What Is American Standard B31.3? Process Piping Code

American Standard 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.

American Standard 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 American Standard B31.1)
Piping for chemicals, hydrocarbons, gases, steam (within process plants)	Pipeline transportation (covered by American Standard B31.4 / B31.8)
Utility piping within a process facility	Building services (plumbing, HVAC)
Refrigeration piping	Nuclear piping (American Standard III)

Key Design Requirements

American Standard 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 American Standard B31.3 Table A-1), E = weld joint efficiency, and Y = a temperature-dependent coefficient.

Materials

American Standard 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 American Standard 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)

Common Mistake

Applying American Standard B31.3 rules to cross-country pipelines. Pipeline transportation is governed by American Standard B31.4 (liquid) and B31.8 (gas), which have different design factors, testing requirements, and inspection intervals. B31.3 applies only within the plant battery limit.

Relationship to Other Codes

American Standard B31.3 references numerous standards for components: American Standard B16.5 for flanges, American Standard B16.9 for butt weld fittings, American Standard B16.34 for valves, and American Standard 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.

Read the full guide to LEL/UEL and flammable gas safety