FEA for 3750 m³ LPG Mounded Storage Tank
Project Background
LPG mounded bullets are considered critical safety assets in any refinery or terminal. Unlike above-ground vessels, a mounded LPG storage tank is partially or fully covered with soil, designed to offer passive fire protection and improved safety in case of external events.
For this 3750 m³ LPG mounded storage tank, the client wanted more than just code hand-calculations. They needed a detailed finite element analysis (FEA) to confirm:
- The shell and head stresses are within ASME Section VIII Division 2 limits
- The vessel behaves safely under combined design load cases
- The analysis methodology and results can withstand third-party and regulatory audits
Ideametrics carried out a full FEA study to verify stress levels, triaxial stress limits, and overall structural adequacy.
Engineering Objectives
The main objectives of the analysis were:
- Check primary and bending stress limits as per ASME Sec.VIII Div.2 (Class 2)
- Evaluate triaxial stress criteria using PL + Pb combinations
- Validate hoop stresses from FEA against code equations
- Use appropriate surface elements to represent internal pressure and soil interaction
- Document a traceable, auditable workflow for client and statutory bodies
Design Basis & Code Framework
Equipment: 3750 m³ horizontal LPG mounded storage tank (bullet)
Code: ASME Section VIII Division 2, Edition 2021 – Class 2
FEA Platform: ANSYS
Assessment Focus:
- Membrane, bending and peak stresses
- Linearization along Stress Classification Lines (SCLs)
- Triaxial stress check as per Clause 5.3.2
The design load combinations included internal design pressure, soil/mound effects, temperature, and other relevant loads as per project specification.
Triaxial Stress Check – Design Load Combinations
As per ASME Sec.VIII Div.2, Clause 5.3.2, the algebraic sum of the three principal linearized stresses (S1 + S2 + S3) from the Design Load Combination (1) must be checked against the allowable limit: PL + Pb ≤ 4 × S
Where:
- PL = Primary membrane
- Pb = Primary bending
- S = allowable stress for material at design temperature
For this tank, the allowable limit used was:
- 4 × S = 752 MPa
Sample Result Highlights (LC-7 & LC-8)
Two critical load cases (LC-7 and LC-8) were taken from the design envelope and evaluated in detail:
| Load Case | Governing Location | PL + Pb (MPa) | Allowable (4S) (MPa) | Result |
|---|---|---|---|---|
| LC-7 | Critical shell section | -44.80 (approx) | 752 | Pass |
| LC-8 | Same region under alternate combination | -45.21 (approx) | 752 | Pass |
The negative sign simply indicates the direction of the principal stresses; the magnitude is what matters for comparison.
Conclusion: Triaxial stress checks confirmed a very comfortable margin to allowable, with no risk of plastic collapse or code violation.
FEA Validation – Hoop Stress Cross-Check
To build confidence in the FEA model, hoop stress from the shell away from discontinuities was validated using classical ASME formulas:
- Outer diameter: 8062 mm
- Inner diameter: 8003 mm
- Internal Pressure: 1.3566 MPa
- Joint Efficiency: 1
Using ASME Sec.VIII Div.2 Part 4, Clause 4.3.10.2 (Eq. 4.3.32):
- Calculated Hoop Stress ≈ 184.01 MPa
- FEA Hoop Stress ≈ 184.17 MPa
The near-perfect match between hand-calculated and FEA results confirms:
- The input loads and boundary conditions are correctly modeled
- The mesh and element formulation are appropriate
- The model is trustworthy for downstream code assessment
Use of Surface Effect Elements (SURF154)
Because this is a mounded LPG bullet, it is not just a “simple pressure vessel in air”. Certain loading conditions, like internal pressure distribution and interaction at the soil interface, required the use of ANSYS SURF154 surface effect elements.
These elements allowed us to:
- Apply pressure correctly on curved surfaces
- Represent special surface effects with stiffness and added mass where needed
- Maintain compatibility with 3D solid elements underneath
In short, SURF154 helped capture realistic surface load behavior without overcomplicating the model.
Global Model Behavior & Robustness
Along with the stress checks, the FEA model was constructed to be:
- Stable – no artificial rigid body motions, no unrealistic constraints
- Accurate – verified through hoop stress and code correlation
- Auditable – stress linearization, SCL locations, and load combinations all documented
This is essential for LPG storage, where safety authorities often ask for detailed engineering justification.
Final Conclusion
From the performed FEA and code-based evaluation:
- All critical stress locations on the vessel shell and heads satisfy ASME Sec.VIII Div.2 triaxial stress limits
- Membrane + bending stresses (PL + Pb) are well below 4S allowable
- Hoop stress validation confirms strong agreement between FEA and analytical design
- Modeling approach using SURF154 and solid elements is technically sound and defensible
The 3750 m³ LPG mounded storage tank is structurally adequate and compliant for the defined design conditions.
Download the Full Technical Case Study
The full technical report includes:
- Detailed stress linearization tables for all governing sections
- Complete load combination definitions (pressure, soil, temperature, etc.)
- Stress plots and contour images for LC-7, LC-8, and other envelope cases
- Element selection rationale and modeling notes
- References to ASME clauses used in each check