Upon an increase in speed of the hydraulic turbine runner, the sleeve of centrifugal governor 1 moves upward, shifting the spool of valve 2 downward. Hydraulic fluid from valve 2 is directed to the right end of servomotor 3, moving piston 4 to the left. This changes the position of the regulating member and the speed is reduced. At the same time, when piston 4 is moving in the closing direction, bell-crank lever 5 and rod 6 move piston 7 of the idle-drain cataract downward. The cataract housing moves rapidly downward and, through lever 13, shifts valve spool 8 upward. With the guiding device stationary, idle-drain valve member 9 is held closed by piston 12 of the servomotor, under which valve spool 8 directs fluid under pressure. For this purpose, the valve has negative overlap, i.e. a narrow opening between the land of the spool and the delivery port. When valve spool 8 moves upward, the lower end of the servomotor is connected to the tank, piston 12 moves downward and idle-drain valve member 9 opens. At this, the required amount of water is drained from the inlet volute chamber to prevent water hammer. Feedback levers 10 and 11 return valve spool 2 to its central position. When valve spool 8 returns to its initial position, fluid under pressure is delivered again under piston 12, moving it upward. Valve member 9 closes slowly because it must overcome hydraulic resistance. Upon a reduction in turbine runner speed, valve member 9 remains closed. This is accomplished by the provision of check valve a in cataract piston 7.
$4244$CHP,Rg$