# Investigation of the Fluid Flow in a Large Ball Valve Designed for Natural Gas Pipelines
## 2 Methods  
Metal-metal sealing is well known to be one of the safest methods, and according to API6D and ISO 14313 this type experiences very little leakage [19; 20]. According to the standard SR EN 3317:2015, the composition of the natural gas is at least 85% methane (CH\({}_{4}\)), and the remaining 15% is composed of the major components ethane (C\({}_{2}\)H\({}_{6}\)), propane (C\({}_{3}\)H\({}_{8}\)), butane (C\({}_{4}\)H\({}_{10}\)), pentane (C\({}_{5}\)H\({}_{12}\)), hexane (C\({}_{6}\)H\({}_{14}\)), nitrogen (N\({}_{2}\)), and carbon dioxide (CO\({}_{2}\)), and the minor components hydrogen (H\({}_{2}\)), oxygen (O\({}_{2}\)), carbon monoxide (CO), and helium (He) [21]. In the studied case, a density of 0.717 kg/mwas taken into consideration for the simulation--a value calculated for the previous composition, which is in accordance with the standard and with the Romanian Energy Regulatory Authority (ANRE) regulations [22]. For this fluid, the critical temperature is 190.56 K, the critical pressure is 4,599,200 Pa, and the critical compressibility factor is 0.286243. The principal components of the ball valve are the ball, the principal body, the upstream secondary body, the downstream secondary body, two identical seats, the acting stem, two identical guiding plates, and the connecting piece (see Figure 1). The studied valve was a trunion type, which differs from other categories by virtue of the fact that the ball is held firmly by two guiding plates. The closing of the ball valve takes place as follows: when the fluid flow moves from upstream to the closed valve, the upstream seat is pushed on the ball by the pressure through the gasket and by the springs; in that moment, the metal sealing surface from the seat covered with tungsten carbide is in contact with the metal sealing surface from the ball (also covered with tungsten carbide), and the sealing is completed. Tungsten carbide has several important advantages, including high strength, high resistance, high precision, good temperature resistance, and good corrosion resistance [23]. The producers of balls and seats coated with tungsten carbide claim that the coated balls and seats keep their high sealing qualities after more than 70,000 cycles [24]. The sealing of seats using the DPE design solution can provide highly efficient sealing, due to the two O-ring gaskets situated at two stages to ensure the same sealing performance upstream and downstream. Other sealing construction designs, such as the SPE (single-piston effect), provide efficient sealing only from the outside to the inside of the valve [25]. The case studies analyzed using SolidWorks Flow Simulation 2023 are presented in Section 3. One hypothesis assumed for the simulations was that the ball valve was  
Figure 1: Ball valve components (front section). constructed from rigid elements, which would mean that there were no deformations of the shape generated by compression. The defined boundary conditions were as follows: at the inlet section (upstream), the nominal pressure was 6.3 MPa and the velocity was 20 m/s; at the outlet section (downstream), the pressure was atmospheric pressure and the velocity was 0 m/s. SolidWorks Flow Simulation employs transport equations for the turbulent kinetic energy and the \(k-\epsilon\) model for the dissipation rate. To provide more robust and accurate simulations, nested iterations are used by the SolidWorks solver to allow treatment of the mutual influence of flow parameters and boundary conditions. These iterations are run step by step and repeated until the solver convergence is obtained, a maximum number of nested iterations is reached, or each conservation law's convergence condition is satisfied. The convergence solver allows that identification of the goals that converge, in contrast to those that do not converge or have no solution [26].