Principles of naval engineering - Historic Naval Ships Association

^Chapter 15. -PUMPS AND FORCED DRAFT BLOWERSIDLERROTOR HOUSINGFigure38.105X15-20.— Double-screw low-pitch pump.are in mesh. Everything in the chamber rotatesexcept the crescent, causing liquid to be trappedin the gear spaces as they pass the crescent. Thetrapped liquid is carried from the suction to thedischarge, where it is forced out of the pump bythe meshing of the gears. As liquid is carriedaway from the intake side of the pump, the pressurethere is diminished, thus forcing otherliquid into the suction side of the pump. The directionof flow in the internal gear pump can bereversed by shifting the position of the crescent180 degrees.CENTRIFUGAL PUMPS.-Centrifugal pumpswidely used aboard ship for pumping waterareand other nonviscous liquids. The centrifugalpump utilizes the throwing force of a rapidlyrevolving impeller. The liquid is pulled in at thecenter or eye of the impeller and is dischargedat the outer rim of this impeller. By the timethe liquid reaches the outer rim of the impeller,it has acquired a considerable velocity. The liquidis then slowed down by being led through avolute or through a series of diffusing passages.As the velocity of the liquid decreases, its pressureincreases— or, in other words, some of themechanical kinetic energy of the liquid is transformedinto mechanical potential energy. In theterminology commonly used in discussion ofpumps, the velocity head of the liquid is partiallyconverted to pressure head .Figure 15-21.LOWER THRUSTPLATE47.80-Triple-screw high-pitch pump.Centrifugal pumps are not positive-displacementpumps. When a centrifugal pump is operatingat a constant speed, the amount of liquiddischarged (capacity) varies with the dischargepressure according to the relationships inherentin the particular pump design. The relationshipsamong capacity, total head (pressure), and powerare usually expressed by means of a characteristicscurve.Capacity and discharge pressure can bevaried by changing the pump speed. However,centrifugal pumps should be operated at or neartheir rated capacity and discharge pressurewhenever possible. Impeller vane angles andthesizes of the pump waterways can be designedfor maximum efficiency at only one combinationof speed and discharge pressure; under otherconditions of operation, the impeller vane anglesCharacteristics curves are generally given in themanufacturers' technical manuals or in the outline assemblydrawings of pumps.407