Align Pumps and Drivers to Maximise Energy-PumpAlignment

8/13/2019 Align Pumps and Drivers to Maximise Energy-PumpAlignment

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COMBINED CYCLEJOURNAL, Third Quarter 2008 47

SPECIAL ISSUE:OUTAGE HANDBOOK ROTATING EQUIPMENT

The focal point of activity dur-ing inspection/maintenanceoutages at combined-cycleplants is the gas turbine. Heat-

recovery steam generators, steamturbines, and generators also get

their share of attention. Howev-er, you dont hear much at user-group meetings about plant-lev-el experience with all the otherequipmentpumps, for example.

Thats not to say O&M staffsarent paying the necessary atten-tion to pumps and other criticalcomponentsmost probably are.But the gas-turbine-based sectorof the electric generation industrycontinues to expand despite theeconomic downturn and many newemployees lack in-depth experiencewith equipment they at least shareresponsibility for.

What percentage of plant personnelwould you think have fewer than fiveyears of hands-on experience? Andwhat fraction of that group under-stands how critical proper alignmentof pump and motor are to availability,power consumption, and other keyfactors? The editors guess: A largerpercentage than you might think.

Dieter Seidenthal, VP, Ludeca Inc,Doral, Fla, who has exhibited vibra-tion-monitoring and laser-alignmentinstrumentation at many user-groupmeetings over the years, says thatthe biggest contribution one can

make to lower the operating cost ofrotating equipment is to align it cor-rectly in the first place.

Ludecas Pedro Casanova addsthat when misaligned, the load-ing of shafts increases dramaticallybecause of the reaction forces cre-ated within the coupling (Fig 1). Theresult:n Bearings, seals, shafts, and cou-

plings fail prematurely.n Bearing and coupling tempera-

tures run hothigh enough toidentify misalignment using infra-red thermography.

n

Vibration becomes excessive.

n Power consumption increases sig-nificantly.

A survey conducted by one of theworlds leading service organizationsspecializing in rotating equipment,continued Casanova, found thatfewer than 10% of the 160 machines

randomly chosen for measurementwere aligned within acceptable lim-its. Nearly one-third of the samplehad offsets from 8.1 to 20 mils, closeto one-third exhibited even greatermisalignment.

To quantify the benefits of properalignment, Seidenthal took out somecharts developed by PrueftechnikAlignment Systems GmbH, Isman-ing, Germany, from data providedprimarily by users. Ludeca is Prue-ftechniks exclusive representative inthe US for the German firms align-ment and vibration monitoring tools.

Effects on power consumption are

illustrated in Fig 2. If you add up thepower consumption of large pumps(circulating water, condensate, feed-water, etc) in an F-class 2 1 com-bined cycle, the dollars-and-centsargument for holding alignment tomanufacturer specs is compelling.

Fig 3 presents the relationshipbetween mechanical-seal life andoffset misalignment. Note that thescale for seal life is logarithmic. Itdoesnt take much misalignment toreduce seal life by 90%. Figs 4 and 5show that the rate of seal and pumprepairs declined dramatically withthe full-scale use of precision align-ment in Year Six.

Seidenthal, who has been help-ing engineers at power and processplants for decades, discussed the evo-lution of alignment tools during hisprofessional career. He recalled the

straight-edge/feeler-gauge methods

Align pumps and drivers to

maximize availability, minimizeenergy consumption

Offset, mils10 20 30 40 50 60

Increaseinelectric

consumption,

%

10

8

6

4

2

0

1. Misalignment can cause premature bearing, seal, shaft, and/or coupling failures

2. Significant power savings can beachieved by correcting misalignment

Offset, mils0 50 100

2 mils

Mechan

ical-seallife

100

10

1

0.1

3. Life of mechanical seals decreas-es quickly as misalignment increases


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48 COMBINED CYCLEJOURNAL, Third Quarter 2008

SPECIAL ISSUE:OUTAGE HANDBOOKROTATING EQUIPMENT

that were state-of-the-art for years(Fig 6). They rely on the limited reso-lution of the human eye and are accu-rate to only 4 mils, which is not goodenough for most rotating machines.

Dial indicators represented a majoradvancement in alignment measure-ment (Fig 7). They offer a resolutionof 4 tenths (0.4 mils), but it takesconsiderable experience to mastertheir use and to obtain good results.Rounding error, sticking/jumpingdial needles, play in mechanical link-ages, reading errors, tilted dials, sag-ging indicator brackets, etc, all canhave a harmful impact on measure-ment results.

Todays laser-alignment tools areclaimed to provide resolution to 4hundredths of a thousanth (0.04 mils)

quickly, accurately, and reliably (Fig8). There is no human factor to dealwith, no sag influence, no readingerrors. You can change technicians,but the results remain the same.

The handheld alignment instru-ment/computer in the photo (thetop-of-the line Rotalign Ultra) showsa one-cable connection to the laser-beam emitter and receiver, butwireless is an option. Horizontaland vertical coupling and foot val-ues are computed automatically anddisplayed on the screen. A diagramof the shaft system also can be dis-

played while you make adjustments

so you can track your progress live.The device alerts when shimming iscorrect. If you have never seen oneof these instruments in use, watchthe video at www.ludeca.com/prod_rotalignultra.php.

Training is required, of course, butmost technicians can become expertenough in a day or two of classroom/field instruction to handle most taskson their own. If they get stuck, theresa help line to call.

With the value plant owners placeon availability, its difficult to believe

any combined-cycle facility would bewithout such critical diagnostic toolsas a borescope, vibration monitor,laser alignment, ultrasonic NDE,multimeters, etcand techniciansonsite capable of using them to theirfull capability.

User viewpointChuck Hildebrand, who has beentrouble-shooting rotating equipmentfor more than 30 years, agrees. Withthe convenience and accuracy offeredby some of todays laser-based align-

ment tools, he believes their useprovides a healthy return on invest-ment. However, he points out thatsome plants, having made substan-tial investment in laser tools, havenot committed the time or moneyessential for developing the neces-sary onsite knowledge to use themeffectively.

Hildebrand should know, havingspent most of his career at a majorEast Coast electric utility as themulti-plant companys vibration andalignment problem-solver. He recent-ly started his own firm, DynamicBalancing Co, Lighthouse Point, Fla([email protected]), to help

Numberof

me

chanical-sealrepairs

Year1 2 3 4 5 6 7 8

50

40

30

20

65%

Year1 2 3 4 5 6 7 8

100

80

60

40

20Num

berofpumprepairs

30%

4. Mechanical-seal repairs declinedby 65% after precision alignment wasintroduced. Longer maintenance inter-vals increase availability

5. Rate of pump repairs decreased aslaser shaft alignment became an inte-gral part of the pump repair process.Reduced parts consumption stretchesyour maintenance budget

7. Dial-indicator method works well,but you essentially need machinist-level skills to obtain the quality ofresults required for precise alignment

6. Straight-edge/feeler gauge align-ment method is reminiscent of amuseum and far too inaccurate formost machines today

8. Laser alignment tools are fast, accurate, and reliable. Plus, they docu-ment results which then can be exported to produce all manner of customizedreports on a personal computer


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SPECIAL ISSUE:OUTAGE HANDBOOK ROTATING EQUIPMENT

users solve alignment problems oncritical rotating machinery.

Focusing on pumps for a moment,Hildebrand says the need to accu-rately measure the degree of shaftmisalignment between a pump andits driver often is a result of exces-sive vibration at some time

during normal operation.Electronic laser-alignmenttools are a most convenientway to obtain the exact infor-mation needed to provideefficient repositioning ofrotating machinery. It takesonly a few minutes to obtainaccurate shaft-alignmentmeasurements for a typi-cal horizontally mountedpump and driver in relatively goodmechanical condition.

Some sophisticated instruments,such as Ludecas Rotalign-Ultra, he

continues, can be considered expertadvisors, because they also providea simplified evaluation of the manypossible ways to correct misalign-ment.

Although the latest generation ofthese diagnostic tools provides excel-lent information regarding the posi-tion of the shafts centers of rotation,achieving and/or maintaining goodalignment conditions during opera-tion still poses many challenges. Toillustrate: High-energy boiler feedpumps are connected to heavy-walledpiping systems. Normal thermalexpansion of this piping during plantoperation can produce forces largeenough to move the pump in abnor-mal directions.

Various methods have been usedto measure this dynamic alignmentbehavior. Once known, the mea-sured dynamic alignment change canbe compensated for by intentionalmisalignment during the alignmentprocess. In some cases, however, theonly effective solution for achiev-ing the desired operating reliabil-ity requires modifications to piping,foundations, couplings, etc.

Alignment measurements on

vertical pumps are treated a littledifferently. Although the measure-ment concept is essentially the same,vertical pumps do not normally haveflexible shaft couplings. Misalign-ment is measured with the rotorand pump shaft hubs disconnectedand with the pump shaft centeredin its upper bushing; the pump shaftremains stationary during align-ment measurements. Although thesetup of the laser alignment toolis a little more complicated whenmeasuring vertical pumps, the samestraightforward alignment correc-

tions are provided.

State-of-the-art laser alignmenttools can be used to accommodatea wide variety of alignment tasks.To illustrate: Relative alignment ofbearings, bores, and rotational cen-terlines of shafts can all be measuredand compared in any combination.

Theres no machinery

alignment task I can thinkof where laser measure-ments would not providethe desired position infor-mation in a most conve-nient and accurate way,Hildebrand says.

But he cautions that themere purchase of thesetools does not make onean instant expert. A good

understanding of alignment-mea-surement concepts and deep knowl-edge of specific rotating equipmentare necessary for determining the

most effective solutions for machinetrains troubled by misalignment.

Theres no substitute for experi-ence, Hildebrand adds.

T h e c o n v e r s a t i o n m o v e d t oaeroderivative GTs with Hildebranddescribing equipment idiosyncrasieshe has observed while working on aparticular model. Although identi-cal by design, no two units on which

he has worked have behaved in thesame manner. Troubled by leakinghot exhaust gases (not a problem initself), dynamic alignment changeduring operation varied by as muchas 25 mils from unit to unit.

Furthermore, the ever-changingdirection of exhaust leaks on theseunits caused misalignment thatcould not be compensated for reliablyusing the cold offset misalignmentprocedure described earlier for boil-er-feed pumps. In at least one case,reliable operation was achieved onlyafter redesign of the flexible shaft

coupling.Hildebrand also shared his expe-

rience in aligning shafts of large-frame GT and steam-turbine gen-erator sets. Unlike typical auxiliarymachinery, synchronously rotatingthe shafts of these prime movers toacquire misalignment measurementscan be difficult.

The same laser-alignment toolscan be used, but it is extremelyimportant to assure measurementsof the highest accuracy and repeat-ability. Statistical evaluation of mea-surement points allows exclusionof abnormal data, thereby result-ing in highly reliable shaft-positioninformation. Because adjustments ofthese behemoths can be labor inten-sive, this measurement technique isappreciated for minimizing the num-ber of position corrections.

Alignment preparationsDont let Fig 8 fool you. Set-up anduse of electronic laser-alignmenttools may be a clean coverallsassignment, but the actual deck-plates-level task of machine realign-ment is of the old-fashioned dirty-

fingernails variety that youre moreused to.

Figuring out what must be doneto realign a pump, compressor, orfan and its driver may be far simplertoday because of laser alignment,but the devil remains in the details.Making permanent adjustmentsthat will hold the equipment trainin alignment requires know-how andgood mechanic skills.

The editors asked Casanova, whooften can be found on the deck plateswith users, to suggest an alignmentprocedure and provide some details

on how to prepare equipment for the

Hildebrand

What is soft foot,anyway?A gap between one foot of amachine and the foundation gener-ally is referred to as soft foot. Apump or motor typically has fourfeet and generally rests on three,leaving a gap under the fourth.Recall from high-school geometrythat three points determine a plane,

which is why dairy-farm milkingstools only have three legs.A soft foot left unshimmed

means that a machine frame willdistort from its resting position asthe hold-down bolts are tightenedto secure it in place. This distortionbinds the shaft and preloads thebearings. The latter is one of theleading causes of premature bear-ing failure.

Thus a soft-foot check is vital tothe pre-alignment process. Keep inmind that there are different kindsof soft foot. In its simplest form, afoot can either be parallel or bent.

Its relatively easy to locate a paral-lel foot (think of it as a short leg) andcorrect for it. A bent foot is moredifficult to fix.

One of the benefits of a sophis-ticated laser alignment system isthat it will have a soft-foot wizard,or diagnostic routine, built intothe firmwear to determine if thisproblem exists and if it must beaddressed immediately to assuresatisfactory alignment. The wizardrecommends to the technician thequickest path back to production(maximum availability).


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50 COMBINED CYCLEJOURNAL, Third Quarter 2008

work required. The macro proce-dure he suggested has six steps:n Pre-alignment checks.n Rough alignment to eyeball

clean (with bolts loose).n Rough soft-foot: Loosen all bolts

and fill any obvious gaps.n Initial alignment: Get to within 5

to 15 mils at the coupling or lessthan 20 mils at feet.

n Final soft foot: All feet to less than2 mils.

n Final alignment within tolerances.Safety first. Before beginning pre-

alignment checks, make sure themachine youre working on is isolatedfrom the fluid circuits served andfrom the plant electric systemandtagged out. That done, you can safelycheck for deficiencies which couldprevent the attainment and retentionof good alignment.

Begin with the foundation. It

should be of adequate size and ingood condition. Good engineeringpractice suggests that a concretefoundation be three times the weightof rotating driver and driven equip-ment, five times for reciprocatingmachines.

Check grout condition; no voidsshould be in evidence below thebaseplate. If grout recently has beenpoured to fix a battered foundationor to accommodate a new machine, besure it properly cured prior to start-ing alignment work.

The baseplate must be rigid, so nopart of any foot moves more than 1mil out of plane with the other feeton the same machine. Use a straightedge and feeler gauge to verify thatmounting pads are level, flat, paral-lel, coplanar, and clean. Be sure toselect hold-down bolts of proper size,ones with enough clearance to permitany corrective movement required.

Foot pads should be sufficientlyhigh so theres enough room underthe machine to use lifting jacks.Mount jackscrews on rigid padsones that wont deflectmade fromstainless steel to resist corrosion.Coat jackscrews with an anti-seize

compound such as Never-Seez.Piping should be well fitted and

supported, and sufficiently flex-ible, so that shaft movement is nomore than 2 mils when the last pipeflanges are tightened (Sidebar). Ifpipe-flange angular misalignmentexists, a dutchman or taperedfiller piece may be necessary. Spiral-wound gaskets can be helpfulinaddition to, or instead of, a taperedfiller piece.

Excessive parallel offset at thepump flange connection cannot becured with a filler piece. However, it

may be possible to absorb it by offset-

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ting several successive joints slight-ly, taking advantage of the clearancebetween flange bolts and their holes.If excessive offset remains, bend thepiping to achieve a better fit.

In some cases, pipe expansion ormovement may cause pump move-ment, leading to misalignment and

increased vibration. Better pipe sup-ports, stabilizers, or flexible spoolpieces may be needed in such situa-tions.

Shims must be clean for best resultsand made from a corrosion- andcrush-resistant material. Caution:If commercial pre-cut shims areused, verify actual thickness with amicrometer before use. Most manu-facturers supply shims in thickness-es of 50 mils and above only in nomi-nal sizes subject to standard materialvariations.

Make sure that shims are free of

burrs, bumps, nicks, and dents. Alsothat size numbers or trademarks areetched into the shimnot printed orstamped. Generally speaking, try touse the smallest commercial shimthat will fit around the hold-downbolt without binding. The smaller theshim, the more accurate the align-ment corrections will be.

Cleanup is important. Use a powerwire brush to remove all rust, scale,paint, dirt, etc, from the bottoms ofthe machines feet, and from the base-plate flats or pads. Take special careto remove all dirt in the area of hold-down bolt heads; otherwise, it is sureto get knocked onto the shimmingarea by wrenches, jacks, etc. Likewise,clean any jacking or lifting surfaces tokeep any loose dirt from finding itsway under the machines feet.

Rough alignment. Use a straightedge and feeler gages (there areother methods) to position the pumpand its driver within some reason-able distance from their desiredfinal aligned location. A rule ofthumb for close-coupled machines:Coupling offset should be less thana tenth of an inch. The angles orcoupling faces should be nearly eye

clean, or within 10 mils per inch ofcoupling diameter.

Straight-edge and feeler-gaugealignment can be much more pre-cise than this, but the extra time itwould take to be more exacting iswasted. The idea is to just get theshafts within the measuring range ofthe laser alignment system and useits guidance to finish the job withindesired specifications. Interestingly,many experienced mechanics consid-er themselves good enough to roughalign by eyeball alone, without evena straight edge. More often than not

they are correct.CCJ

Documents

Align Pumps and Drivers to Maximise Energy-PumpAlignment