In any procurement conversation, you will hear flange specifications described as either "ANSI" or "ASME." Many engineers treat these terms as interchangeable, while others worry about subtle differences. The truth is both simpler and more interesting than either assumption. Understanding the history and practical reality of these designations will eliminate confusion and streamline your specification process.
The Historical Origin of ANSI Designations
ANSI stands for American National Standards Institute, the organization that accredits and publishes technical standards in the United States. In the 1970s, ANSI published flange standards under the designation ANSI B16.5, ANSI B16.47, and others, covering industrial piping.
These standards were developed through consensus among engineers, manufacturers, and end users. The technical content was rigorous and comprehensive. For decades, industrial specifications simply referenced "ANSI B16.5" as the governing standard for small-diameter, general-service flanges.
ASME, the American Society of Mechanical Engineers, also developed and published flange standards. ASME B16.5 and ASME B16.47 were the ASME publications, parallel to the ANSI versions. In many cases, the ANSI and ASME versions were technically identical or had only minor differences, and the standards were updated in coordination.
The Consolidation and Modern Approach
Over time, ASME became the primary standards-developing organization recognized by ANSI for piping standards. Instead of maintaining parallel versions, ASME became the author and publisher, with ANSI accrediting the process. Today, when you purchase or reference a flange standard, you are typically using an ASME standard that is accredited by ANSI.
The modern designation is "ASME B16.5" or "ASME B16.47." Some older documents and legacy specifications still reference "ANSI B16.5," but these refer to the same technical content as modern "ASME B16.5" standards. The difference is organizational structure, not flange dimensions or pressure ratings.
In practice, a flange manufactured to ANSI B16.5 (from the 1980s or 1990s) and one manufactured to modern ASME B16.5 are dimensionally compatible and functionally identical. The bolt hole spacing, face diameter, and pressure rating align perfectly.
No Practical Difference for Specification
From a procurement standpoint, ANSI and ASME flanges are not different products. A supplier quoting you a "Class 300 ANSI B16.5 flange" and another quoting a "Class 300 ASME B16.5 flange" are quoting the same item. The flange will have identical dimensions, pressure rating, temperature limits, and material specifications.
The only reason to distinguish between them is historical record-keeping. If your engineering drawings were created in 1995 and specify "ANSI B16.5," your documentation is accurate and does not need updating. If you are creating new specifications today, using "ASME B16.5" reflects current standards conventions.
Some specifications require "ASME" because the facility was constructed to newer documentation standards. Others allow either designation because the technical content is identical. Neither creates compatibility issues or requires design changes.
Why Both Terms Persist in Industry
Legacy documentation, distributed across plants, refineries, and engineering firms, continues to reference ANSI standards because that was the current designation when the documents were created. Updating every old specification to say "ASME" instead of "ANSI" requires effort with no technical benefit, so many organizations have not made the change.
Additionally, international procurement occasionally encounters references to older ANSI standards in third-party documentation or purchasing guides. A distributor's catalog might list items as "ANSI" because their source material was created before the ASME transition became complete.
Regulatory bodies and inspection authorities are also accustomed to both designations. A pressure equipment directive in a Western nation will recognize flanges manufactured to either ANSI or ASME standards, provided the material certificates and test reports demonstrate compliance with the technical requirements.
Best Practices for Specification
When specifying a flange, use whichever designation is already in your documentation system. If your plant standard says "ASME B16.5," continue using that. If your legacy specifications say "ANSI B16.5," there is no need to change it for compatibility or safety reasons.
For new specifications, modern conventions favor "ASME" because it reflects the current standards organization. However, this is a documentation choice, not a technical requirement. A supplier will understand either designation and provide the identical product.
If you are consolidating specifications from multiple sources—perhaps a company merger or integrating an acquired division—standardize on "ASME" for clarity going forward, but do not reject bids or equipment because they reference "ANSI." Dimensional compatibility is guaranteed.
When creating a purchasing specification, be thorough about other parameters: pressure class, size, facing type, material, and temperature rating. These are the characteristics that actually matter. Whether you write "ANSI B16.5" or "ASME B16.5" is essentially a documentation standard with no impact on the flange you receive.
International Considerations
Outside the United States, ANSI and ASME standards may be less familiar. European standards like EN 1092-1 and ISO 7005 define different flange dimensions and pressure ratings. If you are specifying globally, always include the full standard designation and pressure class, not just "ANSI" or "ASME," because those abbreviations are primarily American conventions.
Similarly, if you receive flange specifications from international suppliers, verify that they are referencing ASME or ANSI standards (U.S. convention) versus ISO or EN standards (international convention). Mixing these creates incompatibility problems that are real and require design work to resolve.
Conclusion
ANSI and ASME flange designations refer to the same technical standards, with ASME being the modern convention. The two terms are interchangeable from a technical and procurement standpoint. No compatibility issues, design changes, or performance differences arise from using one designation versus the other. When specifying flanges, focus on the critical parameters—standard number, pressure class, material, size, and temperature—and do not worry about whether your specification says "ANSI" or "ASME." Your supplier will provide the correct flange either way.