and close. The O-ring can become trapped between the sharp edges of the mating surface.
Harder seal materials can help to reduce elastomer extrusion and the use of backup devices to alleviate of the pressure on the original O-ring is also advisable in applications of high pressure.
Explosive decompression
Explosive decompression is the most common cause of O-ring failure in the oil and gas industry and is easily identifiable.
The seal surface will be blistered, cracked, marked with deep splits or completely ruptured in the worse examples. In some cases it may be necessary to inspect the inner of the seal.
During a rapid decompression cycle, gas may permeate into the seal due to high pressure conditions as it tries to escape.
Damage to the seal can be exacerbated at higher temperatures.
A quick fix to prevent this failure is to increase the time for decompression and reducing the temperature to reduce damage risk.
Seals should be made with ED-resistant material and should meet NACE, TOTAL, NORSOK and ISO international standards.
The above examples are only a handful of failures that are possible in applications which require O-rings for sealing. Being able to identify some of the common problems will drastically reduce downtime and increase your knowledge of potential problems within your process.
