Entgasung von Wasser-Luft-Gemischen in Unterwasserkanälen von Pelton-Wasserkraft- anlagen : Spezielle Anwendung in Pumpspeicherkraftwerken

The deregulated electricity market in Europe opens new potential for the construction of pumped storage hydropower plants. Optimum turbine efficiency under high head conditions for the various operation modes is achieved by the use of Pelton turbines. In order to reduce the shaft length between the storage pump and pelton runner in the power cavern (or powerhouse), It is possible to place the Pelton runner below the water level of the lower reservoir. This kind of arrangement requires that the water level below the runner is pressed down by means of overpressure. When the water -flowing out from the nozzles - hits the runner buckets and falls down into the tailwater channel, considerable amounts of air are dragged into the water. This air must be separated from the fluid before the flow reaches the pressurized conduit system. In case that the de-aeration length of the tailwater channel is too small, air is conveyed towards the reservoir. In order to maintain air volume in the tailwater channel, additional air has to be pumped into the tailwater channel which may cause disadvantageous technical and economical situations for the whole system. The paper describes the general flow pattern in tailwater channels of Pelton turbines starting from the runner bucket. General effects of overpressure on the de-aeration process is discussed. Results of prototype measurements for air-water velocities and void fraction for three tailwater channels are presented. An extension of an existing formula for the de-aeration length is presented.