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Magnetically driven (magdrive) centrifugal pumpshave been in use since1947 when the first mag-

drive pump was developed. Magdrives have always been the work-horse pump in applications withcorrosive and often toxic fluid ap-plications, and particularly over thelast 20 years they’ve become morecommon in the chemical process in-

dustries (CPI). A typical mag-drive pump is com-

prised of a magnetically coupledrotor-and-drive assembly separatedby a containment shell that hermet-ically seals pumpage from the at-mosphere (Figure 1). The mag-drivepump’s key feature is a seallessdesign, which eliminates a mode offailure. This often makes it a strongsolution for pumping applicationswith mechanical seal problems.The mag drive is commonly used to

pump hazardous and high intrinsic value pumpage where the processcannot or should not be diluted byflush media (typical in a traditionalmechanical-seal pump). Mag driveshave many growing applicationsin global industries, such as petro-chemicals in petroleum refineries,pharmaceutical applications, andpulp-and-paper mills, which employcaustic, acid and solvent servicesduring production. However, thistype of pump is somewhat under-utilized in many of these industriesdue to common misconceptionsabout the technology.

Mag-drive fundamentalsMagnetically driven pumps haveseveral design features that extendpump life, especially in common CPIapplications with hazardous scenar-ios. The mag drive’s key design fea-ture is the absence of a traditionalrotating mechanical seal (compareFigure 1 and Figure 2). Instead, afull hermetic seal with no rotatingcomponent reduces the risk of leaks.This sealless design is the most well-

known feature of mag drives and isa primary reason why they are oftenused for hazardous or valuable flu-ids. Another advantage is avoidanceof mechanical seal maintenance andflush plan maintenance.

Mag-drive pumps are availablein either ANSI or ISO dimensionalstandards. They are available inmost metallurgies, as well as non-metallic designs, such as with apolymer lining. A polymer design of-fers improved corrosion resistance,utilizing polymer coatings such asEFTE (ethylene tetrafluoroethyl-ene). All mag drives, whether metal

or lined, have very limited solidshandling capability. The nature ofthe mag-drive design has circuitsthat facilitate process lubricationof internal bearings, and these pas-sageways are typically very small.If solids are introduced, the circuitscan easily get blocked, leading topump damage. The passage waysare sometimes so small that it willtake only a minute amount of verysmall solids to erode the compo-

nents of a mag-drive pump.

Design and usageIn addition to the sealless advan-tages, there are some key consid-erations when deciding whetherto use a mag-drive pump. Becausethey utilize magnets to transferpower and torque from the driveassembly to the driven rotor as-sembly, it’s important to recognizethat there are temperature limita-tions for magnet’s materials. Whenexposed to temperatures abovetheir threshold they can begin tolose their magnetism. Over time,

Feature Report

56 CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM SEPTEMBER 2014

Feature Report Part 1

FIGURE 1. The main componentsof a mag-drive pumpare shown in thiscross section

Magnetically DrivenPumps: An overviewUnderstanding sealless pump technologies

and their potential applications

Richard TymITT Goulds Pumps

Source:ITT GouldsPumps

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this loss of magnetism results inpump failure due to degradation ofthe magnetic coupling between the

drive and the driven component.Therefore temperature characteris-tics of the application are a criticalfactor when considering applicationof a mag-drive pump.

Another factor to weigh is thetorque limitation due to the mag-netic coupling of the drive and com-ponents. It’s important to make surethat the magnets are sized correctlyso magnetic-coupling breakawaytorque is not exceeded during unitstartup or steady-state operation. If

breakaway torque is exceeded, themagnetic coupling between the driveand the driven assemblies is lost, andthe impeller will cease to spin, whichmeans the pump must be shut downto allow the magnets to recouple.If there are numerous instances ofmagnet decoupling due to impropermagnet selection or other circum-stances, such as clogging or processupset, magnets will degrade overtime to the point where the pumpwill no longer operate properly.

In metallic mag-drive pumps,eddy currents can also affect perfor-mance. Eddy currents are electricalcurrents generated by the motion ofthe magnets moving past an electri-cally conductive containment shell.The inner workings of a metallicmag-drive pump have a drive mag-net and a driven assembly separatedby a metal barrier. As the magnetsstart rotating around this barrier,eddy currents form and generateheat. The design of a mag-drivepump must take into account theremoval of this heat to avoid boilingand possible flashing or vaporizing

of process lubricants, which willcause pump failure through bear-ing damage. Furthermore, there is

a loss of horsepower due to eddycurrents. Eddy-current effects canbe mitigated by correct sizing of themagnets and the motor.

A final consideration when utiliz-ing mag-drive pumps is to be awarethat they are extremely sensitiveto dry-run and dead-head condi-tions. In the case of dry run, a lossof liquid in the pump system cancause the process lubricated bear-ings to overheat and crack, leadingto pump failure. Dead head occurs

when running the pump againsta closed valve or a blockage in theline results in the loss of fluid cir-culation. This results in a buildupof heat and excessive thrust loadingthat can cause unit failure.

Overview of mag-drive designs A common misconception aboutmag-drive pumps is that their de-signs are complicated. These mis-conceptions are based on oldermag-drive designs that commonly

had a large number of componentparts, including individual heat-shrunk bearings, spacers and vari-ous O-rings. More than a dozencomponents were typically involvedand assembling and disassemblingrequired longer maintenance time.However, the industry has madegreat strides in recent years, result-ing in newer, significantly simplertechnologies (Figure 3). Improve-ments to bearing-cartridge designsreduce and consolidate components,such as bearings and spacers, mak-ing for easier maintenance, reduceddowntime and less inventory.

Operationally, there are severalfundamental components in a mag-drive pump design. There are two

separate rotating assemblies (driveand driven) connected by a mag-netic coupling. The motor transferspower through the drive magnetassembly to the driven magnet as-sembly, which is connected to theimpeller and ultimately moves thefluid in the pump system. Betweenthe two assemblies is a contain-ment shell that keeps all of thefluid within the pump and servesto maintain pressure, acting as ahermetic seal that prevents fluids

and vapors from escaping to theatmosphere. The magnets interactthrough magnetic flux lines thatare translated across the contain-ment shell. Within these two assem-blies are alternating rings of northand south magnets, which both at-tract and oppose each other basedon positioning, preventing slippagefrom occurring. This type of designis known as a coaxial synchronousmagnetic drive, and ensures thatboth the pump and motor will spin

at the same rate.Some high-temperature mag-

drive designs (>500°F) use aslightly different design to alleviatedemagnetization effects. The drivemagnet assembly outside the con-tainment shell is typically the sameas a standard design. However, thedriven assembly consists of a me-tallic torque ring that couples withthe drive magnet to spin the impel-ler (Figure 4). This design protectsagainst excessive temperatures byremoving driven magnets from im-mersion in high-temperature media,and is often used in heat-transfer-

CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM SEPTEMBER 2014 57

FIGURE 2. One advantage of mag-drive pumps is the absence ofa mechanical seal (shown here), which reduces the risk of leaks

FIGURE 3. Unlike earlier designs, today’s mag-drivepumps are simpler and have fewer components

Source: ITT Goulds PumpsSource: ITT Goulds Pumps

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Feature Report


media applications, as well as manyother high-temperature chemical,oil-and-gas and general industryapplications. This design allows for

a small amount of slippage due tothe use of the drive metallic torquering, but it typically does not ex-perience issues with decoupling,although a slight loss of efficiency(when compared to traditional mag-drive designs) may result.

Finally, the rare earth materialsfrom which magnets are made areimportant to understand. The mostcommon is neodymium iron boron(NdFeB), which has a high mag-netic field strength per volume, but

cannot withstand high tempera-tures. Other materials commonlyused in mag drives are samariumcobalt (SmCo) and aluminum nickelcobalt (Alnico). SmCo has slightlylower magnetic strength thanNdFeB, but can withstand highertemperatures. Alnico can handlemuch higher temperatures thanboth NdFeB and SmCo, however itlacks strength and cannot handlethe torque present in some of themore demanding pumping applica-

tions. Magnet material selection istherefore critical when specifyingmag-drive pumps.

Typical applicationsHazardous and regulated servicesare the most common applicationsfor mag drives as they are well-suited for pumping liquids thatmay pose threats to people and theenvironment. The mag-drive designoffers heightened safety for work-ers and better protection for the

environment by eliminating leaksof hazardous fluids (see boxes on p.58 and 59). Sealed pumps appliedto these types of services may leakover time, or require complex doubleseals to prevent hazardous liquidsand vapors from escaping to the at-mosphere, which can lead to safetyhazards, downtime and increasedmaintenance requirements.

Some examples of primary mag-drive applications are liquid pump-ing of strong acids, strong bases,and solvents, such as acetone, hy-drochloric acid, sulfuric acid andsodium hydroxide. Many of these

would pose a serious health risk toplant personnel if there is a fluidor vapor leak. Some materials caneven auto-ignite when exposed tothe atmosphere.

In these situations, a mechani-cally sealed unit can be riskier andmore expensive than a mag drive.Typically a traditional mechani-

cally sealed pump in a hazardousapplication process would utilizecomplex double-seal systems thatare a significant capital investment.They also require much more main-tenance and additional monitoring.

Other mag-drive applications canbe where liquid is hard to seal witha traditional mechanical seal. Forexample, pulp-and-paper mills em-ploy sodium hydroxide applicationswhere pumpage can crystallize onseal faces, which then can cause

seal failure. To avoid this, a flushmust be run to the seal that canincrease installation, maintenance,plant water and energy consump-tion costs.

The need for environmentalregulation adherence in the mar-ketplace has also driven aware-ness of mag-drive pumps. The U.S.Environmental Protection Agency(EPA) regulates emissions andwaste, and instituted the Clean Air Act of 1990, which requires certainchemicals or services to utilize asealless pump. Furthermore, manychemical plants have implemented

their own set of guidelines based oninternal hazard classifications.

Valuable pumpage, such as mer-cury and printer ink, along withother industry services wheredowntime brings significant costs,can also be viable candidates formag-drive usage. Single seals leakfluid upon failure, and once a pump

leaks, the fluid is not usually recov-erable. This results in lost moneyand environmental cleanup time. A double seal is an expensive andhigher-maintenance solution. Mag-netically driven pumps can protectprecious pumpage and eliminaterisk of leakage due to their her-metically sealed design and limitedmaintenance needs.

Finally, remote locations are sce-narios that often call for seallessmag-drive pumps. As plants ex-

pand, sometimes over miles of land,some services are located remotelyand are not conducive to routinemonitoring and maintenance. Ex-amples include wastewater-treat-ment facilities where pH correctionis needed because water is going tobe introduced back into a systemor a nearby river or lake. This re-quires technology to eliminate po-tential leaks and limit maintenanceneeds. Another example would be asealed pump in a remote area thatwould require flushing. This mayrequire running a significant lengthof flush line to the pump’s location

REDUCING ENVIRONMENTAL CONCERNS

AT PAPER MILLS

Pulp-and-paper mills are now able to drastically reduce environmental concerns byeliminating a primary potential point of failure by replacing sealed pumps withsealless mag drives. The latest mag-drive designs and technologies feature fewer

parts and are robust enough to withstand the caustic chemicals commonly used in papermills, including sodium hydroxide, sulfuric acid, sodium hypochlorite, chlorine dioxide,and hydrogen peroxide. Any of these chemicals may cause damage to and leakagethrough the seal faces, ultimately damaging pumps and posing safety and health risks to

workers. Plant operators can streamline maintenance requirements and utilize mag-drive

pumps to handle hazardous sulfuric acid services commonly found in almost any papermill, minimizing potential environmental hazards and saving time and money. ❏

FIGURE 4. Thishigh-temperature(>500°F) mag-drivedesign uses a metallictorque ring instead ofa drive-magnet as-sembly to couple withthe drive magnet. Re-moving magnets fromthe high-temperaturepumpage eliminates

the possibility ofdriven magnetdemagnetization

Driven torque ring

Motor (drive) Pump (driven)

Drive magnetassembly

Source: ITT Goulds Pumps

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CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM SEPTEMBER 2014 59

and also carries high water-usagecosts. Introducing a sealless pump

to a remote location eliminates theneed for seal checks and flushing,and provides a reliable system withminimal maintenance.

By taking the seal out of the sys-tem, a major failure point is elimi-nated and reliability is enhanced.Eliminating bearings is a secondfeature mag-drive pumps offer thatimproves reliability. Close-coupledmag-drive designs eliminate thepower end, removing the possibil-ity of oil-lubricated bearing failure,

coupling misalignment, and in mostcases, the need for a baseplate.

Comparison to other seal options A mag-drive pump is comparablein cost to single-sealed and canned-motor pump designs, and is lesscostly than double-sealed pumpsthat utilize barrier-fluid systems.There is no replacement of sealsand no running of flush lines, lead-ing to reduced maintenance anddowntime. When evaluating differ-

ent pump sizes and duty points onemust look at total lifecycle costs.The chart shown in Figure 5 provides a breakdown across the vari-ous pump designs. Canned-motorpumps require more maintenancetime and dollars, as they are gener-ally very difficult to work on, andrequire a specialized maintenanceshop on site. Also, operators mayneed to send the motor out for ser- vice if there is a breach of the pri-mary barrier to the internal motorwindings. Double-sealed pumpshave higher costs across pumpsizes compared to mag drives. Mag

drives also deliver optimalperformance, with metallic

mag-drive efficiencies oftenequivalent to that of metal-sealed ANSI pumps. Alsoworth noting is that non-metallic (lined) mag-drivepumps can be up to 30%more efficient than metallic seal-less pumps.

Lined versus metallicLined mag-drive designs (Figure 6)protect the metal casing from cor-rosion that is common in chemical

applications. Some common liningsare PTFE (polytetrafluoroethyl-ene), PFA (perfluoroalkoxy alkanes),ETFA (ethylene tetrafluoroethylene)and PVDF (polyvinylidene fluoride). All offer corrosion resistance fromchemicals; however, they are notuniversally resistant to all media,which is why various coating optionsexist. Some components can also befiber reinforced with carbon or glass.For example, the impeller can oftenbe reinforced with these materials to

provide greater protection from hy-draulic erosion. In a lined mag-drivepump there is no eddy-current heatbuildup or potential power loss fromcurrents as the containment shell istypically fiberglass-reinforced plas-tic (FRP) that is coated with one ofthe linings listed above.

One thing to consider with linedmag-drive pumps is that they aregenerally limited in the areas oftemperature and pressure. Thelinings can only accommodatetemperatures in the mid-200°Frange, with various casings typi-cally capable of sustaining design

pressures in the mid to upper 200psi range. Overall, lined mag-drivepumps are very good for workingwith various acids or bases as longas they are below certain tempera-ture and pressure thresholds.

A metallic mag-drive-pump de-

sign (Figure 7) is capable of with-standing higher pressure and tem-peratures limits. These pumps arewell suited for solvents, heat-trans-fer fluids and other non-conductivefluids that typically run hotter.Some metallic mag drives can easilyhandle over 500°F for liquid-servicetemperatures. Metallic mag-drivepumps also are strong solutionsfor pumping non-conductive fluids,such as benzene, that can build upan electrostatic discharge, which

can be an issue for designs employ-ing an FRP polymer-lined contain-ment shell. When using an FRP-lined shell design, the electrostaticdischarge can “arc” through thenonmetal containment shell, caus-ing a pinhole leak that will causecomplete pump failure over time,as well as introduce environmentaland personnel hazards that may bedifficult to observe at their onset.In a metallic mag-drive pump thisarcing charge will not penetrate thealloy containment shell.

Finally, metallic mag-drive de-signs have better solids-handling

SAFER WORK ENVIRONMENT FOR

ALUMINUM MANUFACTURER

ANorth American architectural aluminumstore-front and door manufacturer usedseveral anodizing tanks to treat and color

its aluminum products. Each tank contained17% sulfuric acid that needed to be maintainedat 70°F for optimal results. During the courseof the anodizing process, heat was generatedas the sulfuric acid reacted with the aluminum

products, and the acid was pumped out of thetanks through chillers. Each of the pumps wasa mechanically sealed ANSI pump with a largesheet of Plexiglas leaning up against the pumpunit, effectively acting as a “spray shield.” A finemist of sulfuric acid would emit from each pumpseal, barely noticed by plant personnel. Thishazardous condition resulted in workers findingmultiple holes in work clothing where the acidhad splashed during the workday. Needless tosay, a change was needed in order to increase

worker safety. Initially, the plant agreed to installone non-metallic magnetically driven pump as atest on one of these tanks. The mag-drive pumpcut maintenance, increased safety and reduceddowntime. The replacement was such a successthat mag-drive pumps replaced all mechanicallysealed ANSI pumps, providing a safer and moreefficient work environment for plant personnel.❏

Pump size

150

100

50

0

L i f e c y c l e c o s t s ,

%

50-32-160 85-50-200

Singlemechanicalsealed pump

Doublemechanicalsealed pump

Mag drive

Cannedmotor pump

Discharge Q = 10 m3 /h 40 m3 /h

Head H = 28 m 41 m

Speed n = 2,900 rpm 2,900 rpm

Power P = 1.5 kW 7.0 kW

FIGURE 5. This chart compares the lifecycle costs for centrifugal pumpsusing various sealing options

Source: InfractorDEGUSSA-Hüls

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Feature Report


capabilities compared to their linedcounterparts. Metal is more resilientto solids erosion than a lined pump,although only up to a point. Also,

with metal-mag-drive designs thereare typically options for providingadditional internal bearing flushsince concerns over breaching andcompromising polymer linings areeliminated. Metal mag drives alsooffer more options for monitoring andcontrols. Instrumentation devices,such as thermocouples, resistancetemperature detectors (RTDs), levelswitches and temperature switchesare all more readily available to useon a metallic mag drive, once again

because concerns about breachinglined components are eliminated.

Avoiding failure modesWhen using mag drives, one canencounter failure modes not pres-ent with a sealed-pump applica-tion. The most common mag-drivefailure mode is dry run. Simplyput, dry run occurs when a pumpis run without fluid. Mag-drivedesigns employ process lubricatedbearings often made of carbon or

silicon-carbide material. During dryrun, the lack of fluid in the systemcauses the bearings to lose lubrica-tion, which can result in cracking,fracture and eventual failure due tothe nature of the bearing material. Another cause of dry run is whenfluid overheats, boils and flashes offin the lubrication circuit, effectivelyremoving process lubrication. Thiscan lead to chipping, cracking andincreased temperatures that canmelt plastic linings and lead to a

pump seizing.Some solutions to avoid dry run

are to attach a power monitor to thepump, which can alert the operatoror control system of the condition,or just automatically shut the pumpdown after a short period of time.There are also specialized methodsof treating or coating rotating andstationary silicon-carbide compo-nent surfaces that reduce their co-efficient of friction and can, for ashort time, minimize the effects ofdry run.

The other common failure modeof mag-drive pumps is dead head-

ing, which can occur by running thepump against a closed discharge valve. Fluid temperatures beginto rise quickly because of energybuildup of impeller rotation in aclosed system, leading to boiling,cavitation and bearing damage.

Dead heading can be avoided byflow and temperature monitoringand controls.

Processing solids can also providea set of challenges due to erosion orclogging. Mag-drive pumps are notmade to handle solids. Introducingsolids will cause accelerated ero-sion, especially within lined mag-drive pumps. Solids can also collectin low-pressure areas in the bear-ing circuits, typically at the rearcorners of the containment shell,

causing accelerated erosion. Clog-ging of process lubrication and re-circulation circuits can also occurwhere the bearings can lose lubri-cation and fail. A primary solutionin metallic mag drives is a bear-ing flush where added pressure isintroduced to a pump system andcan help to keep solids from build-ing up in these circuits. Improvedpump designs in both lined andmetallic mag drives can also leadto more fluid movement (and thus

easier solids removal from “problemareas”), achieved by introducingmore channels or adding pumpingactuators in the lubrication circuit.

A final important consideration ismagnetic decoupling. In mag-drivepumps, the magnetic field betweenthe magnets spins the impeller.The magnetic coupling has a torquelimit, so the pump must be prop-erly sized, and designs must takeinto consideration the proper mag-netic materials as well as sizing. Ifmagnet materials and size are notproperly specified, the torque limitis exceeded and the magnetic cou-

pling connection can be broken, re-sulting in downtime and possibleunit failure. Some common causes ofmagnetic decoupling are hard startswith higher rates of start-up torquethan seen during steady operation,wet end clogging due to an unex-

pected high shear or fibrous fluid, ora process upset, such as encounter-ing colder temperatures than antici-pated, which causes a spike in fluid viscosity. Employing a power moni-tor can detect magnet decouplingand mitigate these effects.

Final remarksCPI plant operators continue to gaina deeper understanding of the latestmagnetically driven pump technolo-gies, which offer simpler and more ef-

ficient fluid-process solutions. Usingmagnetically driven pumps canyield significant benefits by improv-ing safety, reducing plant downtime,slashing maintenance time and as-sociated costs, cutting onsite partsinventory and eliminating potentialfailure points through a hermeticsealless design. Today’s mag-drivepumps have ideal processing ap-plications in industries that handlehazardous fluids. Mag-drive pumpdesigns offer distinct advantages

over traditional sealed models whenused in the proper environmentsand appropriately specified. ■

Edited by Gerald Ondrey

AuthorRichard Tym is globalproduct manager for thenon-metallic and mag-driveproduct lines at ITT GouldsPumps (240 Fall St., SenecaFalls, NY 13148; Phone: 315-568-7378; Fax: 315-568-7076;Email: [email protected]).During his six years with ITTGoulds Pumps, Tym has held

various positions in engineer-ing and product marketing.He earned a bachelor’s degree in mechanical en-gineering from Wilkes University and an MBAfrom the Rochester Institute of Technology.

FIGURE 7. Metallic mag-drive pumpsare more suited for higher temperatureand pressure applications

FIGURE 6. Mag-drive pumps can besupplied with polymer linings for coro-sion protection

Source: ITT Goulds Pumps Source: ITT Goulds Pumps

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