Analisis respons mekanik torsional dan deformasi stem ball valve 4” Class 1500 RTJ material XM-19 pada fase transisi closed-to-open

The reliability of ultra-high-pressure fluid control instruments in the oil and gas piping sector is a critical parameter to ensure operational safety and environmental sustainability. This study aims to analyze the torsional mechanical response, angular twist deformation characteristics, and equivalent von Mises stress distribution on a 4-inch Class 1500 RTJ ball valve stem made of UNS S20910 (XM-19 / Nitronic 50) stainless steel during the critical closed-to-open transition phase (breakaway torque). The evaluation was conducted using a combined approach of classical analytical mechanics and multi-step non-linear Finite Element Analysis (FEA) in ANSYS Student 2026R1. Actual material properties obtained from the Material Test Certificate (MTC No. 2023051702) with a yield strength of 435 MPa were incorporated. The analytical results indicated a maximum shear stress of 218.50 MPa, while the FEA simulation (optimum mesh 5.5 mm) recorded a peak von Mises stress of 402.14 MPa (maximum shear stress of 231.85 MPa) concentrated at the keyway fillet transition region. Model validation produced a relative deviation of +6.11%, complying with the ASME V&V 20 criteria (under 10%). The structural integrity evaluation confirmed that the peak stress remained below the yield strength with a Safety Factor of 1.082 (SF > 1.0), satisfying the drive train requirements of API 6D. Although the angular twist deformation of 1.0328° is macroscopically safe, the hydrostatic thrust force (311.17 kN) induces local micro-tilting, which acts as the primary mechanism for scratch formation on the sealing surface.