What are Flange facings and its Types, Surface Finish, Standards

Flange facings are essential components in the design of flanged joints used in piping systems and equipment connections. They play a critical role in ensuring a secure and leak-free connection between two flanges.

Flange Facings Definition:

Flange facings are the flat surfaces on the flange that come into contact with each other when two flanges are bolted together. These facings provide the sealing surface for the gasket and help prevent leakage from the joint.

Types of Flange Facings:

There are several types of flange facings, with the choice depending on the specific requirements of the application. The most common types include:

• Flat Face (FF): These facings have a flat and smooth surface and are suitable for low-pressure applications. They require the use of full-face gaskets.
• Raised Face (RF): Raised face facings have a small raised ring around the bolt holes. They are compatible with a wide range of gasket types and are suitable for moderate-pressure applications.
• Ring Type Joint (RTJ): RTJ facings feature a groove and a specially designed ring gasket. They are used in high-pressure and high-temperature applications, such as those found in the oil and gas industry.
  • Ring Joint Flanges: Ring joint flanges, designed for use with ring joint gaskets per ASME B16.20, have specific applications. They are used in Class 900 and higher-rated flanges, underwater pipelines with Class 300 and higher ratings, high-temperature services exceeding 480°C, API SPEC 6A Type 6B flanges, and hydrogen service in Class 300 and higher-rated flanges. The selection of ring joint flanges is based on the demanding requirements of these applications.
• Tongue and Groove (T&G): These facings have a tongue on one flange and a matching groove on the other. They are used in applications where alignment and centering are critical.
• Male and Female (M&F): Male and female facings have a protruding “male” flange and a corresponding “female” flange. They are used in applications where quick alignment and ease of assembly are required.

Gasket Selection:

The choice of flange facing influences the type of gasket that should be used. For example, flat face flanges require full-face gaskets, while raised face flanges can use both full-face and ring gaskets.

Surface Finish:

The surface finish of the flange facing is crucial for gasket sealing. It is typically specified in terms of roughness (Ra) and should meet industry standards. Proper surface finish ensures that the gasket can create an effective seal.

Surface Finish Standards: The surface finish of flange facings must meet the requirements of industry standards such as ASME B16.5, ASME 16.47, or API 6A, depending on the application. These standards specify the allowable roughness (Ra) values, ensuring a proper sealing surface. In hydrogen service, a very smooth finish with an Ra not exceeding 3.2 micrometers (125 microinch) is required to prevent leakage.

Flange Facing Standards:

Flange facings are designed and manufactured in accordance with industry standards such as ASME B16.5 for pipe flanges and ASME B16.47 for large-diameter flanges. These standards specify the dimensions, materials, and facing types for various applications.

Technical Considerations for Flange Facings

Here are some important consideration points about flange facings:

1. Sealing and Leakage Prevention: The primary function of flange facings is to provide a sealing surface that prevents leakage of fluids or gases from the joint. Proper gasket selection and bolt tightening are also critical factors in achieving a reliable seal.
2. Maintenance and Inspection: Flange facings should be inspected regularly to ensure they are free from damage, corrosion, or deformation. Any defects in the facing can compromise the integrity of the joint and lead to leaks.
3. Low Strength Flanges: Flanges made of low-strength materials like cast iron typically have a flat face configuration. This type of flange is paired with full-face gaskets to create a reliable seal. It’s important to match low-strength flanges with low-strength bolts to maintain uniform strength throughout the joint.
4. Temperature Limits for Raised Face Flanges: Raised face flanges are commonly used in various pressure ratings up to Class 600. These flanges are suitable for design temperatures ranging from minus 50 to 425°C. This temperature range covers a wide range of operating conditions.
5. Compatibility of Flange Faces: It is crucial to ensure that the assembly of different types of flange faces is not permitted. Each flange face type has its specific application and gasket sealing mechanism. Mixing different types can lead to leaks and safety issues. However, with proper approval, a raised face may be removed to convert it to a flat face flange, provided it meets ASME codes’ requirements.
6. Tongue-and-Groove or Male-and-Female Facings: The use of tongue-and-groove or male-and-female facing joints is not permitted. These configurations are not considered suitable for standard flanged joints due to their differences in design and sealing mechanisms.

In summary, flange facings must conform to industry standards for surface finish, and their type should be selected based on factors such as material strength, temperature, pressure rating, and specific application requirements. Mixing different types of flange faces is not allowed to maintain the integrity of the joint and prevent leakage.

Flange facings are crucial components in the design of flanged joints, and their selection should be based on the specific requirements of the application to ensure a secure and leak-free connection. Proper maintenance and adherence to industry standards are essential for reliable performance.

FAQs about Flange Facings