Case Study: Pressure Vessel Fatigue Analysis Using Design-By-Analysis

Project Overview

MechSourcing conducted a detailed fatigue analysis of a pressure vessel using the Design-By-Analysis (DBA) approach as per ASME Boiler and Pressure Vessel Code Section VIII, Division 2, Part 5.

Objectives of the Study

The primary objective of this case study is to demonstrate the application of Design-By-Analysis (DBA) methodology for evaluating fatigue damage in pressure vessels subjected to cyclic operating conditions.

Specifically, the study aims to:

• Evaluate cyclic stresses due to pressure and temperature fluctuations

• Identify fatigue-critical regions such as nozzle-to-shell junctions and weld toes

• Quantify alternating stress ranges using FEA-based stress linearization

• Estimate fatigue life using ASME-approved S–N curves

• Verify compliance with ASME VIII-2 Part 5 acceptance criteria

• Support safe operation, life extension, and inspection planning

Geometry and Computational Domain

The reference system modelled in this case study represents a generic industrial pressure vessel, commonly used in chemical and process industries.

Pressure Vessel System

Key geometric features:

• Cylindrical shell with ellipsoidal heads

• Reinforced nozzles with full-penetration welds

• Modeled as a full 3D solid domain

• CAD geometry imported into FEA software

The model captures geometric stress concentrations critical for fatigue evaluation rather than relying on simplified analytical solutions.

Material Properties

The vessel and nozzles are fabricated from carbon steel pressure vessel material with temperature-dependent properties.

Material inputs include:

• Elastic modulus (E)

• Poisson’s ratio (ν)

• Yield strength as a function of temperature

• Coefficient of thermal expansion

• Fatigue S–N curve data as per ASME VIII-2

Welded regions are evaluated using structural stress methods consistent with ASME fatigue provisions.

Contact and Structural Modeling

• Fully bonded connections between shell, head, and nozzle regions

• Weld geometry implicitly represented through stress linearization paths

• No contact separation assumed under normal operating conditions

• Structural stress approach used to avoid mesh-dependent notch effects

Governing Equations:

Equilibrium (Static / Quasi-Static)

Constitutive Relationship (Linear Elasticity)

Strain–Displacement Relation

Thermal Strain

Equivalent Alternating Stress

Fatigue Damage (Miner’s Rule)

Acceptance criterion:

Simulation Methodology

Step 1: Geometry Import

• CAD geometry imported into FEA environment

• Weld-critical regions identified for stress linearization

Step 2: Meshing

• 3D solid elements used throughout

• Local mesh refinement near nozzles and shell intersections

• Mesh convergence verified for stress accuracy

Step 3: Load Definition

• Internal pressure cycling (minimum to maximum design pressure)

Step 4: Boundary Conditions

• Fixed and displacement boundary conditions were applied

• Realistic vessel supports modeled

• Operating pressure cases were simulated with different load case options

Step 5: Solver Execution

• Linear elastic analysis per ASME requirements

• Stress linearization performed along critical paths

• Extraction of membrane, bending, and structural stresses

Post-Processing and Key Results

Key outputs analyzed:

• Stress intensity and von Mises stress distributions

• Structural stress ranges at weld locations

• Alternating stress amplitude for fatigue assessment

• Number of allowable cycles from ASME S–N curves

• Cumulative fatigue damage index

Results confirmed that all evaluated locations satisfied ASME VIII-2 fatigue acceptance criteria for the specified service life.

Engineering Insights Gained

• Nozzle-to-shell junctions govern fatigue life

• Thermal transients significantly influence alternating stress magnitude

• Design-By-Analysis provides higher confidence than Design-By-Rule for cyclic service

• Structural stress approach avoids mesh-dependent fatigue errors

• Fatigue damage well below allowable limits for intended operating cycles

Industrial Applications

Oil & Gas Industry

• Pressure vessels in cyclic separator and compressor service

• Thermal fatigue assessment during frequent startups

Chemical & Process Industry

• Reactors and columns subjected to batch operation

• Fatigue evaluation of nozzle connections

Power & Energy

• Heat recovery vessels and drums

• Thermal cycling due to load variation

Hydrogen & High-Pressure Equipment

• Life assessment of vessels under pressure cycling

• Support for life extension and rerating studies

Benefits to Industry

• Code-compliant fatigue life assessment

• Early identification of fatigue-critical locations

• Reduced risk of in-service cracking

• Optimized inspection and maintenance planning

• Extended equipment life with engineering justification

• Reduced conservatism compared to rule-based design