564 HYDKAULICS AND ITS APPLICATIONS 



respect. Either type may be used in connection with a suction tube, 

 though the pressure turbine lends itself more readily to this construction 

 and has the further advantage that it may be drowned without loss of 

 efficiency. The efficiency of the pressure wheel is not so sensitive to 

 changes of supply pressure as that of the impulse wheel, and therefore 

 this type is better fitted for work under a variable head. On the other 

 hand, except when fitted with swivelling guide blades, its part gate 

 efficiency is low. Apart from the conditions outlined, the possibilities of 

 accurate speed regulation are about equal in the two types. 



While the cost of the machine depends largely on the type and form of 

 construction, the capital costs of a pressure and an impulse turbine to 

 give equally good results as regards efficiency and speed regulation are 

 practically equal for a head of about 175 feet. For greater heads the 

 pressure turbine, and for lower heads the impulse turbine, becomes the 

 more expensive. 



ART. 152. THE STAND PIPE. 



The advantages of a stand pipe in increasing the possibilities of 

 accurate speed regulation on an increasing load have already been 

 indicated, and, as will be readily understood, the larger the area of this 

 pipe the more satisfactory are the results likely to be. Mechanical 

 difficulties, as well as considerations of first cost, however, limit the 

 maximum permissible size, and it becomes important to determine what 

 minimum size of pipe will enable satisfactory speed regulation to be 

 performed. 



The following investigation, though only approximate, gives results 

 which are sufficiently near to enable the necessary size to be estimated 

 with fair accuracy. 



In this investigation, which will take the form of a specific example, 

 the horse power of the turbine, its efficiency, the diameter of the penstock, 

 its length, and the working head, are assumed as being known, us is the 

 maximum increase in load likely to occur at any one time. This enables 

 the necessary velocity of flow along the penstock to be determined both 

 before the increase in load and after the velocity has again become con- 

 stant after this increase. By applying the equation of energy in the two 

 cases, the pressure at the stand pipe, and thus the free level may be- 

 obtained, from which the fall in level, and hence the volume; of water 

 leaving the stand pipe during the change, may be obtained in terms of 

 the area. The energy entering the wheel from the stand rJipe may then 

 be calculated, and, if it be assumed that the acceleration in J che penstock i 



