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API 6A Gate Valve Body (51.7 MPa Hydrotest)
API 6A hydrostatic shell proof pressure qualification and flange neck stress linearization for high-pressure sour gas service.
API Spec 6A requires wellhead gate valves operating at 5,000 psi rated working pressure to undergo a mandatory 1.5× hydrostatic shell proof test at 51.7 MPa (7,500 psi). Severe internal fluid pressure acts on the internal crotch region between the flow bore and the rectangular bonnet cavity, creating intense stress concentrations. This analysis evaluates primary membrane and bending stress categories to ensure compliance with ASME Section VIII Division 2 stress limits and prevent plastic deformation during factory acceptance tests.
1. Geometry & FEA Mesh
- •Forged low-alloy steel AISI 4130 (75K class: E = 206 GPa, ν = 0.29, yield strength 517 MPa).
- •Monolithic valve body with two 3-1/16" 5K end flanges, top bonnet cavity flange with BX-154 gasket ring grooves, and 24 drilled M24 stud holes.
- •Complex internal intersection between Ø78 mm flow passage bore and rectangular gate cavity with 15 mm transition fillets.
- •High-resolution tetrahedral mesh with 249,150 elements and 162,300 active degrees of freedom.
2. Loads & Boundary Conditions
- •Symmetric axial bolt-face constraint applied to the left end flange mating ring face.
- •Internal hydrostatic proof pressure P_test = 51.7 MPa (7,500 psi) applied uniformly across all internal wetted cavity surfaces.
- •Equivalent hydrostatic end-thrust blow-off traction applied to the flange gasket sealed diameters.
3. Simulation Results & Findings
- ✓Peak von Mises stress σ_vm = 453.2 MPa localized at the internal crotch fillet between the bonnet cavity and the downstream bore.
- ✓Maximum displacement magnitude |u| = 0.360 mm at the top bonnet flange outer rim.
- ✓Safety factor SF = 1.14 against yield during 1.5× proof hydrotest, satisfying ASME Section VIII Div 2 limits for hydrotest conditions.
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