PRINCIPLES OF NAVAL ENGINEERING 



increase in internal energy of the fluid arising 

 from friction and turbulence. In some cases of 

 fluid flow, all elements in the bracketed term are 

 of such small magnitude that they may be safely 

 disregarded. 



Consideration of Bernoulli's theorem indi- 

 cates that the term pressure head , as used in 

 connection withpumps and other hydraulic equip- 

 ment, is actually a measure of mechanical poten- 

 tial energy ; that velocity head is a measure of 

 mechanical kinetic energy ; and that friction head 

 is a measure of the energy which departs from 

 the system as thermal energy in the form of heat 

 or of the energy which remains in the liquid, gen- 

 erally unusable, in the form of internal energy. 



Types of Pumps 



Pumps are so widely used for such varied 

 services that the number of different designs is 

 almost overwhelming. As a general rule, how- 

 ever, it may be stated that all pumps are designed 

 to move fluid substances from one point to an- 

 other by pushing, pulling, or throwing, or by some 

 combination of these three methods. 



Every pump has a power end and a fluid end. 

 The power end may be a steam turbine, a recip- 

 rocating steam engine, a steam jet, or an elec- 

 tric motor. In steam-driven pumps, the power 

 end is often called the steam end . The fluid end 

 is usually called the pump end. However, it may 

 be called the liquid end , the water end , the oil 

 end, or some other term to indicate the nature 

 of the fluid substance being pumped. 



Pumps are classified inanumber of different 

 ways according to various design and opera- 

 tional features. Perhaps the basic distinction is 

 between positive-displacement pumps and con- 

 tinuous-flow pumps. Pumps may also be classi- 

 fied according to the type of movement that 

 causes the pumping action; by this classification, 

 we have reciprocating, rotary, centrifugal, pro- 

 peller, and jet pumps. Another classification may 

 be made according to speed; some pumps run at 

 variable speed, others at constant speed. Some 

 pumps have a variable capacity, others discharge 

 at a constant rate. Some pumps are self-priming, 

 others require a positive pressure on the suction 

 side before they can begin to operate. These and 

 other distinctions are noted as appropriate in the 

 following discussion of specific types of pumps. 



RECIPROCATING PUMPS.-A reciprocating 

 pump moves water or other liquid by means of 

 a plunger or piston that reciprocates inside a 



cylinder. Reciprocating pumps are positive- 

 displacement pumps; each stroke displaces a 

 certain definite quantity of liquid, regardless of 

 the resistance against which the pump is operat- 

 ing. 



The two main parts of a reciprocating pump 

 are the water end, which consists of a piston and 

 cylinder arrangement and appropriate suction 

 and discharge valves, and the steam endl, which 

 consists of another piston and cylinder and ap- 

 propriate valves for the admission and release 

 of steam. 



Reciprocating pumps in naval service are 

 usually classified as: 



1. Direct-acting or indirect-acting, 



2. Simplex (single) or duplex (double). 



3. Single-acting or double-acting. 



4. High pressure or low pressure. 



5. Vertical or horizontal. 



The reciprocating pump shown in figure 15-7 

 is a direct-acting, simplex, double-acting, high 

 pressure, vertical pump. Now let us see what all 

 these terms mean, with reference to the pump 

 shown in the illustration. 



The pump is direct-acting because the pump 

 rod is a direct extension of the piston rod; thus 

 the piston in the power end is directly connected 

 to the plunger in the liquid end. Most reciprocat- 

 ing pumps used in the Navy are direct-acting. An 

 indirect-acting pump may be driven by means of 

 a beam or linkage which is connected to and mo- 

 tivated by the steam piston rod of a separate re- 

 ciprocating engine; or it may be driven by a crank 

 and connecting rod mechanism which is operated 

 by a steam turbine or an electric motor. An in- 

 direct-acting pump might appear to have only 

 one end— that is, the pump end. However, this 

 pump, as all others, must have a power end as 

 well; the separate engine, turbine, or motor 

 which drives the pump is the actual power end of 

 the pump. 



The pump shown in figure 15-7 is called a 

 single or simplex pump because it has only one 

 liquid cylinder. Simplex pumps may be either 

 direct-acting or indirect-acting. A double or 



1 Practically all reciprocating pumps in naval use are 

 steam driven. However, a few low pressure, motor- 

 driven reciprocating pumps are used for fresh water, 

 sanitary, bilge, ballast, and fuel oil transfer services. 

 These pumps are generally horizontal. When driven by 

 an electric motor, reciprocating pump is usually re- 

 ferred to as a power pump. 



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