ASME B31.3

Covers — piping typically found in petroleum refineries, chemical, pharmaceutical, textile, paper, semiconductor, and cryogenic plants, and related processing plants and terminals. The scope is divided into fluid services:

• Normal Fluid Service — standard piping subject to B31.3 base requirements.
• Category D Fluid Service — nonflammable, nontoxic fluids not exceeding 150 psig (1,035 kPa) or 366 °F (186 °C).
• Category M Fluid Service — highly toxic fluids requiring stricter design and examination.
• High Pressure Fluid Service — designated by the owner (Chapter IX applies).
• High Purity Fluid Service — requiring alternative methods for cleanliness (Chapter X applies).

Does NOT cover (exclusions) — the following are governed elsewhere:

• Power boilers and boiler external piping → ASME BPVC Section I and B31.1.
• Pipeline transportation systems for liquids → ASME B31.4.
• Gas transmission and distribution piping → ASME B31.8.
• Building services piping → ASME B31.9.
• Piping systems designed for internal gage pressure at or above zero but less than 15 psi (105 kPa), provided the fluid is nonflammable, nontoxic, and not damaging to human tissues.

The boundary between B31.3 and its sibling B31 codes is a common source of code mis-application — always confirm which code has jurisdiction before design.

B31.3 sets minimum requirements across the full lifecycle of a piping system:

• Materials — selection of materials qualified for process service (allowable stresses drawn from Mandatory Appendix A).
• Design — pressure design of pipe wall thickness, branch connections, flanges, and other components.
• Fabrication & Erection — cutting, bending, forming, welding (procedures/welders qualified to BPVC Section IX), and installation.
• Examination, Inspection & Testing — nondestructive examination (NDE) of welds tailored to fluid service, and pressure testing (hydrostatic or pneumatic) before commissioning.
• Operation & Maintenance — B31.3 applies primarily to new construction, although it provides some guidance for repair. In-service inspection is usually governed by API 570.

The technical evaluations below determine whether a process piping system is safe for its design pressure and temperature.

Pressure Design

Calculation of the minimum required wall thickness to contain internal pressure (Equation 3a), including corrosion/erosion allowance and mill (manufacturing) tolerance.

Stress Analysis

• Sustained loads — pressure plus the weight of pipe, fluid, insulation, and components.
• Thermal (displacement) loads — stresses from thermal expansion/contraction over the operating range, checked against the allowable displacement stress range.
• Occasional loads — temporary stresses from wind, seismic, and dynamic events such as relief-valve discharge.

Flexibility & Supports

Stress intensification factors (SIF) assess the fatigue life of fittings and layout flexibility; supports must guide, anchor, or support the pipe without causing excessive localized stresses.

Where to find it in the code: allowable stress tables → Mandatory Appendix A · SIF / flexibility factors → Appendix D.

Engineering Note: B31.3 governs the overall system; component standards such as ASME B16.5 (flanges) and ASME B16.9 (fittings) define the parts used within it.