This case study presents the finite element analysis (FEA) and verification process conducted for the pressure-containing parts of the F35-340 actuator, developed by Actuator Technology Company. The primary goal was to demonstrate the structural integrity of the actuator under specified operating conditions and to ensure compliance with the requirements of the ASME Boiler and Pressure Vessel (B&PV) Code, Section VIII, Division 2, as well as the Pressure Equipment Directive (PED). The assessment considered all relevant failure mechanisms, including plastic collapse, fatigue, buckling, and bolt failure.
Modeling Approach
A detailed three-dimensional model of the F35-340 actuator was generated using PTC Creo, based on client-supplied drawings. Finite element calculations were conducted in Ansys 2019 R3. The actuator’s main pressure boundary comprises two flanges and a cylindrical shell, assembled using eight tie rods, which also provide the necessary sealing force at the flange interface. The actuator operates with a moving piston, which divides the internal volume into a pressurized upper chamber and an ambient lower chamber. For the analysis, the pressure in the lower chamber was assumed to be atmospheric.
Bonded interfaces were defined for the following surfaces: at bolt-to-flange, bolt-to-tie rod (threads), and flange-to-cylinder surfaces, while all other contacting surfaces were treated as frictionless. The analysis simulated multiple load cases in a subsequent manner from initial bolt pretension to full design pressure, operational torque (in both directions), and temperature gradients representative of service conditions. Both piston-high and piston-low positions were evaluated, with reported results always reflecting the most severe stress condition.
Material Properties
The materials of which the actuator was fabricated were following EN standards. To perform an ASME study equivalent ASME materials with similar properties were used. materials. Materials included:
- Cylindrical shell, top flange, piston: A–516 Gr.70 (P355N)
- Bottom flange: A-656 (S355J2)
- Tie rods: A547 4140 (42Cr)
- Bolts: A574 4340 (34Cr)
Critical material properties such as yield stress, ultimate tensile strength, Young’s modulus, and allowable stresses (at both ambient and design temperatures) were used in the analysis.
Boundary Conditions and Loading Scenarios
The design conditions are:
- Design pressure: 12 bar(g)
- Maximum operating pressure: 6.9 bar(g)
- Design temperature: 82°C (with ambient at 20°C)
- Bolt pretension: Set to induce 240 MPa stress (196 kN per bolt)
Transient thermal and mechanical loads were considered in a stepwise manner. The most critical fatigue loading was the daily heating cycle (temperature from 20°C to 82°C), while the torque cycle (arising from valve actuation) occurred less frequently. The actuator was designed for a minimum service life of 25 years, with load cycles calculated accordingly.
Code Assessment Criteria
The actuator design was validated against ASME VIII-2 Part 5 requirements. The principal failure modes assessed included:
- Plastic collapse: Evaluated using calculated primary and local membrane and bending stresses versus code-allowable values.
- Fatigue: Assessed using stress range and cycle count in accordance with ASME fatigue curves and Miner’s rule for cumulative damage.
- Buckling: Specifically checked for the cylindrical shell under compressive bolt loads.
- Bolt failure: Average and bending stresses in bolts were compared to code-allowable limits.
Each actuator component (cylinder, tie rods, piston, top flange, bottom flange, and bolts) was individually analyzed for these criteria.
Results and Discussion
Cylinder
The cylinder experienced compressive loads through bolt and tie rod pretension, with internal pressure acting on the wall between the piston and top flange. Maximum calculated stresses under operating pressure were well within allowable limits for plastic collapse (149 MPa vs. 161.3 MPa allowable membrane stress). Buckling checks confirmed that the maximum compressive stresses were well below the critical buckling stress (912 MPa). Fatigue assessment for the daily heating cycle showed the maximum amplitude (48.9 MPa) was far below the endurance limit, allowing for over 10¹¹ cycles, ensuring fatigue failure was not a concern. Ratcheting stress ranges were also within code limits.
Tie Rods
Tie rods carry the pressure thrust and connect the top flange to the rest of the actuator. Maximum primary membrane stress was 128 MPa (allowable: 233.1 MPa). The highest fatigue stress amplitude, after accounting for stress concentration at the notch (total 114.5 MPa), corresponded to an allowable cycle count of 1.1 × 10⁷, well above operational requirements. Ratcheting and plastic collapse criteria were satisfied, ensuring robust performance under both static and cyclic loads.
Piston
The piston was modeled with a rigid spring connection to the bottom flange to simulate pressure thrust transfer. Maximum membrane (120 MPa) and combined (123 MPa) stresses were comfortably under the respective allowable values. For fatigue, the heating cycle produced a maximum amplitude of 90.3 MPa, with an allowable cycle count of 8 × 10⁵, more than sufficient for the expected service life. Ratcheting stress was within the permissible range.
Top Flange
The top flange was primarily loaded at bolt locations, with maximum membrane stress of 90 MPa (allowable: 161.3 MPa). Fatigue due to heating cycles resulted in a stress amplitude of 134.3 MPa, corresponding to an allowable cycle count of 1.1 × 10⁵, which is adequate given the expected number of load cycles. Ratcheting checks passed without requiring further stress linearization.
Bottom Flange
The bottom flange’s critical load came from the pressure thrust via the piston. The highest membrane stress recorded was 106 MPa (allowable: 117.9 MPa), and the fatigue amplitude of 33.5 MPa allowed for 2.5 × 10⁸ cycles. Ratcheting and plastic collapse criteria were also met.
Bolts
Bolts joining the top flange to tie rods were evaluated for both plastic collapse and fatigue. Maximum primary membrane stress was 343 MPa (allowable: 466.2 MPa), and the combined stress was 609 MPa (allowable: 699.3 MPa). For fatigue, using a conservative strength reduction factor and a material-specific S-N curve for 4340 steel, the bolts could withstand at least 16,700 cycles at the highest stress amplitude encountered (470 MPa), which covers the expected operating regime. Ratcheting stress was also within allowable limits.
Key Takeaways
The finite element analysis validated the F35-340 actuator design against all relevant ASME VIII-2 failure modes. All primary pressure-retaining components and critical fasteners demonstrated compliance with plastic collapse, fatigue, buckling, and ratcheting limits, ensuring safe long-term operation under the specified thermal, pressure, and mechanical loadings. Special attention was given to the bolts, where a more applicable fatigue curve (4340 steel) was employed for a realistic assessment, confirming an adequate margin for the expected service life.
The detailed FEA-driven verification process not only demonstrated code compliance but also provided high confidence in the actuator’s structural reliability for the intended application.