PUMP VALVES 593 



the delivery side. Also considerable shock takes place when these valves 

 close, this shock being due, not only to the valve itself dropping suddenly 

 to its seat, but also to the fact that a large mass of water partakes of the 

 return motion. 



The violence of the shock depends on the kinetic energy possessed 

 by the valve and accompanying water at the instant of closing, and 

 will therefore increase with the weight of the valve, with the mass of the 

 accompanying water, and with the maximum distance of the valve from 

 its seat, since the latter factor will determine the velocity of closing. 



The mass of water accompanying the valve is found to be proportional 

 to the mass included between the valve and the water level in the 

 corresponding air vessel, so that to minimise this effect, the difference in 

 level between valve and air vessel should be reduced as far as possible, 

 while the lift of the valve should be as small as is consistent with ample 

 inlet and outlet areas. 



With self-closing valves, the lift may be reduced either 



(1) by increasing the weight of the valve ; 



(2) by increasing the spring loading of a light valve ; 



(3) by limiting the lift by stops ; 



(4) by increasing the number or diameter of the valves. 



The first method, while reducing the lift and giving a quicker closing 

 effect, increases the hydraulic resistance ; while the increased weight of 

 the valve is in itself productive of shock. On the other hand, a light 

 valve is more subject to vibration while opening or closing, and this may 

 cause large oscillations of pressure in the pipe line. This tendency to 

 vibration increases with the speed of the pump, and diminishes with an 

 increase in the delivery pressure. On the whole, however, the balance 

 of advantage lies with the fairly light spring-loaded valve. 



The method of limiting the lift by stops is decidedly unsatisfactory, as 

 giving rise to oscillations of the valve, and hence of the pressure in the 

 pipe line. 



The best method is to increase the effective valve area by an increase 

 in the number or diameter of the valves. In the ordinary disc valve 

 (Fig. 280 a and [>), 



If r = radius and I = lift of valve, 

 the area of valve seat = IT r 2 (neglecting the effect of guides, etc.). 



But the effective valve area is the area of the cylindrical surface 

 generated by the perimeter of the valve during its lift = 2 TT r I. 



The most effective lift is thus obtained when these values are identical, 

 i.e., when 



. H.A. Q Q 



