1 - Gas Turbine Functional Description GFD71FAS

GE Power SystemsGas Turbine

GFD71FASRevised, September 2001

These instructions do not purport to cover all details or variations in equipment nor to provide for every possiblecontingency to be met in connection with installation, operation or maintenance. Should further information be desired orshould particular problems arise which are not covered sufficiently for the purchaser’s purposes the matter should bereferred to the GE Company. 2001 GENERAL ELECTRIC COMPANY

Gas Turbine Functional Description

I. INTRODUCTION

A. General

The MS–7001FA is a single-shaft gas turbine designed for operation as a simple-cycle unit or in a com-bined steam and gas turbine cycle (STAG). The gas turbine assembly contains six major sections orgroups:

1. Air inlet

2. Compressor

3. Combustion System

4. Turbine

5. Exhaust

6. Support systems

This section briefly describes how the gas turbine operates and the interrelationship of the major compo-nents. Typical illustrations and photographs accompany the text.

The flange-to-flange description of the gas turbine is also covered in some detail. A separate section isdevoted to the air inlet and exhaust systems. Support systems pertaining to lube oil, cooling water, etc.are also covered in detail in individual sections.

B. Detail Orientation

Throughout this manual, reference is made to the forward and aft ends, and to the right and left sides ofthe gas turbine and its components. By definition, the air inlet of the gas turbine is the forward end, whilethe exhaust is the aft end. The forward and aft ends of each component are determined in like manner withrespect to its orientation within the complete unit. The right and left sides of the turbine or of a particularcomponent are determined by standing forward and looking aft.

System DescriptionGas Turbine

2

C. Gas Path Description

The gas path is the path by which gases flow through the gas turbine from the air inlet through the compres-sor, combustion section and turbine, to the turbine exhaust.

When the turbine starting system is actuated and the clutch is engaged, ambient air is drawn through theair inlet plenum assembly, filtered and compressed in the multi-stage, axial-flow compressor. For pulsa-tion protection during startup, compressor bleed valves are open and the variable inlet guide vanes are inthe closed position. When the high-speed relay actuates, the bleed valves begin operation automaticallyand the variable inlet guide vane actuator energizes to position the inlet guide vanes for normal turbineoperation. Compressed air from the compressor flows into the annular space surrounding the combustionchambers, from which it flows into the spaces between the outer combustion casings and the combustionliners, and enters the combustion zone through metering holes in each of the combustion liners.

Fuel from an off-base source is provided to flow lines, each terminating at the primary and secondary fuelnozzles in the end cover of the separate combustion chambers. On liquid fueled machines, the fuel is con-trolled prior to being distributed to the nozzles to provide an equal flow into each liquid fuel distributorvalve mounted on each end cover and each liquid fuel line on each secondary nozzle assembly. On gasfueled machines, the fuel nozzles are the metering orifices which provide the proper flow into the combus-tion zones in the chambers. The nozzles introduce the fuel into the combustion zone within each chamberwhere it mixes with the combustion air and is ignited by one or more of the spark plugs. At the instantwhen fuel is ignited in one combustion chamber flame is propagated, through connecting crossfire tubes,to all other combustion chambers where it is detected by four primary flame detectors, each mounted ona flange provided on the combustion casings.

The hot gases from the combustion chambers flow into separate transition pieces attached to the aft endof the combustion chamber liners and flow from there to the three-stage turbine section. Each stage con-sists of a row of fixed nozzles and a row of turbine buckets. In each nozzle row, the kinetic energy of thejet is increased, with an associated pressure drop, which is absorbed as useful work by the turbine rotorbuckets, resulting in shaft rotation used to turn the generator rotor to generate electrical power.

After passing through the third-stage buckets, the gases are directed into the exhaust diffuser. The gasesthen pass into the exhaust plenum and are introduced to atmosphere through the exhaust stack.

II. BASE AND SUPPORTS

A. Turbine Base

The base that supports the gas turbine is a structural steel fabrication of welded steel beams and plate. Itsprime function is to provide a support upon which to mount the gas turbine.

Lifting trunnions and supports are provided, two on each side of the base in line with the two structuralcross members of the base frame. Machined pads on each side on the bottom of the base facilitate itsmounting to the site foundation. Two machined pads, atop the base frame are provided for mounting theaft turbine supports.

B. Turbine Supports

The MS7001FA has rigid leg-type supports at the compressor end and supports with top and bottom pivotsat the turbine end.

Gas TurbineSystem Description

3

On the inner surface of each support leg a water jacket is provided, through which cooling water is circu-lated to minimize thermal expansion and to assist in maintaining alignment between the turbine and theload equipment. The support legs maintain the axial and vertical positions of the turbine, while two gibkeys coupled with the turbine support legs maintain its lateral position. One gib key is machined on thelower half of the exhaust frame. The other gib key is machined on the lower half of the forward compressorcasing. The keys fit into guide blocks which are welded to the cross beams of the turbine base. The keysare held securely in place in the guide blocks with bolts that bear against the keys on each side. This key-and-block arrangement prevents lateral or rotational movement of the turbine while permitting axial andradial movement resulting from thermal expansion.

III. COMPRESSOR SECTION

A. General

The axial-flow compressor section consists of the compressor rotor and the compressor casing. Withinthe compressor casing are the variable inlet guide vanes, the various stages of rotor and stator blading, andthe exit guide vanes.

In the compressor, air is confined to the space between the rotor and stator where it is compressed in stagesby a series of alternate rotating (rotor) and stationary (stator) airfoil-shaped blades. The rotor blades sup-ply the force needed to compress the air in each stage and the stator blades guide the air so that it entersthe following rotor stage at the proper angle. The compressed air exits through the compressor dischargecasing to the combustion chambers. Air is extracted from the compressor for turbine cooling and for pulsa-tion control during startup.

B. Rotor

The compressor portion of the gas turbine rotor is an assembly of wheels, a speed ring, tie bolts, the com-pressor rotor blades, and a forward stub shaft (see Figure 1).

Each wheel has slots broached around its periphery. The rotor blades and spacers are inserted into theseslots and held in axial position by staking at each end of the slot. The wheels are assembled to each otherwith mating rabbets for concentricity control and are held together with tie bolts. Selective positioningof the wheels is made during assembly to reduce balance correction. After assembly, the rotor is dynami-cally balanced.

The forward stubshaft is machined to provide the thrust collar which carries the forward and aft thrustloads. The stubshaft also provides the journal for the No. 1 bearing, the sealing surface for the No. 1 bear-ing oil seals and the compressor low-pressure air seal.

The stage 17 wheel carries the rotor blades and also provides the sealing surface for the high-pressure airseal and the compressor-to-turbine marriage flange.

C. Stator

1. General

The casing area of the compressor section is composed of three major sections. These are the:

a. Inlet casing

System Description

Gas Turbine

4

Figure 1. C

ompressor R

otor Assem

bly

CompressorRotor Blades

No. 1 BearingJournalThrust

Collar


5

b. Compressor casing

c. Compressor discharge casing

These casings, in conjunction with the turbine casing, form the primary structure of the gas turbine.They support the rotor at the bearing points and constitute the outer wall of the gas-path annulus. Allof these casings are split horizontally to facilitate servicing.

2. Inlet Casing

The inlet casing (see Figure 2) is located at the forward end of the gas turbine. Its prime function isto uniformly direct air into the compressor. The inlet casing also supports the No. 1 bearing assembly.The No. 1 bearing lower half housing is integrally cast with the inner bellmouth. The upper half bear-ing housing is a separate casting, flanged and bolted to the lower half. The inner bellmouth is posi-tioned to the outer bellmouth by nine airfoil-shaped radial struts. The struts are cast into the bellmouthwalls. They also transfer the structural loads from the adjoining casing to the forward support whichis bolted and doweled to this inlet casing.

Variable inlet guide vanes are located at the aft end of the inlet casing and are mechanically positioned,by a control ring and pinion gear arrangement connected to a hydraulic actuator drive and linkage armassembly. The position of these vanes has an effect on the quantity of compressor inlet air flow.

3. Compressor Casing

The forward compressor casing contains the stage 0 through stage 4 compressor stator stages. Thecompressor casing lower half is equipped with two large integrally cast trunnions which are used tolift the gas turbine when it is separated from its base.

The aft compressor casing contains stage 5 through stage 12 compressor stator stages. Extractionports in aft casing permit removal of 13th-stage compressor air. This air is used for cooling functionsand is also used for pulsation control during startup and shutdown.

4. Compressor Discharge Casing

The compressor discharge casing is the final portion of the compressor section. It is the longest singlecasting, is situated at midpoint — between the forward and aft supports — and is, in effect, the key-stone of the gas turbine structure. The compressor discharge casing contains the final compressorstages, forms both the inner and outer walls of the compressor diffuser, and joins the compressor andturbine casings. The discharge casing also provides support for the combustion outer casings and theinner support of the first-stage turbine nozzle.

The compressor discharge casing consists of two cylinders, one being a continuation of the compres-sor casing and the other being an inner cylinder that surrounds the compressor rotor. The two cylindersare concentrically positioned by fourteen radial struts.

A diffuser is formed by the tapered annulus between the outer cylinder and inner cylinder of the dis-charge casing. The diffuser converts some of the compressor exit velocity into added static pressurefor the combustion air supply.


6

Figure 2. Compressor Inlet Casing and No. 1 Bearing

BearingNo. 1 Stationary Oil SealAssembly

BearingNo. 1 OilFeed & Orifices

BearingNo. 1 FWDStationaryOil Seals

Tilting PadJournalBearing

Thrust Bearing


7

5. Blading

The compressor rotor and stator blades are airfoil shaped and designed to compress air efficiently athigh blade tip velocities. The blades are attached to the compressor wheels by dovetail arrangements.The dovetail is very precise in size and position to maintain each blade in the desired position andlocation on the wheel.

The compressor stator blades are airfoil shaped and are mounted by similar dovetails into ring seg-ments in the first five stages. The ring segments are inserted into circumferential grooves in the casingand are held in place with locking keys. The stator blades of the remaining stages have a square basedovetail and are inserted directly into circumferential grooves in the casing. Locking keys hold themin place.

IV. DLN–2 COMBUSTION SYSTEM

A. General

The combustion system is of the reverse-flow type with the 14 combustion chambers arranged around theperiphery of the compressor discharge casing as shown on Figure 3. Combustion chambers are numberedcounterclockwise when viewed looking downstream and starting from the top left of the machine. Thissystem also includes the fuel nozzles, a spark plug ignition system, flame detectors, and crossfire tubes.Hot gases, generated from burning fuel in the combustion chambers, flow through the impingementcooled transition pieces to the turbine.

High pressure air from the compressor discharge is directed around the transition pieces. Some of the airenters the holes in the impingement sleeve to cool the transition pieces and flows into the flow sleeve. Therest enters the annulus between the flow sleeve and the combustion liner through holes in the downstreamend of the flow sleeve. (See Figures 4 and 5). This air enters the combustion zone through the cap assemblyfor proper fuel combustion. Fuel is supplied to each combustion chamber through five nozzles designedto disperse and mix the fuel with the proper amount of combustion air.

The DLN–2 combustion system shown in Figure 4 is a single stage,dual mode combustor capable of op-eration on both gaseous and liquid fuel. On gas, the combustor operates in a diffusion mode at low loads(<50% load), and a pre-mixed mode at high loads (>50% load). While the combustor is capable of operat-ing in the diffusion mode across the load range, diluent injection would be required for NOx abatement.Oil operation on this combustor is in the diffusion mode across the entire load range, with diluent injectionused for NOx.

B. Outer Combustion Chambers and Flow Sleeves

The outer combustion chambers act as the pressure shells for the combustors. They also provide flangesfor the fuel nozzle-end cover assemblies, crossfire tube flanges, and, where called for, spark plugs, flamedetectors and false start drains. The flow sleeves (Figure 5) form an annular space around the cap and linerassemblies that directs the combustion and cooling air flows into the reaction region. To maintain the im-pingement sleeve pressure drop, the openings for crossfire tubes, spark plugs, and flame detectors aresealed with sliding grommets.


8

Figure 3. MS7001FA DLN-2 Combustion System Arrangement.

Dynamic Pressure Probe(Typ. all Chambers)

Flame DetectorsML-112)

CLCL

CrossfireTube (Typ)

CL

SparkPlugs

CL

FalseStartDrains

FalseStartDrains

CL

FalseDet.

CL

TurbineCL

TurbineCL

Gas Turbine

System Description

9

Figure 4.

MS

7001FA D

LN-2 C

ombustion A

rrangement.

Multi-NozzleCover

Fuel NozzleCasing

Cap Assembly

Transition Piece

Flow Sleeve

Combustion Liner

Tertiary NozzleCooling Air

Typical Chamber Cross Section

System Description

Gas Turbine

10

FloatingSeal Ring

SpringSeal

MountingFlange

LinerStop

XfireTube

19.660

Figure 5. Flow Sleeve Assembly


11

C. Crossfire Tubes

All combustion chambers are interconnected by means of crossfire tubes. The outer chambers are con-nected with an outer crossfire tube and the combustion liner primary zones are connected by the innercrossfire tubes.

D. Fuel Nozzle End Covers

There are five fuel nozzle assemblies in each combustor. Figures 6, 6A shows a cross-section of a DLN–2fuel nozzle. As noted, the nozzle has passages for diffusion gas, premixed gas, oil, and water. Whenmounted on the endcover, as shown in Figure 6, the diffusion passages of four of the fuel nozzles are fedfrom a common manifold, called the primary, that is built into the endcover. The premixed passage of thesame four nozzles are fed from another internal manifold called the secondary. The premixed passagesof the remaining nozzle is supplied by the tertiary fuel system; the diffusion passage of that nozzle is al-ways purged with compressor discharge air and passes no fuel.

E. Cap and Liner Assemblies

The combustion liners (Figure 7) use external ridges and conventional cooling slots for cooling. Interiorsurfaces of the liner and the cap are thermal barrier coated to reduce metal temperatures and thermal gradi-ents. The cap (Figures 8, 8A) has five premixer tubes that engage each of the five fuel nozzle. It is cooledby a combination of film cooling and impingement cooling and has thermal barrier coating on the innersurfaces (Figures 8, 8A).

F. Spark Plugs

Combustion is initiated by means of the discharge from spark plugs which are bolted to flanges on thecombustion cans and centered within the liner and flowsleeve in adjacent combustion chambers. A typicalspark plug arrangement is shown in Figure 9. These plugs receive their energy from high energy-capacitordischarge power supplies. At the time of firing, a spark at one or more of these plugs ignites the gases ina chamber; the remaining chambers are ignited by crossfire through the tubes that interconnect the reac-tion zone of the remaining chambers.

G. Ultraviolet Flame Detectors

During the starting sequence, it is essential that an indication of the presence or absence of flame be trans-mitted to the control system. For this reason, a flame monitoring system is used consisting of multipleflame detectors located as shown on Figure 3. The flame detectors (Figs. 10 and 11) have water cooledjackets to maintain acceptable temperatures.

The ultraviolet flame sensor contains a gas filled detector. The gas within this detector is sensitive to thepresence of ultraviolet radiation which is emitted by a hydrocarbon flame. A DC voltage, supplied by theamplifier, is impressed across the detector terminals. If flame is present, the ionization of the gas in thedetector allows conduction in the circuit which activates the electronics to give an output indicating flame.Conversely, the absence of flame will generate an output indicating no flame.

The signals from the four flame detectors are sent to the control system which uses an internal logic systemto determine whether a flame or loss of flame condition exists.

System Description

Gas Turbine

12

Figure 6. DLN–2 Fuel Nozzle Cross-section


13

Figure 6A. Fuel Nozzle Arrangement.

Fuel NozzleCover

SteamConnection

QuaternaryGas

Fuel NozzleCase

Liquid FuelInjector

Gas FuelInjector

System Description

Gas Turbine

14

Figure 7. Combustion Liner AssemblyC L

Liner Stop (3)

SparkPlugFlameDetector

CrossfireTube Collar Cooling

HolesSpringSeal

Spark Plugand FlameDetector

Spring Seal

Turbulators

CrossfireCollar

FLOW

CoolingSlots

Combustion Liner Details

Gas Turbine

System Description

15

SpringSealCL

OuterRadial

Liner Stop

Figure 8. Cap Assembly – View from Upstream


16

CL

OuterRadial

Figure 8A. Cap Assembly-View From Downstream


17

Figure 9. Spark Plug Assembly

Spark Plug

Forward FlangeOuter Comb.Case

Gasket

Downstream

CL Chamber

CL Chamber

Liner

CL Spark Plug

(2.500) Stroke


18

For detailed operating and maintenance information covering this equipment, refer to the vendorpublications.

V. TURBINE SECTION

A. General

The three-stage turbine section is the area in which energy in the form of high temperature pressurizedgas, produced by the compressor and combustion sections, is converted to mechanical energy.

MS7001FA gas turbine hardware includes the turbine rotor, turbine casing, exhaust frame, exhaust diffus-er, nozzles, and shrouds.

B. Turbine Rotor

1. Structure

The turbine rotor assembly, shown in Figure 12, consists of the forward and aft turbine wheel shaftsand the first-, second- and third-stage turbine wheel assemblies with spacers and turbine buckets.Concentricity control is achieved with mating rabbets on the turbine wheels, wheel shafts, andspacers. The wheels are held together with through bolts mating up with bolting flanges on the wheelshafts and spacers. Selective positioning of rotor members is performed to minimize balance correc-tions.

2. Wheel Shafts

The turbine rotor distance piece extends from the first-stage turbine wheel to the aft flange of the com-pressor rotor assembly.

The turbine rotor aft shaft includes the No. 2 bearing journal.

3. Wheel Assemblies

Spacers between the first and second, and between the second and third-stage turbine wheels deter-mine the axial position of the individual wheels. These spacers carry the diaphragm sealing lands. The1–2 spacer forward and aft faces include radial slots for cooling air passages.

Turbine buckets are assembled in the wheels with fir-tree-shaped dovetails that fit into matching cut-outs in the turbine wheel rims. All three turbine stages have precision investment-cast, long-shankbuckets. The long-shank bucket design effectively shields the wheel rims and bucket root fasteningsfrom the high temperatures in the hot gas path while providing mechanical damping of bucket vibra-tions. As a further aid in vibration damping, the stage-two and stage-three buckets have interlockingshrouds at the bucket tips. These shrouds also increase the turbine efficiency by minimizing tip leak-age. Radial teeth on the bucket shrouds combine with stepped surfaces on the stator to provide a laby-rinth seal against gas leakage past the bucket tips.

Figure 13 shows typical first-, second-, and third-stage turbine buckets for the MS7001FA. The in-crease in the size of the buckets from the first to the third stage is necessitated by the pressure reductionresulting from energy conversion in each stage, requiring an increased annulus area to accommodatethe gas flow.


19

Figure 10. Flame Detector Assembly


20

Figure 11. Water-Cooled Flame Detector


21

Figure 12. Turbine Rotor Assembly

2

5 3

6

4

7

8

1

1. Compressor-to-TurbineDistance Piece

2. 1st Stage Turbine Rotor Wheel3. 2nd Stage Turbine Rotor Wheel4. 3rd Stage Turbine Rotor Wheel5. Stage 1-2 Turbine Rotor Spacer6. Stage 2-3 Turbine Rotor Spacer7. Through Bolt Assemblies8. Turbine Rotor Aft Shaft


22

Figure 13. MS7001FA First, Second and Third-Stage Turbine Elements


24

Figure 14. Turbine Section-Cutaway View Showing Cooling Air Flows

Nozzle ArrStg. 1

DiaphragmStg 2

DiaphragmStg 3

NozzleArr.Stg. 3

NozzleArr.Stg. 2 B

lade

Stg

. 3

Bla

deS

tg. 1

1-2 Spacer

2-3 Spacer

Aft Shaft


25

C. Turbine Stator

1. Structure

The turbine casing and the exhaust frame constitute the major portion of the MS7001FA gas turbinestator structure. The turbine nozzles, shrouds, and turbine exhaust diffuser are internally supportedfrom these components.

2. Turbine Casing

The turbine casing controls the axial and radial positions of the shrouds and nozzles. It determinesturbine clearances and the relative positions of the nozzles to the turbine buckets. This positioningis critical to gas turbine performance.

Hot gases contained by the turbine casing are a source of heat flow into the casing. To control the cas-ing diameter, it is important to reduce the heat flow into the casing and to limit its temperature. Heatflow limitations incorporate insulation, cooling, and multi–layered structures. 13th stage extractionair is piped into the turbine casing annular spaces around the 2nd and 3rd stage nozzles. From therethe air is ported through the nozzle partitions and into the wheel spaces.

Structurally, the turbine casing forward flange is bolted to the bulkhead flange at the aft end of thecompressor discharge casing. The turbine casing aft flange is bolted to the forward flange of the ex-haust frame.

3. Nozzles

In the turbine section there are three stages of stationary nozzles (Figure 14) which direct the high–ve-locity flow of the expanded hot combustion gas against the turbine buckets causing the turbine rotorto rotate. Because of the high pressure drop across these nozzles, there are seals at both the inside andthe outside diameters to prevent loss of system energy by leakage. Since these nozzles operate in thehot combustion gas flow, they are subjected to thermal stresses in addition to gas pressure loadings.

4. First–Stage Nozzle

The first–stage nozzle receives the hot combustion gases from the combustion system via the transi-tion pieces. The transition pieces are sealed to both the outer and inner sidewalls on the entrance sideof the nozzle; this minimizes leakage of compressor discharge air into the nozzles.

The Model 7001FA gas turbine first–stage nozzle (Figure 17) contains a forward and aft cavity in thevane and is cooled by a combination of film, impingement and convection techniques in both the vaneand sidewall regions.

The nozzle segments, each with two partitions or airfoils, are contained by a horizontally split retain-ing ring which is centerline supported to the turbine casing on lugs at the sides and guided by pinsat the top and bottom vertical centerlines. This permits radial growth of the retaining ring, resultingfrom changes in temperature, while the ring remains centered in the casing.

The aft outer diameter of the retaining ring is loaded against the forward face of the first–stage turbineshroud and acts as the air seal to prevent leakage of compressor discharge air between the nozzle andturbine casing.


26

Figure 15. MS7001FA First–Stage Bucket Cooling Passages


27

Figure 16. MS7001FA Stage–2 Bucket Cooling Flow


28

Figure 17. MS7001FA First–Stage Nozzle Cooling.


29

On the inner sidewall, the nozzle is sealed by a flange cast on the inner diameter of the sidewall thatrests against a mating face on the first–stage nozzle support ring. Circumferential rotation of the seg-ment inner sidewall is prevented by an eccentric bushing and a locating dowel that engages a lug onthe inner sidewall.

The nozzle is prevented from moving forward by the lugs welded to the aft outside diameter of theretaining ring at 45 degrees from vertical and horizontal centerlines. These lugs fit in a groove ma-chined in the turbine shell just forward of the first–stage shroud T hook. By moving the horizontaljoint support block and the bottom centerline guide pin and then removing the inner sidewall locatingdowels, the lower half of the nozzle can be rolled out with the turbine rotor in place.

5. Second–Stage Nozzle

Combustion air exiting from the first stage buckets is again expanded and redirected against the se-cond–stage turbine buckets by the second–stage nozzle. This nozzle is made of cast segments, eachwith two partitions or airfoils. The male hooks on the entrance and exit sides of the outer sidewall fitinto female grooves on the aft side of the first–stage shrouds and on the forward side of the second–stage shrouds to maintain the nozzle concentric with the turbine shell and rotor. This close fittingtongue–and–groove fit between nozzle and shrouds acts as an outside diameter air seal. The nozzlesegments are held in a circumferential position by radial pins from the shell into axial slots in thenozzle outer sidewall.

The second–stage nozzle is cooled with 13th stage extraction air.

6. Third–Stage Nozzle

The third–stage nozzle receives the hot gas as it leaves the second–stage buckets, increases its velocityby pressure drop, and directs this flow against the third–stage buckets. The nozzle consists of castsegments, each with three partitions or airfoils. It is held at the outer sidewall forward and aft sidesin grooves in the turbine shrouds in a manner similar to that used on the second–stage nozzle. Thethird–stage nozzle is circumferentially positioned by radial pins from the shell. 13th stage extractionair flows through the nozzle partitions for nozzle convection cooling and for augmenting wheelspacecooling air flow.

7. Diaphragm

Attached to the inside diameters of both the second and third–stage nozzle segments are the nozzlediaphragms. These diaphragms prevent air leakage past the inner sidewall of the nozzles and the tur-bine rotor. The high/low, labyrinth seal teeth are machined into the inside diameter of the diaphragm.They mate with opposing sealing lands on the turbine rotor. Minimal radial clearance between station-ary parts (diaphragm and nozzles) and the moving rotor are essential for maintaining low interstageleakage; this results in higher turbine efficiency.

8. Shrouds

Unlike the compressor blading, the turbine bucket tips do not run directly against an integral ma-chined surface of the casing but against annular curved segments called turbine shrouds. The shrouds’primary function is to provide a cylindrical surface for minimizing bucket tip clearance leakage.


30

The turbine shrouds’ secondary function is to provide a high thermal resistance between the hot gasesand the comparatively cool turbine casing. By accomplishing this function, the turbine casing coolingload is drastically reduced, the turbine casing diameter is controlled, the turbine casing roundness ismaintained, and important turbine clearances are assured.

The first and second–stage stationary shroud segments are in two pieces; the gas–side inner shroudis separated from the supporting outer shroud to allow for expansion and contraction, and thereby im-prove low–cycle fatigue life. The first–stage shroud is cooled by impingement, film, and convection.

The shroud segments are maintained in the circumferential position by radial pins from the turbinecasing. Joints between shroud segments are sealed by interconnecting tongues and grooves.

9. Exhaust Frame

The exhaust frame is bolted to the aft flange of the turbine casing. Structurally, the frame consists ofan outer cylinder and an inner cylinder interconnected by the radial struts. The No. 2 bearing is sup-ported from the inner cylinder.

The exhaust diffuser located at the aft end of the turbine is bolted to the exhaust frame. Gases ex-hausted from the third turbine stage enter the diffuser where velocity is reduced by diffusion and pres-sure is recovered. At the exit of the diffuser, the gases are directed into the exhaust plenum.

Exhaust frame radial struts cross the exhaust gas stream. These struts position the inner cylinder andNo. 2 bearing in relation to the outer casing of the gas turbine. The struts must be maintained at aconstant temperature in order to control the center position of the rotor in relation to the stator. Thistemperature stabilization is accomplished by protecting the struts from exhaust gases with a metalfairing that forms an air space around each strut and provides a rotated, combined airfoil shape.

Off–base blowers provide cooling air flow through the space between the struts and the wrapper tomaintain uniform temperature of the struts. This air is then directed to the third–stage aft wheelspace.

Trunnions on the sides of the exhaust frame are used with similar trunnions on the forward compressorcasing to lift the gas turbine when it is separated from its base.

VI. BEARINGS

A. General

The MS7001FA gas turbine unit has two four–element, tilting pad journal bearings which support the gasturbine rotor. The unit also includes a thrust bearing to maintain the rotor–to–stator axial position. Thrustis absorbed by a tilting pad thrust bearing with eight shoes on both sides of the thrust bearing runner. Thesebearings and seals are incorporated in two housings: one at the inlet casing, one in the exhaust frame. Thesemain bearings are pressure–lubricated by oil supplied from the main lubricating oil system. The oil flowsthrough branch lines to an inlet in each bearing housing.


31

1. Lubrication

The main turbine bearings are pressure–lubricated with oil supplied, from the oil reservoir. Oil feedpiping, where practical, is run within the lube oil drain lines, or drain channels, as a protective mea-sure. In the event of a supply line leak, oil will not be sprayed on nearby equipment, thus eliminatinga potential safety hazard.

When the oil enters the housing inlet, it flows into an annulus around the bearing. From the annulus,the oil flows through machined holes or slots to the bearing rotor interface.

2. Lubricant Sealing

Oil on the surface of the turbine shaft is prevented from being spun along the shaft by oil seals in eachof the bearing housings. These labyrinth seals are assembled at the extremities of the bearing assem-blies where oil control is required. A smooth surface is machined on the shaft and the seals are as-sembled so that only a small clearance exists between the oil seal and the shaft. The oil seals are de-signed with tandem rows of teeth and an annular space between them. Pressurized sealing air isadmitted into this space to prevent lubricating oil vapor from exiting the bearing housing. The air thatreturns with the oil to the main lubricating oil reservoir is vented to atmosphere after passing throughan oil vapor extractor.

VII. LOAD COUPLING

A rigid, hollow coupling connects the forward compressor rotor shaft to the generator. A bolted flangeconnection forms the joint at each end of the coupling.

1 - Gas Turbine Functional Description GFD71FAS

Documents

gas turbine assembly

turbine exhaust

gas turbine cycle stag

singleshaft gas turbine

left sides ofthe gas

combustion section

forward end

compressed air