210 HYDRAULIC EQUIPMENT 



For impulse turbines of the Pelton type, specific speeds down 

 to very low values may be obtained with good results. The 

 highest efficiencies may perhaps be obtained with specific speeds 

 varying from 1 to 4, and will then be increasingly reduced as the 

 speed is increased up to about 6J or 7, which might be taken as the 

 extreme limit, the figures applying to single-nozzle wheels. The 

 maximum obtainable efficiencies with an impulse wheel may be 

 taken as between 85 and 89 per cent, these being figures to the 

 center line of the nozzle. If two or more nozzles are used on the 

 same wheel, a reduction of several per cent will result, and of course 

 the power will be increased in proportion to the number of nozzles. 



Where, therefore, the specific speed characteristics exceed the 

 above values, multi-runner turbines, more than one nozzle or 

 smaller units must be used. 



There is no hard and fast rule for the choice of a reaction tur- 

 bine or an impulse turbine, the field of their respective usefulness 

 overlapping to a considerable degree. The thing which limits the 

 specific speed of reaction turbines under high heads is the risk of 

 corrosion of the buckets, while the reason for the non-use of 

 impulse turbines under very low heads is the lack of economy 

 due to large dimensions and low speed required. The reaction 

 type of turbine is generally used for heads from 10 to 600 feet 

 and the impulse type between 300 and 3000 feet. The proper 

 system seems, therefore, to be very well fixed for low or high 

 heads, while for the intermediate range between about 300 and 600 

 feet, the proper system must be determined by referring to the 

 specific speed characteristics. 



Assume, for example, an installation having a head of 1900 

 feet and where the generators would require turbines of 10,000 

 H.P. capacity running at 375 R.P.M. What type of wheel 

 should be installed? 



1900 5 / 4 ~"' 



and, consequently, an impulse turbine should be selected. 



On the other hand, with a 10,000 H.P. wheel to operate at 

 57.7 R.P.M. under a 32-foot head, we get 



and a reaction turbine must be used. 



