Past installments of this column have generated a fairamount of interest among end users looking toreduce their total cost of pump ownership and

improve their reliability records. This includes specificinterest in how to address excessive maintenance costsand lack of reliability associated with the use of magnet-ic drive pumps. Why is this such a common complaint?

Start with a challenging pump situation and/or adifficult-to-seal liquid. Add a double helping of anurgent desire for a quick-fix panacea. Then mix in a fewcloudy misconceptions of magnetic coupling technology.Abracadabra! The rabbit you pull out of this hat may notalways be what you were hoping for. Let’s go behind thescenes and learn why.

Mechanically Sealed or MagneticDrive

Both technologies--mechanically sealed and magnet-ic drive (magnetic coupling) pumps--have places wherethey perform well, and both technologies have weakness-es and limitations. In considering a switch from mechan-ically sealed to magnetic drive it is important to under-stand that you won’t necessarily be solving a problem perse. Rather, you simply may be trading one set of prob-lems and limitations for another. And, if your underlyingproblem is a mismatch of the pump to the systemrequirement, you’ll find that magnetic couplings will beeven more vulnerable than mechanical seals to the result-ing effects of dead heading or cavitation.

Magnetic drive pumps can be an attractive alterna-tive to their mechanically sealed counterparts in the fol-lowing situations:

• With very difficult-to-seal liquids such as acrylni-trile, styrenated compounds, or sodium hydroxide;

• Where there is a desire to eliminate the requirementfor seal support systems such as pressurized barrierfluids, flush liquids or quench fluids;

• When no product contamination can be toleratedby barrier fluid in leakage or flush liquid dilution.

Advantages and DisadvantagesThe advantage of magnetic drive pumps is that the

need for mechanical seals and seal support systems iseliminated. Compared to using a double mechanicalseal, this can be quite significant. A double mechanicalseal will typically require a pressurized vessel, a source ofpressure to keep the barrier fluid at a higher pressurethan the fluid in the stuffing box, and associated pipingand controls. The barrier fluid will be gradually used up,and requires replenishment on an ongoing basis. Also,

the barrier, in relatively small amounts, will end up in theproduct.

The disadvantage with a magnetic drive is that amagnetic coupling and product-wetted bearings areintroduced. In many designs, there also will be product-wetted thrust washers that serve as thrust bearings. Boththe magnetic coupling and the product-wetted bearingshave weaknesses and limitations that must be designedand operated around to get reliable performance.

Simplifying the Magnetic driveDiscussion

To keep things simple, we will describe magneticcoupling technology based on designs using permanentmagnets.

One set of magnets is in a housing that is attached tothe shaft that drives the impeller, rotor or vanes. That isreferred to as the inner magnet assembly. The outer mag-nets are attached to a shell that is driven by the motor.The inner and outer magnets are separated by a contain-ment “can” that serves to keep the liquid in the pump.

Motor torque is transferred by the magnetic fieldset-up that attracts the inner and outer sets of magnets.The containment “can” may be constructed out of metalor non-metallic materials. A general arrangement of amagnetic coupling is shown in Figure 1

If the containment “can” is constructed of metal,eddy currents created by the alternating magnetic fluxlines will be present. These eddy currents generate heatthat must be carried away by a fluid circulation loopdesigned to promote flow through the inner magnetarea. (Refer to the flow path illustrated in Figure 2.) Theamount of heat generated is proportional to the speed ofthe pump squared. Choice of metallurgy for the con-

The Magic Of Magnetic DrivePumps: Part I

Kevin Delaney, Contributing Editor

Pumping Profits & Productivity

Considering a switch from mechanically sealed?You may be trading one set of problems and limitations for another.

PUMPS & SYSTEMS www.pump-zone.com JULY 2003 13

Figure 1. General arrangement of a magnetic coupling.(Image courtesy of ITT-Goulds Pumps)

JULY 2003 www.pump-zone.com PUMPS & SYSTEMS14

tainment canister is also a consideration. For example,Hastelloy will have substantially lower eddy current-gen-erated heat compared to stainless steel.

Where a metal containment shell is used, adequateremoval of heat generated by eddy currents is always animportant consideration. Because the heat is proportion-al to the square of the speed, heat removal for motorspeed pumps is especially critical. If the liquids beingpumped are heat sensitive, this is even more crucial.

The magnetic drive pump manufacturer can calcu-late what the flow through the containment can area is,and what the temperature rise is. Magnetic drive manu-facturers often cite what the overall temperature rise is,and the overall number is relatively low, usually just a fewdegrees.

For heat-sensitive liquids, the more critical numberis what the temperature rise is on the relatively small flowthat is in the containment area recirculation path. Therelatively small flow through the containment can areacould result in a temperature rise of 30-40∞F.

Excessive heat rise can cause product degradation.Even worse, the product may react and set up in theinner magnet area. If the pumped liquid is a high-vaporpressure liquid, such as a solvent, the heat absorbed maybe enough to vaporize the liquid in the inner magnetcooling circuit. Vapor, instead of liquid, in the coolingcircuit does not carry away heat. The result is that thewhole pump overheats, and the liquid lubricating theproduct-wetted bearings vaporizes. This may lead tobearing failure, and ultimately, to catastrophic failure ofthe pump.

The moral of the story is not to look at just theoverall temperature rise of the pumped liquid. Pay atten-tion to the temperature rise in the cooling circuit, andmake sure that you have a safe margin to avoid vaporiza-tion.

The Problem of DecouplingIt is important to know that like mechanical cou-

plings, magnetic couplings are rated by torque. Torqueexpressed in inch pounds is 63,025 times horsepower,and divided by operational speed. The upper torque rat-ings are a limit to stay below for the purpose of avoidingdecoupling.

Decoupling occurs when the magnets no longeroperate synchronously, i.e., at the same speed. If thepump operates with the magnets in a decoupled state forvery long, the magnets will permanently demagnetize. Asthere is no way to restore the magnets, this necessitates avery expensive set of replacement magnet assemblies.

A higher than expected specific gravity or viscositycan cause decoupling. With positive displacement pumps,a high starting torque situation with a viscous liquid or aline blockage downstream can create an overload situation.With centrifugal pumps, running out on the curve cancause decoupling.

When using standard mechanical couplings, servicefactors are used. For positive displacement pumps, near-ly all of the mechanical coupling manufacturers recom-mend a 1.5 service factor on the coupling-rated torquedivided by the maximum anticipated operating torque.

The purpose of the service factor is to keep the cou-pling from being damaged in the event of an overloadsituation. If a 1.5 service factor is universally recom-mended for mechanical couplings on positive displace-ment pumps, why would less of a service factor beacceptable for magnetic couplings?

A lesser service factor is probably not a good idea.With that said, however, the difference between a 1.0and a 1.5 service factor in a mechanical coupling mayadd up to only a few dollars. With a magnetic coupling,though, the difference could run into hundreds or thou-sands of dollars. In the absence of a specification, thepump supplier may be tempted to put in a 1.0, and per-haps even more marginal service factor magnetic cou-pling to keep the cost down.

To justify the practice of using little or no servicefactor, some magnetic drive pump manufacturers makethe claim that this decoupling upper limit is a goodthing, preventing damage to the pump and motor in theevent of an overload situation. Based on my own expe-rience in the field, I maintain that proper pump protec-tion is accomplished by use of relief valves for positivedisplacement pumps and by power monitors for all mag-netic drive pumps--not the sacrificial use of expensivecomponents that can cost up to 80% of the replacementvalue of the pump.

Minimizing Damage & RiskSound design practice calls for a hierarchy of failure,

with the minimum amount of damage and risk causedby the first modes of failure. Recognizing that damageto the magnetic coupling is a critical failure that willrequire taking the pump out of service, it makes no senseto design the magnetic coupling with a lower service fac-tor than the electric motor.

Most plants have clear specifications calling for a1.15 service factor on electric motors. Yet, those sameusers often have no idea of what the service factors are ontheir magnetic drive pumps. Furthermore, many endusers are not even aware that magnetic drive manufac-turers have multiple magnetic coupling sizes for the samepump!

As a first step toward avoiding the damage andexpense of running decoupled, users should specify amagnetic coupling service factor higher than the motor

Figure 2. Flow path through inner magnet area (Image courtesy of DESMI Inc./Rotan Pumps)

PUMPS & SYSTEMS www.pump-zone.com JULY 2003 15

service factor. Ideally, the motor trips out before thecoupling decouples. The overload circuit breaker in themotor starter may not be fast acting enough, in all cases,to prevent decoupling. Thus, as will be discussed later,the recommendation for a power monitor to protect thepump still holds.

Identifying the Right ServiceFactor

For fixed speed pumps, the magnetic coupling servicefactor should be based on the motor nameplate horse-power and speed. Some magnetic drive pump manufac-turers base the service factor on calculated load. Theweakness of this approach is that motor load will increasewith higher viscosity or product specific gravity. The actu-al load then can be significantly higher than the calculat-ed load, in effect, subtracting from much or all of the ser-vice factor. A good rule of thumb to use is a minimum of1.25 service factor for the magnetic coupling. Otherwise,users should specify at least the same service factor aswould be recommended for a mechanical coupling.

Temperature ConsiderationsAs shown in Figure 3, the magnets on magnetic

drive pumps have upper temperature limitations.Consequently, the magnets will demagnetize if exposedto temperatures exceeding the upper limit. The degree ofdemagnetization may range from partial to full, depend-ing on how excessive the temperature is.

When selecting the magnet material, a safety factorof 25-50∞F should be added to the maximum operatingtemperature of the liquids being pumped. In addition,upset conditions that generate heat--such as run dry ordead heading--should be avoided. The best protectionagainst running dry or dead heading is a properlyinstalled and set up power monitor.

Materials Are ImportantThe two most common types of permanent mag-

nets used in magnetic drives are neodymium iron boron,and samarium cobalt. There is a misconception thatsamarium cobalt, because it is more expensive thanneodymium iron boron, is a “better” magnet material.The truth is that samarium cobalt has only one mean-ingful advantage--a higher temperature rating. (Notecomparisons in Figure 3.) Although samarium cobalt istwice as expensive, it only has 60% of the strength ofneodymium iron. Why should this be of interest to you,the user?

Here’s a typical scenario. You’re pumping sodium

hydroxide. The maximum temperature that the sodiumhydroxide will see is 120∞F. You want to make surethat this application will work, so you specify “premium”magnet material, samarium cobalt. The magnetic drivepump supplier, to keep costs down, supplies a unit witha .93 service factor on a motor nameplate magnetic cou-pling. The motor has a 1.15 service factor to meet theplant standards. On a cold day, the pump is turned on,as usual. After about an hour, someone notices that noproduct is being unloaded. What happened?

Your pump experienced an overload condition withthe cold sodium hydroxide and the coupling broke freeupon start-up. With no power monitor to automatical-ly shut the pump down, the unit, inadvertently, wasallowed to run decoupled for an entire hour. The pumpwas stopped and restarted, but would not pump again.You send the pump back to the manufacturer for war-ranty evaluation, whereupon you are told that the pumpran decoupled and that the magnets were demagnetized.The result? You have to replace the inner and outer mag-net assemblies at a cost of $10,0000.

Was anything learned? Because there is an urgentdesire to get the pump back in service, it is rebuilt per theoriginal configuration with the samarium cobalt magnetassemblies and a .93 service factor.

In this case, because the operating temperature waswell within the range of neodymium iron boron, youwould have been better off using the less expensive, yetstronger neodymium iron, and specifying a 1.5 service

factor on motor nameplate for themagnetic coupling, and investing in apower monitor. The total cost wouldhave been about the same (and per-haps less) than what your companyspent on the “premium” materialsamarium cobalt. You also wouldhave avoided the problem of runningdecoupled.

End users would be well advisedto request manufacturers’ data sheetson magnetic coupling torque ratings

and magnet temperature ratings, and to specify a manu-facturer’s guarantee of minimum torque ratings.Magnetic drive pumps represent an expensive invest-ment, thus, it’s worth taking a little extra time study thedetails. Check back next month for a few more magnet-ic drive troubleshooting tips.

Editor’s NoteThis magnetic drive discussion will continue in the

August issue of Pumps & Systems. In the meantime,because of the strong interest in magnetic drive technol-ogy, Kevin Delaney will be conducting a special three-hour, hands-on workshop entitled “TroubleshootingMagnetic Drive Pumps: Myths & Mysteries,” onWednesday afternoon, September 17, at PumpUsersExpo 200, in Baton Rouge, LA. Enrollment will be lim-ited, though. So, if you have problems with your mag-netic drive pumps and want to attend, you’ll need to actfast. Please refer to your registration packets and/or visitwww.pump-zone.com for details. Or call 205/212-9402to sign up. P&S

Figure 3. Magnet characteristics (Source: Magnet Sales & Manufacturing)