468 



HYDRAULICS AND ITS APPLICATIONS 



tangential to the wheel at entrance, and the necessity for the escaping 

 stream to clear the discharge from the following bucket render it 

 impossible to deflect the jet through 180. If a is the angle which the 

 incoming jet makes with the plane of the wheel in an axial flow turbine 

 and with the tangent plane to the wheel at the point of impact in a radial 

 flow machine, and if y be the angle which the discharge tips of the vanes 

 make with the same plane (Figs. 210 and 211), a is generally made about 

 24 and y about 21. These values, however, depend upon the head 

 increasing from about 12 and 13 respectively, with large heads and small 

 volumes of water, to about 30 and 28 with low heads and large volumes. 

 j3, the angle of inclination of the vanes at entrance is then made so that 

 the entering stream slides along the vanes without shock 3 



FIG. 211. 



The construction for determining the vane angles is shown in Fig. 211. 

 Here a b represents v, the velocity of the stream leaving the guide vanes, 

 in magnitude and direction, while h b = a c represents u, the velocity of 

 the wheel buckets at the point of entry, c b = a h then represents the 

 relative velocity at entrance to the buckets, and marks the correct 

 inclination of the bucket tips. 



In order to take advantage of the full head any impulse turbine working 

 under atmospheric pressure should be located at as small an elevation 

 above the tail-race as is possible. While keeping this in view, however, 

 it is highly important that the situation of the wheel be such that it is not 

 liable to become submerged by a raising of the tail-race level in time of 

 flood. If this should occur the buckets run full, the wheel works as a 

 pressure turbine, the conditions for which it is designed are entirely 

 violated and efficient working becomes impossible. 



