PELTON WHEEL 



449 



some suitable protecting device, such as a stand pipe, relief valve, or 

 pressure regulator, to the pipe line, this being placed as near to the valve 

 as possible. 



(3) A portion of the jet may be cut off at the nozzle by means of a 

 sharp-edged sluice sliding across the orifice, or the section of the jet may 

 e reduced by means of a needle regulator (Figs. 200 202). This 

 consists of a cylindrical needle of tapering section fitted inside the 

 nozzle axially with the jet. The water flows through the annulus 

 between the needle and the nozzle, forming a solid cylindrical jet on 



To Governor 



fo Governor Link 



Fixed Jtioveable 



FIG. 201. Needle Regulator and Pressure Regulating Device for Pelton Wheel. 



aving the needle. By axial regulation, the latter may be adjusted so 



to fill the orifice either partially or wholly. 



While giving a slightly greater loss by friction than the sluice regula- 

 on, a more stable jet is obtained, and on the whole needle regulation is 



be preferred. 



It is highly important, however, that the position of the needle in the 

 ozzle should be perfectly central, or the form and efficiency of the jet 

 ay be seriously affected. Also the needle must be supported so as to 

 re vent all vibration and consequent distortion of the jet. A further 

 )int to be noted is that the minimum section of the discharge channel 

 lould occur exactly at the tip of the nozzle for all positions of the 

 aedle. This may be illustrated by reference to Figs. 200 a and b. In a 



is condition is satisfied. In b this minimum cross section occurs at 



H.A. a Q 



