522 HYDRAULICS AND ITS APPLICATIONS 



space behind this piston is connected through the pipe A to the waste 

 valve B. This is a balanced valve held closed by means of the spring S 

 against the pressure of the penstock water which acts through the pipe C 

 on the piston D. When the pressure in the penstock becomes greater 

 than normal the waste valve B is forced open, allowing water to escape 

 and relieving the pressure behind the piston P. The relief valve V then 

 opens, relieving the pressure in the main. When this pressure falls 

 below normal the valve B again closes, and the pressure behind the 

 piston P gradually increases to that in the penstock, and again closes the 

 valve V. The rate of closing can be regulated to prevent surging by 

 suitable adjustment of the throttle valve E on the connecting pipe F. 



Special consideration of the circumstances of each installation is 

 necessary to determine which of these regulating devices is likely to give 

 the best results. 



Dealing firstly with pressure turbines, the easiest type of plant to 

 govern is one in which the turbine is placed in a forebay of ample 

 dimensions fed directly from the supply canal (Fig. 238). Here a demand 

 for power is instantly met by an increased flow, at the velocity corre- 

 sponding to the supply head, while, when the gates are closed, inertia 

 effects are unimportant. Under these circumstances the speed may be 

 regulated with great nicety, and in general the more nearly the arrange- 

 ment of any power plant approximates to this the easier does it 

 become to get good regulation. Both head and tail-race should be of 

 ample size, so that any fluctuation in flow may not cause an appreciable 

 difference in either level, while all approach channels and passages should 

 have easy curves and well-finished surfaces so that the production of any 

 periodic wave motion may be prevented. 



Wherever possible the use of a long penstock should be avoided, and 

 the water brought as near as possible to its work in an open channe 

 of ample area, for it may be laid down as a general principle that the 

 easiest plants to govern are those in which the slope from open head- 

 water to open tail-water is as steep as possible, and in which the ratio 

 of kinetic energy to total energy is as small as possible. 



A few specific types of plant will now be considered in more detail, with 

 respect to their possibilities in the way of speed regulation. 



(a) Turbine fed by a Supply Pipe ivhicJi is almost vertical, i.e.) uitli a 

 slope of 60 or over. 



Here a demand for energy on an increasing load receives an immediate 

 response, and speed regulation in this direction, with a supply pipe oJ 



