Chapter 15. -PUMPS AND FORCED DRAFT BLOWERS 



CONNECTING 

 ROD 



CONTROL SHAFT 



SHAFT TRUNNION 

 BLOCK 



SOCKET RING 



DIAGRAM-TILTING BLOCK POSITIONS 



38.103 



Figure 15-14.— Cutaway view of axial-piston 

 variable stroke pump. 



The floating ring is so constructed that it can 

 be shifted offcenter from the pump shaft. When 

 it is centered, or in the neutral position, the pis- 

 tons do not reciprocate and the pump does not 

 function, even though the electric motor is still 

 causing the pump to rotate. If the floating ring 

 is forced offcenter to one side, the pistons re- 

 ciprocate and the pump operates. If the floating 

 ring is forced offcenter to the other side of the 

 pump shaft, the pump also operates but the di- 

 rection of flow is reversed. Thus both the di- 

 rection of flow and the amount of flow are 

 determined by the position of the cylinder body 

 relative to the position of the floating ring. 



ROTARY PUMPS.— Rotary pumps, like re- 

 ciprocating pumps, are positive-displacement 

 pumps. The theoretical displacement of a rotary 

 pump is the volume of liquid displaced by the 

 rotating elements on each revolution of the shaft. 

 The capacity of a rotary pump is defined as the 

 quantity of liquid (in gpm) actually delivered 

 under specified conditions. Thus the capacity is 



equal to the displacement times the speed (rpm), 

 minus whatever losses may be caused by slip- 

 page, suction lift, viscosity of the pumped liquid, 

 amount of entrained or dissolved gases in the 

 liquid, and so forth. 



All rotary pumps work by means of rotating 

 parts which trap the liquid at the suction side and 

 force it through the discharge outlet. Gears, 

 screws, lobes, vanes, and cam-and-plunger ar- 

 rangements are commonly used as the rotating 

 elements in rotary pumps. 



Rotary pumps are particularly useful for 

 pumping oil and other heavy, viscous liquids. 

 This type of pump is used for fuel oil service, 

 fuel oil transfer, lubricating oil service, and 

 other similar services. Rotary pumps are also 

 used for pumping nonviscous liquids such as 

 water or gasoline, particularly where the pump- 

 ing problem involves a high suction lift. 



The power end of a rotary pump is usually an 

 electric motor or an auxiliary steam turbine; 

 however, some lubricating oil pumps that supply 

 oil to the propulsion turbine bearings and to the 

 reduction gears are attached to and driven by 

 either the propulsion shaft or the quill shaft of 

 the reduction gear. 



Rotary pumps are designed with very small 

 clearances between rotating parts and between 

 rotating parts and stationary parts. The small 

 clearances are necesarry in order to minimize 

 slippage from the discharge side back to the suc- 

 tion side. Rotary pumps are designed to operate 

 at relatively slow speeds in order to maintain 

 these clearances; operation at higher speeds 

 would cause erosion and excessive wear, which 

 in turn would result in increased clearances. 



Classification of rotary pumps is generally 

 made on the basisof the type of rotating element. 

 In the following paragraphs the main features of 

 some common types of rotary pumps are discus- 

 sed. 



The simple gear pump (fig. 15-15) has two 

 spur gears which mesh together and revolve in 

 opposite directions. One gear is the driving gear, 

 the other is the driven gear. Clearances between 

 the gear teeth and the casing and between the 

 gear faces and the casing are only a few thou- 

 sandths of an inch. The action of the unmeshing 

 gears draws the liquid into the suction side of 

 the pump. The liquid is them trapped in the 

 pockets formed by the gear teeth and the casing, 

 so that it must follow along with the teeth. On the 

 discharge side, the liquid is forced out by the 

 meshing of the gears. Simple gear pumps of this 



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