Failure Modes

COMPRESSION SET

Probably the most common cause of O-ring failure is compression set. An effective O-ring seal requires a continuous “seal line” between the sealed surfaces. The establishment of this “seal line” is a function of gland design and seal cross section which determines the correct amount of squeeze (compression) on the O-ring to maintain seal integrity without excessive deformation of the seal element.

(See Section II in ORD 5700 titled Basic O-Ring Elastomers, for an in-depth discussion of compression set and Section IV, Static O-Ring Sealing, for information on correct gland design.)

Many factors can contribute to compression set failure of an O-ring seal, including:

• Selection of O-ring material with inherently poor compression set properties.
• Improper gland design.
• Excessive temperature developed causing the O-ring to harden and lose its elastic properties. (High temperatures may be caused by system fluids, external environmental factors, or frictional heat build-up.)
• Volume swell of the O-ring due to system fluid.
• Excessive squeeze due to over tightening of adjustable glands.
•  Incomplete curing (vulcanization) of O-ring material during production.
• Introduction of fluid incompatible with O-ring material.

The following suggestions may improve the rate of compression set:

1. Use “Low Compression Set” O-ring material whenever possible.
2. Select O-ring material compatible for the intended service conditions (a material with higher temperature capability or greater chemical resistance).
3. Reduce system operating temperature.
4. Check frictional heat build-up at seal interface and reduce if excessive.
5. Verify groove dimensions are consistent with recommended Parker practices.