Understanding and Using Sealless Rotary Pumps 1998 Imo Pump. All rights reserved.

by: James R. BrennanImo Pump, Monroe, NC, USA

There are many sealless centrifugal pump designsthat have been well established for many years.When handling viscous liquids, shear sensitive liquidsor flow/pressure requirements outside of centrifugalpump ranges, rotary, positive displacement pumpsprovide reliable, efficient, economic solutions.Sealless rotary pumps are available in an ever-widening range of configurations. The need forsealless pumps ranges from cosmetic housekeepingissues and maintenance costs to the pumping oftoxic, corrosive, crystallizing or other dangerous ordifficult to seal liquids. Sealless refers to the absenceof a dynamic seal where the pump shaft penetratesthe pump casing. Dynamic seals such as lip seals,mechanical seals, packing rings and similararrangements, are prone to wear and leakage. Staticseals such as o-rings or gaskets generally providevery long term, high reliability sealing due to theabsence of relative motion. Excessive mechanicalseal leakage or failure represent over 50% of pumprepair incidents.

CONVENTIONAL SHAFT SEALINGShaft packing rings are designed as controlledleakage devices and must be permitted to leak in theorder of 10 to 20 drops per minute to stay lubricatedand to minimize shaft or sleeve wear and damage.Lip seals will frequently “weep” liquid and generallyhave a life expectancy of a few thousand hoursunder good seal conditions.

Mechanical seals, figure 1, are in extensive use onindustrial pumps and can provide good life. They,too, however, are controlled leakage devices. Thesealing faces cannot be permitted to contact in a dryenvironment. Some liquid must pass between therotating and stationary face in order to minimizefriction and to carry away the heat generated. If thisleakage evaporates when reaching the atmosphere,then there is no visible leakage. If it doesn’t, leakage

in the order of 10 drops per hour might be the bestachievable. Figure 2 illustrates the mechanical sealdilemma. Some seal manufacturers rely on high

face loading to minimize leakage but at the expenseof seal longevity. Others do the opposite, whichprovides longer life but greater leakage.

Tandem and double seals as well as gas-purgedseals have been used in difficult or dangeroussealing services. These arrangements can be veryelaborate and expensive. High skill and high cost

maintenance is a requirement that goes along withusing pumps with these shaft sealing arrangements.

UNSEALED PUMPSUnsealed pumps are those with no shaft seal thatare permitted to leak in an environment that does notcause the leakage to be a problem. Figure 3 is anexample of such an arrangement. Shown is a three-screw oil hydraulic elevator pump/motor package.The pump shaft plugs into the motor shaft. Thepump shaft end bearing also acts as the motor shaft

I M M R STATIO N AR Y M EM BE R(SE AT)

RO TAT N G E BE(HE AD )

O U TSID E P U M PATM O SPH E RE

A N TI-R O TATIO N PIN

P UM P S HA FT

O -RING STATIC SEA LS

STATIO N AR Y C O VERO R SEAT C AR RIER

RU N NING FAC ESSET SCR EW

IN SID EPU M P

(LIQ U ID )

Figure 1 – Conventional mechanical seal

0 50 150SEAL FACE SEPARATIOIN IN MICRO INCHES

LEAKAGEWEAR

Figure 2 – Mechanical seal leakage vs life

Figure 3 – Submersible hydraulic elevator pump and motor(Courtesy Imo Pump)

For more information, please mail to marketing@tushacopumps.com

end bearing. This entire package is bolted to thebottom of an oil reservoir. The motor is of openconstruction and operates submerged in hydraulicoil as does the pump. Leakage from the pump shaftend merely returns to the reservoir. There are,perhaps, half a million such units in operationworldwide. They handle pressures to 1000 PSI (70BAR) and power levels up to 100 HP (75 KW).

Figure 4 shows main and emergency lube oil pumpsand drive motors mounted to a tank top fabricationwith relief valves and discharge piping. Wheninstalled on a lubricating oil reservoir, the pumps willbe vertically submerged with the electric motorsoutside the tank. The flexible couplings betweenmotors and pumps are below the tank top. Again,leakage at the pump shaft is allowed to drain backinto the reservoir. Figure 5 shows a lube oil pumpmounted to an otherwise unused shaft of a 600 HP(450 KW) speed reducer. The pump is driven by thegear box shaft. Leakage from the pump goesthrough the gear box bearing drains to the bottom ofthe gear case which acts as the oil reservoir.

SEALLESS PUMPSCompletely sealed rotary pumps are available froma number of manufactures. Styles include magneticdrive pumps and canned motor/pumps as well as theperistalic (hose or flexible tube) pump. The peristalicpump, figure 6, is available from dosing size flows,fractions of a milliliter per minute, to about 150 GPM(570 L/M) or more at pressures as high as 220 PSI(15 BAR). They operate at relatively low speed, 20 to200 RPM, and are used for metering, pumpingliquids with some abrasives and handling easilydamaged sensitive liquid products. Rollers compressthe hose driving the liquid ahead of the compressionpoint. Metering accuracy is very good. Chemicalcompatibility is entirely a hose issue. A good varietyof hose materials can be used with peristalic pumps.Hose life depends on material and operatingconditions but can range from a few hundred toseveral thousand hours. Unless the hose fails,peristalic pumps are completely leak free. Some

Figure 5 – Unsealed lube pump driven by lubricatedmachine (Courtesy Imo Pump)

Figure 4 – Unsealed tank mount main and emergency lubepumps (Courtesy Imo Pump)

TANK TOP

PUMP

MOTOR

COUPLING

OUTLET FLANGE

OPTIONALOUTLETPIPING

PUMPINLET

Figure 6 – Peristalic (hose or flexible tube) pump(Courtesy Allweiler AG)

For more information, please mail to marketing@tushacopumps.com

units are available designed such that a hose leakinside the pump is still contained while an alarm orshutdown is initiated.

OPERATION – MAGNETIC DRIVESWhile existing for more than 25 years, magneticdrive rotary pumps have come into prominent use inthe last 7 to 8 years. All the magnetic couplingsoperate on the same principles, figure 7, althoughdesign details will differ among pump manufacturers.The pump shaft has a cylindrical hub mounted to it.The outside diameter of the hub contains highstrength magnets with the entire hub either potted orcanned to protect the magnets from the liquid beingpumped. Around the pump hub, a can or containmentshell is installed. This can seals the pump fromexternal leakage. The can must be a non-magneticmaterial, frequently stainless steel, Hastelloy or anon-metallic material. The non-metallic caneliminates nearly all eddy current losses. The metallic

cans will incur some temperature rise and minorpower losses. Outside of the can, a driven cylindricalhub surrounds the can. Its inside diameter is linedwith high strength magnets as well. As this hub isrotated, its magnetic fields cut through the can andengages the inner driven hub magnetic fields. Thus,the pump shaft rotates without physical connectionto the drive and therefor no dynamic shaft seal isneeded. Cans tend to be thin walled to minimize thedistance between the inner and outer magnets.Since magnetic field strength decays with the squareof distance from it, the closer together the magnetsoperate, the more cost effective the coupling will be.Can-to-hub clearances will vary with manufacturerbut are in the order of 0.060 to 0.150 inches (1.3 to3.8 mm) radially. Can wall thickness is typically inthe order of 0.100 to 0.150 inches (2.5 to 3.8 mm).Some coupling designs incorporate a rub ring on theouter drive hub. It will contact the coupling framebefore the hub can contact the containment can.

This feature provides additional protection to the canin the event there is a problem with the drive hub/shaft bearing system.

Magnet materials are usually Neodymium orSamarium Cobalt. The Neodymium material is lesscostly per unit of torque transmission capability(stronger per unit volume) but limited to about 225°F(107°C). The Samarium Cobalt material, while moreexpensive, is usable for liquid temperatures to 400°F(204°C) or more. Both materials loose strength withincreasing temperature and both will sufferirreversible strength loss if operated much beyondthese temperatures.

A magnetic drive as just described is a synchronousdevice. That is, there is no slip between the inner andouter magnetic hubs. If the torque required by thepump exceeds the torque capability of the magneticdrive, the hubs will “disengage” driving, the pump willstop and, without instrumentation, the drive hub willcontinue to be rotated by the pump driver.Overheating and possible coupling damage canresult if the decoupling is not detected and the drivershut down. Note that with the pumps stopped, as thedriver is de-energized and slows, the magneticcoupling will re-engage at a low speed and cause thedriver to stop more abruptly. This is characteristicand should not be cause for alarm.

Magnetic drive pumps are not suited to handlingliquids with contaminant or hard material. Therelatively close can-to-inner hub running clearanceand the relatively thin can wall could result in a canfailure if contaminant scores the can inside diameter.Consideration should be given to detection of leakagedue to containment shell damage, especially if thepumped liquid is dangerous. Various arrangementsare possible including optical switch, float switch,double wall containment can with space between innerand outer walls instrumented for leak detection, etc.

GEAR PUMPSFigure 8 is a magnetic drive external gear pump.This particular design is for precision metering ofvery small flow rates between 6 milliliters and 19liters per minute (0.36 in3/minute to 5 GPM). Variablespeed drive would be a common arrangement forthese pumps. Pumped liquid is trapped in the spacebetween each gear tooth and the casing walls. Thistrapped volume is rotated to the discharge side ofthe pump where it is forced to exit by the meshing ofthe gear set. Each tooth space that is not open to theinlet or discharge chambers effectively acts like astage causing a gradual pressure rise. Internal gearpumps have a bearing on both ends of the shaft tosupport loading due to hydraulic pressure acting on

Figure 7 – Magnetic drive operation(Courtesy Micropump, Inc.)

For more information, please mail to marketing@tushacopumps.com

the gears from discharge towards inlet. Figure 9 isan internal gear type pump using magnetic drive. Itincludes a built-in discharge pressure relief valve.Note the temperature probe mounted to the outerframe. It is in contact with the containment shell andcan detect excessive temperature and be used toalarm or shut down the drive. This style pump isavailable with wetted materials of cast iron, steel,

stainless steel, composite and Hastelloy. Internalgear pumps, figure 10, trap pumped liquid betweenthe gear teeth and an internal fixed crescent. Again,this trapped volume is rotated to the discharge sideof the pump where it is forced to exit by the re-engagement of the gear mesh. Like external gear

pumps, there are normally two bearings, one oneither side of the gear set to support radial loads dueto pressure acting on the gears from dischargetowards the pump inlet area. Flow and pressurecapabilities are illustrated in figure 11. Magneticdrives for gear pumps are available for torque up toabout 450 lbf-ft (610 N-m).

Larger gear pumps need to run relatively slowly tokeep filling velocities reasonable for good inletpressure capability. They thus require larger magneticdrives for the same power range vs centrifugalpumps for example. Small displacement gear pumpsrunning at four or six pole motor speeds make betteruse of magnetic couplings from a power (and couplingcost) point of view (power = torque X speed).

Vane PumpsVane pumps are available in magnetic drivearrangements. Like gear pumps, flow is radial andlarger displacement pumps must run more slowly toprovide good inlet pressure capabilities. A rotorcontaining vanes in slots rotates on a center offsetfrom the larger diameter case bore center. Vanesmay be spring loaded outward or may rely uponcentrifugal force to keep them against the case bore.Liquid pumped is trapped between adjacent vanes,figure 12, and the inner pump casing. It is expelledfrom the pump by the succeeding trapped volumes.Like internal and external gear pumps, vane pumpswill have a bearing on either side of the rotor tosupport the shaft against hydraulically inducedloading from discharge towards inlet. Vane pumps

Figure 8 – Magnetic drive external gear metering pump(Courtesy Micropump, Inc.)

Figure 9 – Magnetic drive internal gear pump with temperatureprobe (Courtesy Viking Pump)

Figure 10 – Positive displacement internal gear pump

0100 200 300 400 500

10

15

20

25

30

5100

200

300

400

500

500 1000 1500 2000

NOMINAL FLOW/PRESSURERANGE BASED ON 500 SSU,110 cst LIQUID

Flow/ressure range of internal gear magnetic drive pumps

PR

ES

SU

RE

– P

SI

FLOW – LITERS/MIN

PR

ES

SU

RE

– B

AR

FLOW – GPM

Figure 11 – Magnetic drive internal gear pumppressure/flow range (Courtesy Viking Pump)

For more information, please mail to marketing@tushacopumps.com

are available in flow rates of 4 to 330 GPM (15 to 1250L/M) and suitable for pressures in the 100 to 125 psi(6.9 to 8.6 BAR) range. Circumferential casing orliner wear is somewhat compensated by the ability

of the vanes to extend further radially as either thecasing or vane wears. Some units are manufacturedwith a replaceable liner against which the vanesslide. These rotary vane pumps are available inductile iron or 316 stainless steel construction. Sincethe number of vanes is small, there is little ability tostage the pressure rise and so vane pumps aregenerally suited to low pressure applications.Balanced vane pumps are produced for hydraulicfluid power applications. While such designs are notavailable in sealless configurations, they are capableof several thousand psi operation on good qualityhydraulic fluid.

Screw PumpsThree screw pumps have also been successfullyadapted to magnetic drive technology. Figure 13illustrates typical construction. Flow in a screwpump is axial rather than radial as in a gear pump.Pumped liquid is trapped in the space between thescrew mesh and the casing or liner. As the screw setis rotated, the trapped volume is pushed towards thedischarge port by the center screw. The two outsidescrews acts as seals between each otherwiseseparated volumes of liquid; they do no pumping.The volumes of liquid are expelled by the nexttrapped volumes behind it. In practice, one volumeis opening while another is closing so

the effective discharge area is constant and pressurepulsation and noise are minimal. As in gear pumps,each successive volume acts as a pressure stage.Pumps have been manufactured with twelve or morepressure stages for high pressure operation. Axialloads due to hydraulic pressure are normally balancedby the use of a balancing piston at the discharge endof the center screw and by bringing dischargepressure into a cavity at the inlet end of the twooutside rotors. Radial loads are self-canceling dueto the radial symmetry of the meshed screw set.Carbon fiber wound composite containment shellsare available for the magnetic drive screw pumpdesigns. The composite can virtually eliminateseddy current losses in the can yet provides goodcorrosion resistance on aggressive liquids. Coolingis provided to the driven internal coupling hub byallowing a small amount of pump discharge flow tocirculate through the can and return to pump inlet viaa drilled passage in the shaft. Figure 14 shows thepressure/flow capability of three screw pumps. Theirmagnetic couplings have been supplied to 300 lbf-ft (405 N-m).

Because three screw pumps typically operate at twoor four pole electric motor speeds, the 300 lbf-ft (405N-m) coupling is capable of handling powerrequirements up to 200 HP (150 KW) at 3500 rpm or100 HP (75 KW) at 1750 rpm.

MAGNETIC DRIVE SERVICE PRECAUTIONSThe larger torque magnetic drive couplings need tobe handled with great care during disassembly/repair/reassembly. The attracting and repulsingforces can be very high and can easily result in injuryor damage if both coupling hubs are not keptconstrained at all times. Follow the pumpmanufacturer’s instructions with extra care to avoidproblems. Plan the work area before beginningdisassembly. A wooden table or thick sheet ofplywood over a metal table should be used. Know

Figure 12 – Positive displacement rotary vane pump

1000900800700600500400300200100

350030002500200015001000500

0

175

150

125

100

75

50

25

2500

2000

1500

1000

500

Flow/ressure range of three-screw magnetic drive pumps

PR

ES

SU

RE

– P

SI

FLOW – LITERS/MIN

PR

ES

SU

RE

– B

AR

FLOW – GPM

Figure 14 – Magnetic drive three-screw pump pressure/flowrange (Courtesy Imo Pump)

CONTAINMENT SHELLINNER HUB/MAGNETS

OUTER HUB/MAGNETS

Figure 13 – Magnetic drive three screw pump(Courtesy Imo Pump)

For more information, please mail to marketing@tushacopumps.com

where hand tools will be placed after each use.Know where each pump component will be placedupon removal so that the magnets will not “leap”together or attract a hand tool and cause damage.Jacking magnetic coupling hubs apart or together isa common method of keeping them under control.Do not place hands or fingers between componentsparts while the hubs are near enough to each otherto still attract or repulse. Do not allow the magnetichubs to be near electronic equipment,instrumentation, wrist watches, credit or magneticidentification cards, computers, floppy diskettes,etc. The intense magnetic field can damage ordestroy such devices. People with pace makers orother implanted metallic, electric or electronic devicesshould not be allowed near magnetic hubs unlessthey are contained within an assembled pump oradequately boxed to keep their fields at a reasonabledistance from container handling. The hubs shouldbe immediately wrapped in clean cloth after removalto prevent magnetic debris such as machining chips,filings, etc. from attaching to the magnets. If allowedwithin the coupling, such debris can begin scoringthe containment can wall and weaken or fail the can.Magnetic components (magnets, hubs, completepumps) are potentially hazardous to aircraft. Checkwith your carrier for packaging requirements. Partsor units packaged for air shipment may be requiredto be within a specific gauss measurement limit.

OPERATION – CANNED MOTOR/PUMPSRotary pumps can be canned adopting conceptssimilar to centrifugal pumps. The hydraulic elevatorpump in figure 3 is almost a canned motor/pump. Bysupplying an inlet cover for pipe connection in lieu ofthe inlet strainer shown and completely closing theelectric motor, the result is a canned motor/pump.Figure 15 shows the construction of a three screwcanned motor pump. It is of the wet stator designmeaning that the liquid pump, which also circulateswithin the motor for cooling, must be compatible withthe winding insulation as well as the rotor laminations.This compatibility issue limits the wet stator design

to applications on relatively non-aggressive liquids.By placing non-magnetic, corrosion resistantcontainment barriers around both the rotor andstator, a dry stator design is created with broaderapplication range for more aggressive liquids. Currentwet stator designs reach 6 HP (4.5 KW) at 3500 rpmand can deliver flows to 100 GPM (378 L/M) orhandle differential pressures to 1000 PSI (69 BAR).Canned rotary motor/pumps are currently beingdesigned in the 100 HP (75 KW) range. Full loadspeeds are somewhat slower for canned motor/pumps due to the drag of the motor rotor in the liquidpumped i. e. 3450 RPM and 1725 RPM nominal forsmall 60 HZ, two and four pole motors respectively.Like their magnetic drive counterparts, canned motor/pumps are not well suited to handling liquidscontaining solids, contaminants or abrasive particles.

Sealless pump technology has come a long way butstill needs improvement especially in the area ofprice/cost. Magnetic drive pumps are relativelyexpensive. Canned motor technology promisessome improvement in sealless pump costs as moresizes and designs are developed. The future ofpump users will see more and more regulation onthe amounts and types pump emissions permitted.The use of sealless technology as well as gassealing technology will be driven but such regulation.Given the frequency and cost of conventional sealreplacement, sealless pumping may very wellcompete with the life cycle costs of conventionalsealed pumps. Including fines for emission violations,there may be no contest at all for the pumping ofhighly regulated liquids.

The author thanks the following pump manufacturerswho contributed information and illustrations:Allweiler AG, Imo Pump, Micropump, Inc., TuthillCorp., and Viking Pump, Inc.

~James R. Brennan is currently Market ServicesManager for Imo Pump, Monroe, NC, USA. Hisresponsibilities include worldwide service andtechnical support for the company’s rotary screwand gear pump product lines. Brennan is a 1973graduate of Drexel University in Philadelphia, amember of Society of Petroleum Engineers, SPE,has over 30 years of application and trouble shootingexperience and has authored numerous articlesand spoken at a variety of technical conferencesand seminars.

This is the unedited text of a technical presentationmade at Pump Users Expo ’98, Cincinnati, OH, USA,29 October 1998.Figure 15 – Canned sealless three screw pump

(Courtesy Imo Pump

~

~For more information, please mail to marketing@tushacopumps.com