steam turbine overhaul1

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LESSON

1LECTURE

INTRODUCTION TO TURBINEMAINTENANCE

SUB - OBJECTIVE

At the end of the lesson the Trainee ill !e a!le to de"onstrate an #nderstand theIntrod#$tion to T#r!ine Maintenan$e%

1%& MAINTENANCE '(ILOSO'()

Break downs and unscheduled outages on large modern turbine plant areexpensive, both in direct repair costs and replacement costs. aintenance isthere!ore directed towards obtaining sustained reliable operation between ma"oroverhauls. The scope !or carr#ing out routine preventive maintenance duringperiods o! operation is limited except on peripheral items, such as automaticgreasing e$uipment, oil !ilters and coolers, etc. This section is there!ore concernedmainl# with work carried out at times o! ma"or overhaul. The extent and !re$uenc#

o! turbine maintenance is governed b# a number o! re$uirements. %hen planningturbine overhauls, it is essential to consider he interactions between the varioustasks and to cater ade$uatel# !or the need to maintain a clean and tid# workingenvironment. Clean&condition working areas should be set up whenever turbinec#linders, valve, pipework and bearings are opened. 'de$uate checks must beincorporated when rebuilding plant to ensure that no !oreign material remains inturbine components on completion.

TURBINE ROTATIN* 'ARTS

Turbine rotating parts here comprise sha!ts, bearings, coupling and barring gear,together with the related sub"ects o! alignment, coupling concentricit# andlubrication.

S(A+TS

't times o! ma"or strip down, it is usual to carr# out a comprehensive surve# o!turbine sha!ts. The degree and t#pe o! inspection varies between the di!!erentturbine stages. (ha!ts sub"ected to high temperatures )*+ and + reheat- areexamined !or creep e!!ects as well as !atigue, whereas, on the high&stressed L+sha!ts, stress corrosion andor corrosion !atigue e!!ects predominate. 'll sha!ts areexamined at gland areas !or signs o! corrosion pitting and at the "ournal areas !orwear and scoring. ! necessar#, the sur!aces are skimmed or polished at thesepositions.

*igh temperature creep is monitored b# carr#ing out dimensional checks o! sha!tdiameter, together with truth checks. t ma# be necessar# to remove scale b# gritblasting, using either sharp sand or !ine aluminum oxide. The prepared sur!acescan then be examined !or !atigue cracking )using either magnetic particle inspection)+- techni$ues or d#e +enetrant-, particularl# at changes in section, or at blade&root !ixings.

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(ome machines have experienced stress corrosion andor corrosion !atiguecracking o! L+ sha!ts, with the presence o! condensing steam providing asu!!icientl# corrosive environment. The problem can be accentuated on reheatmachines undertaking to shi!t operation b# contamination !rom boilerreheater leaksduring vacuum raising. Ultrasonic techni$ues are now available which permitinspection !or cracking at vulnerable areas, such as changes o! section anddiscke#wa# locations.

JOURNAL BEARIN*S

The sha!ts o! a turbine generator are supported in "ournal bearings. These arewhite metal lined, thick&wall bearings and ma# be either plain or sphericall# seated,dependent on their position and dut#. 5ig. 3&3 shows a t#pical sphericall# seatedturbine bearing, with shimmed pads !or ad"ustment. The bearing is lined with a tinbased white metal. The bore is machined to an elliptical shape to combat an#tendenc# !or sha!t whirl6 this shape is achieved b# machining the bore with shims inthe hal! "oint to suit the sha!t "ournal si7e, as recommended b# the turbinemanu!acturer. 2nce machined, there is no re$uirement to bed the sha!t or ad"ustclearances, except that horn clearances ma# need slight ad"ustment.

aintenance o! bearing covers )keeps- and top halves. 's with other parts o! theturbine, it is important to record the 8as !ound9 condition, as this ma# assist indetermining the work to be carried out and in accomplishing a success!ul rebuild.Readings are taken o! the bearing top and side oil clearances and o! the pedestalbridge gauge readings. Bearing top hal! clearances are taken b# assembling thebearing with a piece o! appropriatel# si7ed lead wire on top o! the "ournal andmeasuring the thickness to which it is compressed. Each bearing pedestal is !ittedwith a bridge gauge !or the measurement o! the side and top clearances o! the"ournal, this determining precisel# how the sha!t is l#ing with respect to the pedestal.These datums are use!ul as checks o! correct re&assembl#.

*aving obtained these initial measurements, the sha!t can be supported either b#

overhead crane or b# special sha!t raising gear to permit removal o! the bearingbottom hal!. 't this stage, various inspections are carried out on the bearing.

The bearing sur!ace is !irst examined !or signs o! deterioration. The relativel# lowmelting point white metals undergo a rapid loss o! !atigue strength with increasingtemperature, so the sur!ace should be inspected !or cracking or cra7ing. This losso! strength ma# also result in extrusion or wiping under adverse loadinglubricationconditions. 'lthough, in less severe cases, the bearing ma# subse$uentl# bedressed !or re&use, it is important to !ind the cause o! the damage be!ore doing so.

/uring manu!acture, the white metal !orms a metallurgical bond to the steelhousing. This adhesion should be checked, using ultrasonic techni$ues6 an#

signi!icant deterioration o! the bonding renders the bearing un!it !or !urther service.

The sur!ace contour must be inspected at "acking oil holes, lubricating oil !eeds andat an# anti&whirl grooves that m# be included. ! necessar#, the metal is dressed.The condition o! the anti&rotation devices, housing hal!&"oints and spherical pads,should be examined and an# roughness removed with a !ile. 5retting o! thespherical sur!aces ma# indicate incorrect bedding o! the pads to the pedestal6 thiscan be checked using engineers9 blue marking and the pad shims ad"ustedaccordingl#.

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%hen rebuilding the bearing, all sur!aces must be kept scrupulousl# clean. Bridgegauge readings and clearances are recorded !or !uture re!erence. %ith sphericall#seated bearings, the weight the weight o! the sha!t must correctl# seat and align thebearing in the pedestal. The keep is then re$uired to 8nip9 the bearing in thisposition6 this is checked b# means o! lead wire on the top pad with shims at thekeep "oints. ' ;.;:&;.;< mm 8nip9 is generall# acceptable. Be!ore the keep is !inall#re!itted, the pedestal is cleaned and an oil !lush carried out.

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T(RUST BEARIN*S

Two main t#pes o! thrust bearing are used on turbine generators. The high dut#tilting pad variet# is used to absorb the residual end&thrust o! the main turbine sha!ts#stem, whilst maintaining the correct axial alignment o! !ixed and moving blades.The bearing consists o! two rings o! white&metal !aces pads on either wide o! a sha!tthrust collar. ndividual pads are able to tilt to allow !ormation o! a stableh#drod#namic oil !ilm. The other !orm o! thrust bearing incorporates white&metalthrust !aces at each end o! a "ournal bearing. These are associated with lighterduties, t#picall# being !itted to exciters and gearboxes.. The thrust !ace metal ma#be grooved radiall# to !eed oil and to aid cooling o! the sur!ace. 5ig. 3&:)a- and )b-illustrate the two t#pes.

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+i,% 1-/0a% (i,h d#t tiltin, .ad%

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+i,% 1-/0!% 2hite "etal thr#st fa$es%

n both, it is important that the load is transmitted evenl#6 this re$uires the collar tohave a !lat, true sur!ace, perpendicular to the axis o! the sha!t )i.e. %ithout swash-,and an e$uall# true and !lat sur!ace o! white metal. The bedding is checked usingengineers9 blue and the white metal scraped accordingl#. The thickness o! the tiltingpads must be matched. n the tilting pad bearing, the !loat between thrust !aces isad"ustable b# means o! shims and liners attached to the pad carriers. The liners arealso used to position the turbine sha!ts to give the correct axial blade clearances.

/uring maintenance, the condition o! the white metal lining is inspected !or wear,scoring and adhesion. To ensure !reedom to tilt, the ribs or steps on the backs o!pads must not be worn or damaged. 'll oil !eed galleries must be clear be!ore re&assembl#.

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COU'LIN*S

The high power transmission associated with steam turbines necessitates the use o!solid couplings, with !itted bolts, between individual sha!ts. 0er# little maintenanceis involved unless the holes su!!er scoring, or concentricit# checks indicate thatcoupling hole re&boring is necessar#. Bushes are o!ten !itted to couplings whichenable badl# scoring or oversi7e holes to be recovered to near normal si7e.

Bolt&hole re!urbishing is carried out with the coupling halves clamped togetherconcentricall#. The hole are bored or drilled to remove an# step between the twohalves. ' honed !inish is used to give a smooth sur!ace. The recesses !or the boltheads and nuts must be !aced perpendicular to the hole to ensure that there is nodistortion o! the bolts as the# are tightened. The nuts must be locked, asrecommended b# the turbine manu!acturer.

5lexible couplings, which are able to accommodate the large axial movementscaused b# turbine expansion, are o!ten used to couple exciters, where powertransmission duties are less onerous.

5ig. 3&= illustrates a claw&t#pe coupling which drives through pads on each claw.These pads need checking periodicall# !or wear or !or ridging o! the sur!ace, as thisma# cause the coupling to resist expansion movement and overload the thrustbearing. Badl# worn pads should be renewed and bedded to the driven sur!aces o!the coupling mu!! or sleeve. This is best accomplished b# means o! a special "igthat holds the claw and sleeve concentricA each pad is bedded in turn. The padsare ad"usted so that the# all contact the sleeve sur!aces in this concentric condition.

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S(A+T ALI*NMENT

Large, modern turbine generators consists o! up to six solidl# coupled rotors thatmust be aligned such that the shear !orces and bending moments at the couplingsare 7ero. Each sha!t exhibits elastic de!ormation due to its own weight and thrustthe per!ectl# aligned s#stem !orms a curve, the sha!t catenar#. 5ig. 3&> illustrates asha!t catenar# !or a @@; % six rotor machine. The rotors retain their naturalde!lections at all speeds except when passing through a sha!t critical. To ensurethe smooth running o! a machine, it is !irst necessar# to build the sha!t s#stem to thestatic de!lection curve and later to check the alignment periodicall#. Compensation,b# biasing the heights o! particular bearings, is build into the sha!t alignment to cater!or the e!!ects o! A

Changes o! bearing height due to thermal expansion.

Large di!!erences in "ournal diameter between ad"acent bearings, such as ma#

exist, !or example, between the generator rotor and exciter.

(ha!t whirl associated with lightl#&loaded bearings.

'n initial approximate alignment o! bearing pedestals and c#linders has traditionall#been obtained during erection b# means o! a taut piano wire stretched along the

axis o! the machine. The various pedestals are positioned with respect to this wire,their height being ad"usted in line with the expected catenar#. odern methodsemplo# optical s#stems, using either precision telescopes or lasers.

5inal alignment o! the rotors is obtained b# ad"usting the bearings to give parallelismand concentricit# at each pair o! couplings. This procedure is !ollowed at times o!maintenance when previous condition monitoring observations, or changes o! rotorsor bearings, have dictated that sha!t alignment should be checked. The alignmentis carried out b# measuring the !ace gaps and peripher# errors between couplings.4ap measurements, using suitable gauges, are taken at the top, bottom and twoside positions and a repeat set taken with both sha!ts rotated through 3;D. Theaverage o! the two sets indicates the true parallelism o! the coupling !aces,

eliminating an# errors due to out o! truth between coupling and sha!t.

Concentricit# is checked b# measuring between the peripheries o! the twocouplings, using either dial indicators or !eeler gauges, to a !inger attached to one o!the couplings. To eliminate coupling errors, both sha!ts are turned together andreadings taken at the top, bottom and two side positions. Calculations, usingra?tios and similar triangles, taking account o! sha!t length, distance betweenbearings and coupling diameter, determine the ad"ustments re$uired at the bearingsto obtain sha!t alignment. %ith solidl# coupled sha!ts, it is usual to work to ver#

close tolerances o! the order o! ;.;:? mm.

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+i,% 1-7% T.i$al shaft $atenar for a 88& M2 t#r!ine ,enerator6 shoin, the 9erti$alhei,hts a!o9e dat#" of ea$h !earin,%



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'lthough it is possible to calculate the ad"ustment needed !or both gap andperipher# errors at the same time, the correction o! gap errors can re$uiresubstantial bearing movement and so it is o!ten advantageous to correct them !irst.t should also be remembered that such corrections will a!!ect blade and glandclearances and consideration should be given to the removal o! c#linders to checkthese.

%here a ma"or deviation !rom the catenar# is suspected, the vertical alignment o!the sha!ts can be checked in situ, using a laser and suitabl# designed sightingtargets mounted on the sha!t "ournals. (uch a s#stem has been developed andused success!ull# b# the CE4B.

COU'LIN* CONCENTRICIT)

%hen two turbine sha!ts are coupled together, it is important to ensure that the# arevirtuall# concentric with one another, i.e. no misalignment between the axes o! thetwo sha!ts, and that the# remain so during subse$uent operation.

5ailure to achieve good concentricit# ma# result in unacceptable bearing vibrationset up b# an out o! balance due to the two sha!t masses being on di!!erent axes.5itted bolts are used at couplings to maintain alignment and to transmit the hightor$ue loading.

'n# error in pitch circle diameters between couplings will adversel# a!!ectconcentricit#. The !ollowing procedure is adopted to ensure concentricit# to within;.;: mm at ad"acent bearing "ournals )Re!er to 5ig. 3&?-.

+i,% 1-:% Con$entri$it "eas#re"ent !eteen ad;a$ent shafts%

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The coupling halves are bolted together. ! the concentricit# is suspected or knownto be in error then onl# a !ew undersi7e bolts are used. This enables the couplingsto be moved relative to one another !or correction purposes.

2ne bearing bottom hal! is removed and dial indicators rigidl# mounted at both"ournals.

The sha!ts are rotated and the indicators are read at >?D intervals. The indicator atthe unsupported "ournal responds to an# misalignment o! the sha!t axes. Theindicator at the supported "ournal is purel# !or control purposes, recording an#movement due to irregularities.or ovalit# o! the sha!t. The di!!erence between theindicators represents the amount o! eccentricit# present.

t must be recogni7ed that an# slight ovalit# or de!ormation o! the "ournal sur!aceswill a!!ect the readings when such small de!lections are being observed. +lottingthe net de!lection versus angular position can aid anal#sis o! the results.Theoreticall#, the plots should !orm a smooth c#clic curve that reduces to a straightline !or per!ect concentricit#. B# matching the best curve to the measuredde!lections, an# errors due to de!ormation can be eliminated and the eccentricit#determined.

(mall errors ma# be corrected within the tolerances o! the !itted bolts6 larger errorswill re$uire the use o! undersi7e bolts to obtain concentricit#, and then the boring o!the coupling holes to !it new bolts.

LUBRICATION S)STEM

aintenance o! lubricating oil s#stem components consists o! stripping the variousregulating valves, oil pumps and their drive arrangements !or inspection. There areno speci!ic re$uirements other than to ensure that all items are in good order,replacing an# worn parts as necessar#. 2il coolers and !ilters must be cleaned andprocedures established !or !lushing the oil s#stem on completion o! maintenance

work to remove an# contamination o! the pipe work, etc. This best accomplishedb# installing temporar# pipe&work at the pedestals to b#pass the bearings and thuspromote !lows greater than those under normal operation. The oil !ilter should becleaned again a!ter !lushing.

TURBINE CASIN*S AND SU''ORT ARRAN*EMENTS

2perational !lexibilit# o! a turbine is largel# dependent on accommodating the largethermal expansions that take place whilst, at the same time, maintaining thealignment and clearances o! the rotating parts. This necessitates regularmaintenance o! turbine casings and their associated support arrangements.

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TURBINE CASIN*S

The opportunit# to inspect and carr# out work on turbine casings usuall# arises !romthe need to do other work, !or example, blade or high temperature bolt inspections.aintenance consists mainl# o! renewal o! de!ective thermocouples, checking !ordistortion, and inspection, including non&destructive testing )1/T- o! the structure.

Large modern turbines emplo# double&c#linder arrangements at the *+ and +stages to reduce the operating stresses and conse$uentl# the si7e o! the c#linder!langes. This means that components, such as bolts, ke#s and thermocouples,operating at the highest temperature conditions are not easil# accessible, withoutresorting to an expensive strip&down o! the outer c#linder. (o, when the opportunit#exists, a through examination o! all components is carried out. This includes 1/T o!the casings themselves, particularl# at changes o! section, to examine !or thermal!atigue or creep cracking6 inspection o! inner c#linder support and retainingarrangements is also undertaken. The double&c#linder con!iguration necessitatesspecial steam inlet connections incorporating a piston ring "oint at the inner c#linder6the condition o! these piston rings must also be checked.

'!ter several #ears o! operation, c#linder distortion ma# arise, resulting in di!!icultiesin making leak !ree "oints and in obtaining glandblading clearances, due to ovalit#.The degree o! distortion is checked b# bolting the c#linder halves together with thespindle removed and checking the bore with internal micrometers. Usuall# thema"orit# o! the distortion is removed b# the bolting operation and the boremeasurements will indicate the allowances that need to be made during the rebuildto cater !or an# small amount o! residual distortion. n more extreme cases,machining o! the "oint !aces andor bores is necessar# to remove the distortione!!ects.

'ttachment o! components to casing materials must be carried out to themanu!acturer9s approved procedures. n particular, the welding o! insulationretaining pins and the !ixing o! thermocouples to high temperature casings needs

care!ul attention to avoid stress raising.

Li!ting attachments are o!ten le!t on c#linders during operation and there!ore re$uirecare!ul examination be!ore use.

L.+. turbines ma# be either single or double casing designs, with the !ixeddiaphragms located into a cast c#linder section6 exhaust sections and outerc#linders are o! welded and bolted construction. nspections o! the welded casing,ba!!les and supports should be made at times o! overhaul. Bled steam connectionso!ten incorporate expansion bellows within the condenser steam space and thesealso re$uire periodic inspection !or damage.

The "ointing o! turbine c#linders re$uires special care i! problems o! steam leakageand air ingress are to be avoided during operation. oint !aces need to be scrapedclean, with particular attention being paid to areas around studs where old "ointingmaterial ma# accumulate.

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TURBINE SU''ORT ARRAN*EMENTS

(upport arrangements associated with turbine c#linders and pedestals areimportant in maintaining alignment o! the sha!t s#stem during expansion andcontraction. 'n# mal!unction ma# lead to crabbing o! the pedestals with aconse$uential e!!ect on sha!t eccentricit#. This, in turn, ma# result in axial and radial!ouling o! the !ixed and moving components. 5ig. 3&@ illustrates t#pical turbinesupport arrangements which compriseA

+edestal guide ke#s to maintain bearing and sha!t alignment.

Bearing plates on which pedestals slide.

C#linder centerline guide ke#s to maintain c#linder alignment.

C#linder palm transverse ke#s to allow transverse expansion and to transmit axial

push&pull !orces during expansion and contraction.

C#linder palm bearing plates.

The !irst two items accommodate the largest movements and are lubricated withhigh temperature grease, usuall# !rom an automatic greasing !acilit#. The higheroperating temperatures o! the others make lubrication di!!icult to achieve but,because o! the smaller movements, this is not usuall# necessar#. 'll ke#s and

bearing plates re$uire examination !or excessive wear and periodic cleaning toremove corrosion products. The !re$uenc# o! these inspections depends on thet#pe o! operation o! the machine and maintenance experience. 5or instance,turbine undergoing two shi!t operation, or sub"ected to high pipe work loading,re$uire more !re$uent inspections. (ome o! the ke#s ma# onl# be accessible withthe c#linder thermal insulation removed and so opportunit# should be taken toexamine these items at least at ever# ma"or overhaul. %hen removing ke#s !orexamination it is important to anchor the components concerned securel#, using theturbine manu!acturer9s approved methods. 5ailure to do this ma# result in pipework !orces a causing movement o! c#linders, making it di!!icult to re!it the ke#sinvolved.

C#linder palm transverse ke#s need to be examined !or both excessive clearanceand tightness. The push pull action on the ke#s can result in apparentl# slight wearon the !lanks being accentuated at the corners, allowing the ke#s to roll as thec#linder expands and contracts. The lost motion caused b# rolling ke#s is o!tengreater than that due to uni!orm excessive clearance and both t#pes o! de!ect mustbe corrected i! reduction o! axial clearances )leading to rubbing- is to be avoided.The un&lubricated ke#s ma# also experience sei7ure during transmission o! axialmovement. This can cause c#linder distortion and abnormall# high locating o! thec#linder center guide ke#s, leading ultimatel# to radial rubbing. This the palm ke#smust be cleaned and checked !or !reedom o! movement with the minimum o!ke#wa# clearance, t#picall# ;.;< mm.

The c#linder&palm bearing plates are usuall# o! a bron7e material support the steelc#linder palm. The movements and loading on these plates are generall# small andmaintenance usuall# onl# consists o! removal, cleaning and re!itting. 'n# hardscoring is dressed out and the cause ascertained )this ma# be due to corrosionproducts on the steel palm-.

C#linder centerline guide ke#s are also onl# sub"ect to small movements and sowear should be light unless other !actors, such as pipe loading or sei7ed transverseke#s, are involved. These items o!ten onl# re$uire a !eeler clearance check.

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+edestal guide ke#s and bearing plates experience movement e$uivalent to the !ullc#linder expansion6 as their operation is essential in keeping sha!t alignment, the#re$uire special attention at times o! maintenance. Each ke# is remove din turn, withthe pedestal anchored to prevent movement. The ke# and ke#wa# are cleaned toremove old grease and each si7ed to obtain clearances. Fe#s having excessiveclearances can lead to crabbing o! the pedestals with conse$uential operationaldi!!iculties. %hen renewing ke#s, consideration should be given to the likel# e!!ecton alignment. ! it is not apparent on which side the wear has taken place, in theabsence o! initial si7e data, a replacement ke# could hold the pedestal out o!alignment. ! necessar#, there!ore, alignment checks should be carried out whenrenewing ke#s, the grease wa#s associated with the ke# cleaned and the ke#re!itted with a new charge o grease. 2nce all the ke#s have been inspected, thepedestals can be "acked to release the bearing plates. These again should bethoroughl# cleaned o! old grease and inspected !or wear be!ore being re!itted.

TURBINE BLADIN* AND *LANDS

Turbine c#linders are opened at times o! ma"or overhaul to allow maintenanceinspections o! the blading and glands. The need !or blade examinations ma# alsobe indicated b# per!ormance monitoring6 when blade damage is suspected, anen!orced outage is necessar# !or investigation and repair. /uring minor overhauls,it is usual to carr# out partial inspections o! the easil# accessible areas, such as theglands land the last row o! L+ blades, the latter to monitor !or the e!!ects o! erosion.The se$uence o! operations involve din a ma"or strip&down is as !ollowsA

3. The c#linders are opened and the 8as !ound9 blade and gland clearancesrecorded.

:. oving and !ixed blades are inspected !or mechanical damage, erosion,corrosion and chemical deposits.

=. The components are cleaned and sub"ected to non destructive testing.>. Repairs and modi!ications are carried out, including the re!urbishment o!

sealing !ins.

?. Radial and axial clearances are ad"usted.@. The turbine rotorc#linder is reassembled, recording the !inal clearance data.

'n# ma"or realignment o! the sha!t s#stem should take place prior to step ?6 minoralignment corrections ma# be carried out with the c#linder assembled.

MEASUREMENT O+ CLEARANCES

Clearances are measured at strip down to evaluate an# remedial work that ma# benecessar# and, on re&assembl#, to provide a record !or $ualit# control purposes and!or !uture re!erence. 'xial clearances are measured at the two hal! "oint positions6radial clearances are taken at the sides, top and bottom positions, although the

latter ma# be omitted i! there is no other reason to remove the turbine spindle. (ideclearances are measured using !eeler gauges. Top and bottom radial clearancesare obtained b# lowering the c#linder or spindle onto appropriatel# si7ed lead wireand measuring the compressed material with suitable dial calipers.

'll clearances should be related to the turbine normal running position, !or example,when checking axial clearances, the sha!t s#stem should be located against thethrust pads. Because o! the support arrangements, some designs o! c#linder ma#necessitate the bottom halves to be raised to obtain running radial clearances6in!ormation concerning this will be contained in the manu!acturer9s maintenanceprocedures.

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+i,% 1-8% T.i$al t#r!ine s#..ort arran,e"ents%



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INS'ECTION6 CLEANIN* AND NDT

nitiall# inspections are carried out at the time o! strip down, when a generalimpression o! the condition o! the blading components becomes apparent. oredetailed visual examinations are undertaken o! the critical areas, in particular, bladeroots and associated securing arrangements, blade support s#stems )such as lacingwire and shrouding- and inter&stage sealing arrangements. The condition o! L+blade and erosion shields is also examined and the securit# o! diaphragm retainingand anti&rotation arrangements are checked. The presence o! blade deposits has adetrimental e!!ect on turbine e!!icienc#, due to a reduction in area or steam !low anda deterioration o! sur!ace !inish and pro!ile. The improved boiler water $ualit#associated with modern generating units has greatl# reduced the problem o! blade!ouling and thus the need !or routine cleaning. *owever, at times o! ma"or overhaulor inspection, it ma# be necessar# to remove an# such deposits. (olubleaccumulation is easil# removed b# water washing, but the more !irml# adheringinsoluble material, mainl# composed o! silica and iron oxide, re$uires the use o!mechanical methods !or its removal.

nsoluble deposits are removed b# a blasting process which, !or turbine rotors andremovable diaphragms, is carried out remote !rom the turbine in a !ull# enclosedtented area. 5ixed blading o! bottom hal! c#linders is usuall# cleaned in situ !orconvenience, in which case special precautions are necessar# to prevent theingress o! deposits and abrasives into bled&steam branches, etc. The blastingmedium used depends on the severit# o! deposits and the sur!ace !inish re$uired.'lumina is commonl# used, being both an e!!ective abrasive and also providing asur!ace !inish suitable !or 1/T.

/etailed examination o! blading is carried out to monitor !or de!ects, particularl#cracking. + and d#e&+enetrant 1/T techni$ues are used at areas o! concern6 thecritical areas are those that are most highl# stressed and where construction "ointsare located. Thus L+ blading )including blade roots, lacing wire holes, !errules andbra7ing, erosion shields and blade shroud riveting- are items that receive attention.

2bviousl#, experience o! similar machines or previous !ailures will in!luence theinspection polic# o! the maintenance engineer.

RE'AIRS AND RE+URBIS(MENT

%ork on blading o!ten necessitates the use o! specialists !or the diagnosis o!de!ects and method o! repair, because o! the skill and expertise re$uired to carr#out the work. 1 particular, since an# de!ect on moving blading can havecatastrophic conse$uences, expert advice should alwa#s be sought. (mall sur!acede!ects, such as cracks or impact damage, can sometimes be merel# dressed outb# care!ul grinding. ore serious de!ects ma# re$uire the renewal o! bladecomponents.

This work, including lacing wire and shrouding repairs which ma# entail removal o!blades, needs the services o! appropriatel# trained personnel who processes theskills and e$uipment re$uired to under take the tasks. Renewal o! erosion shieldsalso necessitates special techni$ues to ensure secure bonding o! the shield b# !ullpenetration o! the bra7e. The method !or re&establishing blade clearance dependson the condition o! the sealing strips and the extent o! ad"ustment necessar#. ! thestrip is undamaged ad clearances are re$uired to be increased, then eithermachining o! the seal areas on the spindle or machining o! the !ixed strip, using aboring bar arrangement, will su!!ice.

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%here clearances are expressive or the strip has been severel# damaged, thespecialist operation o! !itting new strip is the onl# option. '!ter ad"ustment o!clearances, all sealing !ins are dressed to give a kni!e edge. This ensures thatshould a rub occur, the edges o! the sealing strip will de!orm with the minimumproduction o! !riction heat.

5inal setting o! axial clearances is carried out with the c#linders assembled. Thesha!t is !loated in the bearings )with the thrust bearing removed- until the !ixed andmoving blades contact. The thickness o! the thrust bearing liners or couplingspacers is then ad"usted to give the designed o!!set !rom this contact position.

TURBINE *LANDS

aintenance associated with the modern turbine spring back lab#rinth gland issimilar to that alread# described !or blade sealing strips.

The main di!!erence arises due tot he ring o! sealing strips being made up o! anumber o! segments mounted in an ad"ustable carrier ring, and held in closeproximit# to the sha!t b# springs (ee 5ig. 3&@. (hould the sha!t contact an# o! thesegments, the springs allow movement, limiting an# !riction heating e!!ect on thesha!t.

4lands are dismantled, cleaned and inspected checking !or damage to the segment!ins and springs. The segments must be wedged to prevent movement that wouldotherwise be allowed b# the springs. The use o! small "acking screws through thesegment against the carrier ring is a convenient and positive method o! obtainingthis condition.



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+i,% 1-8% T.i$al t#r!ine ,land%

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/amaged or excessivel# worn segments must be renewed. Those shoring onl#slight wear can be re!urbished !or re&use b# machining their backs to allow them tospring into a smaller diameter. (ubse$uent operations are common to both newand re!urbished components. The ends are machined to maintain the designclearance between ad"acent segments. The whole ring is then assembled in thecarrier, each segment is wedged or "acked to its normal running position and theassembl# is machined at the sealing !ins to the re$uired sha!t diameter, plus theclearance allowance. This !inal machining ma# need to allow !or centrali7ation o!the gland to the sha!t, although ad"ustment is o!ten possible b# means o! shimmedpads in a similar wa# to "ournal bearings.

+rior to !inal assembl#, the segment !ins are dressed to give a kni!e edge and"acking holes are plugged. Each segment o! the assembled gland is checked !or!reedom o! movement. %hen the gland ring is !itted to the sha!t, the axial positionmust be checked. Errors in this position ma# lead to damage when di!!erentialexpansion takes place. Liners are provided to enable the axial position to bead"usted.

TURBINE *OVERNIN* S)STEMS

odern turbine generators are o!ten e$uipped with electronic governing s#stems inwhich steam admission valves respond to electronic speed or load signals viaelectro&h#draulic rela#s. aintenance o! both mechanical and electronic s#stems isdescribed, with man# o! the procedures being common to both. 1ormall#maintenance is restricted to times o! statutor# unit overhaul, with onl# de!ects orbreakdown situations being dealt with outside these periods. The t#pes o! problemencountered with governing s#stems !all mainl# into two distinct categoriesA a greatdeal o! the s#stem is, because o! its dut#, continuall# moving, leading to problems o!excessive wear6 other parts tend to run at a single position, when excessive !rictionbecomes the main !actor.

Electronic governing s#stems generall# use a phosphate ester !ire resistance !luid

as the h#draulic operating medium. The rapid speed o! response to match that o!the electronic governor itsel! is achieved b# using the !luid at pressures o! up to 3?;bar. The s#stems are there!ore high pressure and low capacit#6 ver# tightclearances are necessar# at the various components to reduce leakage and wear.(#stem response and capacit# is increased b# the use o! h#draulic accumulators.The !luid is maintained to a ver# high $ualit# b# means o! conditioning plant,incorporating !ilters and vacuum chambers !or the removal o! moisture, particulatematter and gums. +hosphate ester !luid has the advantage o! a higher temperaturecapabilit# than lubricating oil, but must be handled with care due to its toxicit#.

STEAM ADMISSION VALVES

The maintenance turbine valvegear is most convenientl# considered in two parts6the steam side and the operating rela# e$uipment

'lthough detailed designs var# between turbine manu!acturers, man# o! themaintenance aspects are similar. 5ig. 3& a t#pical governor valve and operatingrela# o! a mechanical s#stem. 5ig. 3&)a- illustrates a throttle valve o! an electronics#stems, and 5ig. &3 )b- its electro&h#draulic rela#.

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't times o! ma"or overhaul, it is usual to dismantle the steam valves !or inspection.0alve cover nuts are removed, using procedures described in (ection


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+i,% 1-


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+i,% 1-=0a% *o9ernor 9al9e and o.eratin, ,ear for an ele$troni$ ,o9ernin,sste"%

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+i,% 1-=0!% O.eratin, for an ele$troni$ ,o9ernin, sste"%

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! the turbine blading is to be protected !rom damage, cleanliness duringmaintenance o! valve gear is o! prime importance. %ooden blanks )or similar- mustbe available and strictl# used to cover apertures where debris could enter theinterconnecting pipe work during operations, such as cleaning "oint !aces, etc. 2nre&assembl#, the valve internals must be checked as clear o! all !oreign material.

%here linkages are connected to the valve spindle b# a crosshead arrangement,the assembl# must be securel# !ixed to prevent !atigue !ailure o! the spindle.%here the spindle incorporates a taper !itting to the crosshead, this should belapped to ensure !irm contact.

The h#draulic operating rela#s and valve power pistons associated with mechanicalgoverning s#stems tend to need less maintenance than the steam valves, especiall#i! the operating oil is kept clean and !ree !rom moisture. *owever, it is usual todismantle them at ma"or overhauls and to inspect !or wear at pistons and bushesthat could eventuall# a!!ect their operation. ost designs o! power pistonincorporate strong springs re$uiring special care during dismantling to ensure thatthe stored energ# is released in a controlled manner. %ith tight clearances betweenprecision components, cleanliness is again an important !actor. (prings andspindles are checked !or corrosion, which ma# occur i! moisture control o! the oil ispoor.

The pins and bushes associated with operating linkages o!ten re$uire considerablere!urbishment due to wear resulting !rom the transmission o! large operating !orces.(ome designs use +T5E lubricated bushes, whilst others use grease lubrication!rom the turbine automatic greasing !acilit#.

The servo valves and operating rela#s associated with electronic governing s#stemsoperate with a high $ualit# h#draulic !luid and are there!ore less likel# to re$uireregular maintenance. The high precision components are also ver# susceptible todamage and when maintenance is re$uired it is usual to interchange completemodules. There!ore, maintenance re$uirements are determined b# thorough pre&

outage !unction checking o! the s#stem, so that an# components identi!ied asde!ective or suspect can be exchanged. +iston spools and sleeves are changed asa complete assembl# to prevent damage to sur!aces. (ervo valves are removed tocheck !or internal leakage and, i! this is !ound to be excessive, the# would beexchanged. The re!urbishment o! linkage pins and bushes is similar to thatdescribed above !or mechanical governor s#stems.

Routine maintenance o! the valve servo s#stems is restricted to !ilter changes andaccumulator checks. Each steam valve rela# has its own !ilter, t#picall# = um, whichshould be changed at each ma"or overhaul. 'ccumulator pressure are checkedperiodicall# and their bags should be changed at a !re$uenc# advised b# themanu!acturer, t#picall# ever# > G @ #ears.

/e!ective servo and rela# valve components are usuall# returned to themanu!acturer !or re!urbishment. (trict clean condition workshop !acilities arere$uired with the application o! special working practices. +olished and mirror!inishes are emplo#ed on components. These are ver# susceptible to bothmechanical damage and corrosion6 the latter ma# be initiated b# corrosive !luids oreven perspiration. +arts need to be scrupulousl# cleaned, using an approvedsolvent and non&linting cloth, then stored, coated with oil. *#draulic !luid must beused on components during re&assembl#.

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*OVERNORS

This section is restricted to the work associated with mechanical governors6 themaintenance and calibration o! the electronic components !ollows the philosoph#described in (ection 3: o! this chapter.

' t#pical bob&weight governor is illustrated in 5ig. 3&3;. The centri!ugal !orce o! thespinning weights is balanced b# the springs such that the weight position isproportional to the rotational speed. The lever s#stem translates the weight positionto an oil rela# plunger which in turn produces an oil pressure proportional to speed.The relationship is a !unction o! spring rates and rela# design and is so arranged togive the re$uired governing characteristic )droop-. 2verhaul o! governors mustensure that their relationship are maintained whilst eliminating an# h#steresis e!!ectdue to !riction or backlash. t is o!ten pre!erable to return governors to themanu!acturers !or ma"or re!urbishment, where the necessar# special e$uipment andcalibration rigs are available.

%ear o! the linkage arrangements and oil rela# sleeves are the areas usuall#re$uiring attention. Linkage bushes are usuall# +T5E impregnated, allowing ver#tight clearances. These ma# be renewed, provided the necessar# tools areavailable to allow controlled dismantling o! the spring assembl#. (pecial care isneeded when pressing in the new bushes so as not to damage the bearing sur!ace.2il rela# clearances are checked and an# worn components renewed. %henreassembling the governor to the drive sha!t, checks are carried out to ensure thatthe assembl# rotates concentricall#.

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*ENERAL

There are man# other components re$uiring inspection during ma"or overhaul.These include boiler pressure and vacuum de&loading e$uipment and various otherancillar# oil rela#s. The maintenance associated with these is similar to thatpreviousl# described !or valve gear rela#s.

The tripping s#stem arrangements must be inspected to ensure reliable operation.The impact loads that occur when these s#stems operate can cause bruisting o!latching sur!aces and excessive wear at ke#wa#. ! such damage is allowed to gounchecked, the e!!icienc# o! operation ma# be seriousl# impaired.

2n completion o! maintenance and be!ore return to service, the manu!acturer9srecommended procedures must be !ollowed to set the e$uipment !or correctoperation. 0alve linkage settings must be ad"usted and the valves operated tocheck that the opening characteristics are correct. ' check o! valve position againstcontrol oil pressure in both the opening and closing directions will indicate whetherexcessive !riction is present.

TURBINE (I*( TEM'ERATURE BOLTS

Bolts and studs used on turbine c#linder and valve steam chest "oint, which aresub"ect to high temperatures )H=


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aintenance aspects o! high temperature bolts are predominantl# concerned withA

Routine tightening at recommended intervals.

Renewal o! li!e expired !asteners.

nspection and 1/T o! !asteners sub"ected to abnormal circumstances or when

approaching the limit o! their basic li!e.

aintaining records o! operating hours, ad"usted to take account o! re&tightening

and temperature gradients.

5astener li!e depends upon the controlled application o! the initial strain at eachtightening operation. 5or Cro0, this initial strain is usuall# in the range ;.3:J to;.3@?J6 !or nickel&based allo#s, ;.;J to ;.33J. the percentage strain is thatexisting in the shank o! the !astener between the !irst engaged threads )thee!!ective length-. Controlled application o! strain ideall# means direct measuremento! the applied extension or application o! a measured h#draulic load !rom which thestrain can be derived. %here these methods are not possible, the strain ma# bederived either b# measurement o! the angle o! rotation o! the nut or b# tor$uetightening o! the !asteners whilst using an appropriate thread lubricant. /irectmeasurement o! the extension o! large bolts and studs necessitates the use o!either ultrasonic instruments or special measuring e$uipment6 the latter re$uired a

hole through the length o! the !astener. 5ig. 3&33 illustrates such an arrangement. tconsists o! a tube with a s#stem o! collates at its end which engages the bottom o!the stud )or stud&bolt- hole. ' sleeve locates at the top o! the stud and the changein the relative position o! sleeve and tube is a measure o! the stud extension.

The actual tightening operation must be achieved without impacting the !astenerand one o! three methods is usuall# emplo#edA

4radual tightening, using a h#draulic tor$ue wrench.

Rotation o! the nut, !ollowing extension b# heat.

Rotation o! the nut, !ollowing extension b# h#draulic stretching.

+rior to tightening, the "oint !aces are closed and an# "ointing material crushed b#tightening the nuts su!!icientl# to give metal to metal contact. ! direct measuremento! extension is being used, the initial lengths o! the !asteners in their unstressedcondition will have been measured also. Extension b# heat and h#draulic stretchingre$uires the use o! speciall# adapted studsbolts. 5or heat, an electric heatingelement is inserted into a hole running through the length o! the stud. %hen theapplied heat has caused su!!icient elongation, the nut is rotated through an anglecalculated to give the re$uired strain. This can be checked b# measurement whenthe stud has cooled. *#draulic stretching uses studs with a lengthened threadedportion on to which is crewed the stretching e$uipment. *#draulic pressure is usedto stretch the stud and the nut is then tightened. The h#draulic pressure is

proportional to the stress in the stud and so the s#stem can be calibrated to tension!asteners accuratel#. 5ig. 3&3: illustrates an arrangement !or h#draulic stretching.%hichever method is used, nuts must be tightened in accordance with themanu!acturer9s recommended se$uence. This will involve slackening each nut inturn !rom is tor$ued condition and then appl#ing the controlled tightening procedure.The $ualit# management o! !asteners in a power station demands the maintenanceo! ade$uate records, so that the integrit# o! operating "oints can be con!irmed andreplacement o! !asteners can be arranged at the appropriate time.

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Each "oint involving high temperature bolts should be identi!ied in the recordstogether with data on the t#pe, material, si7e, design, strain and operatingtemperature o! the !asteners. The records are up&stated during the li!e o! the "oint,incorporating the appropriating li!e penalties associated with retightening andorexcessive temperature gradients. (uch data can to used to initiate 1/T o!!asteners when the# reach ;J o! their basic material li!e and planned renewalwhen approaching their li!e expir#. n the event o! discover# o! cracked orcompletel# !ailed !asteners in a particular "oint, all !asteners should be renewed andthe old ones examined metallurgicall# to determine the cause o! !ailure. +remature!ailure ma# be associated with stress corrosion or thermal gradients which wouldre$uire additional measures being taken to avoid a recurrence o! the !ailure. Undersuch circumstances, consideration must be given to other similar "oints that ma# begiven to other similar "oints that ma# be a!!ected.

+i,% 1-11% E5tenso"eter for "eas#rin, !olt e5tension% The le9er arran,e"entis #sed to en,a,e the $ollets in the !otto" of the st#d hole %

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+i,% 1-1/% Bolt e5tension tool (dra#li$ .ress#re at the load $ell is #sed tostret$h the !olt% The n#t is then r#n don to "ate ith the s#rfa$e%

TURBINE T(ERMAL INSULATION

Thermal insulation o! turbine components is o! the utmost importance in reducing

temperature gradients across thick&walled !orging and along !asteners. (uchgradients can cause severe distortion, leading to operational di!!iculties and anincreased likelihood o! !ailures due to thermal !atigue. nsulation on turbinec#linders must also minimi7e the di!!erences in metal temperature between top andbottom halves at start&up and shutdown.

MATERIALS

Two !orms o! man&made mineral !iber )5- are commonl# used in the insulationo! turbine components, either as a spra# material or in mattress !orm. %hichever isused, it is important that the insulation is applied to approved procedures b# skilledapplicators to ensure a high $ualit# installation.

(pra#ed 5 consists o! chopped rock&based !iber pre&mixed with cementationbinders. t is applied with speciali7ed spra# e$uipment whereb# the !iberbinder ismixed with water at the spra# no77le. %hen properl# applied, it !orms a continuousla#er o! small !ibrous bundles bonded together with no "oints or voids. /egradationo! spra# material occurs with age and is accelerated b# vibration and steamwaterleakage. The external coating can sometimes hide the poor condition o! the spra#material beneath and the taking o! core samples to check material condition willassist in determining maintenance re$uirements.

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5 mattress consists o! rock&based !iber woven or !ormed into a $uilt andbonded to :? mm stainless&steel wire mesh on one side. This relative so!t and!lexible arrangement is ideal !or insulating complex shapes although it has thedisadvantage o! compressing easil# and o! slipping, i! not ade$uatel# supported.oints are inevitable and special techni$ues are needed to ensure the absence o!voids.

A''LICATION

nsulation material is supported b# means o! steel studs attached to the valve chestor c#linder, usuall# b# welding. The studs are ? G@ mm diameter, with a maximumpitch when measured between stud tips o! =;; mm. Their length is such that theperpendicular distance !rom the tip to the sur!ace e$uals the re$uired thickness o!insulation. (pecial arrangements are re$uired at !lange bolts to ensure the absenceo! voids and to maintain the insulation thickness, particularl# when mattresses areused. 1utsstuds are wrapped with glass cloth and stainless steel wire is loopedaround them to be used later to pull the outer insulation mesh into contact with theinsulation. 4aps between nuts are !illed with compressed 5 mattress so thatvoids do not occur when mattresses are applied. The applied thickness isdependent upon the highest casing metal temperature and is usuall# calculated togive an outer cold !ace temperature o! ?;DC !or an ambient temperature o! =;DC.5igure 3&3= shows a t#pical relationship between insulation thickness and a hot&!acetemperature. The insulation thickness is built up in one, two or three la#ers, eachretained b# stainless steel wire mesh. Bottom&hal! c#linders have their insulationtemperature di!!erences between top and bottom halves.

(pra# material should onl# be applied b# speciall# trained personnel, using materialthat has been stored in a dr# location in sealed bags. The material should not bemore than 3 #ear old. deall#, the process should be continuous6 special attention isre$uired to ensure good adhesion o! the material to the casing and the achievemento! the speci!ied densit#. This ma# be checked b# using hand pressure to seewhether there is an# movement o! the material6 the resilience will give an indication

!or the densit#. /ensit# can also be checked b# taking core samples. The !inalthickness is checked using a suitable probe.

attress is applied with the wire mesh !acing outwards. The insulation must be inintimate contact with the sur!ace and radiation paths prevented b# ensuring that all"oints in the lower la#er are covered b# mattress in the subse$uent la#er, the overlapbeing ideall# at least !our times the insulation thickness.

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+i,% 1-13% Relationshi. !eteen ins#lation thi$@ness and hot fa$e te".erat#re%

't the ends o! bottom hal! c#linders, the thermal insulation is tapered to !inish !lushwith the gland housing to avoid the possibilit# o! !orming packets which could trapoil. Casing location ke#s are kept !ree o! insulation.

The insulation sur!ace is coated with a la#er o! sel!&setting cement, @& mm thick,vent holes being provided to aid dr#ing out o! the material. %hen thoroughl# dr#,the insulation s#stem is !inished with an oil and water resistant glass rein!orced

sealant.

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INS'ECTION AND UALIT) CONTROL

's with all turbine work, $ualit# assurance is o! paramount importance i! thereliabilit# o! plant is to be assured. Checks are re$uired at ever# stage o! theinsulation process to ensure that approved procedures and materials are beingused. The check list shown in 5ig. 3&3> is an example o! documentation re$uired toinsure this.

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+i,% 1-17% Che$@list for ins#lation .ro$ess%

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+i,% 1-1:% T#r!ine 'lant Nor"al O.eration%

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+i,% 1-18% (ollo (' and I' rotors%

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+i,% 1-1% Bearin, S.an 73


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+i,% 1-1


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+i,% 1-1=% Stea" T#r!ine Se$tional Drain,%

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+i,% 1-/&% Sin,le A#to"ati$ E5tra$tion Condensin, Stea" T#r!ine%

EC*'1C'L '1TE1'1CE '/0'1CE/ C2UR(ELE((21 3 +'4E >: (TE' TURB1E 20ER*'UL


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+i,% 1-/10a% T.i$al Strai,ht Condensin, t#r!ine ith +a!ri$ated Lo-'ress#reCasin,%

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+i,% 1-/10!% T.i$al Sin,le-E5tra$tion Non-$ondensin, t#r!ine%

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