516 



HYDBAULICS AND ITS APPLICATIONS 



possible to throw the whole external load off with an instantaneous 

 increase in speed not exceeding 2 per cent, of the normal and with a final 

 increase of less than 1 per cent., the time to attain this normal speed not 

 exceeding 5 seconds. This, then, is the ideal to be aimed at in the 

 governing of a water-power plant, and while such close regulation as this 

 is practically impossible, an examination of the special difficulties to be 

 overcome in this case will indicate in what direction their most satisfactory 

 solution is to be found. 



In the first place, the motive fluid (water) is almost incompressible 



...ft. 



FIG. 247. Governor with Mechanical Relay as fitted to Girard Turbine. 



and contains a much smaller store of energy per unit weight than in th 

 case of steam, so that a similar demand for energy must be followed by 

 largely increased mass flow. The only force available to give the wate 

 this increased velocity is that of gravity, and it follows that even in th 

 most favourable circumstances, i.e., when the turbine is set directly in a 

 open forebay of ample dimensions, this velocity cannot exceed that du 

 to the supply head. Where the turbine is supplied through a long pij 

 line of small slope the state of affairs is much worse, since the force 

 gravity has now not only to produce increased mass flow and hence 

 give increased kinetic energy to the moving column of water, but h 



