All flanges above ANSI Class 250 work in a similar fashion. They use the bolts arranged in a circle around the flange to pull the faces together with a gasket in between. The gasket gets compressed by the force exerted by the bolts, and a seal between the flanges occurs.
Several different flange face and gasket designs use different mechanics to accomplish a desired, leak tight, connection.
ANSI 16.5 Raised Face (RF) flanges compress a flat, relatively soft gasket, between two serrated raised portions located adjacent to the flange bore. The tightness of the bolts must exert enough pressure on the gasket to compress the gasket and hold it in place, not allowing the flanges to separate under pressure, thus achieving a seal.
For more information about flanges that seal with a flat gasket, see Graphonic Technical Manual.pdf .
ANSI 16.5 Ring Type Joint (RTJ), and API 6B flanges, sealed with an R type gasket, seal in much the same way as raised face flanges, except they utilize an oval or hexagonal steel ring gasket contained in a groove machined into the face of the flange adjacent to the flange bore. Unlike Raised Face flanges of the same size and pressure rating, the compression does not need to hold the gasket in place, as the grooves in the flanges accomplish this retention. The tightening of the bolts compresses the gasket, coining the flanks of the gasket, and with sufficient tightness prevents flange separation under pressure, achieving a seal.
ANSI 16.5 Ring Joint Type, and API 6B flanges, sealed with an RX type gasket, seal in much the same way as the flanges above, except the steel ring gasket has a cross-section profile that facilitates coining only on the outside flanks of the flanges’ seal groove. The gasket profile creates a self sealing feature due to the mechanical force created against the inside of the gasket by the contained pressure, forcing tighter contact between the gasket and the outside flanks of the flanges’ seal grooves.
API 6BX flanges seal with a BX hexagonal steel gasket fitting in machined grooves in raised faces (on at least one of the mating flanges) on the flanges adjacent to the flange bores. The depth of the grooves and the size of the gasket allow the gasket to coin and seal. With sufficient applied bolt torque, the raised faces will contact and react in such a manner as to reinforce the bending strength of the flange connection.