Consider a hydraulic fitting with a Buna-N O-ring that has been stored in a parts bin for who knows how long—probably years. As is often the case, it’s no longer sealed in its original plastic bag. Now, what if that storage bin is in an outdoor tool shed in Florida, where the average humidity in July can reach 80%?
Under the proper conditions a Buna-N O-ring has a shelf life of 15 years—again, under proper conditions. One of those conditions is the humidity must be less than 75%. You can pretty much bet the fitting in question has been exposed to frequent humidity. Pull it out of the bin, put it into service, and you may be asking for trouble—leaks, damage to equipment, or worse.
Not all O-rings are created equal. Too often they are just an afterthought, even though they are essential components that the system depends on. A lot rides on each O-ring you purchase and put into service. If even one fails, an entire system may have to be shut down, equipment could be damaged, and even personal injury can result.
The Shelf Life of a Fitting is Only as Long its O-Ring
The shelf life of an O-ring or other elastomeric seal begins with the manufactured date and depends on being properly packaged and stored under specific conditions, even when in the body of a fitting. The shelf life of rubber is not an exact science because it is significantly impacted by storage and handling. A common misconception is that fittings made of metal last forever in the box, provided they are not stored in a corrosive environment. Although that is true, to a point, if an unused fitting has an O-ring, its shelf life is only as long as that of the O-ring.
Standards for Shelf Life
The conditions of shelf life for O-rings other elastomeric seals are governed by SAE International Standard ARP5316D. ARP5316D encompasses:
Humidity. Relative humidity ratings assume that fluctuations in ambient temperature do not result in condensation. An O-ring, or fitting with an O-ring, should be sealed in a moisture-proof bag where the ambient relative humidity is less than 75% and less than 65% for polyurethanes.