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Fundamentally, a canned motor isintegral with the pump,eliminating the shaft extension

through the pump back cover andmaking the motor itself part of thepressure boundary with the samepressure retaining requirements as thepump casing, (Figure 1.) Fluid iscirculated through the motor to cool itand lubricate the bearings. This istypically achieved by taking a smallportion of the process fluid from thepump discharge and circulating itthrough the motor back to the pumpend via the product lubricated bearings.Some canned motors allow for severaloptional internal circulating systemsutilising an auxiliary impeller in variouscombinations.

Money and purpose

When considering applying a CMP,issues of fluid lubricity, heat removalcapacity, vapour pressure and flashing,corrosiveness, and solids content of theprocess fluid circulated in the motor allhave to be addressed. These not onlyimpact on the materials ofconstruction, but also with the flowpath options and controls on thecirculating fluid through the motor.

With an increasing focus on thefinancial, safety and environmentalconcerns associated with seal failures ofmechanically sealed centrifugal pumps,it has become prudent to investigatethe alternatives available with today’s

CMPs. More pump users have nowincluded the cost of ownership into thebuying process, which is good news forthe CMP manufacturer. The cost ofpump ownership over a twenty yearpump life has been identified for conventional mechanical sealedpumps, as 95% maintenance, operating& installation costs and only 5% purchase price. Significantlymaintenance costs make up the bulk ofthe 95% with mechanical seals beingthe number one cause of breakdownswith bearing failure coming second.

These two components have a greatinfluence on the reliability ofconventional pumps and are where theCMP triumphs. As there are nomechanical seals in a CMP thiseliminates the number one problemwhile the number two issue, bearingfailure, is less of a concern in a CMP.Bearing failures are minimized since asstandard there is no externallubrication required eliminating therisk of bearing failure due to poormaintenance practices or failure ofexternal lubrication systems.Additionally, CMP bearings run on acommon shaft that eliminates the riskof shaft mis-alignment, which is acommon cause of excessive vibrationresulting in reduced bearing life. Whenone considers the ease at which a CMPcan be installed payback starts evenbefore the CMPs are run.

Ultimately a pump is judged on how itperforms over the long term and whobetter to decide how reliable thesepumps are than customers. Several

Considerations for a cannedmotor pump There are three basic types of sealless pump technology available today:Magnetic Drive, Canned Motor and Hybrids. While a cost benefit analysis foreach of the three categories can be made based on the requirements of afacility, Calum Scott C.Eng M I Mech E, product specialist at Hayward Tyler FluidHandling, argues that a canned motor pump (CMP) can offer the highestdegree of containment (double containment), reduced maintenance cost andimproved reliability when properly applied.

f e a t u r e c a n n e d m o t o r p u m p s

Figure 1. A canned motor is integral withthe pump, eliminating the shaft extensionthrough the pump back cover and makingthe motor itself part of the pressureboundary with the same pressure retainingrequirements as the pump casing.

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years ago a customer coined the phrase‘fit and forget’ when referring toCMP’s. The story was that they hadcarried out a ‘head count’ of all theircentrifugal pumps on site and couldn’taccount for one of our CMPs. Aftercontacting a retired maintenanceengineer to see if he knew where it washe directed them to the right part ofthe plant and there was the pump —still running. The simple reason theycouldn’t find it was that nobody inmaintenance needed to work on it andthrough time forgot all about it.

Materialsconsiderations

Most canned motors are constructedof materials that are universallycompatible with a majority of processfluids, yet are not overly exotic orexpensive. Typically the metallurgy ofthe wetted components of a ‘standard’unit is 316 stainless steel (orequivalent). For larger units the statorand rotor liners (‘cans’) are made fromHastelloy C276. Although HastelloyC, a nickel based alloy, is more exoticand expensive compared with 300Series stainless steel, its relativelyhigh electrical resistivity significantlyreduces the ‘eddy current’ losses (canlosses) in the stator ‘can’ (and to asmaller degree in the rotor can),improving the efficiency of the motor.Also, since the stator and rotor cansare normally fabricated from a thinmaterial to minimize electrical lossesand reactivity, it is important to use ahighly corrosion-resistant material.High internal rotor cavity pressurescan be withstood as the cans aremechanically supported by theelectrical motor laminations and bybackup sleeves over the end regions.

Another consideration of materialcompatibility with the motor fluid isthat of the hydrodynamic bearings. Astandard configuration is that of asofter corrosion resistant non-metallicbearing material running on a harder

polished metal surface. This allows forsome wear of the bearing materialover time due to an occasionaladverse running condition orincidental presence of fine abrasiveparticles.

If a worn condition is reached, onlythe one bearing surface has to bereplaced, which is usually designed foreasy replacement. A good durable,forgiving, and highly corrosionresistant bearing material is a grade ofpure carbon where the binder itself iscoked, making it highly corrosionresistant, stable and non-porous(important for high bearing loads).PTFE bearings are the choice in lowerosive and hygienic applicationswhile silicon carbide running ontungsten carbide shaft sleeves or acoated shaft, is used in highly erosiveapplications ie raw sea watercontaining sand particles. From thevast number of CMPs operating in awide variety of pump media andconditions material & bearingselections are commonly made onprevious experience rather than whaton paper would seem the obviouschoice.

Realising thebenefits

When investigating the use andbenefits of a CMP as a sealless pump

Figure 2. Boiler Feed Booster Pump - Design Pressure 280 bar DesignTemperature 300 deg C, Unit supplied for pumping boiler feed-water in a bio-waste power plant.

Figure 3. 'Reactor Cooling Pumps - Design Pressure 60 bar andDesign Temperature 98°C , units supplied for nuclear applications'.

Figure 4. Hayward TylerStandard C Range Canned Motor

Pumps - completely eliminatesthe need for shaft seals.

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f e a t u r e c a n n e d m o t o r p u m p s

option, whether a new installation ora retrofit of an existing pump, it isessential to be aware of the fluidproperties and actual operatingconditions, including expectedabnormal or possible upset conditions,which can affect its performance andreliability. This requires a mutualunderstanding of the applicationconsiderations by the system designer,or end user, and the CMPmanufacturer. With this approach acost effective and reliable installationwill be realised.

A second article will review the use ofretrofitting canned motors to existingpump installations, known as CannedMotor Retrofits (CMR™), and will bepublished shortly. ■

CONTACTHayward Tyler Limited1 Kimpton RoadLuton, Bedfordshire. UKLU1 3LDTel + 44 (0) 1582 731144Fax + 44 (0) 1582 393403E-mail: haywardtyler@haywardtyler.comhttp://www/haywardtyler.com

Client case study – Afval Energie Bedrijf (AEB)A client who recently chose a Hayward Tyler CMP is Afval Energie Bedrijf (AEB).The application is for pumping condensate in a reheat system that is part of awaste power plant located on the outskirts of the City of Amsterdam, TheNetherlands.

A sealless pump design was specified by the client due to the high pressure &temperature regimes that the pumps would be operating in. A high degree ofreliability and analysis of cost of ownership was part of the tender process, asthe units will be running continuously three-off (two operating and one spare)special CMPs of a design pressure of 180 bar and 350°C design temperaturewere selected by AEB as the best solution.

This is the first time a reheat system (common in conventional steam cyclepower station) is to be used in a waste energy power station. This is usedto increase the station's efficiency from 22% to 30%. The reheat system isutilised in an expansion project that will boost the stations waste handlingfrom 800,000 tonnes per year to 1,300,000. This will mean AEB will beoperating the largest waste power plant in the world, overtaking a plant inJapan.

Hayward Tyler Fluid Handling were short listed as a potential vendor for thesepumps by Afval Energie Bedrijf, due to their well respected reputation as anindustry leader in the design and manufacture of CMPs. Paul Noble ManagingDirector, Hayward Tyler Fluid Handling said “Given our experience andknowledge in this industry we felt in a strong position to offer the best solutionfor Afval Energie Bedrijf. We are delighted to be involved in such a flagshipgreen energy project and to have AEB as a new customer ”.

Egbert Vrijen, AEB Project Leader said “Within our team we had previousexperience of Hayward Tyler products and our dealings with the companythroughout this tender process was very positive. Ultimately we selected themas the supplier in this instance as their bid made the best technical andcommercial fit with our needs ”.