PumpsDOE-HDBK-1018/1-93POSITIVE DISPLACEMENT PUMPSThe complete assembly and the usual flowFigure 17 Two-Screw, Low-Pitch, Screw Pump Figure 18 Three-Screw, High-Pitch, Screw Pumppath are shown in Figure 17. Liquid istrapped at the outer end of each pair ofscrews. As the first space between the screwthreads rotates away from the opposite screw,a one-turn, spiral-shaped quantity of liquid isenclosed when the end of the screw againmeshes with the opposite screw. As thescrew continues to rotate, the entrapped spiralturns of liquid slide along the cylinder towardthe center discharge space while the next slugis being entrapped. Each screw functionssimilarly, and each pair of screws dischargesan equal quantity of liquid in opposed streamstoward the center, thus eliminating hydraulicthrust. The removal of liquid from thesuction end by the screws produces areduction in pressure, which draws liquidthrough the suction line.Three-Screw,High-Pitch,ScrewPumpThe three-screw, high-pitch, screw pump,shown in Figure 18, has many of the sameelements as the two-screw, low-pitch, screwpump, and their operations are similar.Three screws, oppositely threaded on eachend, are employed. They rotate in a triplecylinder, the two outer bores of whichoverlap the center bore. The pitch of thescrews is much higher than in the low pitchscrew pump; therefore, the center screw, orpower rotor, is used to drive the two outeridler rotors directly without external timinggears. Pedestal bearings at the base supportthe weight of the rotors and maintain theiraxial position. The liquid being pumpedenters the suction opening, flows throughpassages around the rotor housing, andthrough the screws from each end, in opposedstreams, toward the center discharge. Thiseliminates unbalanced hydraulic thrust. Thescrew pump is used for pumping viscousfluids, usually lubricating, hydraulic, or fueloil.Rev. 0ME-03Page 25
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