What is Flange Facing Finish?
ASME B16.5 Requirements for Flange Facing Finish shall be in accordance with paras. 6.4.5.1 through 6.4.5.3, except that other finishes may be furnished by agreement between the user and the manufacturer. The finish of the gasket contact faces shall be judged by visual comparison with Ra standards (see ASME B46.1) and not by instruments having stylus tracers and electronic amplification. So what is Flange Facing Finish?
The facing finish of a pipe flange, as per ASME B16.5, refers to the smoothness or roughness of the facing surface, which is in direct contact with the gasket in a bolted flange joint.
Types of Flange Faceing Finish
The primary finishes for ANSI B16.5 flanges include Stock Finish, Spiral Serrated Finish, Concentrated Serrated Finish, and Smooth Finish.
Stock Finish
This type of flange surface finish is widely used because it is suitable for most normal service conditions. When compressed, the soft face of a gasket will mold into this finish, creating a seal and high friction between the mating surfaces.
For flanges up to 12 inches, the finish is created using a 1.6 mm radius round-nosed tool at a feed rate of 0.8 mm per revolution. For sizes 14 inches and larger, a 3.2 mm round-nosed tool is used at a feed rate of 1.2 mm per revolution.
Spiral Serrated Finish
This is a continuous or phonographic spiral groove, but it differs from the stock finish. The groove is typically generated using a 90-degree tool, creating a “V” geometry with 45-degree angled serrations.
Concentrated Serrated Finish
As the name suggests, this finish is comprised of concentric grooves. A 90-degree tool is used, and the serrations are spaced evenly across the face.
Smooth Finish
This type of finish should visibly show tool markings. It is commonly used for gaskets with metal facings such as double jacketed, flat steel, and corrugated metal. The smooth surfaces need to fit together tightly to create a seal, relying on the flatness of the opposing faces. This is usually done by creating a continuous spiral groove (sometimes called phonographic) on the gasket contact surface using a 0.8 mm radius round-nosed tool at a feed rate of 0.3 mm per revolution and a depth of 0.05 mm. This process will result in a roughness ranging from Ra 3.2 to 6.3 micrometers (125 – 250 micro inch).
Flange facing finishes in accordance with ASME B16.5.
| Surface Types | Maximum Roughness Value |
| Ring-type joint flanges (and hard gaskets) | 63 µin AARH (1.6 µm AARH) |
| Spiral wound gaskets. | 125 to 250 µin AARH (3.2 to 6.3 µm AARH) |
| Soft gaskets. | 250 to 500 µin AARH (6.3 to 12.6 µm AARH) |
| Tongue and Groove, and small Male and Female | 125 µ.in. or 3.2 µ.m AARH |
FAQ
The finish on the flange face directly influences the gasket’s ability to seal. A properly matched finish ensures that the gasket deforms to fill the surface imperfections, creating a tight seal that can withstand the intended operating conditions.
Flange facing finish is measured using a profilometer, which traces the surface to quantify the roughness, expressed as Ra (Roughness Average) in micrometers (µm) or microinches (µin).
- For metallic gaskets (e.g., spiral wound), a serrated finish is typically recommended.
- For soft gaskets (e.g., PTFE, rubber), a smoother finish like stock or smooth finish is suitable.
- Consider operating pressure, temperature, and the chemical compatibility of the flange finish with the gasket material.
Generally, both flanges in a connection should have the same type of facing finish to ensure a proper seal. Mismatched finishes can lead to sealing issues or gasket failure.
Too Rough: A rough surface can cause excessive gasket wear, leading to a poor seal and potential leakage.
Too Smooth: A very smooth surface may not grip the gasket well, causing slippage and improper sealing, especially under pressure.
Regular inspection and re-machining can maintain the flange face’s integrity. Over time, erosion, corrosion, or mechanical damage can affect the finish, compromising the seal.
In summary
Understanding flange facing finishes is crucial for selecting the correct gasket, ensuring proper sealing, and maintaining the overall integrity of flange connections in various applications.
Related resource references:
https://www.wermac.org/flanges/flanges_stock-finish_smooth-finish.html
