High Performance Rotary Screw Pumps

1 Rotary Screw Pumps have ex-isted for many years and aremanufactured around theworld. More demanding ser-vice requirements impose chal-lenges on Screw pump manu-factures to provide higher pres-sure or flow capability, betterwear resistance, improved cor-rosion resistance and lowerleakage emissions. Bettermaterials and more precisemachining techniques as wellas engineering innovation haveled to improvements in all first Screw pump built wasprobably of an Archimedesdesign used to lift large vol-umes of water small verticaldistances; they are still manu-factured and used for this ser-vice. Today s three Screw highperformance pump can deliverliquids to pressures above 300 Bar and flows to 750 m3/h withlong term reliability and excel-lent efficiency. Twin screwpumps are available for flowrates to 4000 m3/h, pressuresto 100 Bar and can handle cor-rosive or easily stained materi-als, again at good Screw Pumps , alsoknown as progressive cavitypumps use an elastomeric sta-tor and a flexible joint eccentricrotation metallic Screw .

2 Theyare most commonly used forhigh solids content, viscousflows such as sewage, cementand other difficult slurries. Theyare not included in this discus-sion as their design and appli-cation bear little resemblanceto multiple Screw Screw Pumps PRINCIPLE APPLICATIONST hree Screw Pumps are thelargest class of multiple screwpumps in service today. Theyare commonly used for ma-chinery lubrication, hydraulicelevators, fuel oil transport andburner service, powering hy-draulic machinery and in refin-ery processes for high tem-perature viscous products suchas asphalt, vacuum tower bot-toms and residual fuel Screw Pumps also findextensive use in crude oil pipe-line service as well as gather-ing, boosting and loading ofbarges and ships. They arecommon in engine rooms onmost of the world s commercialmarine vessels and many com-bat ships.

3 Subject to materialselection limitations, threescrew Pumps are also used forpolymer pumping in the manu-facture of synthetic fibers suchas nylon and lycra. Designsare now available in seallessconfigurations such as mag-netic drives and canned ar-rangements, figure 1. Themagnetic drive Screw pump isused extensively for pumpingisocyanate, a plastic compo-nent which is an extremely dif-ficult fluid to seal using conven-tional technology. Three screwpumps are renowned for theirlow noise levels, high reliabilityand long life. They are not,however, very low cost AND OPERATIONT hree Screw Pumps are manu-factured in two basic styles,single suction and double suc-tion, figure 2. The single suc-tion design is used for low tomedium flow rates and low tovery high pressure. The doublesuction design is really twopumps in parallel in one cas-ing.

4 They are used for mediumto high flow rates at low to me-dium Screw Pumps will gener-ally have only one mechanicalshaft seal and one, or perhapstwo, bearings that locate theshaft axially. Internal hydraulicbalance is such that axial andradial hydraulic forces are op-posed and cancel each loads are thus verylow. Another common charac-teristic of three Screw Pumps isthat all but the smallest, lowpressure designs incorporatereplaceable liners in which thepumping screws rotate. Thus,field repair is a simple center Screw , called thepower rotor, performs all thepumping. The meshed out-side screws, called idler rotors,cause each liquid-holdingchamber to be separated fromthe adjacent one except forrunning clearances. This ef-fectively allows staging of thepump pressure rise, figure 3. James R.

5 Brennan, Imo Industries Inc., Monroe, NC, USAHigh Performance Rotary Screw PumpsFigure 2 Conventional Three Screw Pump DesignsFigure 1 Sealless Canned Three Screw PumpINLETOUTLETINLETSINGLESUCTIONDOUBLES UCTIONOUTLETHigh pressure Pumps may haveas many as 12 stages whilelow pressure Pumps may haveonly two or three. Because thecenter Screw is performing allthe pumping work, the drivetorque transferred to the idlerrotors is only that necessary toovercome viscous drag of thecylindrical rotor spinning withinits liner clearance. The theo-retical flow rate of these pumpsis a function of speed, Screw setdiameter and the lead angle ofthe threads. Basically, flow rateis a function of the cube of thecenter Screw diameter. Slip flow,the volumetric inefficiency dueto clearances, differential pres-sure and viscosity, is a functionof the square of the power rotordiameter.

6 This results in largerpumps being inherently moreefficient than smaller Pumps , afact that applies to most is ultimately limited bythe applications capability todeliver flow to the pump inlet ata sufficient pressure to avoidcavitation. This is true of allpumps. Three Screw pumpstend to be high speed Pumps ,not unlike centrifugal pole and four pole motorsare the most common large flows, very highviscosities or low available in-let pressures dictate, slowerspeed may be necessary. Forexample, some polymer ser-vices handle liquid at 50,000centistokes or more. Threescrew Pumps on such servicewould typically be operated inthe 50 to 150 rpm. Gas turbinefuel injection service wouldmore commonly be in the 3000to 3600 rpm range since thefuels tend to be low in viscosity,1 to 20 centistokes, and thepump inlet is normally boostedto a positive pressure from afuel treatment skid pump.

7 Thehigh speed operation is desir-able when handling low vis-cosity liquids since the idlerrotors generate a hydrody-namic liquid film in their loadzones that resists radial hy-draulic loads, very similar tohydrodynamic sleeve bearingsfound in order to achieve the highestpressure capability from threescrew Pumps , it is necessary tocontrol the shape of the screwswhile under hydraulic load. Thisis best achieved by the use offive axis NC profile grindingwhich allows complete dimen-sional control and a high de-gree of repeatability. Opposedloading of the idler rotor out-side diameters on the powerrotor root diameter dictate thatthese surfaces be heat treatedto withstand the cyclic , profile thread grindingallows the final Screw contourto be produced while leavingthe rotors quite hard, in theorder of 58R (58 on theRockwell C scale).

8 This hardsurface better resists abrasivewear from contaminants, typi-cal of crude oil versatile as three screwpumps are, there are someapplications for which they arenot suitable. While many ad-vances in materials engineer-ing are taking place, the stateof the art for three Screw pumpsis such that very corrosion re-sistant materials such as highnickel steels have too great agalling tendency. The rotors ofthree Screw Pumps touch andthus any materials that tend togall are unsuitable. Unfortu-nately, this includes many cor-rosion resistant materials. Vis-cosities too low to allow hydro-dynamic film support genera-tion are also application areasfor which the three Screw pumpis not Screw Pumps PRINCIPLE APPLICATIONSG enerally, two Screw or twinscrew Pumps are more costlyto produce than three screwpumps and thus are not in asextensive use.

9 They can, how-ever, handle applications thatare well beyond many types ofpumps including three screwdesigns. Twin Screw pumpsare especially suited to verylow available inlet pressureapplications and more so if therequired flow rates are similar to three screwpumps include crude oilpipelining, refinery hot, viscousproduct processing, syntheticfiber processing, barge unload-ing, fuel oil burner and transferas well as unique applicationssuch as adhesive manufacture,nitrocellulose explosive pro-cessing, high water cut crudeoil, multiphase (gas/oil mix-tures) pumping, light oil flush ofhot process pumping, cargooff-loading with ballast wateras one of the fluids, tank strip-ping service where air contentcan be high and paper pulpproduction needing to pumpover about 10% AND OPERATIONThe vast majority of twin screwpumps are of the double suc-tion design, figure 4.

10 The op-posed thread arrangement pro-vides inherent axial hydraulicbalance due to its pumping screws do nottouch each other and thus lendFigure 4 Twin Screw Pump ConstructionWRAPSLOW PRESSUREHIGH PRESSUREINLETOUTLETTIMING GEARSLUBE OIL RESERVOIRF igure 3 Staging Effects of Screw PumpsPRESSURE RISE themselves well to manufac-ture from corrosion resistantmaterials. The timing gearsserve to both synchronize thescrew mesh as well as to trans-mit half the total power inputfrom the drive shaft to the drivenshaft. Each shaft effectivelyhandles half the flow and thushalf the power. Each end ofeach shaft has a support bear-ing to react the radial hydraulicloads which are not otherwisebalanced. A few designs leavethe bearings and timing gearsoperating in the liquid this results in a signifi-cantly lower cost pump design,it defeats much of the valuethat twin Screw Pumps bring toapplications.