594 HYDEAULICS AND ITS APPLICATIONS 



2 TT r I = TT r 2 



It follows that, in order to take advantage of the available valve seat 

 area, it is impracticable to use very large valves, since for a given lift the 

 effective discharge area varies as the diameter of the valves, while the 



valve weight varies more nearly as d 2 . Also if the lift were made equal 



(\* 



to -, this would soon become excessive. Because of this, small valves 



2 



should be used, with a lift giving a discharge area approaching that of the 

 valve seat, and for high speed pumps modern practice is opposed to the 

 use of disc valves of more than about 3 inches diameter. Where double 

 beat valves are used this may be increased, but in no case should the 

 valve have a lift exceeding f inch. 



Valve Leakage and Slip. A further point to be noted is that slip, or 

 leakage past the valves while closing, is proportional to the mean effective 

 opening of the valve ; to \f h, where h is the head on the valve ; and to 

 the time of closing, t. Generally we may take t as proportional to 



If, then, a number of small valves replace a single large valve of the 

 same effective discharge area, the slip will be reduced, since to get the 

 maximum discharge effect the lifts of the valves must in every case be 

 proportional to their diameters, and this lift, and therefore the time of 

 closing, will be greater with the larger single valve. 



Thus, whereas the slip past n valves of diameter d is proportional to 



1 5. 



n h' 2 d 2 , that past a single valve of the same effective discharge area, and, 



therefore, of diameter D where D = d */ n, is proportional to h' 2 d 2 ? 

 that by increasing the number of valves to n, the leakage is reduced in 



n 1 

 ratio = -=-. 



n* * 



E.g., where n = 4, the leakage is equal to = '707 times that foi 



4 

 with a single large valve. 



ART. 161. HIGH SPEED BECIPKOCATING PUMPS. 

 Owing to the length of time necessary for an automatic disc valve 

 close, and to the irregularities in its action produced by inertia effects 

 the water in the supply and delivery pipes, a high rotative speed 



