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LESSON

2TURBINE MAJOR COMPONENTS

TURBINE CASING & ROTORS

LECTURE

SUB - OBJECTIVE

At the end of this lesson the t!inee "ill h!#e ! thoo$%h no"led%e of the T$'ineC!sin%s( !nd Rotos)

*)+ TURBINE MAJOR COMPONENTS

The essential parts of the turbine are the rotor and the cylinder. The moving bladesare attached to the rotor and transmit the mechanical energy, produced by theexpansion of the steam, through the shaft to the generator. The cylinder, in additionto enclosing the turbine, houses the fixed blades and nozzles. A turbine may haveone, two or more cylinders according to its output capacity. The rotors are supportedin journal bearings and located by a thrust bearing; other devices are added to

control and protect the turbine.

2)+ TURBINE C,LINERS

2)* ARRANGEMENT O. C,LINERS

Turbine cylinders are made as simple in shape as possible to reduce to a minimumany distortion due to temperature changes. They are divided into two parts on ahorizontal joint so that the machine can be opened for inspection and overhaul andthe rotor removed without disturbing the alignment of the bearings. Fig. !"!# showsa turbine with the top half casing removed. $uitable spaces or belts, as they arecalled are built into the casings to ta%e the necessary connections for bled steam tothe feed heaters.

&oth top and bottom halves of the casing have flanges with holes drilled in them tota%e fixing bolts. 'here there is insufficient clearance for bolts. $tuds are used.

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.i%) /-2-* Lo" Pess$e C0linde C!sin%

2)2 C,LINER JOINTS

The joint faces of both top and bottom half cylinders are accurately machined to

ma%e a good joint. The pressure on the joint being considerable in the highpressure cylinders. /igh tensile steel bolts are used for the higher pressure casings,and these bolts have a hole drilled down their length to allow for the insertion ofcarbon rods or immersion heaters for electrical heating when tightening down. Thismethod allows proper extension of the bolts or studs to be carried out.

Figure !"!" shows a bolted cylinder joint and an illustration of the type of bolt used.This type of fixing is now generally used on large turbines, although the /( cylindersof some of the latest designs do not have a horizontal joint.

3rilling holes in the flanges tends to reduce their mechanical strength and in certainearlier designs of casing, for very high pressures, the two flanges are held together

by means or clamps bolted radially into the cylinder. The outer faces of the flangesare made wedge!shaped so that the tighter the clamps are pulled the tighter the nipon the joint faces. /oles are drilled in the body of the clamps for electrical heatingwhen tightening. Figure !"!7 shows a cylinder joint clamped in the mannerdescribed together with an illustration of the type of clamp used.

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.i%) /-2-2 Bolted 1P C0linde Joint

.i%) /-2- Cl!34ed 1P C0linde Joint

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)+ TURBINE CASING

The turbine casings are designed and made according to the high temperature andpressure conditions under which it will operate.

3esigned to resist reaction forces on stationary nozzles and blades and thecontraction8expansion caused by high temperature swings.

onstructed of heavy casings to withstand steam bursting forces and internalstresses in metal sections due to expansion or contraction with temperature.

These casings are secured by large bolts through relatively strong and massiveflange sections.

These high pressure and temperature stresses necessitate high 9uality andrelatively expensive steel alloy materials. The metals are carefully heat treated andtested to eliminate or minimize temperature distortions, internal voids or otherdefects.

the front high!pressure end is allowed to move axially to accommodate movementbut must retain axial alignment to maintain clearances between stationary andmoving parts.

2odern high pressure and temperature utilize double shell type construction.

The inner casing is a pressure vessel and houses the stationary nozzle diaphragmsand the stationary blades.

The inner shell is heated on both sides which results in a decrease in distortionwhen heated.

The outer casing is a pressure vessel which surrounds the inner casing andtransmits the expansion forces and houses the rotor bearings.

-either shell is as thic% as would be re9uired of a single casing unit.

Turbine casings are placed together metal to metal.

$plit horizontally at shaft centerline and bolted together for easy access.

Flanges can have a groove machined into the face for injection of a sealingcompound.

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Turbine casings obey the laws of expansion of materials with increasingtemperature. This applies to all portions of the shell, thus leading to the desirabilityof heating all parts of the turbine casing as uniformly as possible to result in similargrowth due to temperature expansion in all adjacent sections.

#. A section of metal exposed to local heating will expand if unrestricted;however, if this section is surrounded by massive areas of relatively coolermetal, the heated area cannot expand and therefore will develop extremelyhigh internal compressive stresses which may exceed the permanent yieldingstrength of the material. 3istortion will result and after some number ofheating and cooling cycles, the wea%ened metal will fail. This results incrac%s to relieve the high internal stresses.

". astings of irregular contour and shape involving webs, cored passages,etc... will react to high localized internal stresses caused by nonuniformheating by attempting to bend or flex at their wea%est point. 5f little flexibilityexists due to the thic%ness of metal sections, the necessary flexing to relievestresses may not be possible and crac%s may result. This would be mostprobable if the casting were subjected to many heating and cooling cycleswhich would be caused by fre9uent startups and shutdowns of the turbine,or, if the turbine were operated improperly resulting in rapid nonuniformheating or cooling.

7. 'hen heated due to steam flow inside the casing, the shell should ideallygrow uniformly in all directions and no significant stresses will be introduced ifthis expansion is unrestricted.

5t is necessary to carefully chec% for obstructions which might prevent freeexpansion, such as piping interference or strain, hardened and compressedinsulation, wooden bloc%s, etc.

The use of slotted sections or cored out sections near massive joints, whileappearing to wea%en the structure, actually results in a much improved casting dueto the added flexibility. 3ifferential expansion can occur within limits withoutoverstressing unyielding sections and causing crac%s.

$hutdowns, startups, changes in throttle temperature and to a smaller degree, loadvariations, cause temperature changes in the principle structural elements of aturbine. Temperature changes may vary from one element to the other dependingupon the nature of the changed condition.

Abrupt changes in operating conditions are to be avoided whenever possiblebecause the longest useful service life and minimum re9uired maintenance will

result.

$ince turbine casings must be connected to other %ey structure normally operatingat much lower temperatures such as governor and bearing housings, connectionsmust be designed and maintained in such a manner that expansion of the hottercasing may occur without restriction in order to prevent distortion or misalignment ofthe turbine and their conse9uences.

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.i%) /-2-6 o$'le Shell 1P C!sin%

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) C,LINER CASING RAINS

))* 7ETNESS O. STEAM

-on reheat cycles have exhaust wetness of the order of #"> but largereheat units have about ?> wetness at the exhaust. The shape of thecylinder allows this water to drain to the condenser but special draininggrooves are arranged in the cylinder to help remove this water moreeffectively. An example of this type of draining arrangement is illustrated inFig. !"!.

.i%) /-2-8 C0linde C!sin% !in

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.i%) /-2-/ o$'le .lo" LP T$'ine

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6)+ IAP1RAGMS

A diaphragm is the stationary part of each stage in the turbine. 5t

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.i%) /-2-; I34$lse No

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.i%) /-2-= B$ilt-U4 i!4h!%3

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8)+ TURBINE ROTORS

Turbine rotors may be of four main types as follows D

#. Forged $teel 3rum +otor ". 3isc +otor 7. $olid Forged +otor :. 'elded +otor

8)* RUM ROTORS

5n the drum type rotor the /( steam inlet end of the rotor is a single steel forging andthe exhaust end shaft and disc another separate forging. After machining, the drumis shrun% on to the exhaust end disc forging and secured by bolts and drivingdowels. 0rooves are machined in the body of the drum to ta%e the necessaryblades.

The drum type rotor is limited in its application because of the excessive stresseswhich would occur if it were made in large sizes. 5t is, therefore, suitable for small

machines or the high pressure cylinder of machines with more than one cylinder.The main advantage of this construction is that there is approximately the samemass of metal in the rotor as in the cylinder casing. Therefore, there isapproximately the same response to changing temperature conditions in both rotorand casing and wor%ing clearances can be %ept to a minimum.

3rum rotors are used to carry the reaction type blading in the high pressure cylindersof the (arsons design of turbine. Fig. !"!# illustrates the construction of the drumtype rotor.

8)2 ISC ROTORS

The disc rotor is made up of a number of separately forged discs or wheels and thehubs of these wheel are shrun% and %eyed on to the central shaft. The outer rims ofthe wheels have suitable grooves machined to allow for fixing the blades.

The shaft is sometimes stepped so that the wheel hubs can be threaded along totheir correct positions. $uitable clearance are left between the hubs to allow forexpansion axially along the line of the shaft.

4nder operating conditions the temperature of the wheels may rise 9uic%er than thatof the shaft and this tends to ma%e the wheel hubs become loose and excessiveoverspeed will have the same effect. onsiderable care is ta%en during constructionof the rotor to ensure that the wheels are shrun% on tight enough to satisfy normal

operation within correct stress values. Figure !"!## illustrates a disc type of rotorwhich is the type used in the ( cylinder of most designs of large turbines.

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.i%) /-2-*+ $3 T04e Roto

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.i%) /-2-** is> T04e Roto

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8) SOLI .ORGE ROTORS

+otors of this type have discs and shaft machined from one solid forging, the wholerotor being one complete piece of metal. This results in a rigid construction andtroubles due to loose wheels of the shrun% on type are eliminated. 0rooves aremachined in the wheel rims to ta%e the necessary blading.

$olid forged rotors are used in the /( and ( cylinders for most designs employingimpulses type blading and for the ( cylinder when reaction type blading is used.Figure !"!#" shows a rotor of the solid forged type.

.i%) /-2-*2 Solid .o%ed Roto

8)6 7ELE ROTORS

'elded rotors are built up from a number of discs and two shaft ends. These arejoined together by welding at the circumstances and because there are no central

holes in the discs the whole structure has considerable strength. $mall holes aredrilled in the discs to allow steam to enter inside the rotor body to give uniformheating when coming on load. 0rooves are machined in the discs to carry theblades and Figure !"!#7 shows this type of rotor construction.

'elded rotors are used in the $wiss &rown &overi designs of turbines in the 5( and( cylinders and are now used in some 2' turbines, replacing large solidforged rotors.

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.i%) /-2-* Roto sho"in% is>s 'efoe !nd !fte 7eldin%

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.i%) /-2-*6 o$'le .lo" L)P) T$'ine 7elded Roto

9)+ TESTING O. ROTORS

0reat care is ta%en during the manufacture of turbine rotors because they must besound and free from internal stresses as possible. 2any tests are carried outincluding an overspeed beyond the normal operation speed range to ensure thatany flaws in the metal are detected before final dispatch of the rotor to site forerection in the turbine.

/)+ TURBINE ROTOR ASSEMBL,

Turbine rotors are the part of the rotating shaft that carries the buc%et wheelsBbladesC and bearing journals.

+otors may be either solid or built!up design.

E A solid design is where the buc%et wheels are machined solidly with the rotor.

E A built!up design is where the buc%et wheel are machined separately andthen shrun% fit and %eyed to the rotor.

E A turbine rating and steam conditions such as pressure and temperature playa major role in what design will be used.

A turbine rotor once rolling, can easily be %ept turning or speeded up by an individual

with light pushes since it is almost perfectly balanced and turns on a film of oil.

)nly a very small 9uantity of steam is necessary to accelerate a turbine rotor to fullspeed or beyond without load.

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5n fact, the steam necessary is so small that it is hardly sufficient to cool the turbinebuc%ets and wheels. Turbines running at no load will, therefore, heat up abnormallyon the low pressure and causing the wheels and buc%ets to run hotter thandesirable.

)n some units with large wheels on the ( stage, this cannot be tolerated asexcessive heat on the wheels causes expansion at the wheel bore which might allowthe wheel to loosen on the turbine shaft.

This also illustrates the large excess of power existing in the turbine if it

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.i%) /-2-*8 1ot( Idle Roto Bo"

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.i%) /-2-*; 1i%h Pess$e Roto

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.i%) /-2-*= Medi$3 Pess$e Roto

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.i%) /-2-2+ Lo" Pess$e Roto

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;)+ TURBINE BLAES

Turbine blades convert the %inetic energy of the steam to rotating mechanicalenergy.

Turbine wheels and buc%ets increase in size and diameter progressing to the lowpressure exhaust end.

The greatest velocity of a moving part on the rotor would be at the tips of the laststage wheel.

2aterials for their construction must be selected for their strength and resistance toerosion and corrosion.

0enerally constructed of a corrosion!resistant alloy for a hard, erosion! andcorrosion!resistant blade.

The blades are made in various lengths to accommodate steam volumes as thesteam expands through the turbine.

The shape of the longer turbine buc%ets sometimes change from the root to the tipof the buc%et, so that the steam passes through the buc%et at the most efficientangle anywhere between the root and the tip of the buc%et considering thedifference in speed.

5nefficient buc%ets, either through improper design or damage, results in poorutilization of the steam to produce rotating power, excessive thrust forces on therotating buc%ets and cause large steam flows for a given output.

The centrifugal forces exerted by the rotating parts are one of the primeconsiderations which the turbine designer must consider.

The ( end of the rotor, re9uiring large buc%ets to pass the increased volume of theexpanding steam, becomes the limiting factor in turbine rotor design due to the highcentrifugal forces exerted on the heavier and large diameter buc%ets.

&uc%ets in excess of approximately =." cm B7 inchesC in length exert such highcentrifugal forces at 7= rpm that no %nown turbine wheel material is suitable tosafety withstand these forces.

entrifugal force on a rotating part is directly proportional to it

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SPEE .ORCE

$tandstill "." %g B lbsC

# rpm ",7#7 %g B,# lbsC

#? rpm ,:?: %g B#=, lbsC

7= rpm 7@,@7 %g B==, lbsC

Btrip setpointC 7@= rpm 7=,"? %g B?, lbsC

B#"> speedC :7" rpm :7,7#? %g B@, lbsC

5t can be seen that the centrifugal force increases so rapidly as the speed increasesthat the turbine should never be allowed to run at overspeed conditions.

5t can be seen that the failure or loss of rotating parts will cause high unbalancedforces and, conse9uently, severe vibration.

The variable centrifugal forces exerted on the turbine rotor as it comes to speed maycause vibration of various forms in a turbine rotor although it is perfectly balancedstaticly or at low speed.

This type of unbalance is called dynamic unbalance and is based on the unit

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'elded or riveted in place by tips on the blade end called tenons.

$hrouding is used to prevent tip lea%age of steam and also to dampen bladevibration.

&lade seals are used principally in reaction stages to retard lea%age across bladetips.

;)* T,PES O. BLAING

There is either an impulse force Bimpulse type bladingC or a combination of impulseand reaction forces Breaction type bladingC acting on the blades due to the steamflow. The longer the blade the greater the bending force at the root or fixing point ofthe blade.

5n addition there is a centrifugal force, caused by the speed at which the blade isrotating, trying to throw the blade outward.

These two forces, the bending force and the throwing out force, are at a maximum in

the largest blade wheel at the ( exhaust end of the turbine. Thus the stresseswhich these forces impose, limit the size of the blades and the diameter of the lastwheel. This limitation is one of the reasons why turbines are designed with doubleflow in the ( cylinder, and in large sizes up to three double flows.

The mechanical stresses just described are smaller in the /( moving blading butthis blading is subject to much higher temperatures and the material is subject toexcessive growth %nown as creep.

The fixed blades are secured to the turbine cylinder and in the impulse type ofblading ta%e the form of nozzles set in diaphragms alternately between each row ofmoving blades.

;)2 REACTION T,PE BLAING

5n reaction type blading a pressure drop occurs across both the fixed and movingblades. 5n the high pressure cylinder a very effective seal between fixed and movingblading is essential to prevent steam lea%age which would ma%e the turbineinefficient.

The fixed blades are fitted in grooves in the cylinder and the moving blades ingrooves machined in the rotor.

For blading subject to high temperature in /( cylinders the blades are made

complete with root section and shrouding in one piece and are formed in groups orpac%ets for convenience of handling. The shrouds have a projecting portion which isthinned down to form a single %nife edge on the moving blades. )n the fixed bladesa second strip is added which is tapered to form a double %nife edge. The bladepac%ets are then fitted in the grooves to form a complete row of either fixed ormoving blades. The blade pac%ets are serrated along the roots and secured in thegrooves, which are also serrated, by means of side loc%ing strips.

;) BLAE SEALING

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An illustration of this type of blading is shown in Fig. !"!"# and it will be seen thatthe lea%age of steam is controlled by the axial clearance, that is, the clearance alongthe line of the shaft. This type of sealing is %nown as end tightening.

An additional seal is provided by a radial fin machined into the shroud and set at areasonably fine clearance between cylinder bore or rotor body.

.i%) /-2-2* Re!>tion T04e Bl!din% sho"in% End Ti%htenin%

;)6 IMPULSE T,PE MOVING BLAES

The /( moving blades for impulse type turbines are machined from solid bar BFigure!"!""C and the roots and spacers formed with the blade. $uch construction avoidsthe use of distance pieces or pac%ers when assembling the blades in the wheels.Tangs are left at the tips of the blades so that when fitted in position in the wheel theshrouding can be attached. The shrouding is made up from section of metal strippunched with holes to correspond with the tangs. The strip is passed over the tangswhich are then splayed out to secure the strip in position. The shrouding is fitted inseparate sections to allow for expansion.

There is no pressure drop across the moving blades of an impulse type turbine and,therefore, the sealing arrangements are not of such great importance as in the

reaction type. The shrouding on the impulse blading helps to guide the steamthrough the moving blades, allowing larger radial clearance, as well as strengtheningthe assembly.

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;)9) MET1OS O. .ITTING BLAES

;)9)* ROOT .I5INGS

2any types of root fixing shapes exist for turbine blading to suit both theconditions under which the blade must operate and the preference of theparticular designer concerned. 5n general there are the types which either fitin their appropriate groove or straddle it, whilst other designs are fixed byrivets through the blade root. $ome examples of these blade root fixings areshown in Figure !"!"7.

5n the case of reaction type turbines the /( blading is built up in pac%ets ofup to ten blades and held in the rotor groove with serrated loc%ing strips asshown earlier. A gap is left in the rotor groove to allow the last serratedloc%ing strip to be inserted. This gap is then closed with a plate fixed byscrews to the rotor body.

.i%) /-2-2 E@!34les of Root .i@in%