606 



HYDRAULICS AND ITS APPLICATIONS 



of exhaustion in the pump. If the level of the pump is above that 

 of the suction reservoir so as to give a suction lift of h s feet (Pig. 

 289), the available head is correspondingly reduced, and has a maximum 

 theoretical value of (7r a Ji 8 ) feet, where it a is the height of the water 

 barometer. 



Thus, for continuity of contact between piston and water on the suction 

 stroke, we must have : 



- h, > A !-> \ a + 

 9 a* ' 



v* feet. 

 2 m a, J 



(i) 



Should this condition not be satisfied the piston leaves the water at 

 some point usually at the beginning of the stroke, since here a has its 



Fm. 289. 



maximum value the action being termed Separation or Cavitation. So 

 long as this state of affairs exists, the pressure behind the piston is 

 uniform, being that of water vapour at suction temperature, and in con- 

 sequence the water flows along the pipe with an acceleration which is 

 uniform, except for the increasing effect of friction as its velocity increases. 

 The piston acceleration is, however, continuously diminishing, so that 

 the water overtakes the latter at some point before the completion of the 

 stroke, the meeting being usually accompanied by considerable shock, 

 while pressures considerably in excess of those corresponding to the 

 statical head are produced. 



Separation may also occur between the delivery column and the piston 

 during the second half of the delivery stroke, when the piston is 

 retarded. ^ Thus if a is the magnitude of this retardation, the head 

 necessary to produce the retardation in the delivery column is given by 



