What is the difference between ANSI and ASME flanges?

In terms of steel flange design and manufacturing, there’s essentially no difference between ANSI flanges and ASME flanges. Both are made to comply with the same standards, particularly the ASME B16.5 (sometimes referred to as ANSI B16.5 or ANSI/ASME B16.5).

If that’s the case, why do some prefer to call it “ANSI flange”, while others heavily favor the term “ASME flange”? 

The truth is, it all stems from the history of these two interconnected organizations – and nothing more. So, any sources that tell you otherwise don’t know what they’re talking about or are spitting out inaccurate, unverified information.

Read further as this article will explain further the differences and address other related questions to help you understand better.

What is ANSI?

ANSI is the acronym for the American National Standards Institute, which is an organization that oversees the development of voluntary consensus standards for various industries in the US. 

They provide a framework that ensures fair practices in standards development and conformity assessment so that products are consistent and of high quality on a global scale ^[1].

That said, ANSI does not develop the standards themselves, although they used to in the past. They’ve transitioned much of this responsibility to other organizations, such as the ASME.

What is ASME?

ASME, or the American Society of Mechanical Engineers, is the organization responsible for developing technical standards that ensure safety, reliability, and efficiency ^[2]. This includes the ASME B16 series, which covers flanges, fittings, valves, and other piping components.

Going back in time, ASME originally emerged to address specific engineering challenges. Eventually, it took over the role of developing flange standards from ANSI.

To put everything into context, ASME fully develops, updates, and maintains engineering codes & standards. ANSI, on the other hand, accredits ASME and other organizations that develop the standards.

Why are ASME flanges still referred to as ANSI flanges in some places?

To be clear, “ASME flange” is the correct and current designation for flanges that comply with ASME B16.5 standards (which ANSI continues to accredit).

And “ANSI flange” is simply an outdated term for what is now an ASME B16.5 flange.

Still, ASME flanges are still referred to as ANSI flanges in some instances simply out of habit. For years, ANSI was the main body in charge of developing these standards and professionals have become so familiar with the term that they default to calling them ANSI flanges.

It was in 1988 that the ANSI/ASME designation (as in ASME/ANSI B16.5) was used, and it wasn’t until 1996 that the ASME designation (as in ASME B16.5) became the norm ^[3].

How did the ANSI/ASME flange standards develop?

Here’s a timeline of the development of ANSI/ASME flange standards:

• 1920

The American Engineering Standards Committee (later known as the American National Standards Institute (ANSI) was formed. 

This committee organized Sectional Committee B16 to create unified standards for pipe flanges and fittings, with The American Society of Mechanical Engineers (ASME) among its cosponsors.

• 1923

The subcommittee focused on developing standards for steel pipe flanges was established.

• 1927

The first steel pipe flange standard, ASA B16e, was published. Over the years, standards evolved through revisions, incorporating feedback from industry users and addressing new materials and designs.

For instance, in 1942, a special War Committee updated pressure-temperature ratings for materials due to resource conservation efforts during World War II.

•  1982

ANSI accredited the B16 standards as an ASME committee

• 1988

ASME/ANSI B16.5 was published with nickel alloy ratings extended to higher temperatures

• 1996

 ASME B16.5 was published with revised pressure‐temperature ratings

As of writing, the latest version of ASME B16.5 was published in 2021 and is titled B16.5 – Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24, Metric/Inch Standard.

How do I know if a steel flange is ANSI or ASME compliant?

Standard pipe flanges complying with ASME B16.5 are marked with, at the very least, “B16” or “B16.5”, usually along the circumference of the flange face. However, most flanges today also include “ANSI”, “ASME”, or both marks to make it easier for buyers and users to identify, even though this marking is optional according to the standard ^[5].

Are there any differences between ANSI-labeled flanges and those marked ASME?

In terms of quality and design, there are no differences between ANSI-labeled flanges and those marked ASME. Both flanges meet standard dimensional and material requirements specified in ASME B16.5.

Outside of those aspects, any differences come down to variations in manufacturing practices, which can impact the overall quality and reliability of the flange. That said, you should always go with trusted flange manufacturers, as they often maintain higher quality assurance.

What pressure classes are available for ASME flanges?

ASME flanges are available in various pressure classes, including class designations 150 through 2500. Always refer to the ASME B16.5 standard for the specific pressure and temperature ratings as this value depends on the material composition of the pipe flange.

How does an API flange differ from an ANSI/ASME flange?

ASME and API flanges may seem compatible at first glance, as both types use the same bolt circle and number of bolts. However, for the same size, API flanges are rated to withstand much higher pressures than ASME flanges ^[5].

This higher pressure capacity makes API flanges ideal for use in refinery systems and applications with explosive materials.

Another key difference is in the standards they follow: an API flange adheres to the American Petroleum Institute (API) standards, specifically API 6A, while an ASME flange follows standards set by the American Society of Mechanical Engineers (ASME).

Do ASTM standards matter to ANSI/ASME flanges?

Yes, ASTM standards matter to ANSI/ASME flanges since they dictate the materials that will work best for certain applications and pressure ratings. If you have an ASME flange made from carbon steel, for example, it would need to meet an ASTM material standard like ASTM A105.That said, ASTM (American Society for Testing and Materials) standards often complement ANSI/ASME specifications, providing guidelines on the materials used for pipe flanges.

Reference

[1] About ANSI

[2] About ASME

[3] ASME B16.5 History Rev. 2018‐1‐4 Page 1 of 9

[4] B16.5 – Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24, Metric/Inch Standard[5] Flanges – API vs. ASME/ANSI