Forced Response Analysis of Steam Turbine Inlet Flow Control Valves

This work presents a comparison of two forced response methodologies implemented in commercial finite element software Ansys for describing the dynamic structural response of a steam turbine combined stop and control valve assembly in realistic operation conditions. The first method employs one-way coupling where the pressure field of a selected acoustic mode from an acoustic modal analysis on the valve cavity is scaled based on a pressure probe measurement and mapped onto the structure followed by a harmonic forced response analysis at the structure natural frequency. This method is called the decoupled model — it is fast and conservative as it assumes the acoustic and the structure modes to coincide providing a worst-case forced response estimate. The second method employs two-way coupling between acoustics and structure vibration. It takes five to ten times longer to run than the decoupled model because of the presence of non-symmetric system matrices and must be run multiple times with inputs spanning the operating condition range. However, the coupled model provides the opportunity for a more optimal design as it does not assume the acoustic and structure modes to line up. For the valve geometry studied in this work the effect of two-way coupling seems significant in some conditions where it can cause changes of up to 50% in the forced response.