Turbomachines hydrauliques Essais industriels et méthodes de calcul

Abstract

Static and dynamic load data for steady and transient conditions are basic design requirements for the machines considered in this report. Tests and calculation are complementary as a source of this information, the former in supplying the basic calculation assumptions and confirming the "representativeness" of the mathematical model. Turbomachine structures are subjected to both hydraulic and mechanical loads. Mechanica1 loads mainly result from weight and inertia effects (centrifugal and prestress loads), which are fairly easily determined. Hydraulic loads are of two distinct kinds, one being associated with pressure and its fluctuations caused by water-hammer and wave propagation in pipes, and the other with flow forces through the bladings. Overpressure and overspeed values to take into account in connection with machine disconnection overloads are determined with the aid of a general transient condition calculation program for turbines, pumps and pump-turbines. Stationary pressure distributions around guide-vanes and runner blades are calculated by a blading flow program. Having defined the loads and checked them by model and industrial tests, strength calculations are carried out with general or specific programs, depending on the considered turbine component and type of stress. In addition, calculated data are checked against experimental data obtained on models and, especially, machines operating in the power station. The latter tests take place when a unit is started up, or during operation if a particular problem requires investigation. The first requirement for experimental investigation of dynamic machine behaviour is to determine the natural modes of its main components, e.g. stay-vanes, guide-vanes and runner, in both air and water. This is usually done by harmonic excitation (Figs. 1, 3, 4 and 6). In addition, basic frequency can be determined by impact (Fig. 5). The most important quantities measured in industrial tests include displacement, velocity, acceleration, pressure, stationary and moving component stresses and operating parameters, e.g. guide-vane and runner blade settings, electric power, levels, etc. Typical instrumentation for bulb units and pump-turbines is illustrated in Figures 7, 9 and 11. Runner stresses and pressure (Figs. 8 and 10) and flexural and torsional shaft stresses (Figs. 12 and 13) are determined by radio signal transmission. Radial and axial thrust on vertical-shaft units is measured by strain gauges on the bearing support and thrust bearing. The strain gauges are calibrated by applying radial load to the runner and lifting the rotor. Fig. 14 shows thrust behaviour for the pump-turbine at Sainte-Croix during load rejection when operating as a turbine. After magnetic recording of the data, the energy density spectra and inter-spectra are calculated by a correlator operating in conjunction with a numerical Fourier transformer (Fig. 2). The origin and type of pulsation associated with a given effect are determined by spectrum analysis (Fig. 15). The connection between tests and calculation is illustrated by an example of experimental and theoretical stress determination on a Francis turbine runner model (Figs. 16, 17 and 18). The calculation programs relate mainly to investigation of stress and elastic or plastic strain and determination of natural mode, response and structural stability. The methods used are based on integral equations and finite elements (Fig. 19). These programs are preceded by pre-processing programs, i.e. automatic meshing, band width minimalisation, calculating and positioning of loads, and are followed by data-plotting programs (Figs. 20, 21 and 22).