Some characteristics of transient processes in encapsulated units

Abstract

Transient processes in encapsulated units are characterized by a number of distinctive properties in comparison with axial-flow turbines in vertical units with cylindrical guide apparatus, namely: 1. In the case of load drops the most significant characteristics are: rapid rundown of the unit: increase in discharge at the start of the transient process and, as a consequence, a negative water hammer: major role of the virtual mass of the water: unique character of the relation nmax(Ndr), when the maximum rundown can correspond to a partial load drop: the magnitude of the positive axial forces throughout the transient does not exceed their initial values; the maximum negative axial forces occurring in the braking zone can exceed in absolute value by severalfold the value of Pax corresponding to the rated output, especially in those cases when small opeings of the guide apparatus are combined with a high (0.6–0.8)no unit rotational speed. 2. During stopping the axial forces directed toward the lower pool can be greater than in the rated steady regime; the negative force can exceed the maximum values during drops. 3. Stopping of the unit with deceleration from full speed produces large negative forces, but they are not determining when selecting the design loads. 4. During starting comparatively small axial forces are observed, and the starts, as a consequence of the lightness of the unit, are accompanied by small starting discharges and starting moments. 5. By changing the regulating conditions it is practically always possible to operate the unit in a manner whereby a safe regime is provided from the standpoint of flow discontinuity in the turbine section and the axial loads in both directions do not exceed the prescribed values.