La poussée radiale dans les machines hydrauliques : expériences de laboratoire

Radial thrust in hydraulique machines: laboratory testing

Vice-Directeur du Développement et de la Recherche, Hydroart S.p.A. Milan

Abstract

The radial thrust exerted by the flow on the runner plays an important role among the loads acting on a hydraulic machine at the different operating conditions. It is useful to proceed to a classification, though schematic, of the various types of thrust which may occur. A first type of thrust, the fixed one, is due to dissymmetries in the stationary parts. It just has the effect to bend the shaft line and to load the guide bearings. A second type of thrust is the dynamic thrust which causes vibrations and has consequently more importance in machine designing, at least in the case of machines for hydroélectrical plants. We shall distinguish the dynamic thrusts of the rotating type from those of the random type. Among the former, the thrust rotating at the rope frequency, phenomenon characterising the operation of a Francis turbine at part load or at runaway conditions, is to be stressed. In extreme cases this thrust may cause the sliding of the rotating on the stationary part at the labyrinths. The latter occur in far from normal operating conditions and are to be taken into account in particular when calculating fatigue strenght. The paper includes a series of diagrams showing the results obtained from model tests of high specific speed Francis turbines and pump-turbines. They relate above all to the dynamic thrusts and can be summarised as follows : a) Radial thrust rotating at the rope frequency : the phenomenon is directly associated with that of the pressure pulse below the runner and increases with the specific speed of the unit. Some diagrams show that quite a similar phenomenon occurs in runaway operation : the thrust attains its maximum value in high specific speed Francis turbines. b) Random thrust : it occurs at different operating conditions, reaching in case of pump-turbines - its peak values at runaway, in the braking mode and in pump mode at strongly reduced flow.