PELTON WHEEL 451 



By either method of regulation the velocity of efflux is maintained 

 approximately constant, and the efficiency is therefore only slightly 

 affected at low loads, the quantity of water used being approximately 

 proportional to the load. 



On the other hand, the inertia of the supply column tends to prevent 

 close governing unless a relief valve, or some such device, is fitted near 

 to the nozzle, while care should always be taken, as explained on p. 285, 

 that the closing of the nozzle actually does diminish the supply of 

 energy to the wheel. 



One device which prevents a rise in pressure following any sudden 



closing of the regulating nozzle is shown in Fig. 201. Here the relay 



cylinder C is supplied with oil or water under pressure, this supply being 



egulated by a valve operated by the governor link. Any increase in 



peed is then accompanied by the admission of pressure water to the 



right-hand side of the piston. This forces the needle over to the left, 



reducing the supply of water to the wheel, and at the same time moves 



he cylinder itself to the right against the resistance of the springs at S, 



ind so opens the byepass valve V. In this it is aided by the pressure on 



,he valve itself, so that the pressure is quickly relieved. The motion of 



he cylinder relatively to its valve moreover tends to cut off the supply 



>f pressure water to the right-hand side of the piston, while the 



notion of the needle is utilized to bring the governor link back into its 



central position. This equalizes the pressure on the two sides of the 



)iston, and the cylinder itself, under the action of the side springs S, 



*eturns to its central position, at the same time closing the byepass 



valve. The whole apparatus is now ready to respond to a further change 



>f speed in either direction. Some relay returning device of this nature 



s indispensable if hunting is to be prevented (Art. 139). 



An extremely neat device for the same purpose is illustrated in Fig. 202, 1 

 ind is shown in Fig. 203 1 as fitted to a twin Pelton wheel. 



Here the horizontal governor lever A B is not connected to any fixed 

 ulcrum, but is pivoted at A on the end of a plunger working in the dash- 

 )ot C. At B it is connected to the spindle of the regulating valve, F 

 seing a fixed fulcrum. A subsidiary lever connects the end of the 

 plunger working in the dashpot P with the anchor link L and with the 

 governor collar, this being solely for the purpose of steadying the motion 

 of the governor. 



On a sudden increase in speed, following a reduction in load, the 

 governor collar lifts and the valve spindle is depressed, admitting watei 

 1 By courtesy of Messrs. Gilbert Gilkes & Co,, Ltd., Kendal. 



G G % 



