When specifying flanges, choosing between ASME B16.5 and ASME B16.47 can make the difference between a reliable, cost-effective piping system and an over-engineered or under-specified installation. Both standards are foundational in industrial piping, yet they serve distinctly different applications. Understanding when to use each is essential for any engineer or procurement professional.
The Purpose and Application Scope
ASME B16.5 covers flanges and flanged fittings for use with steel, alloy steel, stainless steel, copper-nickel, and nickel-copper piping, primarily for sizes NPS 1/2 through NPS 24. These flanges are designed for general industrial piping systems operating at moderate pressures and temperatures.
ASME B16.47 is the standard for large-diameter flanges used in industrial applications, with Sections A and B covering different size ranges and pressure ratings. Section A covers sizes from NPS 26 through NPS 60, while Section B covers NPS 16 through NPS 48 with higher pressure ratings. B16.47 flanges are engineered for substantial industrial installations where standard B16.5 sizing becomes impractical.
Size Ranges and When They Overlap
The practical sizing ranges determine initial selection. B16.5 tops out at NPS 24, making it ideal for most petrochemical refineries, power plants, and industrial water systems. When your project requires pipes larger than NPS 24, B16.47 becomes necessary.
The overlap between NPS 16 and NPS 24 exists in both standards, but for different reasons. In this range, you might use B16.5 for standard industrial applications or B16.47 if you need higher pressure ratings or specify large-diameter flange designs. The choice depends on your specific pressure and temperature requirements, not size alone.
Pressure Class Differences
B16.5 defines pressure classes as 150, 300, 400, 600, 900, 1500, and 2500 PSI nominal ratings. These classes apply uniformly across the size range and include specific temperature derating curves for each material group.
B16.47 Section A and Section B each define their own pressure rating systems. The design approach allows for aggressive ratings suitable for large-diameter applications in extreme conditions—think deepwater subsea piping or high-pressure transmission lines. B16.47 flanges can achieve working pressures that would require significantly heavier standard B16.5 designs.
Material Groups and Temperature Derating
Both standards organize materials into groups based on design stress at temperature. A ferrite steel like A105 material occupies the same group across both standards, so temperature derating curves align. However, the absolute pressure ratings differ because the flange face areas and bolt circles scale with the pipe size.
When working at elevated temperatures—say 600°F—the engineer must apply the appropriate temperature derating factor. This is not a B16.5 versus B16.47 question; it's inherent to material selection. Your deration factor for A105 steel at 600°F is the same whether you're specifying B16.5 or B16.47.
Common Specification Mistakes
One frequent error is over-specifying B16.5 flanges for large piping systems. An engineer might specify a Class 600 B16.5 flange at NPS 24 when a B16.47 flange at the same or lower class would be more economical and equally safe. The reverse mistake—specifying B16.47 when B16.5 would suffice—adds unnecessary cost and weight.
Another pitfall is assuming pressure class alone determines adequacy. A Class 300 flange is not automatically "weak." At the right temperature, a Class 300 flange holds its rating perfectly. The issue arises when the design temperature exceeds the rated envelope, requiring a higher pressure class or different material.
Confusion over pressure ratings also occurs when mixing face specifications. Raised Face (RF), Flat Face (FF), and Ring Joint Face (RJF) exist in both standards but have different sealing mechanics. Ensure your gasket specification matches the flange facing type.
Face Types and Sealing Integrity
B16.5 includes RF, FF, RTJ (Ring Type Joint), and Lap Joint options across its range. B16.47 similarly offers these options, though RTJ is predominant in large-diameter subsea and deepwater applications due to its superior sealing under extreme conditions.
Selecting the wrong face type can compromise joint integrity even if the flange pressure class is correct. Ring Type Joint faces, for instance, require precise gasket geometry and bolt tension control. Over-tightening or using an incompatible gasket voids the flange's pressure rating.
Making the Right Selection
Start with the system design pressure and temperature. Calculate the maximum allowable working pressure (MAWP) for your material and temperature. Then select the flange standard and class that safely accommodates this MAWP with appropriate margins.
For pipes NPS 1/2 through NPS 24 in standard industrial conditions, ASME B16.5 is almost always the specification of choice. It is widely available, competitively priced, and proven over decades in millions of installations.
For NPS 26 and larger, or when you need pressure ratings beyond B16.5 Class 2500, specify ASME B16.47. Verify that your supplier stocks the specific size and class before finalizing the design, as large-diameter flanges often involve longer lead times and custom manufacturing.
Conclusion
The distinction between ASME B16.5 and ASME B16.47 is practical and economic, not simply theoretical. B16.5 governs the vast majority of industrial piping; B16.47 handles the exceptional cases where size or pressure demands a different approach. Understanding the purpose, scope, and limitations of each standard ensures your specifications are accurate, cost-effective, and safe. When in doubt, consult your supplier and verify the application requirements against the relevant standard's temperature derating curves and pressure rating tables.