Types of O-Ring Failures

O-rings can fail for various reasons, typically related to environmental conditions, material properties, installation issues, or operational stresses. Here are some common types of O-ring failures and their causes:

1. Compression Set

• Description: The O-ring becomes permanently deformed and fails to return to its original shape after being compressed. The cross-section of the O-ring becomes less circular and may take the shape of the groove/gland.
• Causes:
  • Excessive temperature exposure
  • Long-term static load
  • Improper material selection

2. Extrusion and Nibbling

• Description: Portions of the O-ring material are forced into the clearance gap between the mating surfaces, leading to nibbling or tearing. The edges of the O-ring will have a chipped or nibbled appearance. 
• Causes:
  • High pressure
  • Large clearance gaps
  • Inadequate material hardness

3. Abrasion

• Description: The surface of the O-ring wears away due to friction against mating surfaces. The sliding surface of the O-ring will have lacerations and a grazed finish.
• Causes:
  • Rough or poorly finished surfaces
  • Inadequate lubrication
  • Movement between the O-ring and sealing surfaces

4. Chemical Degradation

• Description: The O-ring material deteriorates due to exposure to incompatible chemicals. The O-ring may exhibit a number of blisters, cracks and discolouration due to chemical attack.
• Causes:
  • Contact with aggressive chemicals
  • Use of unsuitable O-ring material for the chemical environment

5. Thermal Degradation

• Description: The O-ring material degrades due to exposure to excessive heat or cold.
• Causes:
  • Operating beyond the temperature limits of the material
  • Inadequate heat dissipation

6. Explosive Decompression

• Description: Rapid decompression causes trapped gas within the O-ring to expand, leading to blistering or rupturing.
• Causes:
  • Rapid pressure changes
  • High-pressure gas environments

Explosive decompression (ED) is a specific failure mode for O-rings that occur when they are exposed to high-pressure gas environments followed by a rapid decrease in pressure. This can cause the gas absorbed or trapped within the O-ring material to rapidly expand, potentially leading to blistering, cracking, or rupturing of the O-ring. Here are details on this type of failure and how to mitigate it:

Mechanism of Explosive Decompression Failure

1. High-Pressure Gas Exposure: The O-ring is exposed to a high-pressure gas environment. Gases can permeate the O-ring material during this period.
2. Rapid Pressure Drop: When the external pressure is rapidly reduced, the gas trapped inside the O-ring attempts to escape quickly.
3. Internal Stresses: The rapid expansion of gas within the O-ring creates internal stresses that can lead to blistering, cracking, or rupture.

Symptoms of Explosive Decompression Failure

• Blistering: Visible blisters or bubbles on the surface of the O-ring.
• Cracking: Radial or circumferential cracks in the O-ring material.
• Rupturing: Complete breakage or tearing of the O-ring.
• Swelling: Noticeable swelling or deformation of the O-ring.

Causes

• Rapid Depressurization: The primary cause is a rapid decrease in external pressure.
• Material Permeability: Materials with high gas permeability are more susceptible to ED.
• High-Pressure Gas: Certain gasses, such as nitrogen or carbon dioxide, can exacerbate the problem.

Mitigation Strategies

1. Material Selection:
   1. Use ED-resistant materials. Fluorocarbon (FKM), Hydrogenated Nitrile Butadiene Rubber (HNBR), and perfluoroelastomers are known to have better resistance to explosive decompression.
2. Design Considerations:
   1. Minimize the rate of pressure drop if possible to allow the gas to escape gradually.
   2. Ensure proper groove design to avoid trapping gas.
3. Preconditioning:
   1. Precondition O-rings in the high-pressure environment before operation to reduce the amount of gas absorbed.
4. Surface Treatments:
   1. Apply surface treatments or coatings that reduce gas permeability.
5. Regular Inspection and Maintenance:
   1. Regularly inspect O-rings for signs of ED damage and replace them as needed.
6. Pressure Management:
   1. Implement controlled decompression procedures to reduce the risk of rapid pressure drops.

7. Installation Damage

• Description: The O-ring is damaged during installation, leading to cuts, nicks, or overstretching.
• Causes:
  • Improper installation techniques
  • Sharp edges or burrs in the groove or mating surfaces

8. Over compression

• Description: Excessive compression of the O-ring leads to flattening and material extrusion.
• Causes:
  • Incorrect groove design
  • Excessive tightening of mating surfaces

9. Weather and Ozone Cracking

• Description: The O-ring surface cracks due to exposure to ozone or ultraviolet (UV) light. The earliest signs of this type of O-ring failure begins with discolouration leading to subsequent cracking. 
• Causes:
  • Outdoor or high-ozone environments
  • Use of materials not resistant to ozone or UV light

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10. Swelling

• Description: The O-ring swells excessively, altering its dimensions and leading to seal failure. The O-ring may swell or increase from its original dimension consistently across the seal or in localized areas.
• Causes:
  • Absorption of fluids
  • Incompatible material with the sealing environment

11. Spiral Failure

• Description: The O-ring twists in its groove and develops spiral cuts or cracks. The O-ring will have a spiraling pattern around its exterior, with deep cuts at 45° angles where the highest stress levels are apparent.
• Causes:
  • Dynamic applications with reciprocating motion
  • Improper O-ring lubrication
  • Misalignment

If you are facing such O-ring failures, contact ISMAT to help find a solution for them. Our team of engineers will help you identify the right material to use with aggressive chemicals and harsh environments. They are also trained to study gland designs and suggest the accurate O-ring dimensions to use. You can also check out our O-ring Installation Guide that can help you in reducing O-ring failures.