ASME B31.3 (Process Piping) and ASME B31.1 (Power Piping) are two of the most widely used piping codes in the industry. While they share many foundational principles, critical differences exist in material selection, allowable stress criteria, design conditions, and testing requirements. Choosing the wrong code can lead to non-compliance, safety issues, or unnecessary cost.
Scope and Applicability
ASME B31.3 covers piping typically found in petroleum refineries, chemical plants, pharmaceutical facilities, and other process industries. B31.1 covers piping for electric power generation, industrial and institutional boilers, and associated steam systems. The fundamental question: is the piping part of a "process" (B31.3) or part of a "power generation" system (B31.1)?
Allowable Stress Basis
One of the most significant differences is how allowable stresses are determined. B31.3 uses a higher percentage of yield strength for its allowable stress basis — typically 2/3 of yield versus 60% of yield for B31.1. This means for the same material and temperature, B31.3 may allow slightly higher design stresses. However, B31.1 has more stringent requirements for creep range materials due to the sustained high-temperature operation of power piping.
Material Selection Differences
B31.3 includes a broader list of listed materials and allows for unlisted materials through an engineering analysis. B31.1 has a more restricted materials list and requires materials to be listed in Section II of the ASME Boiler and Pressure Vessel Code. For high-temperature power applications, B31.1 is more prescriptive about creep-fatigue interaction and material degradation mechanisms.
Design Conditions and Allowances
B31.3 requires a corrosion allowance based on expected corrosion rates and design life, typically 1/16" to 1/8" for most services. B31.1 does not mandate a specific corrosion allowance but requires consideration of erosion and corrosion effects. B31.3 also provides more flexibility in defining design pressures and temperatures when multiple operating conditions exist.
Testing and Inspection
Both codes require hydrostatic testing, but the test pressure requirements differ. B31.3 typically requires 1.5 times the design pressure, while B31.1 uses a similar factor but with additional considerations for elevated temperature service. B31.3 also provides alternative leak testing methods (pneumatic, sensitive leak test) when hydrostatic testing is impractical, with stricter safety precautions.
Conclusion
Understanding the distinctions between B31.3 and B31.1 is essential for proper code application. When in doubt, consult the specific jurisdiction requirements and review the code's scope clauses carefully. Using the correct code from the start saves time, reduces costs, and ensures regulatory compliance.