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It is obvious that abrasion may occur on an o-ring in a sliding situation such as when an o-ring is used as a dynamic seal on a 1st stage regulator piston. However a more devious condition exists when an o-ring seal configuration is improperly designed from the start. Using a tank valve as an example since the configuration is understandable and visual (a face seal): The high pressure gas is in the tank r.e. the exterior -> the inside diameter of the o-ring "sees" the high pressure. The force acting on the o-ring pushes the o-ring outward towards the low pressure side. If the o-ring groove is sized properly, the resultant displacement would be minimal as the o-ring is readily deformed against the LP sided of the groove to create the seal. However if the o-ring groove outside diamter is excessive, the pressure force will cause the o-ring to displace outward until it contacts the LP side groove wall. This movement of the o-ring against the sealing surfaces (and with high-pressure) will cause increased abrasion. In a proper design (with an outward acting pressure), the groove diameter should be sized such that the free OD of the o-ring should equal the OD of the groove to limit the outward displacement. The reserve volume of the o-ring groove (and there should be some) should be accommodated by the groove ID being somewhat smaller than the O-ring free ID. The reverse is true if the high pressure was to act on the OD of the o-ring (e.g. the ID of the groove sized to match the o-ring ID and the reserve volume accommodated by a larger groove OD). If pressure oscillates between positive and a vacuum, the design gets a little more complex (for minimizing abrasion) to keep the o- ring from migrating back and forth within the groove. Secondly the surface finish should be smoother for a dynamic seal and the precompression load on the o-ring should be relaxed from that of a static seal. A second issue regarding O2 compatibility and o-rings is chemical decomposition and the expected service life. A design that should last a year between service may be much different than one that goes 40 years between service. Its all relative to some degree, but the physical configuration of the groove-oring interface can go a long way to improve reliability. IMHO, Doug -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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