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M A r s f in s r th a c d s s a b b th a m b p o n c a p p m h a c

High-speed balancingPANORAMA

428318 | Sulzer Technical Review 4/2009

All rotors are part of a machinery train,be it a generator, gearbox, compressor, orother mechanical assembly. The presenceof an unbalance in any rotating compo-nent in the assembly may cause the entiretrain to vibrate. This induced vibration,in turn, may cause excessive wear in bear-ings, bushings, shafts, spindles, gears aswell as the civil structures. The vibrationsset up alternating stresses in structuralsupports and housings, which may even-tually lead to total failure. Not surprising-ly, the primary reason given by most ofour clients for balancing a rotating ele-ment at the maximum running speed haslittle to do with the technical aspects ofbalancing or modal analysis. Rather, it is

reliability, availability, profitability, andefficacy of the business unit that isweighed to reduce the overall risks inher-ent in the operation and maintenance ofhigh-speed rotating equipment.

Low-cost insuranceAn at-speed balance of the rotor, as a partof the overhaul, is considered low-costinsurance. Shop balancing, by contrast,is usually undertaken at low speed—generally below 1500 rpm—mostly dueto the perceived initial cost and timeadvantages compared to the effortsrequired to balance a rotating element athigh speeds. Low-speed balance standscome in a variety of types and sizes

depending on the parts being handledand the measuring system being used.Most people are familiar with the bal-ance machine used in a tire repair shop,where a lead weight is attached to therim to compensate for an unbalance. Abalanced tire gives the driver a comfort-able ride on the road throughout thespeed range of the vehicle. For most low-speed, rigid-shaft rotors with no adverse operational history, a low-speedbalance in the workshop is usually suffi-cient . These rotors typically run atspeeds that do not excite the first critical-speed band . Most of the rotors in thisrigid class operate below the influence ofthe second critical-speed band, some-

A large steamturbine for a powergeneration facilityundergoing a low-

speed balance at theworkshop. This low-speed balance stand

has a capacity forrotating elements to50 tons at 500 rpm.

Dynamic balancing of rotating elements is an important aspect of the manufactureand the repair of any turbomachinery. A rotating element that is out of balance cancause major operational difficulties, which may prevent the timely start-up of afacility. Furthermore, the unbalanced element can cause internal damage that willrob a machine of its design efficiency, reduce machine reliability, and increase thecosts of operation and maintenance, or worse.

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PAPANORAMA

times called the first flexural mode,where the shaft takes the shape of a sinu-soidal waveform, with identifiablenodal points outside of the radial journalbearing centerline, as is shown below.

Mandatory for high-speed rotorsAt-speed balancing is necessary for somerotors running below the second critical-speed band and is considered mandatoryfor any rotor that operates within theinfluence of, or above, the second critical-speed band. Some of these high-speedrotors also require additional tuning ofthe low-speed residual unbalance toallow the rotor to come through the firstcritical-speed band without excessivedeflection of the centerline. Excessiveshaft deflections during the start-up andshutdown cycles will open up the clear-ances of the shaft seals, can damage thebearings, and may include severe rub-bing of the rotating element surfaces tothe stationary parts of the casing. Whileall of the original equipment machinerymanufacturers have their own at-speedbalancing facilities, they are dedicated toprocessing the new rotors for their vari-ous product lines and providing a smallnumber of aftermarket services forclients. Very few of these specialized bal-ancing bunkers are available to the inde-pendent repair market at a competitiveprice. With the order books for new equip-ment at historically high levels, it is nowharder than ever to schedule an openingat the OEM balance facilities that coin-cides with the clients’ overhaul cycle.

Extensive experienceSulzer Turbo Services overhauls manyrotating elements where the client hasrequested an at-speed balance as a part ofthe repair work scope or where the oper-ational history suggests a benefit . Italso engages in modernization workwhere seal and bearing design improve-ments can be demonstrated in the bunkerbefore being placed into service. To betterserve its clients, Sulzer Turbo Servicesoperates two at-speed balancing facili-ties, including one of the largest andmost advanced bunkers in the UnitedStates at its workshop near Houston ,Texas, and a second facility in Winterthur,Switzerland. Both facilities undergo con-stant revisions and upgrades to keeptheir capabilities in step with advances inthe industry. These two specialized balance bunkers provide reliable andtimely service for all six of Sulzer TurboServices’ strategically placed regionalrepair facilities, as well as to other OEMsand customers directly. Both areequipped with advanced electronics anddiagnostics to provide state-of-the-arttroubleshooting capabilities.

Generator fields One of the areas we have developedspecifically for generator fields is a proce-dure that allows us to complete shorted-turn detection and analysis work usingan air-gap flux probe that senses changesin the radial flux density as the rotor sur-faces passes the probe. The resultingwaveform highlights the presence of one

or more shorted turns by amplifying theminute variations in the flux slopes test-ing at the operational speed of the field.For this procedure, the field is operatedat its normal running speed with normalexcitation. This procedure identifiesproblems that can affect the proper oper-ation and the formation of hot spots inthe generator field that normally wouldnot be seen until the unit was taken intooperation. In many cases, the discoveryof these issues at the job site wouldrequire a forced outage to make therequired corrections.

Research and development platformBecause of the unique nature of the facil-ities, the bunkers are also utilized by ourresearch and development departmentsof Sulzer Turbo Services, Sulzer MetcoSurface Coatings, and our independentresearch arm Sulzer Innotec to developand operationally test various sealingsystem configuration designs. Testsinclude measurements of the durabilityof various abradable coating systemsapplied to segmented seal rings for use inthe next generation of steam turbines andindustrial gas turbine engines.

Some elements that affect rotorbalanceRotors are built up in a variety of ways toaccommodate the process gas or fluid,the stress levels acting on the parts, andthe materials available for construction.Some examples are listed below:

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F A th in p m m s e r H is t m th it e r

• Solid forgings for steam turbines andhot gas expanders

• Forged shafts with shrunk-on disks, orimpellers, only

• Forged shafts with all of the sleeves,disks, and other trim parts shrunk on

• Forged disks or packs of disks boltedtogether with one central or manyperipheral tie bolts using a Hirth orCurvic type clutch fit to transmittorque between disks and to the shaftstub ends

• Welded cylindrical assemblies—theseare usually found in very large steamturbines or high-speed axial-compres-sor rotors where the shaft body is

made of hollow cylinders seam weld-ed together by lasers to reduce theweight of the rotor assembly

• Lastly, a mix of any of the methodsabove

The excitation of vibration modes is thenfurther complicated by differences inmaterials, material quality, rotor size,and stiffness. Any unbalance locatedalong the rotor that coincides with deflec-tion in the mode shape will amplify thedeflection at those positions. Unless cor-rected, the unbalance creates a vibratingforce on the bearings. The movement ofindividual disks or groups of disks,along the shaft or off centerline during

the operation cycle of the machine canbecome problematic, especially if theshift of the mass encroaches on an areawhere the deflection is amplified.

Quality assurance processWhen a new rotor is manufactured, theOEM often balances the rotor at speed aspart of the quality assurance process. Theinitial balance work provides the ownerwith a benchmark rotor signature to com-pare with any subsequent inspections. Itis also important to consider itemsattached to the shaft, such as couplinghubs and thrust disks, as they can affectthe balance and the response of the rotor.Progressive stacking and balancing of arotor can provide a reasonable level oflow-speed balance, and it is an importanttool to ensure that the rotor is built witha low level of vibration. This method hasbeen used by Sulzer Turbo Services manytimes to fix a rotor with a poor vibrationhistory. Progressive stacking and balanc-ing does not assure a smoothly running,completely assembled rotor because itdoes not account for the vibrationscaused by system excitations as it is accel-erated through the critical speed bandsand up to its operating speed. During the service inspection of a

rotor, it is always worthwhile to checkthe weights added to correct the rotor’sinherent unbalance. The objective of anybalance process should be to use the min-imal number of weights to correct anyunbalances in the rotor.

Running above the first critical speedMany high-speed flexible rotors runabove the first critical speed. With suchrotors, it is also important to correct theunbalance response at the first criticalspeed for two reasons:• The unbalance forces and resultingshaft deflections may be so violent thatthe rotor will not pass the first criticalspeed, or it may do so only after wip-ing out the shaft seals. Even minimalcontact between the shaft and the sealswill open the seal clearance values anddecrease the efficiency of the unit. Thisalso creates localized heating of theshaft, which can amplify the deflectionat the first critical speed and mayresult in a permanent shaft bend.

PANORAMA

20 | Sulzer Technical Review 4/2009

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High-speedsteam turbine,

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PANORAMA

• A rotor could do well at the first criti-cal for too long a time during start-upand shutdown events—even whencontrolled acceleration and decelera-tion rates are employed—causingextensive damage to the unit.

Field balancing vs. at-speed balancingA rotor can be balanced successfully inthe field within its own casing and bear-ing system. However, access to balanceplanes can be a limiting factor. Somemachines accommodate this require-ment by adding a single access hole or aseries of access points at each of the twoend planes, thus eliminating the need toremove the entire upper casing half.However, if the location of the unbalanceis not in an accessible plane, the correc-tion at the end planes is only a compro-mise. When there is no access provision,then the whole upper casing—or parts ofit—must be removed and replaced aftereach balancing run. A minimum of fiveruns is typically needed.

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Balancing on the half shellIn some very extreme instances SulzerTurbo Services has field balanced therotor on the half shell, meaning that theupper half of the casing is left off, thelower half seal rings are removed, thenthe bearings are assembled and fitted.For a steam turbine rotor, an externalfluid, usually dry air ported through anozzle, is used to spin the rotor to thedesired speed where readings will betaken. In the case of a large, motor-driv-en turbomachine, the drive motor is usedto operate the machine to obtain thedesired readings; this type is limited bythe number of permissible motor startsper day. As you might imagine, this is atime-consuming prospect.

Rotating element in the fieldThere are significant risks and costs asso-ciated with balancing a rotating elementin the field. Every time a casing is opened,there is a chance to introduce foreignobjects into the flow path or to damage a

component. As most machines are part ofa process, the process has to be startedand stopped many times to achieve theproper balance level. Imagine having toask the operator of a nuclear facility runthe fuel rods in and out a half dozentimes or so over the course of four or fivedays so that you could accomplish some-thing that should have been done off site,in a controlled setting, with proper equip-ment, during the outage cycle. To makethe situation a little more uncomfortable,field balancing forces the user to adjustthe rotor response to correct the com-bined effect of all the non-rotating items(such as misalignment, foundation reso-nances, piping resonances). If a rotorruns in its casing with undesirable vibra-tion levels after being balanced at-speedit is probably not a rotor issue. A systemissue will require further investigationand correction.

Acceptable levels of unbalanceZero unbalance would always be pre-ferred and many engineers will discussthe acceptable level for hours, given thatzero is seldom economically achievable,a common level is chosen using industrystandards for various types of machinery.For at-speed balancing, the AmericanPetroleum Institute (API) requires vibra-tion levels less than the greater of1mm/sec, or 7400rpm.

An investment that pays wellThere is rarely a more uncomfortabletime in the professional life of a rotatingequipment engineer, maintenance man-ager, or operations manager than when acritical part of their process fails to starton time, will not run, or becomes unreli-able between maintenance cycles. Thecosts of trying to correct the issues, whileaccounting for lost production, can bequite staggering. Sulzer's dynamic field-balancing methods save both time andmoney over conventional site methods.

Shaun WestSulzer Turbo Services Zürcherstrasse 128401 WinterthurSwitzerlandPhone +41 52 262 34 44shaun.west@sulzer.com

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Gas turbine rotor assembly being loaded into the bunker. The compressor and turbine sections being bolted together.

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