612 HYDRAULICS AND ITS APPLICATIONS 



If I* 7 represents the delivery head h d , the total hsad on the piston is 

 given by the ordinates of the curve A E" B A', measured from F F' as 

 datum. 



If E were to represent the delivery head, the pressure would become 

 negative on passing the point E" and the suction valve would open, giving 

 suction on the delivery side of the pump. At high speeds, and with self 

 closing valves, this valve action becomes very irregular, delivery often 

 occurring during the major part of the suction stroke and suction during 

 the delivery stroke. 1 



ART. 164. RISE IN PRESSURE FOLLOWING SEPARATION IN A PUMP. 

 During separation the accelerating force on the suction column 

 = W a s {7T a h s hf ht} Ibs., where h f is the head necessary to 

 overcome friction and therefore depends on the velocity, and h t is the 

 pressure in the cylinder due to vapour tension and to air leakage. 

 Neglecting the effect of friction, the acceleration in pipe line 

 q W a. \ TT h, h t \ a (i7 n h. h f } , 



*/ a ( I > t7 V (I 17 _ 



TJ7 . 7 j l.o.o. 



W l s a s 1 8 



.'. Acceleration of water sur- ) (i: a h s h t ) a s 



face in cylinder J I, 'A 



If the water overtakes the piston at a distance x feet from the beginning 

 of the stroke, the time taken by the water surface to cover this distance 

 under its constant accelerating force must equal that taken by the piston 

 to cover the same space. 



/ 2 x I 4. 



The former of these times is given by T = \f - 



9 



since, with uniform acceleration, T = V j - . 



acceleration 



Assuming S.H. motion, the time occupied by the piston to cover 

 same space is given by - seconds, 

 where x = r (1 cos 0) (Fig. 292) 



(\ 

 1 ). 

 r) 



.'. Equating these times : 



1 / T\ / 9 -r 7 ~A 



J. i / .. ""\ A / a 3G v* *at 



- cos * 1 1 = V r- 



M r ' 9 1. V -*i}* 



1 For a series of diagrams showing this action reference may be made to Professor; 

 Goodman's paper. " Proc. lust. Mech. Engineers," 1903. 



