B31 Pipe Wall Thickness Fundamentals | Pressure Design Engineering Guide

1. Overview

Pipe wall thickness is the fundamental pressure design calculation in pipeline and process engineering. The goal is to determine the minimum wall thickness that safely contains the design pressure at the design temperature, then select the lightest standard pipe schedule that meets this requirement.

Three ASME B31 codes govern pipe wall thickness for different service types:

Code	Scope	Governing Section
ASME B31.3	Process piping within facilities (refineries, gas plants, chemical plants)	Section 304.1.2
ASME B31.4	Liquid transportation pipelines (crude, NGL, refined products)	Section 404.1.2
ASME B31.8	Gas transmission and distribution pipelines	Section 841.1.1

Which code applies? Generally, B31.3 governs piping within a plant fence line (upstream of the custody transfer point). B31.4 or B31.8 governs pipelines between facilities. The applicable code should be specified in the project basis of design.

Design Thickness vs. Nominal Thickness

The calculation proceeds in three steps from code minimum to the orderable pipe:

t_min = Code minimum thickness from the applicable formula (contains pressure only)
t_required = t_min + corrosion allowance (accounts for material loss over service life)
t_nominal = t_required / (1 - mill_tolerance) (accounts for manufacturing variation)

The selected pipe schedule must have a wall thickness equal to or greater than t_nominal.

2. Code Formulas

B31.3 - Process Piping (Section 304.1.2)

Straight pipe under internal pressure: t = P × D / (2 × (S × E × W + P × Y)) Where: P = Design pressure (psig) D = Outside diameter (inches) S = Allowable stress at design temperature (psi) from Table A-1 E = Quality factor (longitudinal joint factor) W = Weld joint strength reduction factor (1.0 below creep range) Y = Coefficient (see table below)

The Y coefficient accounts for stress redistribution in the pipe wall under internal pressure. For thin-wall pipe (t < D/6), it has a modest effect; for thick-wall pipe, it becomes more significant.

Material	Temperature < 900°F	Temperature ≥ 900°F
Ferritic steels	Y = 0.4	Y = 0.7
Austenitic steels	Y = 0.4	Y = 0.7
Nickel alloys, high alloy	Y = 0.4	Y = 0.7

B31.3 allowable stress: Table A-1 provides temperature-dependent allowable stress. For carbon steel below 700°F, the basic allowable is typically 20,000 psi (for SMYS = 35,000 psi material). Above the creep range, allowable stress drops significantly and the W factor may decrease below 1.0.

B31.4 - Liquid Transportation (Section 404.1.2)

Barlow's formula with design factor: t = P × D / (2 × S × E × F) Where: P = Design pressure (psig) D = Outside diameter (inches) S = Specified Minimum Yield Strength, SMYS (psi) E = Longitudinal joint factor F = Design factor (typically 0.72)

Unlike B31.3, the pipeline codes (B31.4 and B31.8) use SMYS rather than an allowable stress at temperature. The design factor F provides the safety margin.

B31.8 - Gas Transmission (Section 841.1.1)

Barlow's formula with location class: t = P × D / (2 × S × F × E × T) Where: P = Design pressure (psig) D = Outside diameter (inches) S = Specified Minimum Yield Strength, SMYS (psi) F = Design factor based on location class E = Longitudinal joint factor T = Temperature derating factor

Design Factor F - Location Class (B31.8)

Class	F	Max % SMYS	Description
Class 1	0.72	72%	≤10 buildings in a 1-mile by 440-yard unit; rural areas
Class 2	0.60	60%	11-46 buildings; fringe areas, industrial, farms
Class 3	0.50	50%	≥46 buildings or near places of public assembly
Class 4	0.40	40%	Areas with multi-story buildings (≥4 stories)

Temperature Derating Factor T (B31.8)

Temperature (°F)	T
≤250	1.000
300	0.967
350	0.933
400	0.900
450	0.867

3. Code Comparison

Although all three codes are based on Barlow's formula, they differ in how they apply safety factors:

Feature	B31.3	B31.4	B31.8
Stress basis	Allowable stress S (from Table A-1)	SMYS	SMYS
Safety factor approach	Built into allowable stress (S ≈ SMYS/1.75 to SMYS/3)	Design factor F	Design factor F (by location class)
Thick-wall correction	Y coefficient (PY term)	None	None
Temperature handling	Reduced allowable stress	N/A (low temp)	Derating factor T
Weld factor at temp	W factor (creep range)	N/A	N/A
Location class	N/A (within facility)	Limited	Classes 1-4 per 49 CFR 192

Practical difference: For the same pipe, material, and pressure, B31.3 typically requires a thicker wall than B31.8 Class 1 because B31.3 allowable stress is approximately SMYS/1.75, while B31.8 Class 1 allows 72% SMYS (equivalent to SMYS/1.39). However, B31.3 includes the PY term which slightly reduces the requirement for thin-wall pipe.

4. Material Selection

Material grade determines the SMYS and allowable stress, which directly controls the required wall thickness. Higher-grade materials allow thinner walls for the same pressure.

Common Pipe Materials and SMYS

Material	SMYS (psi)	Specification	Typical Application
A106 Gr. B	35,000	ASTM A106	Process piping, general service
A333 Gr. 6	35,000	ASTM A333	Low temperature service (down to -50°F)
TP304	30,000	ASTM A312	Corrosive service, high temperature
TP316	30,000	ASTM A312	Marine, chemical, chloride environments
API 5L Gr. B	35,500	API 5L	General pipeline service
API 5L X42	42,000	API 5L	Gathering lines, low-pressure transmission
API 5L X52	52,000	API 5L	Standard transmission pipeline
API 5L X60	60,000	API 5L	High-pressure gas transmission
API 5L X65	65,000	API 5L	High-pressure, large-diameter pipelines
API 5L X70	70,000	API 5L	Major cross-country pipelines
API 5L X80	80,000	API 5L	Ultra-high pressure, heavy wall

Grade vs. cost tradeoff: Higher-grade pipe costs more per ton, but the reduced wall thickness means less steel per foot and lighter weight for installation. For large-diameter, high-pressure pipelines, the material cost savings from thinner walls often outweigh the higher per-ton price of X65 or X70 compared to X52.

Longitudinal Joint Factor (E)

Pipe Manufacturing Method	E Factor
Seamless (SMLS)	1.00
Electric Resistance Welded (ERW)	1.00
Double Submerged Arc Welded (DSAW)	1.00
Flash Welded	1.00
ERW (pre-1970, unverified)	0.85
Furnace Lap Welded	0.80
Furnace Butt Welded	0.60

5. Temperature Effects

Temperature affects pipe wall thickness requirements in two ways: it reduces the allowable stress of the material, and in extreme cases it can change the failure mode from yielding to creep rupture.

B31.3 - Allowable Stress Reduction

For carbon steel (A106 Gr. B), the allowable stress from B31.3 Table A-1 varies with temperature:

Temperature (°F)	Allowable Stress (psi)	% of Room Temp Value
≤400	20,000	100%
500	18,900	94.5%
600	17,300	86.5%
700	14,400	72.0%
800	10,800	54.0%
900	6,500	32.5%
1000	2,500	12.5%

Creep range: Above approximately 700-800°F for carbon steel, the controlling failure mode transitions from yielding to creep rupture. The allowable stress drops dramatically, and the weld joint strength reduction factor W may decrease below 1.0. Always verify the W factor from B31.3 Table 302.3.5 for temperatures above 700°F.

B31.8 - Temperature Derating

For gas transmission pipelines, the T factor applies a simple derating above 250°F. Most gas pipelines operate well below 250°F, so T = 1.0 in the vast majority of cases. Exceptions include lines near compressor discharge (hot gas bypass) and heated pipelines for hydrate prevention.

Stainless Steel Advantage at High Temperature

Austenitic stainless steels (TP304, TP316) retain much higher allowable stress at elevated temperatures compared to carbon steel. At 1000°F, TP304 has an allowable stress of approximately 13,000 psi versus only 2,500 psi for A106 Gr. B. This makes stainless steel the preferred choice for high-temperature process piping despite its higher initial cost.

6. Pipe Schedule Selection

After calculating t_nominal, the engineer selects the lightest standard pipe schedule whose wall thickness equals or exceeds the requirement. Standard schedules per ANSI/ASME B36.10M (carbon steel) and B36.19M (stainless) include:

Schedule	Relative Weight	Notes
5S, 10S	Very Light	Stainless only (B36.19M); gauging/sampling lines
10, 20	Light	Low-pressure utility lines
30	Medium-Light	Available in sizes 8" and larger
STD	Standard	Standard Weight; equals Sch 40 for NPS ≤10"
40	Standard	Most commonly specified schedule
60	Medium-Heavy	Available in sizes 4" and larger
80	Heavy	Extra Strong (XS) for NPS ≤8"
100, 120	Very Heavy	High-pressure service
140, 160	Extra Heavy	Very high pressure or thick CA
XXS	Double Extra Strong	Maximum standard wall; small bore only

Pressure margin: A pressure margin of at least 10% between the MAOP of the selected schedule and the design pressure is considered good practice. Margins of 0-10% are marginal but acceptable. If no standard schedule meets the requirement, consider upgrading to a higher material grade or increasing the pipe size.

Mill Tolerance

Per ASTM A530 and API 5L, the standard manufacturing (mill) tolerance for seamless and welded pipe is 12.5% undertolerance on wall thickness. This means the thinnest allowable pipe in a given shipment can be:

t_minimum_mill = t_nominal × (1 - 0.125) = t_nominal × 0.875 Therefore, the nominal thickness required to guarantee the minimum structural thickness is: t_nominal = t_required / (1 - 0.125) = t_required / 0.875

D/t Ratio

The diameter-to-thickness (D/t) ratio is an important indicator of pipe structural behavior:

D/t < 20: Thick-wall pipe. May require B31.3 Section 304.1.2(b) thick-wall provisions.
20 < D/t < 96: Normal range for most pressure piping applications.
D/t > 96: Thin-wall pipe. Susceptible to external pressure collapse, handling damage, and ovality. Check external loading per applicable code.

7. Worked Example

Given: 8" NPS pipeline, API 5L X52, 1480 psig design pressure, 150°F, Class 1 location (B31.8), seamless pipe, 1/16" corrosion allowance.

Step 1: Gather parameters D = 8.625" (OD for 8" NPS) S = 52,000 psi (SMYS for X52) F = 0.72 (Class 1) E = 1.00 (Seamless) T = 1.000 (150 deg F < 250 deg F) CA = 0.0625" Mill tolerance = 12.5% Step 2: Code minimum thickness (B31.8) t_min = P × D / (2 × S × F × E × T) t_min = 1480 × 8.625 / (2 × 52,000 × 0.72 × 1.00 × 1.00) t_min = 12,765 / 74,880 t_min = 0.1704" Step 3: Add corrosion allowance t_required = 0.1704 + 0.0625 = 0.2329" Step 4: Apply mill tolerance t_nominal = 0.2329 / (1 - 0.125) = 0.2329 / 0.875 t_nominal = 0.2662" Step 5: Select pipe schedule Review 8" schedules: Sch 20: 0.148" -- too thin Sch 30: 0.188" -- too thin STD/40: 0.322" -- meets requirement (0.322 ≥ 0.2662) ✓ Selected: 8" Sch STD (0.322" wall) Step 6: Calculate MAOP of selected pipe MAOP = 2 × S × F × E × T × (t - CA) / D MAOP = 2 × 52,000 × 0.72 × 1.00 × 1.00 × (0.322 - 0.0625) / 8.625 MAOP = 74,880 × 0.2595 / 8.625 MAOP = 2,253 psig Step 7: Pressure margin Margin = (2,253 - 1,480) / 1,480 × 100 Margin = 52.2% > 10% ✓ Step 8: Pipe weight W = 10.69 × (8.625 - 0.322) × 0.322 W = 28.57 lb/ft D/t = 8.625 / 0.322 = 26.8 (normal range) ✓

Result: 8" Sch STD (0.322" wall) in API 5L X52 provides a 52.2% pressure margin over the 1,480 psig design pressure, with a MAOP of 2,253 psig. The D/t ratio of 26.8 is well within the normal range.

References

ASME B31.3-2022, Process Piping
ASME B31.4-2022, Pipeline Transportation Systems for Liquids and Slurries
ASME B31.8-2022, Gas Transmission and Distribution Piping Systems
49 CFR Part 192, Transportation of Natural and Other Gas by Pipeline
API Specification 5L, 46th Edition, Line Pipe
ASTM A530/A530M, General Requirements for Specialized Carbon and Alloy Steel Pipe
ANSI/ASME B36.10M, Welded and Seamless Wrought Steel Pipe
ANSI/ASME B36.19M, Stainless Steel Pipe

B31 Pipe Wall Thickness

Process Piping

Gas Transmission

12.5% Standard

Contents

Try the calculator