In engineering and industrial applications, a flange is a crucial component used for connecting pipes, valves, pumps, and other equipment together. Flanges provide a secure and leak-proof connection while allowing easy access for inspection, maintenance, and modification of the system. This article explores the various types, applications, materials, and standards associated with flanges.

What is a Flange?
A flange is a protruding rim or edge, typically used to strengthen or attach one object to another. It is usually equipped with bolt holes, which achieve tight connections between components through fasteners such as bolts and nuts. In industrial contexts, flanges are primarily used to connect pipes, valves, pumps, and other equipment, facilitating assembly, disassembly, and maintenance.

A typical flanged connection is comprised of three parts:
- Pipe Flanges
- Gasket
- Bolt
History of France
Flange, was first proposed by a British engineer named John Lovekin in 1809. Lovekin also introduced casting methods for flanges during that time. However, for a considerable period afterward, flanges were not widely adopted. It was not until the early 20th century that flanges became extensively utilized in various mechanical equipment and pipe connections.
What is the Purpose of a Flange in Pipeline Systems?
In pipeline systems, flanges serve the primary purpose of providing secure, sealed connections between pipes, valves, fittings, and other equipment. They ensure the integrity and safety of the system by using bolts and sealing materials like washers to prevent leaks. Flanges also facilitate easier assembly, disassembly, and replacement of components, simplifying maintenance without the need for cutting or welding pipes.
How Does the Flange Work?
A flange works by providing a method to connect two pipes or other equipment securely. It consists of a protruding rim or collar that is bolted or welded to the adjoining equipment. This connection creates a strong joint that can withstand the forces and pressures within the system. Flanges also often incorporate gaskets between the mating surfaces to ensure a tight seal, preventing leaks and maintaining the integrity of the fluid or gas being transported.

Materials and Manufacturing
Flanges are typically made from materials such as carbon steel, stainless steel, alloy steel, and sometimes brass or PVC for specific applications. Manufacturing processes include forging, casting, and machining, ensuring the flanges meet dimensional and metallurgical standards suitable for their intended use.
What are the Types of Flanges?
There are many types of flanges, which can be classified into various types based on their different uses, structures, and connection methods. Here are some common types of flanges and their main functions:
Image Example | Flange Type | Description | Applications |
---|---|---|---|
![]() | Weld Neck Flanges | Long tapered hub provides reinforcement, making them suitable for high-pressure and high-temperature applications. | High-pressure and high-temperature systems. |
![]() | Slip-On Flanges | Larger diameter than the pipe, slipped over the pipe before welding. | Low-pressure and non-critical applications. |
![]() | Socket Weld Flanges | Recessed area (socket) fits the pipe end, allowing for fillet welding. | Small-bore piping systems, moderate pressure, and temperature applications. |
![]() | Blind Flanges | Solid discs used to close the end of a piping system or vessel. | Pressure testing, sealing pipe ends. |
![]() | Threaded Flanges | Internal threads match external pipe threads, allowing for connections without welding. | Applications where welding is not feasible. |
![]() | Lap Joint Flanges | Two-piece design with a stub end butt-welded to the pipe, backed by a flange. | Systems with limited space or requiring frequent maintenance. |
Beyond the common types, there are several specialized flanges designed for specific applications. These include:
- Spectacle Blind Flanges
- Loose Flanges
- Nipo Flanges
- Long Neck Welding Flanges
- Reducing Flanges
- Expanding Flanges
- Ring Type Flanges
- Groove and Tongue Flanges
- Orifice Flanges
- High Hub Flanges
Can Threaded Flanges Be Used in High-Pressure Systems?
Threaded flanges are typically used in low-pressure, non-critical applications where welding is not feasible. They are not generally recommended for high-pressure systems due to the potential for thread leakage and the lower strength of the connection.
What is the Difference Between a Socket Weld Flange and a Slip-on Flange?
Socket weld flanges have a recessed area where the pipe is inserted before welding, providing a smoother bore and better fluid flow for small-bore piping systems. Slip-on flanges, on the other hand, are slipped over the pipe and welded both inside and outside, making them easier to install but less suited for high-pressure applications.
What Happens if Flange is Too Big?
If a flange is too big for the pipe it is intended to connect, you should consider using a reducer flange or an adapter to match the flange size to the pipe size. Reducer flanges are designed specifically to connect pipes or fittings of different sizes. Using a reducer flange ensures a proper fit and maintains the integrity and functionality of the piping system.
What Happens if Flange is Too Small?
In this case, it is necessary to replace the flange with a flange that matches the correct size and specifications of the pipeline or equipment. Using flanges that are too small may lead to alignment issues, improper sealing, and potential leaks or failures in the piping system.
How Do I Choose the Right Type of Flange for My Application?
To choose the right type of flange for your application, consider the following factors:
- Operating Conditions: Assess the pressure and temperature requirements of your piping system. Different flange types have varying capabilities in handling high-pressure or high-temperature conditions.
- Pipe Size and Material: Ensure the flange is compatible with the size and material of the pipes or equipment it will connect. Flanges come in various sizes and materials (such as carbon steel, stainless steel, and alloy steel), each suited for different environments and fluid types.
- Installation and Maintenance: Evaluate ease of installation and future maintenance needs. Some flange types, like slip-on or threaded flanges, may be easier to install, while others, like weld neck flanges, offer stronger connections suitable for frequent maintenance.
- Sealing Requirements: Consider the sealing method required for your application. Flanges typically use gaskets to prevent leaks; ensure the flange type supports the gasket material suitable for your operating conditions.
- Application Specifics: Understand any specific requirements or constraints of your application, such as space limitations, frequent dismantling needs (favoring lap joint flanges), or special measurements (requiring orifice flanges).
By carefully considering these factors, you can select a flange type that optimally meets your application’s requirements for performance, durability, and safety.

Various Flange Faces for Optimal Sealing in Piping Systems
Flanges come with different face types, each serving specific purposes and requiring corresponding gaskets for effective sealing:
Tongue and Groove (T&G)
T&G flanges have grooves and raised areas that facilitate self-alignment during installation. They also create a reservoir for gasket adhesive, enhancing the seal’s reliability.
Benefits: Ensures easier alignment during assembly, reducing the risk of misalignment that could compromise sealing effectiveness.
Raised Face (RF)
RF flanges have a small raised portion around the bore where a circular gasket fits. This design ensures the gasket seals effectively under pressure, making RF flanges suitable for a wide range of pressure and temperature ratings.
Applications: Widely used in oil and gas, chemical processing, and other industries where reliable sealing is essential.
Ring Joint Face (RTJ)
RTJ flanges feature a groove into which a metal gasket (ring joint gasket) is placed. This type of flange is designed for high-pressure and high-temperature applications, providing a robust and reliable seal.
Suitability: Ideal for environments where extreme conditions require a gasket material that can withstand high pressures without compromising on sealing integrity.
Male and Female Face (M&F)
M&F flanges feature raised portions and corresponding grooves. The gasket is retained on the female face, allowing for precise alignment and accommodating a variety of gasket materials.
Advantages: Offers improved gasket retention and alignment, making it ideal for critical applications where precise sealing is required.
Flat Face
Flat face flanges have a smooth, flat surface across the entire face. They use a full-face gasket that covers the entire flange surface, providing a uniform and reliable sealing surface.
Use Cases: Commonly used in applications where moderate pressures and temperatures are involved, and a consistent sealing surface is required.
Each type of flange face serves specific purposes and is chosen based on the operational requirements and environmental conditions of the piping system. Selecting the correct flange face ensures optimal sealing performance, longevity, and reliability of the piping infrastructure.
Standard Flange Sizes
Flange dimensions are standardized according to specific specifications, most notably:
- American National Standards Institute (ANSI): Provides a series of pressure classes (150, 300, 400, 600, 900, etc.) and corresponding dimensions for each flange size.
- American Society of Mechanical Engineers (ASME): Offers detailed guidelines on flange dimensions and specifications in ASME B16.5 (for flanges up to 24 inches) and ASME B16.47 (for larger flanges).
- International Organization for Standardization (ISO): Provides international standards for flanges, ensuring uniformity across global applications.
Here are some examples of standard flange sizes based on ANSI/ASME standards:
Flange Size (NPS) | Outside Diameter (OD) | Bolt Circle Diameter (BCD) | Number of Bolt Holes | Thickness |
---|---|---|---|---|
1/2″ | 3.5″ | 2.5″ | 4 | 0.15″ |
1″ | 4.0″ | 3.0″ | 4 | 0.18″ |
2″ | 5.0″ | 4.5″ | 4 | 0.22″ |
4″ | 7.0″ | 6.0″ | 8 | 0.30″ |
6″ | 9.0″ | 7.5″ | 8 | 0.34″ |
8″ | 10.5″ | 9.0″ | 8 | 0.36″ |
10″ | 12.0″ | 10.5″ | 8 | 0.43″ |
12″ | 14.0″ | 12.5″ | 8 | 0.49″ |
16″ | 18.0″ | 16.0″ | 12 | 0.59″ |
24″ | 24.0″ | 23.0″ | 12 | 0.71″ |
Flange Pressure Ratings
Flanges are classified according to their pressure ratings, which determine the maximum pressure they can withstand. These ratings are typically denoted as follows:
- 150#: Suitable for low-pressure applications (up to 285 psi at room temperature).
- 300#: Suitable for moderate-pressure applications (up to 740 psi at room temperature).
- 400#: Suitable for medium-high pressure applications (up to 990 psi at room temperature).
- 600#: Suitable for high-pressure applications (up to 1480 psi at room temperature).
- 900#: Suitable for very high-pressure applications (up to 2160 psi at room temperature).
- 1500#: Suitable for extremely high-pressure applications (up to 5000 psi at room temperature).
Example of flange size chart: Class 300 flange
Nominal Pipe Size (in.) | Diameter of Flanges (in.) | No. of Bolts (in.) | Diameter of Bolts (in.) | Diameter of Bolt Holes (in.) | Diameter of Bolt Circle (in.) |
---|---|---|---|---|---|
1/4 | 3-3/8 | 4 | 1/2 | 0.62 | 2-1/4 |
1/2 | 3-3/4 | 4 | 1/2 | 0.62 | 2-5/8 |
3/4 | 4-5/8 | 4 | 5/8 | 0.75 | 3-1/4 |
1 | 4-7/8 | 4 | 5/8 | 0.75 | 3-1/2 |
1-1/4 | 5-1/4 | 4 | 5/8 | 0.75 | 3-7/8 |
1-1/2 | 6-1/8 | 4 | 3/4 | 0.88 | 4-1/2 |
2 | 6-1/2 | 8 | 5/8 | 0.75 | 5 |
2-1/2 | 7-1/2 | 8 | 3/4 | 0.88 | 5-7/8 |
3 | 8-1/4 | 8 | 3/4 | 0.88 | 6-5/8 |
3-1/2 | 9 | 8 | 3/4 | 0.88 | 7-1/4 |
4 | 10 | 8 | 3/4 | 0.88 | 7-7/8 |
5 | 11 | 8 | 3/4 | 0.88 | 9-1/4 |
6 | 12-1/2 | 12 | 3/4 | 0.88 | 10-5/8 |
8 | 15 | 12 | 7/8 | 1 | 13 |
10 | 17-1/2 | 16 | 1 | 1.12 | 15-3/4 |
12 | 20-1/2 | 16 | 1-1/8 | 1.25 | 17-3/4 |
14 | 23 | 20 | 1-1/8 | 1.25 | 20-1/4 |
16 | 25-1/2 | 20 | 1-1/4 | 1.38 | 22-1/2 |
18 | 28 | 24 | 1-1/4 | 1.38 | 24-3/4 |
20 | 30-1/2 | 24 | 1-1/4 | 1.38 | 27 |
24 | 36 | 24 | 1-1/2 | 1.62 | 32 |
Example of flange size chart: Class 400 flange
Nominal Pipe Size (in.) | Diameter of Flanges (in.) | No. of Bolts (in.) | Diameter of Bolts (in.) | Diameter of Bolt Holes (in.) | Diameter of Bolt Circle (in.) |
---|---|---|---|---|---|
1/4 | 3-3/8 | 4 | 1/2 | 0.62 | 2-1/4 |
1/2 | 3-3/4 | 4 | 1/2 | 0.62 | 2-5/8 |
3/4 | 4-5/8 | 4 | 5/8 | 0.75 | 3-1/4 |
1 | 4-7/8 | 4 | 5/8 | 0.75 | 3-1/2 |
1-1/4 | 5-1/4 | 4 | 5/8 | 0.75 | 3-7/8 |
1-1/2 | 6-1/8 | 4 | 3/4 | 0.88 | 4-1/2 |
2 | 6-1/2 | 8 | 5/8 | 0.75 | 5 |
2-1/2 | 7-1/2 | 8 | 3/4 | 0.88 | 5-7/8 |
3 | 8-1/4 | 8 | 3/4 | 0.88 | 6-5/8 |
3-1/2 | 9 | 8 | 7/8 | 1 | 7-1/4 |
4 | 10 | 8 | 7/8 | 1 | 7-7/8 |
5 | 11 | 8 | 7/8 | 1 | 9-1/4 |
6 | 12-1/2 | 12 | 7/8 | 1 | 10-5/8 |
8 | 15 | 12 | 1 | 1.12 | 13 |
10 | 17-1/2 | 16 | 1-1/8 | 1.25 | 15-1/4 |
12 | 20-1/2 | 16 | 1-1/4 | 1.38 | 17-3/4 |
14 | 23 | 20 | 1-1/4 | 1.38 | 20-1/4 |
16 | 25-1/2 | 20 | 1-3/8 | 1.5 | 22-1/2 |
18 | 28 | 24 | 1-3/8 | 1.5 | 24-3/4 |
20 | 30-1/2 | 24 | 1-1/2 | 1.62 | 27 |
24 | 36 | 24 | 1-3/4 | 1.88 | 32 |
Example of flange size chart: Class 600 flange
Nominal Pipe Size (in.) | Diameter of Flanges (in.) | No. of Bolts (in.) | Diameter of Bolts (in.) | Diameter of Bolt Holes (in.) | Diameter of Bolt Circle (in.) |
---|---|---|---|---|---|
1/4 | 3-3/8 | 4 | 1/2 | 0.62 | 2-1/4 |
1/2 | 3-3/4 | 4 | 1/2 | 0.62 | 2-5/8 |
3/4 | 4-5/8 | 4 | 5/8 | 0.75 | 3-1/4 |
1 | 4-7/8 | 4 | 5/8 | 0.75 | 3-1/2 |
1-1/4 | 5-1/4 | 4 | 5/8 | 0.75 | 3-7/8 |
1-1/2 | 6-1/8 | 4 | 3/4 | 0.88 | 4-1/2 |
2 | 6-1/2 | 8 | 5/8 | 0.75 | 5 |
2-1/2 | 7-1/2 | 8 | 3/4 | 0.88 | 5-7/8 |
3 | 8-1/4 | 8 | 3/4 | 0.88 | 6-5/8 |
3-1/2 | 9 | 8 | 7/8 | 1 | 7-1/4 |
4 | 10-3/4 | 8 | 7/8 | 1 | 8-1/2 |
5 | 13 | 8 | 1 | 1.12 | 10-1/2 |
6 | 14 | 12 | 1 | 1.12 | 11-1/2 |
8 | 16-1/2 | 12 | 1-1/8 | 1.25 | 13-3/4 |
10 | 20 | 16 | 1-1/4 | 1.38 | 17 |
12 | 22 | 20 | 1-1/4 | 1.38 | 19-1/4 |
14 | 23-3/4 | 20 | 1-3/8 | 1.5 | 20-3/4 |
16 | 27 | 20 | 1-1/2 | 1.62 | 23-3/4 |
18 | 29-1/4 | 20 | 1-5/8 | 1.75 | 25-3/4 |
20 | 32 | 24 | 1-5/8 | 1.75 | 28-1/2 |
24 | 37 | 24 | 1-7/8 | 2 | 33 |
Flange Dimensions and Considerations for Proper Sizing
Selecting the right flange dimensions is essential for a reliable piping system. Key considerations include the flange thickness, which ensures structural integrity under pressure; the normal bore size, which must match the pipe diameter for a proper fit; and the bolt circle diameter, which ensures correct bolt placement for secure connections. Ensuring these dimensions align with the pipe and gasket specifications is critical for optimal performance and leak-free operation.

Flange Standards and Markings
Flange design adheres to international standards such as ASME B16.5 and B16.47, established by the American Society of Mechanical Engineers (ASME). These standards facilitate easy comparison and ensure compatibility.
These markings, typically found on the flange’s outer edge, follow a stringent hierarchy. They include information like the manufacturer’s logo or code, ASTM material specification, material type, pressure-temperature rating, size, thickness, heat number, and bore size. Special identifiers may also be present, such as QT for quenched and tempered, or W for weld repair.
Applications of Flanges
Flanges are extensively used in various piping systems, primarily for:
- Connecting Pipes: Flanges allow quick connection and disconnection of pipes, facilitating installation and maintenance.
- Connecting Pressure Vessels and Equipment: Flanges are commonly used to connect various pressure vessels and equipment, ensuring system sealing and stability.
- Branching and Diameter Changes in Piping Systems: Flanges enable branching and changes in pipe diameter to meet different process requirements.
- Connecting Instruments and Valves: Flanges provide easy connections for various instruments and valves, facilitating monitoring and control of piping systems.
Installation and Maintenance of Flanges
The installation and maintenance of flanges directly affect the safety and reliability of piping systems and must be carried out strictly according to standards:
- Pre-installation Inspection: Ensure flange surfaces are flat and undamaged, and that bolts, gaskets, and sealing surfaces meet requirements.
- Bolt Tightening: Bolts should be symmetrically and evenly tightened to avoid flange deformation.
- Gasket Selection: Choose appropriate gaskets based on media, temperature, and pressure to ensure sealing effectiveness.
- Regular Inspection and Maintenance: Regularly inspect flange connections, address issues promptly, and ensure long-term stable operation of the system.
Conclusion
Flanges are indispensable components in piping systems, providing secure and flexible connections between various elements. Understanding the different types of flanges, their purposes, and sizes is crucial for selecting the appropriate flange for specific applications. By adhering to industry standards and considering the specific requirements of the system, engineers and designers can ensure the reliable and efficient operation of piping networks across various industries.
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FAQ
Flanges are classified into different facing types such as raised face (RF), flat face (FF), and ring type joint (RTJ). RF and FF faces are common for most applications, while RTJ faces are used in high-pressure and high-temperature environments where a metal-to-metal seal is required.
Flanges are typically made from materials such as carbon steel, stainless steel, alloy steel, and sometimes brass or PVC for specific applications. The choice of material depends on factors like fluid type, pressure, temperature, and environmental conditions.
Flanges are sized based on nominal pipe size (NPS) and are rated according to pressure-temperature ratings (e.g., ANSI class ratings). Standard dimensions and ratings ensure compatibility and interchangeability across different manufacturers and global standards.
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This article was written by engineers from the BOYI team. Fuquan Chen is a professional engineer and technical expert with 20 years of experience in rapid prototyping, mold manufacturing, and plastic injection molding.