Ring Joint Flanges is a special type of flange connection in
which a metal ring (resting in a groove in the flange) serves as a
pressure seal between the two flanges. RTJ flanges seal when tightened
bolts compress the gasket between the flanges into the grooves,
deforming (or "Coining") the gasket to make Intimate Contact inside the
grooves, creating a metal to metal seal.