FFS Analysis of Reactor Tank 1 API 579 Level 3 | Linear + Elastic-Plastic | Buckling

Figure 1: 3D model with ovalities

Contents

Figure 1: 3D model with ovalities

Figure 1: 3D model with ovalities

1. Executive Summary

During inspection, significant ovalities were detected in Reactor Tank 1. API 579 Level 3 FFS assessment with linear + elastic-plastic FEA and buckling analysis confirmed the tank is safe.

  • All 6 load cases PASS
  • Linear: PL+Pb+Q = 315.61 vs 375.12 MPa (84%)
  • All 5 NL LCs converge
  • Buckling: 0.005 < 0.023 MPa
  • Validated with 0.04% equilibrium error

2. Design Parameters

Parameter Value
Equipment Reactor Tank 1 (Oval)
Design Code API 650
FFS Code API 579-1 (2021) Level 3
Analysis Code ASME VIII Div.2 Part 5 (2023)
Design Temperature 112°C
Design Pressure -5 kPa (vacuum)
Wind Pressure 0.573 kPa
Snow Pressure 0.2 kPa
Fluid Density 1760 kg/m³
Material A-283 Gr.C
Software ANSYS Mechanical

3. Mesh & Model

Figure 2: Mesh with mapped ovality imperfections

Metric Target Achieved Status
Aspect Ratio < 5.0 1.0606 PASS
Jacobian > 0.5 1.0619 PASS
Skewness < 0.70 0.0472 PASS
Element Quality > 0.1 0.9574 PASS

4. Boundary Conditions

Figure 3: Pressure and hydrostatic loading

Figure 3: Pressure and hydrostatic loading

Figure 4: Displacement BC and thermal loading

Figure 4: Displacement BC and thermal loading

5. Linear Stress Results (LC-1)

Figure 5: Total deformation — LC-1

Figure 5: Total deformation – LC-1

Figure 6: Max stress at roof — LC-1

Figure 6: Max stress at roof – LC-1

LC PL+Pb+Q (MPa) Allowable Status
LC-1 315.61 375.12 PASS

6. Elastic-Plastic Results (LC-2 to LC-6)

Per ASME VIII Div.2 Clause 5.2.4, convergence = PASS.
Figure 7: Convergence — LC-2

Figure 7: Convergence – LC-2

Figure 8: Convergence — LC-3

Figure 8: Convergence – LC-3

Figure 9: Convergence — LC-5

Figure 9: Convergence – LC-5

7. Buckling Assessment

Eigenvalue buckling per ASME VIII Div.2 Part 5.4:

Check Value Status
Design vs Allowable 0.005 vs 0.023 MPa PASS
Figure 10: Buckling mode shapes 1 & 2

Figure 10: Buckling mode shapes 1 & 2

8. Validation

Check Value
Weight 61,228 kg
Reaction 600,940 N
Error 0.04%

9. Lessons Learned

Ovality doesn’t mean failure

FFS Level 3 confirmed safety despite geometric deviations.

 

Both linear + nonlinear required

API 579 requires dual analysis for compressed geometries.

 

Buckling is the hidden risk

Vacuum + ovality makes buckling the governing mode.

10. Conclusion

  • All 6 LCs pass
  • Linear: 84% utilization
  • All NL cases converge
  • Buckling safe (4.7x margin)
  • Tank fit for continued service

Download the Full Technical Case Study

The full technical report includes:

  • API 579 Level 3 FFS assessment with linear and elastic-plastic analysis
  • Evaluation of all load cases including vacuum, wind, hydrostatic, and thermal effects
  • Buckling analysis with eigenvalue results and safety margins
  • Stress results for ovality-affected regions and critical locations
  • FEA modeling approach with geometric imperfections and boundary conditions
  • Validation checks including equilibrium, weight, and convergence
  • Code references including API 579-1 and ASME Section VIII Division 2

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