FEATURE • CANNED-MOTOR PUMPS

WORLD PUMPS July 2002 0262 1762/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved2

Amajor problem with cen-trifugal pumps deliveringultra-pure, highly toxic,

sterile or delicate fluids is the shaftseal, which seals the rotating driveshaft against the casing. Accordingto P.C. Barnard from ExxonChemical Ltd, 80% of the pumpsemployed in the chemical industryare withdrawn from service becauseof mechanical seal failures, with the remaining 20% withdrawn as a result of failure of bearings,couplings and other associateditems1. These pumps will have amaintenance cost of approximatelytwice their value in the first fiveyears of life.

One answer to this problem ismagnetic coupling pumps or cannedmotor pumps. The latter have a

hermetic casing separating the rotatingparts of the pump from the stator. Inboth cases the torque is transmittedthrough the canning onto the rotor bymeans of a magnetic rotating field. Inthe magnetic coupling pump therotating field is generated by rotatingmagnets, in the canned motor pump byelectric stator windings.

A fundamental problem with thesehermetically sealed centrifugal pumpsis the rotor sleeve bearings, which arelubricated and cooled by the processfluid. These bearings get destroyedimmediately if the pump runs dry. Alsonot all fluids are good lubricants. Fluidswith low vapour pressure evaporate inthe bearings, and fluids, such as CMP(chemical mechanical planarization)

slurries, that contain abrasive particlescause fast bearing wear. Biologicalfluids like blood get destroyed in thebearing and tend to clot. In thesemiconductor industry, particlesgenerated by the sleeve bearings cancause problems, and in the platingindustry, the plating material tends todeposit in the narrow bearing gap andfinally jams the bearing.

Magneticallylevitated rotor

All these problems can be addressed by magnetic levitation technology.Prototypes of canned-motor pumpsusing magnetic bearings have beenbuilt and tested by several groups2–4.None of these pumps has so far becomea commercial product because of thehigh cost associated with magneticbearings. However, with the revo-lutionary technology of the so-calledbearingless slice motor – developed bythe Levitronix group in cooperationwith the Federal Institute of Tech-nology (ETH) Zürich – a very simplecentrifugal pump with magneticallylevitated impeller becomes feasible.The Levitronix design has a singlemotor/bearing unit that provides driveand magnetic bearing functions at one and the same time, and is capableof stabilizing the six special degrees of freedom of the pump impellermagnetically and without contactthrough the walls of the pump housing.Compared to conventional canned-motor pumps, much larger magneticgaps can be realized with the newtechnology and there is more room forthe canning between the rotor and the

Centrifugal pump withoutbearings or sealsBy combining a canned motor with a magnetically levitated pump rotor, Switzerland’sLevitronix GmbH has achieved a centrifugal pump with neither shaft seals nor rotorbearings. This has many advantages for the handling of a range of sterile, shear-sensitiveor delicate fluids. The feasibility of the concept was first demonstrated in 1995 by a groupat Sulzer Electronics who went on to establish Levitronix in January 2001 in order todevelop and commercialize the technology. CEO Dr Reto Schöb explains the basics of thepump’s design and its advantages for a number of medical and industrial applications.

Figure 1. Schematicshowing the basicprinciples of thebearingless canned-motor pump.

B

Inlet

Outlet

Bearing coils

Drive coils

Stator

Rotor

Pump housing Impeller

FEATURE • CANNED-MOTOR PUMPS

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motor stator. The canning can be madeof plastic materials and form a portionof the pump housing. This makes thepump itself a very simple designcomprising only three parts: an im-peller containing an over-mouldedmagnet ring, a lower housing and anupper housing (Figures 1 and 2). Thepump component can be simply de-tached from the motor/bearing statorenabling it to be cleaned very easily.For certain applications such as forblood pumps, the pump can be pro-duced as a disposable item (Figure 3).

Outstandingfeatures

Due to its unique construction, theLevitronix bearingless canned motorpump has several outstanding features.Simply put it combines all theadvantages of conventional canned-motor or magnetic coupling pumps butavoids the problems related to theprocess-lubricated bearings. There areno narrow gaps and fissures and thepumps do not generate any particles.They can be operated even with suchcritical fluids as blood or ultra-purechemicals and CMP slurries. Becausethere is no mechanical coupling of theimpeller and the pump casing, thepump produces virtually no vibrations,

even if there is a huge imbalance in theimpeller (for example, if solid parts arestuck in an impeller). Thus, thebearingless pumps are very quiet; onlyat high flows can turbulence-inducednoise be heard. The absence of any

mechanical contact between rotatingand stationary parts leads to a virtuallywear-free product. The pump part maywear with time if operated withabrasive fluids or very aggressivechemicals, but if this happens it can beexchanged very easily within a fewseconds or minutes, depending on thetype of mountings and fittings.

In addition, due to its variable speed,the Levitronix pump allows precisecontrol of flow or pressure and a wideoperating range. With just one pumpmodel, a whole range of conventionalfixed frequency pumps can be replaced.Due to its avoidance of bearings andmetal cans, and because of the widegaps and small motor surfaces, veryhigh operating speeds (up to 10 000rpm) are possible. Therefore high pres-sures can be achieved with only onestage. As an example, 6 bar (87 psi) isfeasible with an impeller diameter ofonly 65 mm. This also allows thepumps to be built smaller than fixed-speed pumps of the same power.

The benefits of the Levitronix pumpcan be divided into three categories –those dependent on the canned motordesign, those stemming from the

Figure 2. Cross-section through a bearingless canned-motor centrifugal pump(Levitronix BP 3.1).

Figure 3. Design of abearingless canned-

motor pump withdisposable pump.

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generation, precise continuous flow,and chemical inertness are additionalrequirements in the semiconductor in-dustry. A bearingless pump systemwith high-purity PVDF or PTFE pumpheads has been especially designed forthis demanding industry (Figure 5). Itcan deliver flows of up to 50 litres perminute and pressures of up to 2.5 bar.For more than a year this pump hasbeen successfully used in etching,cleaning and stripping processesduring chip production at semi-conductor facilities in Europe, Asiaand the USA. Smaller as well as largermodels (up to 120 l/min and 6 bar)with PTFE pump heads are currentlybeing tested in the lab.

BiotechnologyA Levitronix pump has no fissures orsmall gaps where bacteria couldsurvive during sterilization. In com-bination with the leak-free design, this makes it an ideal pump forbiochemical applications. It allows thedesign of completely hermetic bio-production systems. With a specialflange and impeller, the pump can alsobe used as a sealless, bearingless stirrer.

The pulse-free, continuous flow makesit an ideal choice for cross-flowfiltration systems.

Plating industryMetal solutions and paints tend tocoagulate in process-lubricated bear-ings and some of them are alsosensitive to high shear. Presumably, thewide-gap, low-shear, hermetic design ofthe Levitronix pump would help tosolve these problems. The pump istherefore currently being evaluated inseveral plating applications.

Summary andoutlook

The bearingless canned motor pumpcombines all the advantages ofconventional canned-motor or mag-netic coupling pumps but avoids theproblems related to process-lubricatedbearings. Initial products have beendesigned for the medical and semicon-ductor industries. Other applicationsin the biotechnology and platingindustry are currently under evalua-tion. Further developments will lead tolarger pumps and the use of other

materials. In addition, the high costwill come down as larger quantities areproduced and electronics costs fall.This might open the way to muchwider applications for these pumps inthe chemical and food industries.

References

(1) P.C. Barnard, ‘The way forward’,Meeting of the British Institute ofMechanical Engineers, 1991(2) P.E. Allaire, ‘Design, constructionand test of magnetic bearings in anindustrial canned motor pump’, WorldPumps, September 1989(3) S. Katsuta et al., ‘Centrifugal pumphaving magnetic bearing’, EP0355796 A2, February 1990(4) M. Brunet et al., ‘Rotary pumpwith immersed rotor’, WO 99/14503,March 1999

CONTACTDr Reto Schöb, CEOLevitronix GmbHTechnoparkstrasse 1CH-8005 Zürich, Switzerland.Tel: +41-1-445-1382Fax: +41-1-445-1392E-mail: schoeb@levitronix.comwww.levitronix.com