An Overview of ASME B31.3 Process Piping Code

Chapter I: Scope and Definitions

Key Content:
Defines jurisdictional boundaries for process piping systems, including what’s covered (chemical plants, refineries, pharmaceuticals) and excluded (power piping, HVAC). Clarifies terms like “owner,” “designer,” and “fluid service categories” (Normal, Category D, Category M).

Takeaways:
– Piping begins at the first flange/valve after equipment and ends at vessel connections.
– Category M (lethal) fluids trigger stricter rules (100% NDE, no threaded joints).

Chapter II: Design

Key Content:
Covers pressure design (wall thickness calculations), flexibility analysis (thermal expansion), and load stresses. Includes:
– Equation 3a for wall thickness (`t = (P×D)/[2(S×E×W + P×Y)]`).
– Branch reinforcement rules (area replacement method).
– Allowable stress limits for sustained (Sh + Sc) and occasional (e.g., seismic) loads.

Takeaways:
– Always verify SIFs (Stress Intensification Factors) for bends/tees (Appendix D).
– Cyclic service (>7,000 cycles) requires fatigue analysis.

Chapter III: Materials

Key Content:
Lists approved materials (carbon steel, stainless, nickel alloys) with temperature limits and restrictions. Mandates:
– Impact testing for low temps (< -29°C for carbon steel).
– PMI (Positive Material Identification) for alloys.
– Banned materials (e.g., pure lead).

Takeaways:
– ASTM A106 Gr. B is the default carbon steel pipe.
– 316L SS requires PMI to avoid material mix-ups.

Chapter IV: Piping Components

Key Content:
Specifies standards for flanges (ASME B16.5), valves (API 600), and fittings (ASME B16.9). Addresses:
– Unlisted components: Require additional qualification per para. 304.7.
– Dimensional tolerances (e.g., pipe ovality ≤5%).

Takeaways:
– Never mix metric/imperial flanges without verification.
– Butt-weld fittings must match pipe material specs.

Chapter V: Fabrication & Assembly

Key Content:
Rules for welding (WPS/PQR per ASME IX), bending (cold/hot), and alignment. Requires:
– Preheat*for carbon steel >19mm thick (Table 330.1.1).
– PWHT (Post-Weld Heat Treatment) for P-No. 1 materials >19mm.

Takeaways:
– Grind flush all welds in Category M service.
– Avoid temporary lugs on pressure-retaining parts.

Chapter VI: Inspection & Testing

Key Content:
Mandates hydrostatic testing (1.5× design pressure) and NDE extent based on fluid service:
– Normal: 5–10% RT.
– Category M: 100% RT + 10% PT/MT.

Takeaways:
– Pneumatic tests (1.1× design pressure) require extra safety precautions.
– Leak tests must hold for 10 minutes (no visible drops).

Chapter VII: Non-metallic Piping

Key Content:
Covers plastic (PVC, FRP) and lined piping (PTFE). Limits:
– Max temperature: 150°C for thermoplastics.
– Design pressure: 0.5× hydrostatic test pressure.

Takeaways:
– Avoid UV exposure** for outdoor FRP piping.
– Support spacing is tighter vs. metallic pipes.

Chapter VIII: Category M Fluid Service

Key Content:
Extra requirements for lethal fluids (e.g., HCN, HF):
– No threaded joints (butt-weld only).
– 100% RT + PWHT for all welds.

Takeaways:
– Double-gasket seals required for flanges.
– No socket welds in Category M systems.

Chapter IX: High-Pressure Piping

Key Content:
Rules for Class 2500+ systems (>10,000 psi):
– Autofrettage allowed for fatigue resistance.
– Leak-before-burst design mandatory i.e., Cracks must grow slowly enough for detection.

Takeaways:
– Avoid sharp corners in fittings (stress risers).
– Proof testing may replace calculations for complex geometries.

Key Appendices

1. Appendix A: Allowable stresses

(Table A-1) and quality factors (Table A-1B).

• Example: A106 Gr. B at 100°C: S = 20 ksi.

2. Appendix D: SIFs (Stress Intensification Factors) for flexibility analysis.
3. Appendix F: Rules for non-metallic linings (e.g., glass-lined pipe).

Takeaways:
– Always cross-check S values in Appendix A for your material/temperature.
– SIFs for elbows vary by R/D ratio (Appendix D).

Key Engineering Takeaways

• Design: Always verify SIFs for bends – undersizing causes fatigue failures.
• Materials: PMI prevents 316L/304 mix-ups in corrosive service.
• Testing: 1.5× hydrotest pressure is non-negotiable for safety.