Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGREVOLVINGIMPELLERSTATIONARY OIFFUSER VANESFigure 15-26.— Volute turbine pump.23.19vanes and the volute reduce the velocity of theliquid, there is in this type of pump an almostcomplete conversion of kinetic energy to potentialenergy.Centrifugal pumps may be classified in severalways. For example, they may be eithersingle-stage or multistage, a single-stage pumphas only one impeller; a multistage pump has twoor more impellershousedtogether in one casing.In a multistage pump, as a rule, each impelleracts separately, discharging to the suction of thenext-stage impeller. Centrifugal pumps are alsoclassified as horizontal or vertical, dependingupon the position of the pump shaft.The impellers used in centrifugal pumps maybe classified as single suction or double-suction.The single-suction impeller allows liquid toenter the eye of the impeller from one directiononly; the double- suction type allows liquid toenter the eye from two directions. Single- suctionand double- suction arrangements are shown infigure 15-27. The double- suction arrangementhas the advantage of balancing end thrust in onedirection by end thrust in the other.Some of the more important centrifugalpumps used on naval ships are the main feedpump, the feed booster pump, the main and auxiliarycondensate pumps, fire pumps, freshwaterpumps, and gasoline pumps.A typical main feed pump is shown in figure15-28. This is a horizontal, high speed, turbinedrivenpump. Main feed pumps on most surfaceships operate at a dischargepressurethat is 100to 1 50 psig above the maximum steam drum pres-23.25Figure 15-27. —Single-suction and doublesuctionarrangements in centrifugal pumps.sure of the boiler. On ships having 1200-psigboilers, the discharge pressure of the main feedpumps is approximately 200 to 300 psig above thesteam drum pressure. A main feed pump mustoperate at varying speeds to maintain a constantdischarge pressure under all conditions of load.A constant-pressure pump governor" is used toregulate the admission of steam to the turbineand thus control the discharge pressure of thepump.PROPELLER PUMPS.— Propeller pumps areused on some ships for pumping water. Althoughthey are often classified as centrifugal pumps,this classification is incorrect because propellerpumps do not actually utilize centrifugal force fortheir operation.A propeller pump consists essentially of apropeller fitted into a narrow, tube-like casing.The propeller pumps the liquid by pushing it ina direction parallel to the pump shaft.Constant-pressure pump governors are discussed inchapter 16 of this text.410