Why do O-rings fail? A brief guide to O-ring failure modes
There are many reasons why O-rings fail, this brief guide provides details of some of the most common failure modes (there are additional failure modes not covered here).
At Precision Polymer Engineering, we have spent decades developing innovative elastomer materials able to resist the most challenging sealing environments. If you would like further assistance please contact our sealing experts who will be happy to help diagnose the problem and suggest possible solutions.
Appearance: The O-ring has a grazed surface, with excessive wear there may be deeper lacerations and breaking in places. Only the sliding contact faces of the seal will demonstrate these signs of wear.
Cause: Common in dynamic applications, abrasion can result from excessive repeat rubbing and friction between the O-ring surface and the housing. Improper lubrication or surface finish of the metalwork can exacerbate the risk, as can ingress of abrasive contaminants into the sealing system.
Solution: Ensuring the correct lubrication for the sealing system is important, but PPE can offer a range of O-ring sealing materials with improved abrasion resistance. Our engineers can also advise on the correct surface finish for the metalwork. Ingress of contaminants can be reduced through the use of wiper or scraper rings.
Appearance: Dependent on the chemical media being sealed, the O-ring may exhibit a number of signs including blisters, cracking, increased hardness, discoloration or change in consistency. Severe cases may be visibly identifiable, but physical measurement may be required for some diagnoses.
Cause: When an elastomer material is incompatible with its environment, the chemical media can attack the rubber and change the polymer’s crosslinking properties. Additional cross-links in the polymer structure cause it to become hard/brittle with a loss of elastic properties and the ability to provide a reaction force. Less cross-links cause the O-ring to become soft and tacky, with a loss of its original shape and integrity.
Solution: Correct elastomer material selection is vital to ensure the seal is compatible with the application media. Chemical attack is accelerated when elastomer seals are under stress, this includes elevated temperatures, excessive stretch and squeeze and mechanical conditions. PPE engineers can recommend the most appropriate sealing material based on your application parameters. For the ultimate in chemical resistance combined with high temperature capability, ask about our Perlast® perfluorolastomers (FFKM). Check the chemical compatibility of the main elastomer material types using our online guide.
Appearance: The O-ring appears larger than its original dimensions. This may be consistent across the whole seal or in localized areas that have been exposed to the chemical media.
Cause: Swell is caused by the absorption of media into the elastomer. The increased seal volume can lead to gland fill, extrusion and loss of sealing. Chemical swell can also result in a loss of physical properties such as tensile strength.
Solution: Switch to an elastomer sealing material with proven resistance against the chemical environment. PPE sealing experts can help you identify a material which will deliver lasting sealing performance in the chemical media within your application. Check the chemical compatibility of the main elastomer material types using our online guide.
Appearance: The cross-section of the O-ring becomes less circular with flattened surfaces that have taken the shape of the groove/gland. The O-ring has taken on a permanent ‘set’ meaning it is less able to recover to its original shape after distortion. ‘Set’ is quantified as a percentage loss in compression compared to the original compression applied.
Cause: Physical and chemical changes can occur to an elastomer at elevated temperatures that cause additional cross-linking. This results in the O-ring losing its elasticity and ability to return back to the original shape. The reduction in cross section results in a lower contact sealing force, which increases the risk of leakage in systems where thermal and pressure cycling, occurs. Other causes include improper gland design, volume swell due to system fluid and incomplete curing of the seal during production.
Solution: Selecting elastomer materials with low compression set and/or higher temperature capability will help to extend seal life. Gland design should also be checked to ensure the O-ring is not over-compressed with too much squeeze applied.
Extrusion and Nibbling
Appearance: The edges of the O-ring on the low pressure side have a nibbled, chipped or ‘frilly’ appearance. Shaving can occur in severe cases where the surface of the O-ring appears to be peeled off.
Cause: High pressure forces the material into, and down, the clearance causing extrusion. Pulses of high pressure in can cause the clearance gap of the mating edges to open and close. This can lead to the O-ring becoming trapped between the sharp edges of the mating surfaces, resulting in physical damage to the seal surface and eventual seal failure through nibbling.
Solution: A harder seal material can help, as can the use of backup devices to alleviate some of the pressure on the original O-ring seal. Your PPE sealing expert can also advise on the installation of correctly sized O-rings for your application, decreasing your clearance gaps and minimizing the risk of extrusion and nibbling.
Appearance: The seal surface may be blistered, cracked, marked with deep splits or completely ruptured in the worst examples.
Cause: Gas which has permeated into the seal under high pressure conditions tries to escape during a rapid decompression cycle resulting in blistering and rupturing, this is exacerbated at elevated temperatures.
Solution: Increasing the time for decompression and reducing the temperature will typically reduce the risk of explosive decompression (ED) damage, as will choosing an ED-resistant material. PPE has been an industry leader in the development of ED-resistant elastomer seals and O-rings – conforming to NACE, TOTAL, NORSOK and ISO international standards, and improving operational safety and efficiency.
Appearance: Damage caused to the seal during installation can often be diagnosed by specific and exact cutting and notching on the seal surface, with all damage restricted to the O-ring surface furthest from the bottom of the groove.
Cause: Installation damage can take a wide range of forms, from ‘skiving’ of the seal with metal components through to damage caused by careless installation of dirty, twisted or improperly lubricated seals. Incorrect sizing of the seal for the application is also a significant factor in installation damage.
Solution: Taking care during the installation of O-rings is key to preventing installation. Covering sharp edges and threads using tape or protective sheaths will prevent nicks to the surface. Ensuring hardware has suitable lead-in chamfers aids the make-up of the hardware, along with suitable lubrication.
Appearance: A key marker is powdery residue on the seal surface, but discoloration and general erosion are also evidence of plasma degradation.
Cause: In the high energies and high temperatures where plasma exists, seals are subjected to super-heating through electron bombardment, along with very high chemical attack.
Solution: With long term exposure to plasma, seal damage is unavoidable. The chemical compatibility of the material can help to resist damage for longer, improving the lifetime of the seal and reducing the impact of equipment downtime. Ask your PPE sealing expert about Nanofluor® materials which provide plasma resistance that is comparable or superior to many high purity FFKM grades.
Appearance: The seal demonstrates a tell-tale spiralling pattern around its exterior, with subsequent deep cutting of the seal surface at 45 degree angles where the highest stress levels are apparent.
Cause: Spiralling of an O-ring can occur during dynamic, reciprocating motion. The levelling of spiralling is affected by many different factors, including but not restricted to uneven surface finishes, improper lubrication, friction, installation errors and eccentric components.
Solution: A harder O-ring material is a good starting point for the prevention of spiralling. You might also consider a different seal profile, with PPE X-rings (quad rings) and D-seals proven to resist spiralling without a reduction in seal performance. If a high pressure seal is required, the T-seal provides a robust sealing solution with complimentary backup rings. X-rings, D-seals and T-seals can usually fit existing O-ring grooves/glands.
Appearance: There may be instances of radial cracking on surfaces bearing the highest temperatures. If the seal material is prone to thermal softening, the surface may also become shinier in places. Thermal degradation is often accompanied by compression set.
Cause: The temperature of the application has exceeded the maximum temperature ceiling of the selected seal material or excessive temperature cycling has occurred. High temperatures can increase the cross-link density in elastomers resulting in an increase in hardness and modulus, making them less elastic.
Solution: Selection of a higher temperature elastomer material is the obvious solution. PPE can provide reliable high temperature sealing up to +325°C (+617°F) with its range of Perlast® perfluoroelastomers (FFKM).
Appearance: There are obvious signs of compression set with the O-ring no longer having a round profile. A ‘frilly’ edge will be evident on two opposing sides or areas of ‘nibbling’ where sections of the O-ring surface have been damaged. In high pressure applications, the extrusion or nibbling may be held off on the high pressure side and only evident on the low pressure side. The O-ring often takes on the shape of the groove.
Cause: The temperature of the application has caused the elastomer material to increase in volume. The material’s coefficient thermal expansion (CTE) has caused the volume of material to exceed the groove/gland volume, forcing the expanded material into the clearance gap between mating surfaces.
Solution: Groove/gland design should be checked to make sure there is sufficient space to accommodate the additional seal volume and avoid ‘gland fill’ - see our online groove/hardware dimensions guide. Careful selection of elastomer materials with correct CTE is the obvious solution.