We recommend that metal flange faces be machined with a concentric-serrated finish of 125-500 AARH, with 250 AARH being the optimum for non-metallic gaskets. Phonographic serrations can also be used with our materials. It should be recognized, however, that their continuous leak path makes them more difficult to seal.
The finish or the condition of the gasket seating surface has a definite effect on the ability of the gasket to create a seal. Sheet gasketing is designed to have a seating stress that allows the gasket material to "flow" into the serrations and irregularities of the flange face. This "bite" aids the gasket in resisting the effects of internal pressure, creep and cold flow.
"Smooth" finishes are usually found on machinery or flanged joints other than pipe flanges. When working with a smooth finish, it is important to consider using a thinner gasket to lessen the effects of creep and cold flow. It should be noted, however, that both a thinner gasket and the smooth finish, in and of themselves, require a higher compressive force (i.e. bolt torque) to achieve the seal.
Therefore, due to the flange design, one may have to resort to a thicker gasket, which requires a lower compressive force to seal the gasket. Another way to seal the gasket, when there is insufficient compressive force available, is to lessen the area of the gasket.