212 



HYDRAULIC EQUIPMENT 



efficiency and maximum output coincide. With low heads and 

 high specific speeds it is, therefore, desirable to operate wheels 

 near their point of maximum output, and to obtain the best re- 

 sults the generator should be designed with consideration to this 

 point. 



Referring again to the curves in Fig. 108, it will be noted that 

 the full-load capacity occurs at about 6 per cent above normal or 

 rated full load in all three cases. This is in accordance with the 

 general practice, the margin being allowed for governing. It is 

 also noted that for curves B and C the efficiency is falling off very 

 rapidly at 6 per cent overload, and that should the gate be opened 

 still further the output would reduce instead of increase. If, 



I* 



60 



5 50 

 30 



20 

 10 



A- Specific Speed =25 Head =210 Ft. 

 B- " =60 = 62 



C- =84 " = 30 " 



5 10 15 20 25 



30 35 40 45 50 55 60 65 70 75 80 85 

 Horse Power in Per cent of Normal Load 



95 100 105 IK 



FIG. 108. Performance Curves of Several Turbines for Various Heads and 

 Specific Speeds as Shown. 



with low specific speed wheels, as represented by Curve A, the 

 gates were still further opened, the power would continue to 

 increase to some extent. 



The point of maximum efficiency for wheels represented by 

 curve A occurs at about 90 per cent of normal full load, in the case 

 of B at 93.5 per cent, while, in the case of C, the maximum effi- 

 ciency occurs just at the point of normal or rated full load. Thus, 

 as stated before, the power at which the maximum efficiency occurs 

 approaches nearer to full load as the specific speed increases. 



For wheels of low or moderate specific speed, as represented by 

 curve A, the efficiency remains very high over a very large range 

 in power, while for wheels of high specific speed, curve C, the 

 efficiency falls off rapidly as the power is reduced below the normal 



