RECIPROCATING PUMPS 



587 



the delivery valve, but of this a volume = (A a) L finds its way to the 

 other side of the piston, and the volume actually delivered == a L. As 



before, for equality of delivery on the two strokes, = 2. In addition 



to an equal delivery on each stroke, the further advantage of an equal 

 suction on each stroke may be obtained by duplicating the suction and 

 delivery valves at each end of the barrel as indicated in Fig. 274, each end 

 becoming in effect a separate 

 displacement pump. 



This, the Double Acting 

 type, may be fitted either 

 as a piston pump, having 

 a packed piston and sleeve, 

 or with a turned plunger 

 working in an unpacked 

 ring, both types being shown 

 in Fig. 274. 



For fairly low pressures, 

 up to about 75 Ibs. per 

 square inch, either type is 

 good, the piston pump giv- 

 ing a slightly larger dis- 

 placement for the same 

 floor space. Where clean 

 water is to be pumped, the 

 unpacked plunger type, 



FIG. 274. Section of 12-inch Duplex Plunger and Ring, 

 or Piston Pump : 10-inch suction ; ,8-inch delivery ; 

 12-inch stroke. For pressures up to 100 Ibs. per 

 square inch. 



however, is advantageous 

 on the ground of its reduced 

 friction. 



A short fitting ring is 

 used, easily renewable, and 

 the ease of refitting such a 

 ring compared with that of re-boring a piston barrel renders its use advis- 

 able, where the condition of the liquid to be pumped permits. In either 

 case the work done on the two strokes is practically the same. 



For very high pressures, or where the water contains gritty matter in 

 suspension, the piston, bucket, or internally packed plunger pump is 

 unsatisfactory, and the type known as the Double -Acting Outside Packed 

 Plunger pump becomes more suitable. 



Here again the pump discharges a volume A L from one end of the 



