TURBINES 221 



relief valves are often provided, either of the pressure or the 

 synchronous by-pass type. The former is analagous to the 

 safety valve on a boiler and does not open until a certain pressure 

 has been obtained. The latter, however, is operated by the gov- 

 ernor at the same time as the turbine gates but in opposite direc- 

 tion, thus affording a by-pass so that there is no reduction in the 

 flow. To prevent waste of water these by-passes may be slowly 

 closed by some auxiliary device. It is obvious, however, that 

 such water-saving relief valves are inoperative when the load is 

 thrown on, and, therefore, cannot then assist the speed governor 

 or prevent surges in the pipe lines caused by the same. For pre- 

 venting these, surge tanks or sufficient flywheel effect of the tur- 

 bine unit must be relied upon. 



For the speed regulation of impulse wheels there are three 

 methods in general use, viz. : 



1. Hand-regulated needle nozzle with jet deflector. 



2. Needle regulating and deflecting nozzle. 



3. Auxiliary relief needle nozzle. 



Either of the above involve the use of the characteristic needle 

 and nozzle tip, a sectional view of which is shown in Fig. 114, the 



FIG. 114. Sectional View of Pelton-Doble Needle Nozzle. 



full lines illustrating the position of the needle when the nozzle 

 is closed and the dotted lines the needle position with jet dis- 

 charging. 



The first system consists of a nozzle body in which is inserted 

 a concentric tapered needle as just described. By means of this 

 needle, which is manually controlled for this type of nozzle, the 

 jet area is adjusted intermittently to correspond to either the 



