Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGincrease in internal energy of the fluid arisingfrom friction and turbulence. In some cases offluid flow, all elements in the bracketed term areof such small magnitude that they may be safelydisregarded.Consideration of Bernoulli's theorem indicatesthat the term pressure head , as used inconnection withpumps and other hydraulic equipment,is actually a measure of mechanical potentialenergy ; that velocity head is a measure ofmechanical kinetic energy ;and that friction headis a measure of the energy which departs fromthe system as thermal energy in the form of heator of the energy which remains in the liquid, generallyunusable, in the form of internal energy.Types of PumpsPumps are so widely used for such variedservices that the number of different designs isalmost overwhelming. As a general rule, however,it may be stated that all pumps are designedto move fluid substances from one point to anotherby pushing, pulling, or throwing, or by somecombination of these three methods.Every pump has a power end and a fluid end.The power end may be a steam turbine, a reciprocatingsteam engine, a steam jet, or an electricmotor. In steam-driven pumps, the powerend is often called the steam end . The fluid endis usually called the pump end. However, it maybe called the liquid end , the water end , the oilend, or some other term to indicate the natureof the fluid substance being pumped.Pumps are classified inanumber of differentways according to various design and operationalfeatures. Perhaps the basic distinction isbetween positive-displacement pumps and continuous-flowpumps. Pumps may also be classifiedaccording to the type of movement thatcauses the pumping action; by this classification,we have reciprocating, rotary, centrifugal, propeller,and jet pumps. Another classification maybe made according to speed; some pumps run atvariable speed, others at constant speed. Somepumps have a variable capacity, others dischargeat a constant rate. Some pumps are self-priming,others require a positive pressure on the suctionside before they can begin to operate. These andother distinctions are noted as appropriate in thefollowing discussion of specific types of pumps.RECIPROCATING PUMPS.-A reciprocatingpump moves water or other liquid by means ofa plunger or piston that reciprocates inside acylinder. Reciprocating pumps are positivedisplacementpumps; each stroke displaces acertain definite quantity of liquid, regardless ofthe resistance against which the pump is operating.The two main parts of a reciprocating pumpare the water end, which consists of a piston andcylinder arrangement and appropriate suctionand discharge valves, and the steam endl, whichconsists of another piston and cylinder and appropriatevalves for the admission and releaseof steam.Reciprocating pumps in naval service areusually 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 15-7is a direct-acting, simplex, double-acting, highpressure, vertical pump. Now let us see what allthese terms mean, with reference to the pumpshown in the illustration.The pump is direct-acting because the pumprod is a direct extension of the piston rod; thusthe piston in the power end is directly connectedto the plunger in the liquid end. Most reciprocatingpumps used in the Navy are direct-acting. Anindirect-acting pump may be driven by means ofa beam or linkage which is connected to and motivatedby the steam piston rod of a separate reciprocatingengine; or it may be driven by a crankand connecting rod mechanism which is operatedby a steam turbine or an electric motor. An indirect-actingpump might appear to have onlyone end— that is, the pump end. However, thispump, as all others, must have a power end aswell; the separate engine, turbine, or motorwhich drives the pump is the actual power end ofthe pump.The pump shown in figure 15-7 is called asingle or simplex pump because it has only oneliquid cylinder. Simplex pumps may be eitherdirect-acting or indirect-acting. A double or1 Practically all reciprocating pumps in naval use aresteam driven. However, a few low pressure, motordrivenreciprocating pumps are used for fresh water,sanitary, bilge, ballast, and fuel oil transfer services.These pumps are generally horizontal. When driven byan electric motor, reciprocating pump is usually referredto as a power pump.396