600 HYDRAULICS AND ITS APPLICATIONS 



pump, having mechanical control to both valves. Tests of such a pump, 1 

 having a differential ram 5 inches and TjV inches diameter by 9 inches 

 stroke, when running at 150 revolutions per minute showed a mechanical 

 efficiency ranging from 82 to 86 per cent, as the head increased from 620 

 to 1.050 feet, with a discharge coefficient of '96. 



AET. 161A. HYDEAULIC RECIPEOCATING PUMPS. 



Where a high pressure water supply is available, the hydraulically 

 operated reciprocating pump has been extensively adopted for pumping 

 water for domestic or other purposes and for pumping sewage. Such a 

 pump is illustrated in Fig. 287. 2 This is driven by two double-acting 

 hydraulic cylinders placed one on each side of the pump body casting to 

 which they are attached, the two plungers P P and the pump rod R being 

 connected to the same crosshead. The pump is operated by means of a 

 piston valve in the valve chest V, this valve being worked from the cross- 

 head by means of the tappet rod T and lever L shown. 



AET. 162. DISPLACEMENT CUEVES. 



Discharge Coefficient. In the theoretically perfect pump the action 

 would be simple. On the suction stroke the delivery valves would be 

 tightly closed while the suction valves would open, admitting sufficient 

 water to entirely fill the barrel. At the commencement of the delivery 

 stroke these valves would immediately close, while the delivery valves 

 would open, and a volume of water equal to the plunger displacement 

 would be discharged. In practice, however, owing to the fact that the 

 valves do not instantaneously close, and because of leakage past the 

 plunger, etc., the volume delivered is not in general equal to the plungei 



displacement. The ratio rr - p , discharge, termed the dischar^ 



coefficient, varies with the type, speed, and condition of the pump, but 

 with moderate speed pumps in good condition and of good design lies 

 between "94 and '99. With high-speed pumps the modifying effects of 

 the inertia of the suction column may be such as to give a discharge 

 greater in volume than the piston displacement, the excess amounting 

 in exceptional cases to as much as 50 per cent. This effect will be 

 considered in further detail at a later stage. 



If a curve, having piston velocities as ordinates be plotted on a time 



1 Mechanical Engineer. June 26, 1908, p. 811. 



2 By courtesy of the Hydraulic Engineering Co., Ltd., Chester. 



