718 HYDRAULICS AND ITS APPLICATIONS 



cylinder, while to prevent the former losses becoming excessive all working 

 velocities must be kept low. 



A further loss of energy occurs in some such machines as presses and 

 riveters, where the maximum force which the plunger can exert is only 

 needed for a short portion of its stroke, but where the expenditure of 

 energy is the same as if this were needed for the whole stroke. Also, in 

 the case of a hydraulic crane, if only one lifting cylinder is provided 

 without any special regulating device, the expenditure of energy is the 

 same whatever load, up to the maximum capacity of the crane, be lifted. 

 Certain devices which have been invented to overcome this difficulty will 

 be considered later. 



ART. 193. ACCUMULATORS. 



Since the delivery from a reciprocating pump is not uniform and since 

 it is necessary to have some reserve of energy to meet a sudden or 

 abnormal demand, some means of storing pressure energy is a necessary 

 adjunct to the hydraulic power station. 



With the high pressures in common use the elevated storage tank is out 

 of the question and the accumulator, devised by Sir W. G. Armstrong, 

 takes its place. 



Pressure water from the pumps, then, is not led directly into the 

 supply mains, but first into an accumulator from which it is taken to 

 feed the pipe line. 



Stripped of unessentials, the accumulator consists of a vertical cylinder 

 fitted with a weighted ram, the weight and area of this being adjusted so 

 as to give the required pressure in the mains. 



Thus, if A = area of ram in square inches, 



W = weight of rani in Ibs., 

 we have p A = W. 



The energy storage capacity of the accumulator is evidently simply 

 equal to the potential energy of the lifted ram and weight, and if L is the 

 length of its travel in feet, is given by L W foot Ibs. 



From another point of view, the storage capacity is given by 



i.e., by the pressure energy in the volume of water stored in the cylinder, 

 and since p A = W, this leads to the same result as before. 



EXAMPLE. 

 Leip = 1,120 Ibs. per square inch, L = 23 feet, A = -n X 9 2 = 81 if. 



