Numerical Analysis of Unsteady Loads on a Steam Turbine Double Seat Control Valve

The quest for the ever-greater operational flexibility of large-scale steam turbines continues to drive enhanced control valve design. Such components, subjected to large static loads, also may experience strong vibrations due to unsteady turbulent fluctuations downstream of the throttling section. Challenging design efforts are required to ensure that these fluctuations are confined far from structural natural frequencies through the entire range of operating conditions. The following study by GE focuses on a computational analysis of the unsteady steam flow developing within a realistic double-seat control valve used in an industrial steam turbine. Actual operating conditions were considered both in terms of steam inflow pressure and temperature, and in terms of flow rates and plug height. Three plug heights were considered: two corresponding to an almost closed plug (thus subjected to choked flow), and the third verified at four different steam rates. To capture the unsteady nature of the flow and verify the fluid-dynamic forcing frequency, the Scale Adaptive Simulation principle was implemented using Ansys® CFX 14.5 code. Calculations were run using a computational time step of 1e−4 s and an effective simulation window of 0.2 s for time-averaged values and pressure time signals. The unsteady response was monitored by analyzing the frequency spectra of both integral variables (forces and moment on plug), as well as punctual pressure oscillations. Analysis of the results showed that it is possible to correlate the principal frequency and amplitude with the operating conditions. The Strouhal number based on plug diameter and bulk flow velocity remains constant independent of operating conditions. Nomenclature