Why this matters
The single most consequential choice on a piping data sheet is the design code. ASME B31.3 and ASME B31.1 drive different wall thicknesses, different NDE percentages, different hydrotest pressures and ultimately different fitting and flange material specifications. Buyers who copy a previous spec without checking which code applies often end up with the wrong wall schedule or unnecessary radiography on the MTR.
Scope comparison
| Aspect | ASME B31.1 (Power Piping) | ASME B31.3 (Process Piping) |
|---|---|---|
| Typical industries | Power generation: utility steam, boiler external piping, district heating | Refining, petrochemical, chemical, pharma, gas processing |
| Allowable stress basis | Lower of 1/3.5 of ultimate tensile strength or 2/3 of yield (commonly described as roughly a 4:1 safety factor on UTS) | Lower of 1/3 of ultimate tensile strength or 2/3 of yield (commonly described as roughly a 3:1 safety factor on UTS) |
| Practical effect | More conservative wall thickness for the same material and pressure | Often allows thinner walls than B31.1 for the same conditions |
| SIFs | Single SIF for in-plane and out-of-plane moments | Separate in-plane and out-of-plane SIFs |
| NDE | Typically 100 percent volumetric for high-pressure / high-temperature steam | Random sampling for Normal Fluid Service; higher coverage for Category M and Severe Cyclic |
| Hydrotest | 1.5 x design pressure | 1.5 x design pressure x stress ratio (allowable stress at test temp / allowable stress at design temp) |
Key differences
- Safety factor philosophy: B31.1 is more conservative on allowable stress, reflecting the high-energy, high-cycle nature of power steam piping. B31.3 allows greater economic optimisation, which is why a refinery line list typically comes out lighter than a power station boiler external piping list of the same conditions.
- Fluid service categories: B31.3 introduces Category D (low hazard), Normal, Category M (highly toxic) and Severe Cyclic. Each tightens NDE and joint design.
- SIFs: For elbows, tees and reducers, B31.3 distinguishes in-plane and out-of-plane bending SIFs, which matters in flexibility analysis.
- Hydrotest: B31.3's stress-ratio adjustment can demand a higher test pressure than B31.1 when the test temperature is much lower than design temperature.
When to choose which
- Boilers and BOP, utility steam headers, large turbine leads, geothermal and combined-cycle steam piping: B31.1.
- Refinery process units, petrochemical reactors, gas plants, LNG, chemical batch lines, hydrogen and amine service: B31.3.
- Pipelines under jurisdiction: usually B31.4 (liquids) or B31.8 (gas), not B31.1 / B31.3.
- Building services: B31.9.
For either code, the matching components are usually seamless butt-welding pipe fittings to ASME B16.9, seamless steel pipes to ASTM A106 / A335, and forged flanges to ASME B16.5 / B16.47.
Procurement / spec checklist
- State the governing code on the data sheet: B31.1 or B31.3 (or B31.4 / B31.8).
- For B31.3, also state the fluid service category (D, Normal, M or Severe Cyclic).
- Confirm the wall thickness calculation uses allowable stress from ASME II Part D for the chosen material at design temperature.
- Define the corrosion allowance and the mill tolerance assumption (typically 12.5 percent on seamless pipe).
- Specify the NDE percentage and acceptance criteria consistent with the code.
- State hydrotest pressure or formula.
- Reference the latest published edition of the code.
Need help converting a B31.1 power-grade BOM into a B31.3 process-grade BOM (or vice versa)? Send the line list to our inquiry page.
Sources
- https://whatispiping.com/difference-asme-b31-3-and-b31-1/
- https://www.alekvs.com/asme-b31-3-vs-asme-b31-1-whats-the-difference/
- https://epcland.com/asme-b31-1-vs-asme-b31-3/
- https://www.littlepeng.com/single-post/asme-b-31-1-asme-b-31-3-code-comparison
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