I. Introduction

Pipe flanges are an integral component of piping systems, used to connect pipes, valves, pumps, and other equipment. They provide a secure and leak-proof seal while allowing for easy maintenance and disassembly of the system. However, with different types of flanges and various standards governing their design and use, it can be challenging to select the right flange for your specific application. In this article, we will discuss everything you need to know about pipe flanges, with a focus on the differences between the two most common standards, ANSI and ASME. By the end of this article, you will have a clear understanding of how to choose the right flange standard for your application, ensuring optimal system performance and safety.

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II. ANSI vs. ASME Standards

A. What is ANSI?

1. History and purpose

The American National Standards Institute (ANSI) is a private, non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, and systems in the United States. It was founded in 1918 and is recognized as the official U.S. representative of the International Organization for Standardization (ISO).

The purpose of ANSI is to promote voluntary standards that enhance product quality, safety, and efficiency, while also improving international trade by ensuring interoperability and compatibility between products.

2. ANSI flange standards

ANSI has developed several flange standards that govern the design, dimensions, and pressure ratings of flanges used in piping systems. The most commonly used ANSI flange standards are:

  • ANSI B16.5: This standard covers pipe flanges and flanged fittings from NPS ½” to NPS 24″ and pressure classes ranging from 150 to 2500. It includes various types of flanges, such as slip-on, weld neck, threaded, lap joint, and blind flanges.
  • ANSI B16.47: This standard covers larger pipe flanges from NPS 26″ to NPS 60″ and pressure classes ranging from 75 to 900. It includes two series of flanges, Series A and Series B, with different dimensions and pressure ratings.
  • Other ANSI flange standards: ANSI also has standards for special-purpose flanges, such as API flanges, orifice flanges, and swivel-ring flanges.

These standards provide guidelines for the design and manufacture of flanges to ensure compatibility and interchangeability between different manufacturers and users of flanges.

B. What is ASME?

1. History and purpose

The American Society of Mechanical Engineers (ASME) is a professional organization that promotes the development of mechanical engineering and related disciplines in the United States. It was founded in 1880 and is known for developing codes, standards, and certifications for various industries, including piping systems.

The purpose of ASME is to advance the engineering profession through the development and dissemination of technical knowledge and standards, as well as the establishment of ethical and professional practices.

2. ASME flange standards

ASME has developed several flange standards that govern the design, dimensions, and pressure ratings of flanges used in piping systems. The most commonly used ASME flange standards are:

  • ASME B16.5: This standard is similar to ANSI B16.5 and covers pipe flanges and flanged fittings from NPS ½” to NPS 24″ and pressure classes ranging from 150 to 2500. It includes various types of flanges, such as slip-on, weld neck, threaded, lap joint, and blind flanges.
  • ASME B16.47: This standard is similar to ANSI B16.47 and covers larger pipe flanges from NPS 26″ to NPS 60″ and pressure classes ranging from 75 to 900. It includes two series of flanges, Series A and Series B, with different dimensions and pressure ratings.
  • Other ASME flange standards: ASME also has standards for special-purpose flanges, such as spectacle blind flanges, orifice flanges, and reducing flanges.

These standards provide guidelines for the design and manufacture of flanges to ensure compatibility and interchangeability between different manufacturers and users of flanges. ASME flange standards are widely recognized and used in various industries, including oil and gas, petrochemical, and power generation.

C. Key differences between ANSI and ASME flange standards

While ANSI and ASME flange standards are similar in many ways, there are a few key differences between them. Some of the most notable differences include:

  1. Scope: While both ANSI and ASME flange standards cover similar types of flanges and pressure classes, ASME standards have a broader scope and cover more types of equipment, such as boilers, pressure vessels, and piping systems.
  2. Design requirements: ASME flange standards have more rigorous design requirements than ANSI standards. For example, ASME flanges must undergo more stringent testing and inspection procedures to ensure compliance with the standards.
  3. Pressure-temperature ratings: The pressure-temperature ratings for ASME flanges are generally higher than those for ANSI flanges. This is because ASME flanges are designed to handle more extreme operating conditions, such as high pressure and high temperature.
  4. Flange facings: ANSI and ASME flanges use different flange-facing types. ANSI flanges typically use raised face (RF) or flat face (FF) flange facings, while ASME flanges may also use ring joint (RTJ) or tongue and groove (T&G) flange facings.
  5. Gasket compatibility: Due to the different flange facing types, ANSI and ASME flanges may require different types of gaskets to achieve a proper seal. This means that ANSI and ASME flanges may not be interchangeable without the use of adapter flanges or special gaskets.

Understanding these differences can help you choose the right flange standard for your specific application, ensuring optimal performance and safety.

III. ANSI Flange Standards

A. ANSI B16.5

1. Overview and scope

ANSI B16.5 is a flange standard developed by the American National Standards Institute (ANSI) that covers the design, dimensions, and pressure ratings of pipe flanges and flanged fittings. It is widely used in various industries, including oil and gas, chemical, and water treatment.

The scope of ANSI B16.5 covers flanges from NPS ½” to NPS 24″ and pressure classes ranging from 150 to 2500. It includes various types of flanges, such as slip-on, weld neck, threaded, lap joint, and blind flanges.

2. Types of flanges covered

ANSI B16.5 covers a wide range of flange types, including:

  • Slip-on flanges: These flanges have a flat face and are slipped over the end of a pipe or fitting before being welded in place.
  • Weld neck flanges: These flanges have a neck that is welded to the end of a pipe or fitting, providing a strong and leak-proof connection.
  • Threaded flanges: These flanges have internal threads that allow them to be screwed onto the end of a pipe or fitting.
  • Lap joint flanges: These flanges are used with a stub end fitting, allowing for easy disassembly and maintenance.
  • Blind flanges: These flanges are used to seal the end of a pipe or fitting, preventing the flow of fluid.

3. Pressure ratings and dimensions

ANSI B16.5 flanges are rated for pressure classes ranging from 150 to 2500, with higher pressure classes corresponding to higher pressure-temperature ratings. The dimensions of ANSI B16.5 flanges vary depending on the flange type and pressure class.

For example, a 150# slip-on flange for a 4″ pipe would have an outside diameter (OD) of approximately 9″, a bolt circle diameter (BCD) of approximately 7.5″, and a thickness (T) of approximately 0.88″. A 2500# weld neck flange for a 4″ pipe would have an OD of approximately 9.5″, a BCD of approximately 6.5″, and a T of approximately 1.38″.

Understanding the pressure ratings and dimensions of ANSI B16.5 flanges is essential for selecting the right flange for a particular application and ensuring a safe and leak-free connection.

B. ANSI B16.47

1. Overview and scope

ANSI B16.47 is a flange standard developed by the American National Standards Institute (ANSI) that covers the design, dimensions, and pressure ratings of large-diameter steel flanges. It is commonly used in industries that require high-pressure, high-temperature piping systems, such as oil and gas, petrochemical, and power generation.

The scope of ANSI B16.47 covers flanges from NPS 26″ to NPS 60″ and pressure classes ranging from 75 to 900. It includes two different series, Series A and Series B, which have different dimensions and pressure ratings.

2. Types of flanges covered

ANSI B16.47 covers a range of large-diameter flange types, including:

  • Weld neck flanges: These flanges have a neck that is welded to the end of a pipe or fitting, providing a strong and leak-proof connection.
  • Blind flanges: These flanges are used to seal the end of a pipe or fitting, preventing the flow of fluid.
  • Slip-on flanges: These flanges have a flat face and are slipped over the end of a pipe or fitting before being welded in place.
  • Lap joint flanges: These flanges are used with a stub end fitting, allowing for easy disassembly and maintenance.
  • Threaded flanges: These flanges have internal threads that allow them to be screwed onto the end of a pipe or fitting.

3. Pressure ratings and dimensions

ANSI B16.47 flanges are rated for pressure classes ranging from 75 to 900, with higher pressure classes corresponding to higher pressure-temperature ratings. The dimensions of ANSI B16.47 flanges vary depending on the flange type and pressure class, as well as the specific series (A or B) of the standard.

For example, a Series A weld neck flange for a 36″ pipe with a 150# pressure rating would have an outside diameter (OD) of approximately 51.38″, a bolt circle diameter (BCD) of approximately 47.5″, and a thickness (T) of approximately 2.75″. A Series B blind flange for a 48″ pipe with a 900# pressure rating would have an OD of approximately 72″, a BCD of approximately 61.75″, and a T of approximately 3.75″.

Understanding the pressure ratings and dimensions of ANSI B16.47 flanges is essential for selecting the right flange for a particular application and ensuring a safe and leak-free connection in high-pressure, high-temperature environments.

C. Other ANSI flange standards

In addition to ANSI B16.5 and B16.47, there are several other ANSI flange standards that are commonly used in various industries. Here are some examples:

  1. ANSI B16.1: This standard covers cast iron pipe flanges and flanged fittings.
  2. ANSI B16.11: This standard covers forged steel fittings, including socket welding and threaded fittings.
  3. ANSI B16.36: This standard covers orifice flanges and flange unions.
  4. ANSI B16.48: This standard covers steel line blanks, which are used to seal the ends of pipelines.
  5. ANSI B16.9: This standard covers factory-made wrought steel butt-welding fittings.

Each of these ANSI standards specifies the design, dimensions, and pressure ratings of specific types of flanges or fittings. By adhering to these standards, manufacturers can ensure that their products are compatible with other components in a piping system and can be safely and reliably used in various applications.

IV. ASME Flange Standards

A. ASME B16.5

1. Overview and scope

ASME B16.5 is a flange standard developed by the American Society of Mechanical Engineers (ASME) that covers the design, dimensions, and pressure ratings of steel pipe flanges and flanged fittings. It is widely used in industries such as oil and gas, petrochemical, and power generation.

The scope of ASME B16.5 covers flanges from NPS 1/2″ to NPS 24″ and pressure classes ranging from 150 to 2500. It includes various types of flanges, such as weld neck, slip-on, threaded, lap joint, socket weld, and blind flanges.

2. Types of flanges covered

ASME B16.5 covers a wide range of flange types, including:

  • Weld neck flanges: These flanges have a long tapered neck that is welded to the end of a pipe or fitting. They are designed to transfer stress from the flange to the pipe, reducing the risk of leakage and providing a strong and reliable connection.
  • Slip-on flanges: These flanges are slipped over the end of a pipe or fitting and then welded in place. They are generally easier to install than weld neck flanges but may be less secure under high-stress conditions.
  • Threaded flanges: These flanges have internal threads that allow them to be screwed onto the end of a pipe or fitting. They are commonly used in low-pressure applications and for pipes with small diameters.
  • Lap joint flanges: These flanges are used with a stub end fitting, allowing for easy disassembly and maintenance. They are often used in applications where the piping system requires frequent cleaning or inspection.
  • Socket weld flanges: These flanges have a socket-like design that allows them to be welded to the end of a pipe or fitting. They are commonly used in high-pressure applications and for pipes with small diameters.
  • Blind flanges: These flanges are used to seal the end of a pipe or fitting, preventing the flow of fluid. They are commonly used in applications where the piping system may need to be expanded in the future.

3. Pressure ratings and dimensions

ASME B16.5 flanges are rated for pressure classes ranging from 150 to 2500, with higher pressure classes corresponding to higher pressure-temperature ratings. The dimensions of ASME B16.5 flanges vary depending on the flange type and pressure class.

For example, a 150# weld neck flange for a 2″ pipe would have a bolt circle diameter (BCD) of approximately 3.38″, a flange thickness (T) of approximately 0.56″, and a raised face height (RF) of approximately 0.06″. A 2500# slip-on flange for a 12″ pipe would have a BCD of approximately 17.5″, a T of approximately 3.75″, and an RF of approximately 0.38″.

Understanding the pressure ratings and dimensions of ASME B16.5 flanges is essential for selecting the right flange for a particular application and ensuring a safe and leak-free connection in various environments.

B. ASME B16.47

1. Overview and scope

ASME B16.47 is a flange standard developed by the American Society of Mechanical Engineers (ASME) that covers large-diameter steel flanges from NPS 26″ to NPS 60″. It is commonly used in industries such as oil and gas, petrochemical, and power generation.

The scope of ASME B16.47 covers two series of flanges:

  • Series A: These flanges are equivalent to MSS SP-44 flanges and have larger bolt holes than Series B flanges. They are commonly used in low-pressure, large-diameter pipelines.
  • Series B: These flanges are equivalent to API 605 flanges and have smaller bolt holes than Series A flanges. They are commonly used in high-pressure, large-diameter pipelines.

2. Types of flanges covered

ASME B16.47 covers a wide range of flange types, including:

  • Weld neck flanges: These flanges have a long tapered neck that is welded to the end of a pipe or fitting. They are designed to transfer stress from the flange to the pipe, reducing the risk of leakage and providing a strong and reliable connection.
  • Blind flanges: These flanges are used to seal the end of a pipe or fitting, preventing the flow of fluid. They are commonly used in applications where the piping system may need to be expanded in the future.
  • Slip-on flanges: These flanges are slipped over the end of a pipe or fitting and then welded in place. They are generally easier to install than weld neck flanges but may be less secure under high-stress conditions.
  • Threaded flanges: These flanges have internal threads that allow them to be screwed onto the end of a pipe or fitting. They are commonly used in low-pressure applications and for pipes with small diameters.
  • Lap joint flanges: These flanges are used with a stub end fitting, allowing for easy disassembly and maintenance. They are often used in applications where the piping system requires frequent cleaning or inspection.

3. Pressure ratings and dimensions

ASME B16.47 flanges are rated for pressure classes ranging from 75 to 900, with higher pressure classes corresponding to higher pressure-temperature ratings. The dimensions of ASME B16.47 flanges vary depending on the flange type, pressure class, and series.

For example, a Series A 150 weld neck flange for a 26″ pipe would have a bolt circle diameter (BCD) of approximately 29.75″, a flange thickness (T) of approximately 2.56″, and a raised face height (RF) of approximately 0.56″. A Series B 900 blind flange for a 60″ pipe would have a BCD of approximately 70.5″, a T of approximately 12.75″, and an RF of approximately 1.19″.

Understanding the pressure ratings and dimensions of ASME B16.47 flanges is essential for selecting the right flange for a particular application and ensuring a safe and leak-free connection in various environments.

C. Other ASME flange standards

In addition to ASME B16.5 and ASME B16.47, the American Society of Mechanical Engineers (ASME) has developed several other flange standards that are commonly used in various industries. Here are some examples:

  1. ASME B16.36: This standard covers orifice flanges, which are used to measure the flow rate of fluid in a pipeline. Orifice flanges are designed with a small hole or orifice in the center, which is typically paired with an orifice plate to measure the flow of fluid.
  2. ASME B16.48: This standard covers line blanks, also known as spectacle blinds or figure-8 blinds. Line blanks are used to block off a section of a pipeline for maintenance or repair, and are designed with a solid plate on one side and an open ring on the other.
  3. ASME B16.1: This standard covers cast iron flanges and flanged fittings, which are commonly used in low-pressure applications such as water and sewage systems.
  4. ASME B16.9: This standard covers factory-made wrought steel buttwelding fittings, which are used to connect pipes and fittings with a permanent, welded joint.
  5. ASME B16.20: This standard covers metallic gaskets for pipe flanges, which are designed to provide a tight, leak-proof seal between two flange faces. The standard covers a variety of gasket types, including spiral wound, corrugated, and jacketed gaskets.

Understanding the various ASME flange standards and their applications is important for selecting the right flange for a specific piping system. Each standard covers a specific type of flange or fitting and has its own pressure ratings, dimensions, and material requirements. Consulting with an experienced engineer or supplier can help ensure that the correct flange standard is selected for a given application.

V. Choosing the Right Flange Standard

A. Factors to consider

Choosing the right flange standard for a given application involves considering several important factors, including application requirements, material compatibility, and pressure and temperature ratings.

1. Application requirements:

  • Type of fluid or gas being transported: Different fluids and gases have different properties, such as viscosity, corrosiveness, and temperature range. The flange standard chosen should be suitable for the specific fluid or gas being transported.
  • Operating temperature and pressure: The flange standard chosen should have appropriate temperature and pressure ratings that meet the specific requirements of the application. The temperature and pressure ratings of the flange should be able to withstand the highest expected temperature and pressure of the system.
  • Environmental factors: Other environmental factors that may affect the system include vibration, shock, and exposure to the elements. Flange standards that are suitable for outdoor or harsh environments may be required.

2. Material compatibility:

  • Chemical composition: The chemical composition of the piping system and the flange should be compatible to avoid galvanic corrosion, which can lead to leaks and failure of the system. For example, using a stainless steel flange with a carbon steel piping system may lead to corrosion.
  • Temperature range: The material used for the flange should be able to withstand the highest expected temperature of the system without degrading or failing.
  • Pressure range: The material used for the flange should be able to withstand the highest expected pressure of the system without degrading or failing.

3. Pressure and temperature ratings:

  • ANSI flanges: ANSI flanges are typically used in low-pressure and low-temperature applications, with pressure ratings ranging from 150 to 900 pounds per square inch (psi) and temperature ratings up to 600 degrees Fahrenheit (F).
  • ASME flanges: ASME flanges are used in high-pressure and high-temperature applications, with pressure ratings ranging from 150 to 2500 psi and temperature ratings up to 1000 degrees F.

B. Examples of when to use ANSI vs. ASME flanges

1. ANSI Flanges:

  • Water supply systems: ANSI flanges are commonly used in water supply systems, where the pressure and temperature requirements are relatively low. For example, ANSI 150 or ANSI 300 flanges may be used in municipal water supply systems.
  • HVAC systems: ANSI flanges may also be used in HVAC systems, where the pressure and temperature requirements are low. For example, ANSI 150 or ANSI 300 flanges may be used in heating or cooling systems in buildings.

2. ASME Flanges:

  • Oil and gas pipelines: ASME flanges are commonly used in oil and gas pipelines, where the pressure and temperature requirements are high. For example, ASME B16.5 or ASME B16.47 Series A flanges may be used in pipelines that transport crude oil or natural gas.
  • Power plant systems: ASME flanges may also be used in power plant systems, where the pressure and temperature requirements are high. For example, ASME B16.5 or ASME B16.47 Series A flanges may be used in steam pipelines in a power plant.

It is important to note that the choice between ANSI and ASME flanges will ultimately depend on the specific requirements of the application, such as the type of fluid or gas being transported, the operating temperature and pressure, and any other environmental factors that may affect the system. Consulting with a qualified engineer or flange supplier can help ensure that the appropriate flange standard is chosen for the application.

VI. Conclusion

In summary, pipe flanges are an essential component of many industrial systems, and the selection of the appropriate flange standard is crucial to ensure reliable and safe operation. The American National Standards Institute (ANSI) and the American Society of Mechanical Engineers (ASME) have developed several flange standards that specify the dimensions, materials, and pressure ratings of flanges used in various applications. While ANSI and ASME flange standards share some similarities, they also have important differences that must be considered when choosing the right flange standard for a specific application.

When selecting a flange standard, it is essential to consider factors such as the application requirements, material compatibility, and pressure and temperature ratings. Consulting with a qualified engineer or flange supplier can help ensure that the appropriate flange standard is chosen for the application, which can help prevent costly downtime, leaks, or even catastrophic failures.

In conclusion, by understanding the key differences between ANSI and ASME flange standards and by carefully considering the application requirements, engineers can select the right flange standard to ensure the safe and reliable operation of their systems.

If you would like to learn more about pipe flanges and their applications, you may find the following resources helpful:


Lewis Liu

Hello, I am Lewis Liu, a professional sales engineer with over ten years of experience in the flange fittings industry. I am highly knowledgeable in flange selection, installation, and maintenance. I am passionate about providing customers with the best solutions to ensure their pipeline systems run smoothly, safely, and reliably.

If you have any questions or concerns regarding flange fittings for your pipelines, whether it’s about selection, material choice, specification requirements, or any other aspect, please feel free to contact me at any time. I am committed to offering professional advice and assistance to help you make informed decisions and meet your needs.

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