What is the difference between ASME B31.3, B31.4, and B31.8 wall thickness calculations?

B31.3 (Process Piping) uses t = PD/(2(SEW + PY)) with a Y coefficient for thick-wall correction. B31.4 (Liquid Transportation) uses t = PD/(2SEF) with a design factor F typically 0.72. B31.8 (Gas Transmission) uses t = PD/(2SFET) where F is the location class design factor (0.72 to 0.40) and T is a temperature derating factor. All codes add corrosion allowance and mill tolerance to the calculated minimum thickness.

How do I select the correct pipe schedule for my design pressure?

After calculating the required nominal wall thickness (including corrosion allowance and mill tolerance), select the lightest standard pipe schedule whose wall thickness equals or exceeds the required nominal thickness. Standard schedules include Sch 5S, 10S, 10, 20, 30, STD, 40, 60, 80, 100, 120, 140, 160, and XXS. A pressure margin of at least 10% above design pressure is recommended for good engineering practice.

What mill tolerance should I use for pipe wall thickness calculations?

The standard manufacturing (mill) tolerance for seamless and welded pipe per ASTM A530 and API 5L is 12.5% undertolerance on wall thickness. This means the actual wall thickness can be up to 12.5% thinner than the nominal value. The required nominal thickness is calculated as t_nominal = t_required / (1 - 0.125) = t_required / 0.875.

What is the Y coefficient in B31.3 wall thickness calculations?

The Y coefficient in B31.3 Section 304.1.2 accounts for the redistribution of hoop stress in thick-wall pipes. For ferritic steels below 900F, Y = 0.4. For austenitic steels below 900F, Y = 0.4 as well. Above 900F, Y increases to 0.7 for ferritic and 0.7 for austenitic materials. The Y factor reduces the required wall thickness compared to simple Barlow formula by accounting for the strengthening effect of internal pressure.

B31 Pipe Wall Thickness Calculator | ASME B31.3, B31.4, B31.8

Applicable Code

ASME B31 Code

Pipe Geometry

Nominal Pipe Size (NPS)

Pipe Material

Material Specification

Design Conditions

Design Pressure

psig

Design Temperature

°F

Allowances

Corrosion Allowance

Typical: 0.0625" (1/16") for carbon steel, 0 for stainless

Mill Tolerance

Standard: 12.5% per ASTM A530 / API 5L

Joint & Weld Factors

Longitudinal Joint Factor (E)

Weld Joint Strength Reduction Factor (W)

B31.3 only. W = 1.0 below creep range; < 1.0 at elevated temperatures.

B31.8 Design Factors

Design Factor (F) - Location Class

Temperature Derating Factor (T)

B31.4 Design Factor

Design Factor (F)

Understanding B31 Pipe Wall Thickness

B31.3 Process Piping
Uses t = PD / (2(SEW + PY)). The Y coefficient (typically 0.4 for ferritic steels below 900°F) accounts for thick-wall stress redistribution. Covers refinery, chemical plant, and gas processing facility piping.

B31.4 / B31.8 Pipelines
Transportation codes use Barlow's formula with design factor F. B31.8 further derates by location class (F = 0.72 to 0.40) and temperature factor T. B31.4 applies to liquid hydrocarbons; B31.8 to gas transmission.

From Minimum to Nominal Thickness:
t_min (from code formula) + corrosion allowance = t_required. Then t_nominal = t_required / (1 - mill_tolerance). The selected pipe schedule must have wall thickness >= t_nominal.

Learn the Theory

Understand pressure design formulas, code comparison, material selection, and temperature effects on allowable stress

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Results

Selected Pipe Schedule

Code Minimum Thickness (t_min) -

Required w/ CA (t_required) -

Nominal w/ Mill Tol. (t_nominal) -

Actual Wall (Selected Schedule) -

Pipe OD -

MAOP of Selected Pipe -

Pressure Margin -

Pipe Weight -

D/t Ratio -

Allowable Stress at Temp. -

Code Equation -

Formulas

B31.3: t = PD / (2(SEW + PY))
B31.4: t = PD / (2SEF)
B31.8: t = PD / (2SFET)

P = Design pressure (psig)

D = Outside diameter (in)

S = Allowable stress at temperature (psi)

E = Longitudinal joint factor

W = Weld strength reduction factor (B31.3)

Y = Coefficient (0.4 for ferritic <900°F)

F = Design factor (B31.4 / B31.8)

T = Temperature derating (B31.8)

Standards & References

ASME B31.3-2022
Process Piping, Section 304.1.2
ASME B31.4-2022
Liquid Transportation, Section 404.1.2
ASME B31.8-2022
Gas Transmission, Section 841.1.1
49 CFR Part 192
Transportation of Natural Gas
API 5L (46th Ed.)
Line Pipe Specification
ASTM A530
General Requirements for Pipe

Engineering Notes

Mill tolerance: Standard 12.5% undertolerance per ASTM A530 and API 5L
Corrosion allowance: Typical 1/16" for carbon steel; 0 for stainless; increase for sour service
D/t ratio: Values >96 indicate thin-wall pipe susceptible to external pressure collapse
B31.3 Y factor: 0.4 for ferritic steels below 900°F; increases at creep temperatures
Weight: Calculated as 10.69 x (D - t) x t lb/ft per ANSI/ASME formula

B31 Pipe Wall Thickness Calculator