PURPOSE:To prevent a runner of a water turbine from being turned reversely by the jet of water impinged onto the back surfaces of buckets, by employing such an arrangement that the ratio of the distance between the top of a water receiving surface of a deflector and the center of the water jet to the diameter of the water jet at the time of rated operation of the water turbine is equal to or greater than a specific value. CONSTITUTION:A deflector 90 to be provided outise in a Pelton wheel is attached to a nozzle 2 in a pivotally movable manner by a pin 90a. Water jet 3 is ejected from the nozzle 2 from the direction away from the center of the runner, and the direction of the jet 3 is deflected from the direction of buckets by the deflector 90 to stop rotation of the runner. Here, arrangement is such that the ratio (y/d0) of the distance (y) between the top of a water receiving surface 90b of the deflector 90 and the center of the water jet 3 to the diameter d0 of the jet 3 at the time of rated operation of the water turbine is selected to be equal to or greater than 0.6. By employing such an arrangement, it is enabled to prevent the flow 3a of the jet 3 causing reverse rotation of the runner from impinging onto the back surfaces of buckets after passing through the deflector 90.
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