Introduction
In piping design, selecting the correct pipe thickness is one of the most fundamental engineering tasks. Pipe thickness directly affects pressure containment, mechanical strength, corrosion life, safety, and project cost.
In Oil and Gas Piping, chemical plants, refineries, and power plants, pipe thickness is not chosen arbitrarily. It is calculated using ASME B31 codes, verified against piping class, and coordinated with P&IDs, line designation, and stress analysis.
Table of Contents
What Is Pipe Thickness?
Pipe thickness is the wall thickness of a pipe required to safely withstand:
- Internal design pressure
- Design temperature
- Corrosion and erosion
- Mechanical and thermal loads during operation
In piping engineering, we normally deal with:
- Calculated thickness (from code formula)
- Nominal thickness (standard pipe schedule)
Basic Pipe Thickness Terminology
Nominal Pipe Size (NPS)
NPS is a dimensionless size designation, not the actual diameter.
For example, NPS 6 pipe does not have a 6-inch OD.
Outside Diameter (OD)
- OD is fixed for a given NPS
- Thickness increases inward (ID reduces)
Pipe Schedule
Schedule defines pipe thickness (Schedule 10, 40, 80, etc.).
- Higher schedule = thicker pipe
- Schedule selection is final output of thickness calculation
Minimum Required Thickness ™
- Thickness required by code calculation
- Does not include corrosion allowance
Corrosion Allowance (CA)
- Extra thickness added to compensate metal loss over time
Nominal Thickness (tn)
- Thickness selected from standard pipe schedules
- tn ≥ (tm + CA + mill tolerance)
Codes and Standards Governing Pipe Thickness
ASME B31 Codes
Pipe thickness is governed by ASME B31 series:
- ASME B31.3 – Process Piping (most common in oil & gas and chemical plants)
- ASME B31.1 – Power Piping
- ASME B31.4 / B31.8 – Pipelines (outside plant limits)
This article focuses on ASME B31.3.
Design Inputs Required for Thickness Calculation
Before calculating pipe thickness, the piping engineer must collect data from:
- P&ID
- Line List
- Line Designation / Line Numbering System
- Piping Class
Key Design Inputs
- Design pressure
- Design temperature
- Material specification (ASTM grade)
- Fluid service
- Corrosion allowance
- Joint efficiency
- Piping class
👉 Example:
Line Number: 10-P-101-CS-150
- Fluid: Process Hydrocarbon
- Design Pressure: 25 barg
- Design Temperature: 200°C
- Material: ASTM A106 Gr.B
- Piping Class: CS-150-HT
Pipe Thickness Calculation Basis (ASME B31.3)
Governing Clause
ASME B31.3 – Clause 304.1.2
“The minimum thickness of straight pipe shall be calculated using the pressure design equations…”
Basic Pressure Design Equation
For internal pressure:
t = (P × D) / [2 × (S × E + P × Y)]Where:
- t = minimum required thickness (mm)
- P = design pressure
- D = outside diameter
- S = allowable stress (ASME Section II, Part D)
- E = longitudinal weld joint efficiency
- Y = temperature coefficient (Table 304.1.1)
Allowable Stress (S)
ASME Reference
- ASME B31.3 Clause 302.3
- Stress values taken from ASME Section II, Part D
Allowable stress depends on:
- Material
- Design temperature
👉 Example:
ASTM A106 Gr.B
- t 200°C → allowable stress ≈ 138 MPa
Weld Joint Efficiency (E)
ASME Reference
- ASME B31.3 Clause 302.3.4
Typical values:
- Seamless pipe → E = 1.0
- ERW pipe → E = 0.85 to 1.0 (depending on examination)
Corrosion Allowance
ASME Reference
- ASME B31.3 Clause 304.1.1
Corrosion allowance is added after calculation.
Typical values used in plants:
| Service Type | Corrosion Allowance |
|---|---|
| Clean gas | 1.0 mm |
| Hydrocarbon liquid | 1.5–3.0 mm |
| Wet / corrosive service | 3.0–6.0 mm |
👉 Plant Example:
Crude oil transfer line → CA = 3 mm
Mill Tolerance
ASME Reference
- ASME B31.3 Clause 304.1.1(b)
Carbon steel pipes are typically supplied with:
- –12.5% mill tolerance
This means:
Nominal thickness must be higher than calculated thickness to account for tolerance.
Selection of Final Pipe Schedule
Steps followed by piping engineers:
- Calculate minimum thickness ™
- Add corrosion allowance
- Account for mill tolerance
- Select next higher standard pipe schedule
👉 Example:
Calculated thickness + CA = 6.2 mm
Available schedules:
- Sch 40 = 7.11 mm ✅
- Sch 80 = 10.97 mm ❌ (overdesign)
Final selection: Schedule 40
Minimum Thickness Limits
ASME Reference
- ASME B31.3 Clause 304.1.1
Even if calculated thickness is very small:
- Code minimum thickness applies
- Handling, threading, and fabrication limits must be satisfied
Special Service Considerations
High Temperature Service
- Lower allowable stress
- Higher thickness required
Low Temperature Service
- Impact testing requirements
- Material toughness becomes critical
Cyclic Service
- Fatigue considerations
- Refer ASME B31.3 Chapter II
Vacuum Service
- External pressure and buckling checks
- Refer Clause 304.1.3
Pipe Thickness and Stress Analysis
Pipe thickness affects:
- Pipe flexibility
- Thermal expansion stress
- Support loads
Stress engineers may request:
- Thinner pipe for flexibility
- Thicker pipe for local overstress control
This coordination is critical during detailed piping engineering.
Common Mistakes by Beginners
- Ignoring corrosion allowance
- Using wrong allowable stress
- Confusing pipe OD with ID
- Selecting schedule directly without calculation
- Overdesigning “to be safe”
Practical Plant Example (Simplified)
Process line on P&ID:
- Line Number: 6-H-205-CS-300
- Design Pressure: 30 barg
- Design Temperature: 180°C
- Material: A106 Gr.B
- CA: 3 mm
After calculation and checks:
- Required thickness = 6.5 mm
- Selected pipe = NPS 6, Schedule 40
Conclusion
Pipe thickness selection is a core skill in piping design and piping engineering. It links P&IDs, line designation, piping class, material selection, and ASME B31 codes into one calculation that directly impacts plant safety and cost.
By understanding:
- ASME B31.3 clauses
- Calculation logic
- Practical plant constraints
a piping engineer can confidently select the right pipe thickness, not just the safe one.