1 GT5-8 Comp & Turbine Design

GT 5-8 Compressor and Turbine Design Frame MS 9001 E

27

1.

G A S T U R DB EI NS EI G N

1.1

G E N E R A BL O U GTA S TU R B I N5 E- 8

GT 5-8 is a package power plant, as furnished for most installations, is comprised of the single-shaft, simple cycle, heavy duty gas turbine unit driving a generator. Fuel and air are used by the gas turbine unit to produce the shaft horsepower necessary to drive certain accessories, compressor and the generator. The turbine unit is composed of a starting device, support systems, an axial-flow compressor, combustion system components and a three-stage turbine. Both compressor and turbine are directly connected with an in-line, single-shaft rotor supported by three pressure lubricated bearings. The inlet end of the rotor shaft is coupled to an accessory gear having integral shafts that drive the fuel pump, lubrication pump, and other system components.

1.2

G A S TU R B I NFUE N C T I O NDEA SLC R I P T I O N

When the turbine starting system is actuated and the clutch is engaged, ambient air is drawn through the inlet plenum assembly, filtered, then compressed in the 17 stages of axial-flow compressor. For pulsation protection during start-up, the 11th stage extraction valves are opened and the variable inlet guide vanes are in the closed position. When the speed relay corresponding to 95 % (2850 rpm) speed actuates, the 11th stage extraction bleed valves close automatically and the variable inlet guide vane actuator energizes at 82 % speed to open the inlet guide vanes (I.G.V.) to (57°) the normal turbine operating position. Compressed air from the compressor flows into the annular space surrounding the 14 combustors, from which it flows into the spaces between the outer combustion casing (sleeve) and the combustion liners. The fuel nozzles introduce the fuel into each of the 14 combustors where it mixes with the combustion air and is ignited by both (or one, which is sufficient) of the two spark plugs. At the instant, one or both of the two spark plugs ignite their combustors, the remaining combustors are also ignited by crossfire tubes that connect the reaction zones of the combustors. After the rotor approaches operating speed, combustion chamber pressure causes the spark plugs to retract to remove their electrodes from the hot flame zone. The hot gases from the combustors expand into the 14 separate transition pieces attached to the aft end of the combustor liners and flow towards the three stage turbine section of the machine. Each stage consists of a row of fixed nozzles followed by a row of rotateable turbine buckets. In each nozzle row, the kinetic energy of the jet is increased, with an associated pressure drop and in each following row of moving bucket a portion of the kinetic energy of the jet is absorbed as useful work on the turbine rotor.

Prepared by: Fazal-ur-Rehman Babar

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After passing through the third stage buckets, the exhaust gases are directed into the exhaust hood and diffuser which contains a series of turning vanes to turn the gases from the axial direction to a radial direction, thereby minimizing exhaust hood losses. Then, the gases pass into the exhaust plenum. The resultant shaft rotation is used to turn the generator rotor, compressor rotor and to drive certain accessories through accessory gear at various speeds. NOTE: By definition, the air inlet of the gas turbine is the forward end, while the exhaust end is the aft end. The forward and aft ends of each component are determined in like manner with respect to its orientation within the complete unit. The RIGHT and LEFT sides of the turbine or of a particular component are determined by standing forward and looking aft.

2. CO M PRESSO R The axial-flow compressor consists of the rotor and the enclosing casing. Included within the compressor casing are the inlet guide vanes, the 17 stages of rotor and stator blades and the exit guide vanes. In the compressor, air is confined to the space between the rotor and stator blades where it is compressed in stages by a series of alternate rotating and stationary airfoil-shaped blades. The rotor blades supply the force needed to compress the air in each stage and the stator blades guide the air so that it enters in the following rotor stage at the proper angle. The compressed air exits through the compressor discharge casing to the combustion chambers. Air is extracted from the compressor 5th stage for bearing sealing and from 11th stage for pulsation control. Since minimum clearance between rotor and stator provides best performance in a compressor, parts have to be made and assembled very accurately. 2.1

CO M

P R E S RS OO TR O R

The compressor rotor is an assembly of 15 individual wheels, two stub-shafts, each with an integral wheel, a speed ring, tie bolts and the compressor rotor blades. Each wheel and the wheel portion of each stub-shaft have slots broached around its periphery. The rotor blades and spacers are inserted into these slots. Selective positioning of the wheels is made during assembly to reduce balance correction. After assembly, the rotor is dynamically balanced to a fine limit. Forward stub-shaft is machined to provide the forward and aft thrust faces and the journal for the no.1 bearing, as well as the sealing surfaces for the no.1 bearing oil seals and the compressor low-pressure air seals. Prepared by: Fazal-ur-Rehman Babar

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2.2

CO M

PR ESS S TOART O R

The stator (casing) area of the compressor section is composed of four major sections: -

inlet casing forward compressor casing aft compressor casing compressor discharge casing

These sections, in conjunction with the turbine shell and exhaust frame 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. 2.2.1

In le t C a sin g

The inlet casing is located at the forward end of the gas turbine. Its prime function is to uniformly direct air into the compressor. The inlet casing also supports the no.1 bearing housing; a separate casing that contains the № 1 bearing. 2.2.2

V a ria b le In le t G u id e V a n e s (V IG V )

Variable inlet guide vanes are located at the aft end of the inlet casing. The position of these vanes has an affect on the quantity of compressor air flow. Movement of these guide vanes is accomplished by the inlet guide vane control ring that turns individual pinion gears attached to the end of each vane. The control ring is positioned by a hydraulic actuator and linkage arm assembly. 2.2.3

F o rw a rd C a sin g

The forward compressor casing contains the first four compressor stator stages. 2.2.4

A ft C a sin g

The aft compressor casing contains the 5th through 10th compressor stages. Extraction ports in the casing permit removal of 5th and 11th stage compressor air. This air from 5th stage is used for cooling and sealing functions and 11th stage air is extracted for starting and shutdown surge and pulsation control. 2.2.5

D isc h a rg e C a sin g

The compressor discharge casing is the final portion of the compressor section. It is the longest single casting consists of two cylinders. It contains the final seven compressor stages, to form both the inner and outer walls of the compressor diffuser and to join the compressor and turbine stators. They also provide support for bearing № 2, the forward end of the combustion wrapper and the inner support of the first-stage turbine nozzle. 2.3

CO M

PR ESR S OO TR O RBL A D E S

The compressor rotor blades are airfoil shaped and designed to compress air efficiently at synchronous speed. The blades of first eight stages are attached to their wheels by dovetail arrangements. The compressor stator blades are also airfoil shaped and are mounted by similar dovetails into ring segments. The stator blades of the last nine stages and two exit guide vanes have a square base dovetail that are inserted directly into circumferential grooves in the casing. Locking keys also hold them in place. Prepared by: Fazal-ur-Rehman Babar

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2 . 4 CO M

PR ESS S UO RR G E

Operating speed of the gas turbine is held constant and very little deviation takes place. The blades of the axial compressor are designed to achieve optimum efficiency at the synchronous speed. To achieve a good performance, the angle and aerofoil shape of the rotor and stator blades are precisely machined and set in place such that the compression of air through the compressor is smooth and efficient.

During startup and shutdown of a gas turbine, the rotor and stator blades do not deliver a smooth progression of air through the compressor. This occurs, simply because the shape and position of the blades is not conducive to low speed and low flow conditions. Speeds less than the permissible range would exert strain on the front stages of the compressor that the result would be separation of the flow at the airfoils, this is, the air is not smoothly compressed. Such separation will cause the delivery to become unstable. The compressor will begin to surge. This is characterised by rapid fluctuations of the compressor discharge pressure combined with heavy vibration of the unit and surging noise in rhythm with the pressure fluctuations with the compressor blades being endangered by the resulting high alternating bending stresses and high temperatures. Figure illustrates the effects of air flow through a gas turbine compressor during a compressor surge. Since on start-up and shut-down of the gas turbine and under-frequency operation, the compressor is bound to run at speeds below the permissible value, air will have to be blown off at specific parts of the compressor as a result of which the volume flow is matched to the blade case cross-sections.

H O W CO M

P R E S SS UO RR G I ES AV O I D ?E D

For pulsation protection during start up, the variable inlet guide vanes (VIGV) are in closed position and the 11th stage extraction bleed valves are open. When the speed relay corresponding to 95 % speed actuates, the 11th stage extraction bleed valve close automatically and variable inlet guide vane actuator energises to open the inlet guide vanes (IGV) to the normal turbine operating position.

2 . 4 . 1C o n t r o l t h r o u g h I n l e t G u i d e V a n e s Variable inlet guide vanes (VIGV) are installed on the compressor to provide compressor pulsation protection during start-up and shutdown and also to be used during operation under partial load conditions. The variable inlet guide vane actuator is a hydraulically actuated assembly having a closed feedback control loop to control the guide vanes angle. The vanes are automatically positioned within their operating range in response either to the control system exhaust temperature limits for normal loaded operation, or to the control system pulsation protection limits during the start-up and shut-down sequences.


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2.4.2

S u rg e C o n tro l th ro u g h B le e d V a lv e s

The pressure, speed and flow characteristics of the gas turbine compressor are such that air must be extracted from the 11th stage and vented to atmosphere to prevent pulsation of the compressor during the acceleration period of the turbine starting sequence and during deceleration of the turbine at shut-down. Pneumatically operated air extraction bleed valves, controlled by a three-way solenoid valve, are used to accomplish the pulsation protection. Eleventh stage air is extracted from the compressor at four flanged connections on the compressor casing. Each of these connections is piped through a normally open (by spring force), piston-operated, butterfly valve, VA 2-1, -2, -3 and -4 to the turbine exhaust plenum. Limit switches 33 CB-1, -2, -3 and -4 are mounted on the valves to give an indication of valve position. Compressor discharge air controlled by solenoid valve 20 CB is used to close the compressor bleed valves. Air from compressor discharge is piped to a porous air filter which removes dirt and water from the compressor discharge air, by means of a continuous blow-down orifice, before the air enters solenoid valve 20 CB. From the solenoid valve, the air is piped to the piston housings of the four extraction valves. During turbine start-up, 20 CB is de-energized and the 11th stage extraction valves are open allowing 11th stage air to be discharged into the exhaust plenum thereby eliminating the possibility of compressor pulsation. Limit switches, 33 CB-1 through -4, on the valves provide permissive logic in the starting sequence and ensure that the extraction valves are fully opened before the turbine is fired. The turbine accelerates to 95% speed and then the 20CB solenoid valve is energized to close the extraction valves and allow normal running operation of the turbine. When a turbine shut-down signal is initiated and the generator circuit breaker is opened, 20CB is de-energized and 11th stage air is again discharged into the exhaust plenum to prevent compressor pulsation during the turbine shut-down period. CAUTION: Under no circumstances should attempts be made to start the turbine if all four extraction bleed valves are not fully opened. Serious damage to the compressor blades may occur if all the valves are not opened during the accelerating and decelerating cycle of the gas turbine.


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3 . C O M B U S T IO N S E C T IO N The combustion system is of the reverse-flow type with 14 combustors arranged around the periphery of the compressor discharge casing. This system also includes fuel nozzles, spark plug ignition system, flame detectors and crossfire tubes. Hot gases, generated from burning fuel in the combustors, are used to drive the turbine. High pressure air from the compressor discharge is directed around the transition pieces and into the combustor liners. This air enters the combustion zone through metering holes for proper fuel combustion and through slots to cool the combustor liner. Fuel is supplied to each combustor through a nozzle designed to disperse and mix the fuel with the proper amount of combustion air. Combustors are numbered counterclockwise when viewed looking down-stream and starting from the top of the machine. 1-

Combustion Wrapper:

The combustion wrapper forms a plenum in which the compressor discharge air flow is directed to the combustors. Its secondary purpose is to act as a support for the combustor’s assemblies. In turn, the wrapper is supported by the compressor discharge casing and the turbine shell.

Combustor Liner GT 5-8

2-

Combustors:

Discharge air from the axial-flow compressor flows into each combustion flow sleeve from the combustion wrapper (see figure). The air flows upstream along the outside of the combustion liner toward the liner cap. This air enters the combustor’s reaction zone through the fuel nozzle swirl tip, through metering holes in both the cap and liner and through combustion holes in the forward half of the liner. The hot combustion gases from the reaction zone pass through a thermal soaking zone and then into a dilution zone where additional air is mixed with the combustion gases. Metering holes in the dilution zone allow the correct amount of air to enter and cool the gases to desired temperature. Along the length of the combustion liner and in the liner cap are openings whose function is to provide a film of air for cooling the walls of the liner and cap as shown in figure. Transition pieces direct the hot gases from the liners to the turbine nozzles. All 14 combustion liners, flow sleeves and transition pieces are identical.


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3-

Crossfire Tubes:

All fourteen combustors are interconnected by means of crossfire tubes. Once a flame is established in one combustor, the difference of pressure existing between a fired combustor basket and an unfired one, is enough to cause a temporary flame through the crossfire tube which fires the combustor basket unfired. 4-

Spark Plugs:

Combustion is initiated by means of the discharge from two high-voltage retractable electrode spark plugs installed in adjacent combustors no. 12 and 13. These springinjected and pressure-retracted plugs receive their energy from ignition transformers at 13 KV. At the time of firing, a spark at one or both of these plugs ignites the gases in a chamber; the remaining chambers are ignited by crossfire through the tubes that interconnect the reaction zone of the remaining chambers. As rotor speed increases, chamber pressure causes the spark plugs to retract and the electrodes are removed from the combustion zone.

Top Cover

Retaining Nut Lock Plate Insulator Gasket

Tie Bolt

4.5" (114 mm) Stroke

Terminal Extension Spring Cylinder

Piston / Rod Assembly

Spark Plug Washers

Core Assembly Equal Gaps within 0.01" (0.25 mm)

0.054" (1.37 mm) 0.000" (0.00 mm)

Figure CI-40 Spark Plug Assembly 13 KV 5-

Flame Detectors:

During the starting sequence, it is essential that an indication of the presence or absence of flame be transmitted to the control system. For this reason, a flame monitoring system is used consisting of four sensors which are installed on four combustors no. 3, 4, 5 and 11, and an electronic amplifier which is mounted in the turbine control panel. The ultraviolet flame sensor consists of a flame sensor, containing a gas filled detector. The gas within this flame sensor detector is sensitive to the presence of ultraviolet radiation which is emitted by a hydrocarbon flame. A dc voltage, supplied by the amplifier, is impressed across the detector terminals. If flame is present, the ionization of the gas in the detector allows conduction of current in the circuit which activates the electronics to give an output defining flame. Conversely, the absence of flame will generate an opposite output defining "no flame". After the establishment of flame, if voltage is re-established to the sensors defining the loss (or lack) of flame a signal is sent to a relay panel in the turbine electronic control circuitry where auxiliary relays in the turbine firing trip circuit, starting means circuit, etc. shut down the turbine. The “Failure to Fire” or “Loss of Flame” is also indicated on the annunciator. If a loss of flame is sensed by two flame detector sensor, the control circuitry will cause an annunciation only of this condition.


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6-

Fuel Nozzles:

Each combustor is equipped with a fuel nozzle that emits the metered amount of the required fuel into the combustion liner. The fuel nozzle functions to distribute the liquid fuel into the reaction zone of the combustion liner, in a manner which promotes uniform, rapid and complete combustion. Purge Air Fuel Gas

GCV Gas Control Valve

SRV Speed Ratio Valve

Combustion Air Gas outlet

Purge air VCK-2

Atomizing Air HSD or Furnace Oil

VCK-1

Liquid Fuel Drain

Liquid Fuel Inlet

To Sump Tank

Combustion Air Atomizing air

Atomizing air is utilized with liquid fuel to assist in the formation of a finely divided spray. The liquid fuel and atomizing air enter the fuel nozzle assembly through separate connections. Then, they are introduced through separate but concentric passages in the nozzle body. Fuel enters the inner passage. Atomizing air enters around the fuel nozzle and split fuel into very fine particles. In this way fuel ignites easily and burns completely. Fuel burning process completes within combustor liner zone and flame finishes after the liner. Therefore, only flue gas enters into the transition piece. 7-

Transition Pieces

Transition pieces direct the hot gases from the liners to the turbine first stage nozzle. Thus, the first nozzle area is divided into 14 equal areas receiving the hot gas flow. The transition pieces are sealed to both the outer and inner sidewalls on the entrance side of the nozzle, so minimizing leakage of compressor discharge air into the nozzles. 8-

False Start Drain

In liquid fuel units, for safety reasons in the event of an unsuccessful start, the accumulation of combustible fuel oil is drained through false start drain valves provided at appropriate low points in the combustion wrapper (VA 17-1), turbine area (VA 17-2) and lower part of the turbine exhaust frame (VA 17-3).

From turbine drains

Compressor discharge air Oil to sump tank

False start drain valves VA 17-1, VA 17-2 & VA 17-5

Water to washing pit

The false start drain valves, normally open, are closed during start-up when the turbine speed reaches a sufficient value. Air pressure from the axial-flow compressor discharge is used to actuate these valves. During the turbine shut-down sequence, the valves open as compressor speed drops (compressor discharge pressure is reduced). When GT is started, these valve are opened at start and closed at about 35-40% speed. False start drain valve is a three way valve. Normally its position is towards sump tank and any leakage of liquid fuel oil goes towards sump tank. During turbine washing their position is changed towards washing pit and water goes to washing pit. After completion of washing three way valve’s position is again changed towards sump tank.


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4 . T U R B IN E S E C T IO N The three stage turbine section (Fig-2) is the area in which energy in the form of high pressurized gas, produced by the compressor and combustion sections, is converted to mechanical energy. Each turbine stage is comprised of a nozzle and the corresponding wheel with its buckets. Turbine section components include; • turbine

• shroud

• turbine

• exhaus

rotor shell

• nozzle

s

4.1

s

t frame • exhaus t diffuse r

T U R B I NROE T O R

The turbine rotor assembly consists of two wheel shafts; the first, second and third-stage turbine wheels with buckets; two turbine spacers and three bearings. The wheels are held together with through bolts. The forward wheel shaft extends from the first-stage turbine wheel to the aft flange of the compressor rotor assembly. The journal for the no. 2 bearing is a part of the wheel shaft. The aft wheel shaft connects the third-stage turbine wheel to the load coupling. It includes the no. 3 bearing journal. Rotor Blades or Buckets: The turbine buckets increase in size from the first to third-stage. Flue gas pressure is decreased due to energy conversion in each stage, therefore, the bucket size must be increased to accommodate the gas flow. In all heavy duty "General Electric Company" gas turbines high energy "impulse" turbines are used as contrasted with lower energy "reactive" design. For an impulse turbine less number of stages are required for a given output, but more important is that, it permits high operating temperature for a given bucket life. In a pure impulse turbine, expansion process takes place only through the fixed nozzles. In the reactive design, expansion takes place both at fixed nozzles and rotating buckets. The turbine buckets


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in MS 9001 E (3 stages, GT 5-8) have impulse airfoil shape at the root and progressively reactive airfoil shape at their top (see Figure). Therefore, the blade’s foil shape is 50% impulse and 50% reaction. The hot gas mixture coming from the combustion system is guided towards the first stage nozzle through transition pieces which provide an as uniform as possible circumferential hot gas flow distribution to the first stage turbine nozzle. Temperature at first stage nozzle is controlled by the gas turbine control system by means of controlling the hot gas temperature in the exhaust casing. This is important for the turbine life because of thermal stresses. At base load operation, temperature on turbine first nozzle must not exceed 1004 °C. The hot gas stream expands through the first stage nozzle and it leaves with a whirl to attack the first stage turbine wheel, where kinetic energy is converted to mechanical energy. This process is repeated in the other turbine stages. Turbine Rotor Cooling: The first-stage buckets are the first rotating surfaces encountered by the extremely hot gases leaving the first-stage nozzle. Each first-stage bucket contains a series of longitudinal air passages for bucket cooling. Air is introduced into each first-stage bucket through a plenum at the base of the bucket dovetail. It flows through cooling holes extending the length of the bucket and exits at the recessed bucket tip. The holes are spaced and sized to obtain optimum cooling of the airfoil with minimum compressor extraction air. Like the first-stage buckets, the second-stage buckets are cooled by spanwise air passages the length of the airfoil. Since the lower temperatures surrounding the bucket shanks do not require shank cooling, the second-stage cooling holes are fed by a plenum cast into the bucket shank. Spanwise holes provide cooling air to the airfoil at a higher pressure than a design with shank holes. This increases the cooling effectiveness in the airfoil so airfoil cooling is accomplished with minimum penalty to the thermodynamic cycle. The third-stage buckets are not internally air cooled; the tips of these buckets, like the second-stage buckets, are enclosed by a shroud which is a part of the tip seal. These shrouds interlock from bucket to bucket to provide vibration damping.


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The turbine rotor must be cooled to maintain reasonable operating temperatures and, therefore, assure a longer turbine service life. Cooling is accomplished by means of a positive flow of cool air radially outward through a space between the turbine wheel with buckets and the stator, into the main gas stream. This area is called the wheel-space. There are 14 thermocouples before and after the nozzles to measure the wheel-space temperature. Six thermocouples are at stage-1, four are at stage-2 and four are at stage-3. Following wheel-space reading are taken from log sheet of GT-5, 14:00 hrs, dt. 05/02/08. Compressor inlet temperature 14 °C and Compressor discharge temperature 330°C. Thermocouple No.

Temp

Thermocouple No.

Temp

Thermocouple No.

Temp

1st ST FWD WS IN TTW SIF/1

350 °C

2nd ST FWD WS OUT TTWS 2F01

397 °C

3rd ST FWD WS OUT TTWS 3F01

366 °C

st

nd

rd

1 ST FWD WS IN TTW SIF/2

391 °C

2 ST FWD WS OUT TTWS 2F02

352 °C

3 ST FWD WS OUT TTWS 3F02

386 °C

1st ST FWD WS OUT TTW SIF01

341 °C

2nd ST AFT WS OUT TTWS 2A01

360 °C

3rd ST FWD WS OUT TTWS 3A01

292 °C

st

1 ST FWD WS TTW SIF02

356 °C

1st ST FWD WS TTW SIA01

400 °C

st

1 ST FWD WS OUT TTW SIA02

nd

2 ST AFT WS OUT TTWS 2A02

356 °C

rd

3 ST FWD WS OUT TTWS 3A02

338 °C

369 °C

The turbine rotor is cooled by means of a positive flow of relatively cool air (≈ 300 °C, relative to hot gas path air) extracted from the compressor. Air extracted through the rotor, ahead of the compressor 17th stage, is used for cooling the 1st and 2nd stage buckets and the 2nd stage aft and 3rd stage forward rotor wheel spaces. This air also maintains the turbine wheels, turbine spacers and wheel shaft at approximately compressor discharge temperature to assure low steady state thermal gradients thus ensuring long wheel life. The 1st stage forward wheel-space is cooled by air that passes through the high pressure packing seal at the aft end of the compressor rotor. The 1st stage aft and 2nd stage forward wheel spaces are cooled by compressor discharge air that passes through the first stage shrouds and then radially inward through the stage 2 nozzle vanes. The 3rd aft wheel-space is cooled by cooling air that exits from the exhaust frame cooling circuit. 4.2

T U R B I NSTE A T O R

The turbine shell and the exhaust frame constitute the major portion of the gas turbine stator structure. The turbine nozzles, shrouds, bearing no.3 and turbine exhaust diffuser are internally supported from these components. 

Turbine Shell

The turbine shell controls the axial and radial positions of the shrouds and nozzles. It determines turbine clearances and the relative positions of the nozzles to the turbine buckets. This positioning is critical to gas turbine performance.


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Hot gases contained by the turbine shell are a source of heat flow into the shell. To control the shell diameter, it is important that shell design reduces the heat flow into shell and limits its temperature. Heat flow limitations incorporate insulation, cooling and multilayered structures. The external surface of the shell incorporates cooling air passages. Flow through these passages is generated by frame blower cooling fans 88 TK 1, 2.



Turbine Nozzles

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

18 Segments, 2 airfoils





First-Stage Nozzles

The first-stage nozzle receives the hot combustion gases from the combustion system via the transition pieces. The transition pieces are sealed to both the outer and inner sidewalls on the entrance side of the nozzle; this minimizes leakage of compressor discharge air into the nozzles. The 18 cast nozzle segments, each with two partitions of airfoils, are contained by a horizontally split retaining ring which is centreline supported to the turbine shell on lugs at the sides and guided by pins at the top and bottom vertical centrelines. This permits radial growth of the retaining ring, resulting from changes in temperature while the ring remains centred in the shell. The aft outer diameter of the retaining ring is loaded against the forward face of the firststage turbine shroud and acts as the air seal to prevent leakage of compressor discharge air between the nozzle and shell. First-stage nozzle is cooled with compressor discharge air.  Second-Stage Nozzle Combustion air exiting from the first-stage buckets is again expanded and redirected against the second-stage turbine buckets by the second-stage nozzle. This nozzle is made of 16 cast segments, each with three partitions or airfoils. The male hooks on the entrance and exit sides of the outer sidewall fit into female grooves on the aft side of the first-stage shrouds and on the forward side of the second-stage shroud, to maintain the nozzle concentric with the turbine shell and rotor. This close fitting tongue-and-groove fit between nozzle and shrouds acts as an outside diameter air seal. The nozzle segments are


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held in a circumferential position by radial pins from the shell into axial slots in the nozzle outer sidewall. Second-stage nozzle is cooled with compressor discharge air. 

Third-Stage Nozzles The third-stage nozzle receives the hot gas as it leaves the second-stage buckets, increases its velocity by pressure drop, and directs this flow against the third-stage buckets. The nozzle consists of 16 cast segments, each with four partitions or airfoils. It is held at the outer sidewall forward and aft sides in grooves in the turbine shrouds in a manner identical to that used on the second-stage nozzle. The third-stage nozzle is circumferentially positioned by radial pins from the shell.

 Diaphragms Attached to the inside diameters of both the second and third-stage nozzle segments are the nozzle diaphragms (see figure at previous page). These diaphragms prevent air leakage past the inner sidewall of the nozzles and the turbine 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 stationary parts of diaphragm and nozzles and the moving rotor are essential for maintaining low interstage leakage; this results in higher turbine efficiency, due to which it can take more load.  Shrouds Unlike the compressor blading, the turbine bucket tips do not run directly against an integral machined 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. The turbine shrouds' secondary function is to provide a high thermal resistance between the hot gases and the comparatively cool shell. By accomplishing this function, the shell cooling load is drastically reduced, the shell diameter is controlled, the shell roundness is maintained and important turbine clearances are assured. The shroud segments are maintained in the circumferential position by radial pins from the shell. Joints between shroud segments are sealed by interconnecting tongues and grooves.

4 . 3 R O T O RAL I G N M

EN T

Generator rotor is 1.25 mm above than turbine rotor. After GT start, when turbine rotor becomes hot then it becomes high. To compensate this misalignment bearing no. 3 has special arrangement of tilting pads. 4.4

E X H A U SFRT A M AE N DDI F F U S E R

On exhaust, at normal load the hot gas temperature is still high at about 500 °C for the gas turbines 5-8. In simple cycle, exhaust system opens in ambient atmosphere through a duct assembly comprising filter and silencer. The exhaust frame assembly consists of the exhaust frame and the exhaust diffuser. The exhaust frame is bolted to the aft flange of the turbine shell. Structurally, the frame consists of an outer cylinder and inner cylinder interconnected by ten radial struts. The inner gas path surfaces of the two cylinders are attached the inner and outer diffusers. The bearing no.3 is supported from the inner cylinder.


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The exhaust diffuser, located at the extreme aft end of the gas turbine, bolts to, and is supported by, the exhaust frame. The exhaust frame is a fabricated assembly consisting of an inner cylinder and an outer divergent cylinder that flares at the exit end at a right angle to the turbine centreline. At the exit end of the diffuser between the two cylinders are five turning vanes mounted at the bend. Gases exhausted from the third turbine stage enter the diffuser where velocity is reduced by diffusion and pressure is recovered. At the exit of the diffuser, turning vanes direct the gases into the exhaust plenum. Exhaust frame radial struts cross the exhaust gas stream. These struts position the inner cylinder and bearing № 3 in relation to the outer casing of the gas turbine. The struts must be maintained at a uniform temperature in order to control the centre position of the rotor in relation to the stator. This temperature stabilization is accomplished by protecting the struts from exhaust gases with a metal fairing fabricated into the diffuser and then forcing cooling air from 88 TK 1,2 into this space around the struts. Turbine shell cooling air enters the space between the exhaust frame and diffuser and flows in two directions. The air flows in one direction into the turbine shell cooling annulus and also down through the space between the struts and the airfoil fairings surrounding the struts and subsequently into the load shaft tunnel and turbine third-stage aft wheel-space.

5 . B E A R IN G S The MS 9001 E gas turbine unit 5-8 contain three main journal bearings used to support the compressor-turbine rotor. The unit also includes thrust bearings to maintain the rotorto- stator axial position. These bearing assemblies are located in three housings: one at the air inlet, one in the compressor discharge casing, and one in the exhaust frame. Bearing metal temperature detectors have been installed on all bearings. High temperature alarm limit is 130 °C. All bearings are pressure-lubricated by oil supplied from the main lubricating oil system. The oil flows through branch lines to an inlet in each bearing housing.

BEARINGS No.

Class

Type

Vibration Pick-ups

1

Journal

Elliptical

BB-1 / BB-2

1

Loaded Thrust

Self-Aligned


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(Equalized) 1

Unloaded Thrust

Tilting Pad

2

Journal

Elliptical

BB-3

3

Journal

Tilting Pad

BB-4 / BB-5

4

Journal

BB-7 / BB-8

5

Journal

BB-9

 Lubrication The three main turbine bearings are pressure-lubricated with oil supplied by the 12,540 litres capacity lubricating oil reservoir. Oil feed piping, where practical, is run within the lube oil reservoir drain line, or drain channels, as a protective measure. This procedure is referred to as double piping and its rationale is that in the event of a pipe-line leak, oil will not be lost or sprayed on nearby equipment, thus eliminating a potential safety hazard. When the oil enters the bearing housing inlet, it flows into an annulus around the bearing liner. From the annulus the oil flows through machined slots in the liner to the bearing surface. Oil is prevented from escaping along the turbine shaft by labyrinth seals.  Oil Seals Oil on the surface of the turbine shaft is prevented from being spun along the shaft by oi1 seals in each of the three bearing housings. These labyrinth packings and oil deflectors (teeth type) are assembled on both sides of the bearing assemblies where oil control is required. A smooth surface is machined on the shaft and the seals are assembled so that only a small clearance exists between the oil and seal deflector and the shaft. The oil seals are designed with two rows of packing and an annular space between them. Pressurized sealing air is admitted into this space and prevents lubricating oil from spreading along the shaft. 

Oil Vent

Some of the sealing air returns with the oil to the main lubricating oil reservoir and is vented through a lube oil vent. A vent pipe takes air from the tank and an air nozzle or venturi sucks air from the lube oil tank. Blower fan 88FX-1 provides air to the venturi. 5.1

B E A R I N№G . 1O U( JR N A&L HTR U SBTE A R I )N G

The № 1 bearing assembly is located in the centre of the inlet casing assembly and contains three bearings; (1) (2)

(3)

active (loaded) thrust bearing inactive (unloaded) thrust bearing journal bearing

Additionally, it contains a floating ring or shaft seal, labyrinth seals and a housing in which the components are installed. The components are keyed to the housing to prevent rotation. The № 1 bearing assembly is centre-line supported from the inner cylinder of the inlet casing. This support includes ledges on the horizontal and an


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axial key at the bottom centre line. The upper half of the bearing housing can be removed for bearing liner inspection without the removal of the upper half inlet casing. The lower half of the bearing assembly supports the forward stub shaft of the compressor rotor. Labyrinth seals at each end of the housing are pressurized with air extracted from 5th stage of the compressor. The floating ring seal and a double labyrinth seal at the forward end of the thrust bearing cavity are to contain oil and to limit entrance of air into cavity. 5.2

B E A R I N G№ . 2 O U (RJ N ABEL A R I )N G

The № 2 bearing assembly is centre-line supported from the inner cylinder of the compressor discharge casing. This support includes ledges on the horizontal and an axial key at the bottom centre-line permitting relative growth resulting from temperature differences while the bearing remains centred in the discharge casing. The lower half of the bearing assembly supports the forward wheel shaft of the turbine rotor assembly. This assembly includes three labyrinth seals at both ends of the housing. The № 2 bearing is located in a pressurized space between the compressor and the turbine, and air leaks through the outer labyrinth at each end of the housing. The space between the two other seals is cooled by air extracted from the 5th compressor stage.

Air flows through this seal into the drain space of the housing and is vented outside the machine via the inner pipe connecting to the bottom of the housing. This drain space vent piping continues to the lubricating oil tank. The middle labyrinth prevents the hot air leakage from mixing with the oil. The mixture of hot air and cool air is vented outside the unit via the outer pipe connected at the top of the bearing housing. 5.3

B E A R I N№G . 3O U( JR N ABEL A R I NW GI T HTI L T I N PGA D )S

The № 3 bearing assembly is located at the aft end of the turbine shaft in the centre of the exhaust frame assembly. It consists of a tilting pad bearing, five labyrinth seals and a


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bearing housing. The individual pads are assembled so that converging passages are created between each pad and the bearing surface. These converging passages generate a high-pressure oil film beneath each pad, which produces a symmetrical loading or "clamping" effect on the bearing surface. The clamping action helps maintain shaft stability. Because the pads are point-pivoted, they are free to move in two directions, which make them capable of tolerating both offset and angular shaft misalignment. The tilting pad journal bearing comprises two major components pads and a retainer ring. The retainer ring serves to locate and support the pads. It is a horizontally split member that contains the pad support pins, adjusting shims, oil feed orifice and oil discharge seals. The support pins and shims transmit the loads generated at the pad surfaces and are used to set the bearing clearance. An anti rotation pin extends from one edge of the lower half of the rectangular ring. This pin locates the bearing within its housing and serves to prevent the bearing from rotating with the shaft.

Bearing No. 3, Upper half with tilting pads

6 . C O U P L IN G S Couplings are used to transmit starting torque from the accessory gear to the gas turbine axial compressor and to transmit shaft horsepower from the turbine to the driven generator. Couplings are of two types; rigid and flexible. 6.1

L O A DCO U P L I N G

A rigid, hollow coupling connects the turbine rotor shaft to the generator rotor. A bolted flange connection forms the joint at each end of the coupling. 6.2

A C C E S S O GRE YA RCO U P L I N G

An oil filled flexible coupling is used to connect the accessory drive to the gas turbine shaft at the compressor end. The coupling is designed to transmit the starting and driving torque of starting motor and turning gear as well as to provide flexibility to accommodate nominal misalignment and axial movement of the turbine rotor relative to the accessory gear box. There are three types of misalignment that is accommodated by the coupling: angular, parallel and a combination of both.

7 . S T A R T IN G S Y S T E M Before the gas turbine can be fired and started it must be rotated or cranked by accessory equipment. This is accomplished by an induction motor, operating through a torque


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converter to provide the cranking torque for speed required by the turbine for start-up. The starting system consists of: Starting motor induction type Torque Converter: Voith Germany Torque Adjusting Motor Turning Gear Motor 7.1

88 CR Voith 88 TM 88 TG

1000 kW, 6600 V, 2975 rpm 0.5 kW, 415 V, 2820 rpm 30 kW, 415 V, 750 rpm

T O R Q UCEO N V E R T E R

A motor driven torque adjustor drive 88 TM, provides the means for adjusting torque output within specified ranges. Control of the torque converter is achieved via a solenoid valve 20 TU-1 and a hydraulically operated dump valve.

After a shut down order, when speed decreases to 99 rpm, the torque converter motor sets it to 11 turns and a turning motor 88 TG specially provided to rotate the turbine for cooldown purpose starts. Then speed is increased and turning speed value is about 120 rpm. The main parts of the torque converter are the impeller driven by the input shaft, the turbine wheel which drives the output shaft and the stator which directs fluid from the impeller to the turbine at the correct angle to produce the required output torque. Starting motor is linked to an impeller called pumpwheel. The turbine shaft is linked to a turbine-wheel. The pump-wheel and turbine-wheel do not have any mechanical contact. Two seconds after the start up of cranking motor, the torque converter is filled with oil, extracted from lube oil circuit at 6.9 bar by energizing solenoid valve 20 TU-1. Pump-wheel transforms the power of the cranking motor into a manometric lift (i.e. increase in pressure) of oil. The turbine-wheel transforms this manometric lift into a rotating power and transmits it to the turbine shaft. That means, the oil transmits the power. Due to great difference of speeds at the beginning of the start up sequence Prepared by: Fazal-ur-Rehman Babar

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(Cranking motor 2975 rpm, turbine shaft 120 rpm) torque converter transmits a very high torque to the turbine shaft and thus breaks it away. To adjust various speed limits for turning, start up, warm up, acceleration, washing etc. the torque converter is equipped with a range of variable vanes, automatically operated by the torque adjusting motor or manually by a hand-wheel drive. These vanes, installed in converter housing, admit circulation of certain oil flow corresponding to their position. That means, output speed is limited by reducing the output power through variable vanes. The torque adjustor drive 88 TM rotates the blades of the impeller. When the impeller blades are fully closed there is no transfer of torque and when blades are fully open maximum torque is transferred. The number of turns, as it can be seen from the top of torque converter, depends on the operation of limit switch. Six switches are provided for torque adjustment; 33 TM-1 33 TM-2 33 TM-3 33 TM-4

Adjusted at 6 turns Adjusted at 3 turns Adjusted at 11 turns Adjusted at 22 turns

33 TM-5

Adjusted at 0 turns

33 TM-6

Adjusted at 30 turns

7.1.1

Washing speed Minimum firing torque for warm up Turning gear operation To obtain the maximum torque; during accelerating phase upto 60 % speed For minimum torque limit; to limit the torque in case of malfunction of the system For high torque limit

F u n c tio n a l D e s c rip tio n o f T o rq u e C o n v e rte r

1. Start-up In the normal starting sequence, fluid is admitted into the torque converter hydraulic circuit from the lubrication system by the integral valve 20 TU-1 two seconds after the starting motor 88 CR is energized. Torque converter is adjusted at 22 turns to achieve maximum torque of the starting motor. The turbine begins to increase in speed and continues to accelerate until firing speed is attained and relay 14 HM picks up. When the turbine has reached this speed (14 HM setpoint), the internal geometry of the torque converter is adjusted to 3 turns by the torque adjustor drive 88 TM to hold firing speed constant throughout the firing and warm-up cycle. Readjustment of the converter geometry (torque adjustment) at the end of warm-up allows the torque converter to assist in accelerating the unit up to self sustaining speed. At this speed, (about 60 % speed), the torque converter hydraulic circuit is drained, by deenergizing solenoid valve 20 TU-1, at the same time cranking motor 88 CR is deenergized, which effects disconnect. A crank and restart can be initiated at any time below 14 HT speed (at turning gear). 2. Shut-down The shut-down order is given and the turbine speed slows down. When relay 14 HP drops out (at about 99 rpm), the turning motor 88 TG starts. Solenoid valve 20 TU-1 is energized and the torque is adjusted to 11 turns allowing to turn the turbine at a speed of about 120 rpm for cool down purposes after shut down. 3.

Turning

The turbine is at standstill and all circuits are ready for turning. The operator turns the operation selector switch 43 of the turbine control panel to position TURNING, then Prepared by: Fazal-ur-Rehman Babar

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gives a START order. The starting motor 88 CR starts, 20 TU-1 is energized and torque converter is adjusted to 22 turns. When the speed reaches about 120 rpm, motor 88 CR is stopped. The speed decreases a little and at about 99 rpm, turning motor 88 TG starts. Torque converter is readjusted at 11 turns and it allows a turning speed of about 120 rpm. NOTE: Torque Converter is drained at 60 % speed by opening the 20 TU-1 solenoid valve, but if does not drained then indication “Torque Converter drain valve trouble” is appeared on the screen and the m/c would take shut down command.

8 . A C C E S S O R Y D R IV E The function of the accessory gear in this system is to drive a number of the control components and to provide connection between the starting motor and the gas turbine compressor. It is permanently coupled to the turbine compressor shaft by a flexible coupling. Location of air ejector

Automizing air compressor shaft TURBINE SIDE Main Oil Pump

Fuel Pump

Driving shaft

Mechanical Overspeed

Main HP Oil Pump

The accessory drive gear, located at the compressor end of the gas turbine, is a gearing assembly. Contained within the gear casing are the gear trains which provide the proper gear reductions to drive the accessory devices at the required speed, with the correct torque values. Its functions are to: Drive accessories of gas turbine at proper speed. • Connect and disconnect the turbine by its starting motor. • In addition it contains the turbine overspeed bolt and trip mechanism. Accessories driven by the gear include: •

• •

• •

The main lube oil pump The main hydraulic supply pump The liquid fuel pump and The main atomizing air compressor

The speeds and functions of shafts are: Shaft 1:

3000 rpm


Driving shaft GT 5-8

47

Shaft 2: Shaft 3A: Shaft 3B: Shaft 4:

3424.2 rpm 1554.2 rpm 6607.2 rpm 1421.9 rpm

A tachometer on turbine side Fuel pump through electromagnetic clutch It drives atomizing air compressor HP hydraulic oil pump on starting motor side and Lube oil pump on turbine side

A high-pressure turbine overspeed trip, capable of mechanically dumping the oil in the trip circuits, is mounted on the exterior casing of the gear. This device can shut the turbine down when the speed exceeds the design speed. The overspeed bolt which actuates the trip upon overspeed is installed in the main shaft. Lubrication of the gear is from the turbine’s pressurized bearing header supply. 9.

C O M P A R T M EE NN CT LS O/ S U R E S

Gas Turbine 5-8 is a package type unit; it is divided into various compartments/ enclosures. Gas turbine enclosures, referred to as compartments, are those partitioned areas in which specific components of the overall power plant are contained. These compartments are built for all weather conditions and designed for accessibility when performing maintenance. They are provided with thermal and acoustical insulation and lighted for convenience. Compartment construction includes removable panels, hinged doors and a thermally insulated roof section with welded frame structuring providing the support for these parts. The panels are thermally insulated and held in place with bolts. Doors are kept tightly closed by sturdy latches. Gaskets between panels and framing maintain a weather-tight condition. Inspection and maintenance are facilitated as the door panels allow easy access for station personnel and the removable panels provide greater accessibility for major inspections and overhauling. There is an inlet plenum between the accessory and the turbine compartments and an exhaust plenum between the turbine and generator compartments. Thus, in the compact integrated gas turbine-generator design, there is an in-line sequence of lagged compartments, the sequence being broken by the inlet plenum and the exhaust plenum. These compartments are named as:


Exhaust Compartment

Compartment

Load Coupling Compartme nt

1Accessory compartment 2Turbine compartment 3Exhaust compartment 4Load coupling compartment Generator 5Generator compartment

Turbine Compartment

Accessory Compartment

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1 0 .G E N E R A L A N D A C C E S S O R Y B A S E Most of the mechanical and electrical auxiliary equipment necessary for starting and operating the gas turbine is contained within the accessory compartment. This enclosure and its components are mounted on a separate structural base located forward of the gas turbine compartment. Many systems are involved in the operation of the turbine. Several of these systems have accessory devices, or mechanisms, located within the accessory section; namely, • • •

Liquid fuel system, Lubrication system, Atomizing air system

•

Gas system,

•

Hydraulic system,

•

On-base portion of the turbine cooling water system

Several major components mounted on the accessory base include; • •

Turning gear motor, torque converter,

•

starting motor,

•

accessory drive gear

Besides being the main link between the starting system drive components and the gas turbine, the accessory drive gear is the gear reduction unit connected directly to the turbine for driving several of the accessory devices of the gas turbine support systems. A pressure gauge and switch cabinet located on the left side of the accessory compartment (looking downstream), contains panel mounted gauges and switches used with the systems mentioned above. 10.1

A C C E S S O BRA YS E

The base of the accessory compartment is a structural steel assembly, fabricated of steel I-beams and plate. It forms a mounting or support platform for the accessory gear, starting equipment and various accessory equipment devices besides forming the lube storage tank (reservoir). The interior of the base forms a self-contained, lube storage reservoir located between the top and bottom plates of the steel beam framework. The bottom plate of the lube reservoir is positioned at a slight angle that slopes toward two drain pipes and plugs, one near each end of the base. A bolted access cover plate is provided at the control compartment end for access to the lube reservoir. Fabricated supports and mounting pads are welded to the upper surface of the base for mounting the accessory gear, starting motor, fuel oil pump, and other accessory


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components. Four lifting trunnions and supports are provided, one near each corner of the base, for lifting the accessory compartment. Finished pads or sole plates, located on the bottom of the base, facilitate its mounting on the site foundation.

1 1 .I N L E T A N D E X H A U S T S E C T I O N S It is necessary to treat incoming atmospheric air before it enters the turbine in order to adapt to the environment and realize the desired machine performance. Specially designed equipment is installed to modify the quality of the incoming air to make it suitable for use in the unit. It is necessary also to attenuate the high frequency noise in the air inlet, caused by the rotating compressor blading. At the exhaust end of the gas turbine, gases produced as the result of combustion in the turbine require specific equipment according to their exhaust to atmosphere or towards heat recovery boilers. 11.1

A I R IN L E TSY S T E M

The air inlet system is at down-stream of the air filtering installation. It consists of an air duct, followed by sections of parallel baffles silencers, then a screen system located in an inlet elbow, and an expansion joint after which air will reach the gas turbine air inlet plenum. The gas turbine inlet plenum contains the compressor inlet casing. The silencers are of baffle-type construction to attenuate the high frequency noise in the air inlet, caused by the rotating compressor blading. 11.2

E X H A U SGAT S SY S T E M

In the exhaust section, the gases which have been used to power the turbine wheels are redirected to be either released to atmosphere, or towards a heat recovery boiler. After leaving the exhaust frame, the hot gases reach the diffuser, located in the exhaust plenum. On the exhaust plenum wall facing the exhaust diffuser there are 24 thermocouples arranged in circular measure exhaust gas temperature. The thermocouples send their signals to the gas turbine temperature control and protection system. From these thermocouples turbine spread is measured. The exhaust plenum configuration is that of a box open on a side and welded to an extension of the turbine base. Insulation in the plenum fabrication provides thermal and acoustical protection. A flow path from the exhaust plenum open side to a duct is provided by an extension plenum and an expansion joint. Two silencers are installed in series in the duct (the first one for the low frequency noises, the second one for the high frequency noises), after which there is another expansion joint, before exhaust either to atmosphere, upwards or to a recovery boiler.


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GAS TURBINE MODEL FRAME 9001 E SIMPLE - CYCLE, SINGLE - SHAFT, HEAVY - DUTY GAS TURBINE

GT 5-8

COMPRESSOR 1 Inlet plenum assembly 2 Inlet casing 3 Magnetic pickup arrangement 4 Thrust bearing 5 No 1 bearing 6 Variable inlet guide vane arrangement 7 Compressor blading 8 Compressor rotor assembly 9 Forward compressor casing 10 After compressor casing 11 Compressor discharge casing 12 Inner compressor discharge casing 13 Turbine forward support 14 Turbine base COMBUSTION 15 Combustion wrapper 16 Fuel nozzle assembly 17 Combustion liner 18 Transition piece 19 Combustion chamber arrangement 20 Spark plug 21 Flame detector TURBINE 22 Turbine casing & shrouds 23 First stage nozzle 24 Second stage nozzle & diaphragm 25 Turbine stage nozzle & diaphragm 26 Turbine rotor assembly -

Forward shaft First stage turbine wheel & bucket assembly Second stage turbine wheel & bucket assembly Third stage turbine wheel & bucket assembly Spacer wheels After shaft

27 No 2 bearing 28 No 3 bearing 29 Turbine after supports

AIR INLET

51

COMPRESSOR

COMBUSTOR

TURBINE

EXHAUST

EXHAUST 30 Exhaust hood 31 Exhaust diffuser 32 Load coupling 33 Turbine vanes 34 Control & regulation thermocouples 35 Exhaust plenum assembly

52

AIR INLET

52

COMPRESSOR

COMBUSTOR

TURBINE

EXHAUST

1 GT5-8 Comp &amp; Turbine Design

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