PRINCIPLES OF NAVAL ENGINEERING 



duplex pump is an assembly of two single pumps 

 placed side by side on the same foundation; the 

 two steam cylinders are cast in a single block 

 and the two liquid cylinders are cast in another 

 block. Duplex reciprocating pumps are seldom 

 found in modern combatant ships but were once 

 commonly used in the Navy. 



In a single-acting pump, the liquid is drawn 

 into the liquid cylinder on the first or suction 

 stroke and is forced out of the cylinder on the 

 return or discharge stroke. In a double-acting 

 pump, each stroke serves both to draw in liquid 

 and to discharge liquid. As one end of the cyl- 

 inder is filled, the other end is emptied; on the 

 return stroke, the end which was just emptied is 

 filled and the end which was just filled is emptied. 

 The pump shown in figure 15- 7 is double-acting, 

 as are most of the reciprocating pumps used in 

 the Navy. 



The pump shown in figure 15- 7 is designed to 

 operate with a discharge pressure which is 

 higher than the pressure of the steam operating 

 the piston in the steam cylinder. In other words, 

 this is a high pressure pump . In a high pressure 

 pump, the steam piston is larger in diameter than 

 the plunger in the liquid cylinder. Since the area 

 of the steam piston is greater than the area of the 

 plunger in the liquid cylinder, the total force ex- 

 erted by the steam against the steam piston is 

 concentrated on the smaller working area of the 

 plunger in the liquid cylinder; hence the pressure 

 per square inch is greater in the liquid cylinder 

 than in the steam cylinder. A high pressure pump 

 discharges a comparatively small volume of 

 liquid against a high pressure. A low pressure 

 pump, on the other hand, has a comparatively low 

 discharge pressure but a larger volume of dis- 

 charge. In a low pressure pump the steam piston 

 is smaller than the plunger in the liquid cylinder. 



The standard way of designating the size of 

 a reciprocating pump is by giving three dimen- 

 sions, in the following order: (1) the diameter of 

 the steam piston, (2) the diameter of the pump 

 plunger, and (3) the length of the stroke. For ex- 

 ample, a 12" x 11" X 18" reciprocating pump has 

 a steam piston which is 12 inches in diameter, a 

 pump plunger which is 11 inches in diameter, and 

 a stroke of 18 inches. Thus the size designation 

 indicates immediately whether the pump is a high 

 pressure pump or a low pressure pump. 



Finally, the pump shown in figure 15-7 

 is classified as vertical because the steam piston 

 and the pump plunger move up and down. Most 

 reciprocating pumps in naval use are vertical; 

 a few, however, are horizontal , with the piston 



moving back and forth rather than up and 

 down. 



The remainder of the discussion of recipro- 

 cating pumps is concerned primarily with direct- 

 acting, simplex, double-acting, vertical pumps, 

 since most reciprocating pumps used in the Navy 

 are of this type. 



The power end of a reciprocating pump con- 

 sists of a bored cylinder in which the steam pis- 

 ton reciprocates. The steam cylinder is fitted 

 with heads at each end; one head has an opening 

 to accommodate the piston rod. Steam inlet and 

 exhaust ports connect each end of the steam cyl- 

 inder with the steam chest. Drain valves are in- 

 stalled in the steam cylinder so that water result- 

 ing from condensation may be drained off. 



Some reciprocating pumps have cushioning 

 valves at each end of the steam cylinder. These 

 valves can be adjusted to trap a certain amount 

 of steam at the end of the cylinder; thus, when 

 the piston reaches the end of its stroke, it is 

 cushioned by the steam and prevented from hit- 

 ting the end of the cylinder. When the pump is 

 operating at high speed, the cushioning valves 

 are kept almost closed so that a considerable 

 amount of steam will be trapped at each end of 

 the cylinder; at low speed, the cushioning valves 

 are kept almost open. Some reciprocating pumps 

 do not have cushioning valves. 



Automatic timing of the admission and re- 

 lease of steam to and from each end of the steam 

 cylinder is accomplished by various types of 

 valve arrangements. Figure 15-8 shows the pis- 

 ton-type valve gear commonly used for this pur- 

 pose; it consists of a main piston-type slide valve 

 and a pilot slide valve. Since the rod from the 

 pilot valve is connected to the pump rod by a 

 valve-operating assembly, the position of the 

 pilot valve is controlled by the position of the pis- 

 ton in the steam cylinder. The pilot valve fur- 

 nishes actuating steam to the main piston-type 

 valve, which, in turn, admits steam to the top or 

 to the bottom of the steam cylinder at the proper 

 time. 



The valve-operating assembly which con- 

 nects the pilot valve operating rod and the pump 

 rod is shown in figure 15-9. As the crosshead 

 arm (sometimes called the rocker arm) is moved 

 up anddownby the movement of the pump rod, the 

 moving tappet slides up and down on the pilot 

 valve operating rod. The tappet collars are ad- 

 justed so that the pump will make the full de- 

 signed stroke. 



The liquid end of a reciprocating pump has a 

 piston and cylinder assembly similar to that of 



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