Principles of naval engineering - Historic Naval Ships Association

CHAPTER 15PUMPS AND FORCED DRAFT BLOWERSThis chapter deals with shipboard pumps andwith the forced draft blowers used aboard manysurface ships to supply combustion air to thepropulsion boilers. In general, we are concernedhere with the driven end of the units rather thanwith the driving end; the auxiliary steam turbinesused to drive many pumps and blowers are discussedin chapter 16 of this text, and the electricmotors used to drive others are discussedin chapter 20.PUMPSAs we saw in chapter 8, the pump is one ofthe five basic elements in any thermodynamiccycle. The function of the pump is to move theworking substance from the low pressure sideof the system to the high pressure side. In theconventional steam turbine propulsion plant,"the pump" of the thermodynamic cycle is actuallythree pumps— the condensate pump, thefeed booster pump, and the main feed pump.In addition to these three pumps which are apart of the basic thermodynamic cycle, there are,of course, a large number of pumps used forother purposes aboard ship. Pumps supply seawater to the firemains, circulate cooling waterfor condensers and coolers, empty the bilges,transfer fuel oil, discharge fuel oil to theburners, supply lubricating oil to main and auxiliarymachinery, supply sea water to the distillingplant, pump the distillate into storage tanks, supplyliquid under pressure for use in hydraulicallyoperated equipment, and provide a variety ofother vital services.Pumps are used to move any substance whichflows or which can be made to flow. Most commonly,pumps are used to move water, oil, andother liquids. However, air, steam, and othergases are also fluid and can be moved withpumps, as can such substances as molten metal,sludge, and mud.A pump is essentially a device which utilizesan external source of power to apply a force to afluid in order to move the fluid from one placeto another. A pump develops no energy of its own;it merely transforms energy from the externalsource (steam turbine, electric motor, etc.) intomechanical kinetic energy, which is manifestedby the motion of the fluid. This kinetic energy isthen utilized to do work— for example, to raisea liquid from one level to another, as when wateris raised from a well; to transport a liquidthrough a pipe, as when oil is carried through anoil pipeline; to move a liquid against someresistance, as when water is pumped to a boilerunder pressure; or to force a liquid through ahydraulic system, against various resistances,for the purpose of doing work at some point.Principles and DefinitionsBefore considering specific designs of shipboardpumps, it may be helpful to examinebriefly certain basic concepts and to define someof the terms commonly used in connection withpumps.FORCE - PRESSURE - AREA RELATION-SHIPS.— When we strike the end of a bar, the mainforce of the blow is carried straight through tothe other end. This happens because the bar isrigid. The direction of the blow almost entirelydetermines the direction of the transmittedforce. The more rigid the bar, the less force islost inside the bar or transmitted outwardat right angles to the direction of the blow.When weapplypressureto theendof a columnof confined liquid, however, the pressure istransmitted not only straight through to the otherend but also equally and undiminished in everydirection. Figure 15-1 illustrates thedifferencebetween pressure applied to a rigid bar and pressureapplied to a column of contained liquid.391