The RX type RTJ gasket is manufactured according to API 6A and ASME B16.20 to suit API 6B and ASME/ANSI B16.5 flanges.
The RX is a pressure energized version of the R octagonal gasket and fits the R type flat-bottomed groove.
The RX has an increased height and utilizes the internal system pressure to energize and improve the seal as internal pressure increases.
Some RX sizes have a pressure relief hole to equalize pressure on both sides of the sealing faces.