684 



HYDRAULICS AND ITS APPLICATIONS 



Water then escapes through this valve into the air vessel, compresses 

 the air, and flows away along the rising main. 



As soon as the momentum of the supply column is destroyed the 

 delivery valve closes, the water below the valve partaking of the backward 

 motion thus instituted. This motion, once set up, can only be checked 

 by a reduction of pressure in the valve box below that corresponding to 

 the statical head and consequently the pressure in the valve box is reduced 

 rapidly until at some instant the waste valve reopens, and the whole 

 cycle of operations is repeated. Fig. 331 shows a typical pressure diagram 



from the valve box of such a 

 ram throughout one complete 

 cycle of operations. 



The idea that the waste 

 valve reopens because the 

 pressure due to the statical 

 head of the supply column is 

 insufficient to keep it to its 

 seat is quite erroneous. In 

 general, the valve will open 

 however light it may be, since 

 the pressure in the valve box 

 is reduced below that of the 

 atmosphere by the reflux 

 action of the water on its re- 

 Tme bound. In practice the waste 

 valve is usually made to weigh 





Mean Pressure in Air Vessel 



FIG. 



331. Pressure Diagram from Valve Box of 

 Hydraulic Ham. 



to 1 



from '30 Ibs. to *45 Ibs. per 

 square inch of area, so that a 

 foot head would be sufficient to 



statical pressure corresponding 

 prevent any opening. 



Advantage is taken of this reduction of pressure to keep the air vessel 

 charged, by the introduction of a snifting valve at Si (Fig. 330 a), air 

 being drawn through this valve into the valve box B when the pressure 

 falls below that of the atmosphere. 



The whole cycle, which may only take a fraction of a second to 

 complete, may be divided into four periods, during which the waste valve 

 is respectively opening, wide open, closing, and closed. 



Since the force tending to open the valve is equal to its weight together 

 with a much larger force due to pressure differences on its two faces, and 

 since the acceleration of the valve will therefore diminish as its weight 



