SPEED REGULATION OF TURBINES 523 



ample area, is comparatively easy. The increase of pressure caused by 

 closing the gates on a diminishing load must, however, be guarded against 

 by the provision of a stand pipe, pressure regulator, or relief valve. 



(b) Turbine fed by a Supply Pipe which is of considerable, length or of 

 slight inclination. 



Here regulation on a diminishing load is easy, the inertia effect on 

 closing the gates being taken care of as in the previous case. Where the 

 statical head is not very great a stand pipe is preferable, since on an 

 increasing load this responds more quickly to an increased demand 

 for energy. 



Where the head is so great as to preclude the possibility of a stand 

 pipe, and where the pipe line is very long, the response to such a demand 

 is slow, this type of plant providing one of the most difficult problems in 

 governing. In such a situation, indeed, the only motor to give satis- 

 factory results is the Pelton wheel fitted with combined hand regulation 

 and deflecting nozzle, and with relief valve or pressure regulator. 



(c) Turbine fed by a long Horizontal Pipe. 



(1) Turbine discharging directly into Tail-race without Draught Tube. 

 As in the previous cases, the difficulty in governing occurs on an 

 increasing load, and may be met by the provision of a stand pipe near 

 the turbine. The efficiency of this stand pipe becomes greater as its 

 area, and in consequence the reserve of pressure water, is increased. In 

 the limit the stand pipe becomes an open forebay in which the turbine 

 works with free access, or to which it is connected by short pipes of 

 ample area. With this arrangement governing is easy, but without 

 it close regulation becomes impossible. 



(2) Turbine fitted with Draught Tube. Where circumstances necessitate 

 the placing of a turbine which works under a low head, at a considerable 

 distance from the supply reservoir and at the same time at some distance 

 above the tail-race level, a suction tube must be used for the discharge, 

 and the problem of successful regulation becomes still more complicated. 

 In such a case, with a very small head above the turbine, if this is 

 installed in an open forebay, a sudden demand for water may result 

 in this head being reduced until air is drawn into the turbine. The 

 vacuum in the suction tube is then destroyed, the suction tube emptied, 

 and the turbine at once stops. To prevent this occurring a minimum 

 depth of water of from 3*5 to 4 feet should be allowed above the turbine, 

 this being increased if the suction head is considerable. Where this 



