604 



HYDRAULICS AND ITS APPLICATIONS 



suction pipes decreases, and also the rate of change of this velocity, /.-., 

 the acceleration, and the smoothness of working in consequence 

 increases. 



The following table indicates how this velocity changes : 



Type of Pump. 



Single cylinder, single-acting .... 



Two single-acting cylinders, cranks at right angles 



Single cylinder, double-acting .... 



Two double-acting cylinders, cranks at right angles 



single-acting 

 double-acting 



Three-throw pump, crank at 120 



Ratio of Maximum 

 to Mean Velocity 

 in Discharge Pipes. 



3-24 

 217 

 1-62 

 1-11 

 1-09 

 1-05 



As will be readily understood, the varying velocity and acceleration in 

 the supply and discharge pipes particularly in the supply pipes produce 

 a tendency to water hammer. 



So long as this hammer is prevented, either by suitably enlarging the 

 pipes ; by reducing the maximum piston velocity or acceleration ; or by 

 the provision of air vessels on suction and delivery pipes, the action of a 

 pump may be predicted very accurately from theoretical considerations. 

 Once water hammer is set up so many factors combine to influence the 

 result, and the subject presents such difficulties of treatment that, except 

 in simple cases, no satisfactory attempt can be made to deal with the 

 matter analytically. This is, however, less important, in that water 

 hammer is not, under any conditions, admissible in a pump, and while its 

 effect may be impossible to predict with any degree of accuracy, save in 

 the most simple cases, the steps necessary to prevent its occurrence are 

 well understood. 



The following demonstration may be relied upon as giving results 

 which are substantially correct so long as the pump is working withoi 

 shock. 



ART. 163. VARIATIONS OF PRESSURE IN THE CYLINDER OF A SINGJ 

 CYLINDER PUMP WITHOUT AIR VESSELS. 



For the water in the suction pipe to follow and maintain contact wit 

 the piston throughout the suction stroke, this column of water must 



