Purpose

To create a seal, gasket has to fill up the voids in the flange surface present because of surface finish and any flange rotation (or relative distortion) between the two surfaces.
The flange surface will thus give a broad indication of which gasket materials are likely to be suitable. Finishes of standard raised face flanges usually fall within the range 3.2 to 12.5mm, but this may be expressed in micro inch or roughness number.

Surface Joining

The recommended surface finish for the compressed fibre jointing (above a thickness of 1 mm) is 3.2mm to 12.5mm Ra (125m in 500min. CLA). These values are also used for graphite laminate (above a thickness of 0.8mm).
A surface finish of 1.6mm to 6.3mm Ra (63 to 200 in. CLA) is possible for tongue and groove flange facings or for very thin gaskets (0.4mm or below).
Surface finishes below 1.6mm are not recommended due to their negative effect on creep resistance of the gasket.

Surface for Spiral Wound Gasket

This type of gasket requires a range of surface finishes dependent upon the application:

• General - 3.2mm to 5.1mm Ra (125min. to 200min. CLA)
• Critical - 3.2mm Ra (125min. CLA).
• Vacuum applications - 2.0mm Ra (80min. CLA)

Solid Flat Metal Surface

A surface finish in the order of 1.6mm Ra is acceptable but for more critical conditions, a finish no more coarse than 0.8mm Ra is preferred. Again for optimum performance, the smoother the flange surface finish, the better the performance.

Surface for Metallic Ring Joint Gaskets

The angled surfaces (typically 23°) of both grooves and octagonal gaskets and the contact faces of oval gaskets should have a surface finish no rougher than 1.6mm Ra.

Machining of Flange Faces

Under no circumstances should flange seating surfaces be machined in a manner that tool marks extend radially across the seating surface. Such tool marks are practically impossible to seal regardless of the type of gasket being used.