What Is ASME B31.3? Process Piping Code
ASME B31.3 is the Process Piping Code published by the American Society of Mechanical Engineers. It establishes mandatory requirements for the design, materials, fabrication, assembly, erection, examination, inspection, and testing of piping systems in petroleum refineries, chemical plants, pharmaceutical facilities, textile plants, paper mills, semiconductor plants, and related process industries.
ASME B31.3 is the most widely referenced piping code in the oil and gas and petrochemical sectors worldwide.
Scope
| Covered | Not Covered |
|---|---|
| Process piping within a plant (from equipment nozzle to equipment nozzle) | Power piping (covered by ASME B31.1) |
| Piping for chemicals, hydrocarbons, gases, steam (within process plants) | Pipeline transportation (covered by ASME B31.4 / B31.8) |
| Utility piping within a process facility | Building services (plumbing, HVAC) |
| Refrigeration piping | Nuclear piping (ASME III) |
Key Design Requirements
ASME B31.3 addresses five service categories, each with different design, examination, and testing requirements:
| Fluid Service Category | Description | Examination Level |
|---|---|---|
| Normal (Category D) | Non-flammable, non-toxic fluids at low pressure/temperature | Reduced (visual) |
| Category M | Lethal fluids; small leak could be fatal | Maximum (100% RT or UT of welds) |
| High Pressure | Pressures exceeding Class 2500 or design stress rules | Special design rules (Chapter IX) |
| Elevated Temperature | Above creep range for the material | Time-dependent allowable stress |
| Severe Cyclic | > 7,000 full-range thermal cycles over design life | Fatigue analysis required |
Design Pressure and Temperature
The code requires that piping be designed for the most severe combination of coincident pressure and temperature expected during operation, startup, shutdown, and upset conditions. Wall thickness is calculated using the Barlow formula modified by code:
t = PD / (2SE + 2YP) + corrosion allowance + mill tolerance
Where P = design pressure, D = outside diameter, S = allowable stress (from ASME B31.3 Table A-1), E = weld joint efficiency, and Y = a temperature-dependent coefficient.
Materials
ASME B31.3 lists approved materials in its Appendix A tables, including carbon steel, alloy steel, stainless steel, and nickel alloys. Materials not listed require special approval by the owner. Each material has temperature-dependent allowable stress values.
Fabrication and Examination
| Requirement | B31.3 Rule |
|---|---|
| Welding qualification | Per ASME Section IX (WPS + PQR + welder performance) |
| NDE requirements | Based on fluid category: 5% spot RT (Normal), 20% RT (severe cyclic), 100% RT (Category M) |
| Post-weld heat treatment | Required for carbon steel above 19 mm (3/4”) wall, Cr-Mo alloys, and per material requirements |
| Hardness testing | Mandatory for sour service (NACE MR0175) and after PWHT |
| Hydrostatic test | 1.5 x design pressure; held for minimum 10 minutes |
| Pneumatic test | 1.1 x design pressure (higher risk; requires safety precautions) |
Relationship to Other Codes
ASME B31.3 references numerous standards for components: ASME B16.5 for flanges, ASME B16.9 for butt weld fittings, ASME B16.34 for valves, and ASME B16.11 for forged fittings. The piping engineer uses B31.3 as the governing code and these referenced standards for component dimensions, ratings, and testing.
Piping inspection during operation falls under API 570, not B31.3. The code governs new construction; API 570 governs in-service inspection.
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