Design Basis

Module from the Piping Engineering Curriculum curriculum.

Module 1 — Piping Design Basis (ASME B31.3) · Learning Objectives · 1. State the scope and applicability of ASME B31.3 Process Piping · 2. Define design pressure, design temperature, and minimum design metal temperature · 3. Identify Fluid Service Categories (Normal, Category D, Category M, High Pressure, High Purity) · 4. Apply the basic pressure design formula for straight pipe under internal pressure · 5. Select appropriate allowable stress values from Table A-1 · Fluid Service Categories (B31.3 §300.2)

Category	Definition	Typical Service	Added Requirements	Examination	Notes
Category D	Nonflammable, nontoxic, ≤150 psig, –20 to 366 °F	Utility air, cooling water	Minimum B31.3 rules	5% visual, 0% RT	Lowest rigor
Normal	All other services not categorized	Most hydrocarbon services	Standard B31.3 rules	5% RT random, 100% VT	Default
Category M	Single exposure could cause serious irreversible harm	HF acid, H2S > 1500 ppm, phosgene	Chapter VIII rules, severe	100% RT typical, stringent	Owner designates
High Pressure	Above B16.5 Class 2500 rating	Synthesis gas, polyethylene plants	Chapter IX rules	100% RT all welds	Rarely used
High Purity	Biopharm, semiconductor (B31.3 Ch. X)	WFI, UPW, BPE	Electropolish, low Ra	Per owner spec	Special
Elevated Temp Fluid Service	>Table A-1 upper temp for that material	Vacuum residue, hot oil	Creep allowable, time-dep	Added	Creep range
Pressure Design — Straight Pipe (B31.3 Eq 3a)
t = P·D / (2·(S·E·W + P·Y)) where t = pressure design thickness (in), P = design pressure (psig), D = OD (in), S = allowable stress (Table A-1), E = joint quality factor (Table A-1B), W = weld strength reduction (Table 302.3.5), Y = coefficient (Table 304.1.1)
Minimum wall = t + c, where c = corrosion/erosion/thread allowance. Add 12.5% mill tolerance for seamless pipe (multiply by 1/0.875).
Allowable Stress — Common Materials at Temperature (ksi)
Material	100 °F	400 °F	700 °F	900 °F	1000 °F
A106 Gr. B (carbon steel)	20.0	20.0	17.3	5.0	Not allowed
A335 P5 (5Cr-0.5Mo)	20.0	20.0	18.9	12.6	7.8
A335 P11 (1.25Cr-0.5Mo)	20.0	19.8	17.5	11.3	4.2
A335 P22 (2.25Cr-1Mo)	20.0	20.0	18.9	15.2	8.0
A312 TP304 SS	20.0	18.7	16.2	14.5	12.4
A312 TP316 SS	20.0	18.9	16.8	15.7	14.1
A333 Gr. 6 (low-temp CS)	20.0	20.0	N/A	N/A	N/A

Source: Piping_Engineering_Curriculum_v1.xlsx · Sheet: Design Basis

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