466 



HYDRAULICS AND ITS APPLICATIONS 



Pressure water supplied through the pipe Q is guided by the con- 

 verging passages P P, its pressure diminishing as its velocity increases, 

 and is discharged at atmospheric pressure into the moving buckets B B. 

 These are ventilated to prevent the jet expanding to touch the rear vane, 

 and since the width of bucket diminishes with the angle of inclination of 

 the vanes, they are also splayed out from the inlet to the discharge side, 

 the breadth at discharge usually varying from 2*5 to 3 times that at inlet. 

 The outward deviation produced by this splaying of the buckets, while 

 slightly diminishing the efficiency of working, cannot be avoided. The 

 guide and vane angles at entry and exit are so designed that water 



FIG. 212. Partial Admission Girard Turbine. 1,000 H.P. at 500 revolutions per minute 



under 1,640 ft. head. 



enters the buckets without shock, and is discharged with an absolute 

 velocity which is only sufficiently great to ensure its ready removal from 

 the wheel. 



When less power is required one or more of the inlet passages may be 

 cut off by means of the slide S, the motion being regulated either by 

 hand or by a governor. By the provision of a series of supply ports 

 which may be completely cut out of action one by one, the loss of energy 

 which is inevitable through contraction and re-enlargement of section 

 whenever a stream is throttled, is reduced to a minimum. Where only 

 one admission port is used, the supply should be regulated by means of si 

 sluice or hood working between the port and the entrance to the buckets \l 

 By this means the velocity of efflux is unaltered, and the only lost*) 



