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38 To: Subject: API Ring Gasket Types Question: I know that ring gaskets have number designations. Can you explain the type differences? Answer: API specifies 5 "Types" of ring gaskets: Type R, Type RX, Type SRX, Type BX and Type SBX. API offers only the Type R ring gasket in two designated cross section profiles: One called "Oval" and the other "Octagonal." API represents these two profiles as interchangeable, neither having advantage over the other. Some very old flanges that utilize Type R ring gaskets have ring grooves with rounded bottoms, and specifications for these flanges called for the use of oval gaskets in these grooves, but no manufacturer makes flanges of this type today. All other API gaskets besides Type R have eight sided cross section profiles, but API does not refer to them as octagonal. Type RX gaskets fit in the same currently specified grooves as Type R gaskets and offer a greater capacity to support weight and resist vibration without leaking. Type RX gasket cross sections seen in profile have 8 "sides", but seal using different forces from Type R octagonal gaskets. API has referred to them as "self-sealing" or "pressure energized" ring gaskets. Type SRX gaskets have all the same dimensions and characteristics as Type RX gaskets, but have special vent holes to allow connection make-up underwater. Type BX gaskets, used only in 6 BX, 17D SS and SV flanges, also have cross sections seen in profile to have 8 "sides." They have a pitch diameter slightly larger than the groove into which they fit; the diameter difference between gasket and groove provides an additional sealing force by maintaining intimate contact with the outside diameter of the groove, even if the flanges move slightly away from each other under pressure. Type SBX gaskets have all the same dimensions and characteristics as Type BX gaskets, but have special vent holes to allow connection make-up underwater. For a detailed explanation of how ring gaskets of all types fit and seal in grooves, see Field Appraisal of Ring Grooves, and Ring Gasket Handling and Inspection.
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