ASME B31.3 Process Piping — Wall Thickness & Thermal Stress Screening

ASME B31.3 — Process Piping — is used to design, size and check the integrity of piping in chemical plants, refineries, pharmaceutical plants and any process facility. It governs pressure wall thickness selection, thermal expansion and flexibility analysis, allowable stress values, weld-joint quality factors, test requirements and the scope of inspection. When a design is described as 'built to ASME B31.3', it means these code requirements have been applied and documented.

ASME B31.3 §304 Eq. 3a gives the minimum wall thickness from the design pressure: t_min = P·D / (2·(S·E_w·W + P·Y)) + c_a, where W is the weld joint strength reduction factor (1.0 below the creep range), P is the design gauge pressure (MPa), D the outside diameter (mm), S the allowable stress from Table A-1, E_w the weld-joint quality factor (1.0 seamless, 0.85 ERW), Y a temperature coefficient (0.4 for ferritic steel at or below 480 °C) and c_a a corrosion allowance. The purchased wall must also account for the mill under-tolerance given in the pipe material specification (−12.5 % for A106/A53 seamless pipe) on top of this minimum.

ASME B31.3 §319.4.1 requires a formal flexibility analysis unless the system duplicates one already in service, can be judged adequate by comparison with a previously analysed system, or satisfies the empirical screening criterion of §319.4.1(c): D·y/(L−U)² ≤ 208 000 in SI, where D is the outside diameter in mm, y the resultant of the total displacement strains to be absorbed in mm, and L and U are the developed pipe length and the straight-line anchor distance in metres. That criterion applies only to a system of uniform size with no more than two points of fixation, no intermediate restraints and no significant cyclic service — it is not a general-purpose test. (Appendix C of B31.3 is the physical-properties tables, not the flexibility criterion.) A formal CAESAR II or equivalent analysis is required for anything outside those limits, for lines on rotating equipment, for lethal or critical service, and for seismic or high-cycle duty. The MechanixCalc pipe stress calculator does not evaluate this criterion — it has no input for the displacement strain y — so work it out from your isometric.

ASME B31.1 (Power Piping) covers steam and utility piping in power plants; it uses slightly different allowable-stress tables and weld-factor conventions. B31.3 (Process Piping) applies to chemical, refinery and general process plant piping and has broader fluid-service categories (ordinary, category D, M, lethal) and fatigue requirements. B31.4 (Liquid Petroleum Transportation) and B31.8 (Gas Transmission) govern cross-country pipelines and use location-class design factors rather than B31.3 weld-joint factors. Select the code specified on the project P&ID or the applicable jurisdictional regulation.

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