The CF (ConFlat) flange is the most widely used for high vacuum and UHV applications. Many gauges, instruments, accessories and feedthroughs are available on this flange system.
The seal mechanism is a knife-edge that is machined below flange's flat surface. As the bolts of a flange-pair are tightened, the knifeedges make annular grooves on each side of a soft metal gasket. The extruded metal fills all the machining marks and surface defects in the flange, yielding a leak-tight seal. The CF seal operates from 760 torr (1013 mbar) to <1 x 10-13 torr (<1.3 x 10-13 mbar), and within the temperature range -196° C to 450° C (depending on material). Flange sizes in North America are determined by outside diameter (O.D.). In Europe and much of Asia, the internal diameter (I.D.)—actually the nominal I.D. of the largest tube that can be welded to a bored flange—is used.
ConFlat flanges are offered in four versions— all relating to the bolt holes.
Fixed flange: once welded to a fitting, it has its bolt-hole orientation fixed with respect to that fitting.
Rotatable flange: when welded to a fitting, it enable the bolt-hole ring to rotate.
Through-hole flange: indicates a mating flange-pair accepts bolts that go through both flanges and is secured by nuts or plate nuts.
Tapped flange: connected to a through-hole flange using only bolts. Vacuum components with a rough axis of symmetry—e.g., a gate valve—can have fixed CF flanges welded with the bolt holes either split by the axis or in-line with the axis.