IMPULSE TURBINES 



469 



This often necessitates the sacrificing of a portion of the available head, 

 and, with a low fall, renders the use of such a turbine inadvisable. With 

 a high fall such proportional loss is in general so small as to be negligible. 



Attempts have been made to remove this drawback to the impulse 

 turbine. Thus Girard, in his system of Hydro-pneumatization, placed the 

 whole turbine in an air-tight casing, the lower' end of which opened out 

 beneath the surface in the tail-race (Fig. 213). By means of an air pump 

 driven by the turbine the air in this casing was maintained at such a 

 pressure as to keep the water level inside the casing below that of the 

 turbine wheel, whatever the tail-race level. The complication and expense 

 thus introduced, however, together with the fact that power is required to 

 work the air pump, prevented the general adoption of this idea. 



By mounting the wheel in an air-tight casing at some distance above the 

 tail-race and coupling this 

 to a discharge pipe or 

 draught tube (Art. 134), 

 delivering below the sur- 

 face of the tail-race, the 

 difficulty may be over- 

 come. On starting up 

 the turbine the escaping 

 water ejects the air from 

 the casing and creates 

 a partial vacuum. An 

 air valve, actuated by a 

 float in a chamber con- 

 nected with both casing 

 and draught tube, then 



FlG. 213. Axiai Flow Girard Turbine with Full Circum- 

 ferential Injection and with Girard's System of Hydro- 

 pneumatization . 



admits sufficient air to prevent the water level from rising as high as the 

 wheel. Fig. 205 shows one of a series of double Pelton wheels of 500 H.P. 

 which work perfectly well under a suction head of 20 feet. 



An older device, due to Meunier, consisted in regulating the discharge 

 by means of a sluice automatically regulated by a float. The required level 

 in the turbine casing was thus maintained, and the addition of a draught 

 tube rendered possible. 



In the Haenel " limit " turbine, which is essentially the same as the 

 Girard, the buckets are so designed that they run full when working as an 

 impulse turbine, the areas of the wheel passages being approximately the 

 same throughout. The flooding of the turbine does not then affect its 

 efficiency except in so far as it affects the available head, since the machine 



