What’s the flange facing finish?

The facing finish or face finish of a pipe flange manufactured in accordance with ASME B16.5 refers to the smoothness or roughness of the facing surface. In a bolted flange joint, the flange face is in direct contact with the gasket. As the bolting is tightened, the softer gasket material flows into the minor machining imperfections of the flange facing, resulting in a fluid-tight seal.

If the flange facing is smooth, a smaller tightening stress exerted on the bolting may result in an impervious seal. If the flange has a rough face, the smaller stress exerted may be inadequate to secure a tight joint.

Hence, the flange facing finish is an important factor in determining the extent to which a gasket must flow to secure an impervious seal.

Different types of flange facing finish:

Generally, there are 4 types of facing finishes as illustrated in the above picture.

STOCK FINISH
The most widely used of any flange surface finish, because practically, is suitable for all ordinary service conditions. Required for flange facings in direct content with a gasket. Under compression, the soft face from a gasket will embed into this finish, which helps create a seal, and a high level of friction is generated between the mating surfaces.

The finish for these flanges is generated by a 1.6 mm radius round-nosed tool at a feed rate of 0.8 mm per revolution up to 12 inch. For sizes 14 inch and larger, the finish is made with 3.2 mm round-nosed tool at a feed of 1.2 mm per revolution.

SPIRAL SERRATED
This is also a continuous or phonographic spiral groove, but it differs from the stock finish in that the groove typically is generated using a 90-° tool which creates a “V” geometry with 45° angled serration.

CONCENTRIC SERRATED
As the name suggests, this finish is comprised of concentric grooves. A 90° tool is used and the serrations are spaced evenly across the face.

SMOOTH FINISH

This finish shows no visually apparent tool markings. These finishes are typically utilized for gaskets with metal facings such as double jacketed, flat steel and corrugated metal. The smooth surfaces mate to create a seal and depend on the flatness of the opposing faces to effect a seal. This is typically achieved by having the gasket contact surface formed by a continuous (sometimes called phonographic) spiral groove generated by a 0.8 mm radius round-nosed tool at a feed rate of 0.3 mm per revolution with a depth of 0.05 mm. This will result in a roughness between Ra 3.2 and 6.3 micrometers (125 – 250 micro inch).

Smooth finish flanges are more common for low pressure and/or large diameter pipelines and primarily intended for use with solid metal or spiral wound gaskets.

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.