PELTON WHEEL 



458 



as to remain closed whatever the pressure in the main. If, however, the 

 spear rod closes the nozzle rapidly, the sudden motion of the dashpot 

 plunger sucks up the dashpot cylinder and with it the small needle valve. 

 This allows water to escape from above the main relief valve, which is then 

 lifted by the excess pressure on its under side and permits of free dis- 

 charge from the body of the nozzle. 



The dashpot cylinder now begins to fall by its own weight, closing the 

 needle valve and thus the relief valve, the time of closing being adjusted 



FIG. 204. Pressure Regulator for Pelton Wheel. 



by regulation of the dashpot orifices to suit the length of the supply pipe 

 line. 



This system has the great advantage that its working is quite indepen- 

 dent of any rise in pressure in the main, but rather anticipates any such 

 possible rise. 



Fig. 204 shows details of a device working on exactly the same principle, 

 and applied in this case to the twin Pelton wheel shown in Fig. 205. 

 Here the cross lever L is connected to the piston rods of the two relay 

 cylinders, and carries the dashpot rod A. Its connected plunger works in 

 the weighted dashpot (7, which itself carries the needle valve Vi. Pressure 



