Investigation of Unsteady Pressure Fluctuations in a Simplified Steam Turbine Control Valve

Abstract

Steam turbines are among the most important systems in the conversion of thermal into electrical power. As the amount of renewable energies increases, existing power plants are experiencing increased times at part load conditions. To control the power output of a steam turbine, the use of control valves is a widely spread method, allowing fast load gradients and a quicker response on variable power requirements. At part load, a significant amount of energy is dissipated across the valve, as the total inlet pressure of the turbine is reduced. At these conditions, the flow across the valve becomes trans- and supersonic and large pressure fluctuations appear within the downstream part of the valve. As a result, unsteady forces can trigger strong vibrations, leading to mechanical stresses and possible valve failures. A spherical valve shape is still used in smaller industrial steam turbines, in which the flow is prone to show strong flow instabilities across a wide range of operating points. Because of these known instabilities, the spherical valve shape was chosen as the experimental test geometry and the evaluation of the unsteady flow and fluid-structure-interaction within the scaled steam turbine control valve. Using numerical methods, the test valve is investigated and the time dependent pressure distribution in the downstream diffuser is evaluated. The evolution of the flow stability will be discussed for different pressure ratios. Pressure signals retrieved from the control valve test rig will be used to compare the numerical results to experimental data.

A Comparison of Steam Turbine Control Valve Geometries and Their Dynamic Behavior at Part Load

“…The power spectral density (PSD) of the same pressure signals is shown in the second row in Figure 7 for 1/12th-octave bands, indicating a continuous increase of low frequency broad band amplitudes as the pressure ratio is decreased for the reference design. In this low frequency region, large vortex structures as discussed in Windemuth et al [13] are observed in CFD results, showing slow and random propagations. The modified Backstep geometry shows a similar distribution of amplitudes outside the range of the annular flow.…”

“…Simultaneously, wall pressures drop and become almost steady, eliminating most of the unsteady pressure forces acting on the valve support. Because the occurrence is far outside the expected operating range, this range is not further discussed, as it is expected to be irrelevant with respect to field applications (refer to Windemuth et al [13] for further details).…”