616 



HYDRAULICS AND ITS APPLICATIONS 



velocity at the open end will still be f.s., while the mean velocity 



4,700 a s 



in. the pipe will be half this. In virtue of this velocity the column 

 possesses a store of kinetic energy which, on stoppage of the piston, is 

 transformed into energy of strain, and which in consequence gives rise to 

 a rapid rise in pressure, of the nature of water hammer. 



Actually this occurs before the end of the stroke at a point where the 



retardation due to the piston becomes equal to 





v being the velocity 



of flow in the pipe at this instant, and the resultant rise in pressure, 

 assuming a rigid pipe line, will equal 63'7 v Ibs. per square inch. As the 

 retardation and velocity can both be calculated in terms of 6, the position 



Atmos 



Line 



FIG. 294. Pump Diagram showing the effect of the elasticity of Suction 

 Column. Air Vessel on Delivery side only. 



of the piston when this occurs, and the magnitude of the shock, can be 

 readily calculated. 



The effect of the elasticity of the suction column is therefore to modify 

 the shape of the diagram as shown in Fig. 294, where the dotted line 

 A' B r C' represents the theoretical curve, neglecting the effect of elasticity, 

 and A B C is the actual curve. 



In the preceding investigations, the effect of loss of energy due to the 

 sudden enlargement of section of the stream on entering the pump barrel, 

 and to valve resistances, has been neglected. In general, however, this 

 will be comparatively small, except where the suction pipe is very short, 

 in which case the difficulties already outlined cease to exist. 



Even with a long suction pipe these may be considerably minimised, 

 if not entirely removed, by the provision of an air vessel or vacuum 

 vessel as it is sometimes termed, since the pressure of the con- 

 tained air is less than that of the atmosphere on the suction side of 



