 Flange fittings are generally used for connections to valves, inline instruments or equipment nozzles.Two surfaces are joined tightly together with threaded bolts, wedges, clamps, or other means of applying high compressive force.Although a gasket, packing, or O-ring may be installed between the flanges to prevent leakage, it is sometimes possible to use only a special grease or nothing at all (if the mating surfaces are sufficiently precisely formed).