The American Petroleum Institute (API) first issued its standard for Ring Joints and Ring Joint Flanges (API Specification 6B) in June of 1936.  This standard (which was adopted by the American Standards Association into the 1939 edition of ASA B16e) had two different groove profiles for ring joint flanges.  Flanges under 6” had a round-bottomed groove and could only use oval rings.  Note that the ring did not contact on the bottom of the groove, but had a wedged contact on the tapered side of the grooves, as is still the case.  The oval style of ring joint gasket was the only option for flanges under 6”.  Flanges 6” and over had a flat-bottomed groove which could use either oval or octagonal ring joints. 

Over the years changes were gradually introduced, so that when the 1957 edition of ASA B16.20 was published, the only groove profile was the flat-bottomed one, and octagonal ring joints were available for all sizes.  This change allowed the oval or octagonal gasket to be used in new (or retrofitted) installations, whereas the oval gasket was still required in existing applications that had the earlier groove design.  Undoubtedly, the fact that the octagonal gasket could not be used in some of the pre-existing flanges was a barrier to its acceptance. 

Both gaskets seal by wedging tightly into the sides of the groove as they are compressed.  However, the octagonal gasket has a broader sealing contact surface, as the entire 23-degree-tapered edge of the gasket ring is in contact with the mating groove face.  (See drawing.)  

While it is generally recognized that the octagonal ring gasket is a superior design that offers higher sealing efficiencies and reliability, oval gaskets still comprise about 90% of the market.  The three factors that most likely account for this surprising fact are:

1. Universality. As noted, the oval gasket will fit into all standard ring joint flanges.  However, as the old round-bottom flanges become rarer, the universality of the gasket becomes less of a benefit. 
   
   
2. Inertia. Long-lived habits die hard.  Many of today’s industrial specifications are rooted in earlier specifications that were written when the change was being made from rounded grooves to flat-bottomed grooves.  The influence of those early decisions still exists – even though the rational they rested on is largely gone.
   
   
3. Price. Most manufacturers charge a 15-20% premium for the octagonal gasket, as there are more surfaces to machine.  
   
   

The deficiencies of the oval gasket primarily arise from its small point of contact with the flange groove.  The oval cross-section generates a single-point contact ring with the flange.  The narrowness of this contact results in very high seating forces.  Even though the gasket is supposed to be softer than the flange material, both the gasket and the flange see some deformation as a result.  This deformation continues until the gasket load is supported.  While this works fine the first time – and maybe even the second and the third – eventually the flange becomes both work-hardened and deformed, making it difficult to get good conformation on the subsequent gasket.  Ultimately, this increases the potential for leaks.  In the following picture, the damage to the groove (“coining” deformation) is clearly visible – a result of the very high seating loads that are generated by an oval ring joint gasket.