GIRARD TURBINE 



467 



introduced is that which may be caused by any alteration in the angle of 

 impact of the jet. 



Fig. 212 shows such a turbine, built by Messrs. Piccard, Pictet & Co., 

 of Geneva, and developing 1,000 H.P. at 500 revolutions per minute, 

 under a head of 1,640 feet. 



In a radial flow machine having full circumferential admission, a 

 cylindrical sluice or gate is often used, this partially cutting off the supply 

 to each bucket. 



While specially well fitted for heads of 100 feet and upwards, this 

 turbine, with full circumferential injection, gives good results with heads 

 from 10 to 50 feet, and has indeed been used with a head as low as 16 J 

 inches. Under the latter head an efficiency of about 55 per cent, may be 

 attained. With low heads and full injection the wheel must be horizontal, 

 since with a vertical wheel the jet velocity at the highest and lowest 

 points would be very different, while at the same time the loss of head 

 due to the difference in level of the guide vanes at exit and the tail-race 

 would become serious. To avoid the latter loss as far as possible in the 

 case of a vertical wheel with partial injection, the guides are placed as 

 near the bottom of the wheel as practicable. The horizontal wheel offers 

 the further advantage for large volumes of water, in that it affords greater 

 facilities for getting rid of a complete circumferential discharge. 



In common with all impulse turbines, the part gate efficiency is high, 

 while under suitable conditions the full load efficiency may amount to 80 

 per cent. In the case of an outward flow Girard turbine described in the 

 " Proceedings of the Institution of Civil Engineers," l and giving 400 H.P. 

 under 594 feet head, the outer diameter of the wheel was 8 ; 11", inside 

 diameter 7' 10J", the vanes, 110 in number, were 4'7 inches wide at the 

 entrance, and were splayed out to 15f inches. A single inlet passage was 

 used 4*31 inches in width. Under this head the jet velocity was 181 '6 

 feet per second, and the efficiencies were as follows : 



In this type of impulse wheel the impossibility of making the jet 



i 1881-2, Part 4. 



II H '2 



