518 



HYDRAULICS AND ITS APPLICATIONS 



solution. Here the centrifugal governor operates a regulating valv 

 which admits eitjier water or oil under pressure to one side or other of ! 

 piston in a relay cylinder, this piston being connected with and operating 

 the turbine gate mechanism. 



Types of this relay mechanism are illustrated in Figs. 201 to 205, 234i- 

 236, 239A, 248 arid 249. 



Even with this relay accurate speed regulation under difficult circum 

 stances cannot be obtained without the provision of what is termed i 



FlG. 248. Thomson Vortex Turbine fitted with Hydraulic Relay Governor, 

 Compensating Device, and Automatic Pressure Regulator. 



relay return or compensating device. The reason for this is evident if 

 consider that as the speed falls, the gates are opened and the supp 

 column is accelerated, this opening going on until the supply of energ 

 per unit time is equal to the demand. But the acceleration of the wate 

 column goes on for an appreciable time after the gate opening has cease 

 and in consequence the supply now becomes too great for the requir 

 menrs of the wheel, the speed rises, and the governor commences to clo 

 the gates. This suddenly checks the motion of the supply column, and 

 virtue of its inertia produces an increased pressure at the valve ai 



