HEAD AND TAIL RACES 527 



turbine runner and shaft itself, and from the electric motor when such 

 is used. 



In the Pelton wheel, however, the turning moment undergoes a slight 

 periodic variation, the period depending on the number of buckets and on 

 the speed of the wheel. On account of this and on account of the 

 comparatively small weight of the wheel for a given power, a flywheel is 

 in general essential for such close governing as is necessary for the 

 driving of. alternators in parallel. 



Figs. 252 and 253 l show the results of governing tests on turbines 

 VII. and IX. (pressure turbines) of p. 508 and Fig. 254 the results of 

 similar tests on turbines XII. and XIII. (Pelton wheels) of p. 444. Each 

 of the latter turbines is fitted with Doble needle regulators, while turbine 



XII. is also provided with an automatic pressure regulator. In turbine 



XIII. a decrease in load is followed by the automatic interposition of a 

 shutter between nozzle and wheel. This shutter is then gradually with- 

 drawn by the action of the relay returning device, and the needle is at 

 the same time gradually moved forward in the nozzle, the operation 

 taking place so slowly that no appreciable increase of pressure takes place. 



ART. 140. DESIGN OF HEAD AND TAIL-RACES. 



The construction of the head and tail-races for a turbine plant requires 

 careful consideration. The forebay should be free from sharp turns 

 and sudden changes of section, the velocity of flow not exceeding 

 from 3 to 4 feet per second. The tail-race should be of the same 

 capacity and should be formed with a well under the turbine outlet, 

 which should contain from 2 to 4 feet of dead water when the turbines 

 are idle. If this be not attended to, a serious loss of head may be 

 caused. 



Where a draught tube is fitted this should be submerged from 3 to 6 

 inches in the standing tail-water, while it is preferably bent at its lower 

 end, so as to discharge in the direction of flow in the tail-race. 



Where a long penstock is installed, this should be supplied with an 

 air pipe (Fig. 237) at its highest point and also at the highest point of 

 any vertical bends in its length, to permit the escape of accumulated air, 

 and also to prevent the collapse of the pipe due to the formation of a 

 vacuum should the turbine gates be opened with the head gates closed. 

 (See p. 271.) 



A strainer is always fitted at the exit from the forebay, or supply canal, 



1 "Proc. lust. Mech. Engineers," 1910-11. 



