Introduction to Gas Turbines

General, Functional Description

 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 and ultimately the driven load generator.

The turbine unit is composed of a starting device, support systems, an axial-flow compressor, combustion system components, a three-stage turbine. Both compressor and turbine are directly connected with an in-line, single-shaft rotor supported by two 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.

About Gas Turbine Units

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 17th stage, axial flow Compressor. For pulsation protection during start-up, the 11th stage extraction valves are open and the variable inlet guide vanes are in the closed position. When the speed relay corresponding to 95 per cent speed actuates, the 11th stage extraction bleed valves close automatically and the variable inlet guide vane actuator energizes to open the inlet guide vanes (I.G.V.) to the normal turbine operating position.

Compressed air from the compressor flows into the annular space surrounding the fourteen combustion chambers, from which it flows into the spaces between the outer combustion casings and the combustion liners.

The fuel nozzles introduce the fuel into each of the fourteen combustion chambers 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 equipped combustion chambers is ignited, the remaining combustion chambers are also ignited by crossfire tubes that connect the reaction zones of the combustion chambers. After the turbine rotor approximates 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 combustion chambers expand into the fourteen separate transition pieces attached to the aft end of the combustion chamber 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 rotatable 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 buckets, a portion of the kinetic energy of the jet is absorbed as useful work on the turbine rotor.

After passing through the 3rd 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, and drive certain accessories.

Design Basis

Simple Cycle Gas Turbine Flow Diagram

The turbine unit is composed of a starting device, support systems, an axial-flow compressor, combustion system components, a three-stage turbine. Both compressor and turbine are directly connected with an in-line, single-shaft rotor supported by two 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.

Gas Turbine Lubrication System

NOTE: Lubricating oil recommendations are included in the "Gas turbine subcontractor’s Literature chapter".

The lubricating requirements for the gas turbine power plant are furnished by a common forced-feed lubrication system. This lubrication system, complete with tank, pumps, coolers, filters, valves and various control and protection devices, furnishes normal lubrication and absorption of heat rejection load of the gas turbine. Lubricating fluid is circulated to the three main turbine bearings, generator bearings, and to the turbine accessory gear and fuel pump. Also, lubricating fluid is supplied to the starting means torque converter for use as hydraulic fluid as well as for lubrication. Additionally, a portion of the pressurized fluid is diverted and filtered again for use by hydraulic control devices as control fluid.

Major system components include:

• Lube reservoir in the accessory base;

• Main lube pump (shaft driven from the accessory gear);

• Auxiliary lube pump and emergency lube pump;

• Pressure relief valve VR-1 in the main pump discharge;

• Lube fluid heat exchanger;

• Lube filters;

• Bearing header pressure regulator VPR-2-1.

Lube fluid temperatures are indicated on the thermocouples which may be located in the bearing header, bearing drains, and the oil tank. For turbine starting, a maximum of 800 SSU is specified for reliable operation of the control system and for bearing lubrication. A thermocouple, LT_OT-1A, prevents turbine start-up if the temperature of the lubricant is lower than the switch setting (only if applicable).

Lubricating fluid for the main, auxiliary and emergency pumps is supplied from the reservoir, While lubricating fluid used for control is supplied from the bearing header. This lubricant must be regulated to the proper, predetermined pressure to meet the requirements of the main bearings and the accessory lube system, as well as the hydraulic control and trip circuits.

Regulating devices are shown on the Lube System Schematic Diagram Figure LS-1. All lubricating fluid is filtered and cooled before being piped to the bearing header.

The reservoir for the lubrication system is the 3300 gallon (i.e. 12 491 l) tank which is fabricated as an integral part of the accessory base. Lubricating fluid is pumped from the reservoir by the main shaft driven pump (part of the accessory gear) or auxiliary or emergency Pumps at a pressure of 25 psig (i.e. 1.75 bar) to the bearing header, the accessory gear and The hydraulic supply system. After lubricating the bearings the lubricant flows back through various drain lines to the lube reservoir.

All lubricant pumped from the lube reservoir to the bearing header flows through the lube fluid heat exchanger(s) to remove excess heat and then through the cartridge type filters providing five micron filtration. The dual heat exchangers are connected in parallel.

Filtration of all lube oil is accomplished by a 5 micron, pleated paper filter installed in the lube system just after the lube oil heat exchanger. Two filters are used with a transfer valve installed between the filters to direct oil flow through either filter and into the lube oil header.

 Lubricating Oil Pumps 

Lubrication to the bearing header is supplied by three lube pumps:

1-The main lube supply pump is a positive displacement type pump mounted in and driven by the accessory gear.

2 -The auxiliary lube supply pump is a submerged centrifugal pump driven by an A.C. motor.

3 -The emergency lube supply pump is a submerged centrifugal pump driven by a D.C. motor.

Main Lube Pump

The main lube pump is built into the inboard wall of the lower half casing of the accessory gear. It is driven by a splined quill shaft from the lower drive gear. The output pressure to the lubrication system is limited by a back-pressure valve to maintain system pressure.

Auxiliary Lube Pump

The auxiliary lube pump is a submerged centrifugal type pump driven by an A.C. motor. It provides lubricant pressure during start-up and shut-down of the gas turbine when the main pump cannot supply sufficient pressure for safe operation. Operation of this pump is as follows:

The auxiliary lube pump is controlled by a low lube oil pressure alarm switch (63 QA-2). This low pressure level alarm causes the auxiliary pump to run under low lube oil pressure conditions as is the case during start-up or shut down of the gas turbine when the main pump, driven by the accessory drive device, does not supply sufficient pressure. At turbine start-up, the A.C. pump starts automatically when the master control switch on the turbine control panel is turned to the START position.

The auxiliary pump continues to operate until the turbine reaches approximately 95 per cent of operational speed.

At this point, the auxiliary (cooldown) lube pump shuts down and system pressure is supplied by the shaft-driven, main lube pump.

During the turbine starting sequence, the pump starts when the start signal is given. The control circuit is through the pressure level of pressure switch 63 QA-2. The pump will run until the turbine operating speed is reached (operating speed relay 14 HS picks up), even though the lube oil header is at rated pressure and the discharge pressure level (63 QA-2) is above alarm level setting.

When the turbine is on the shut-down sequence, this pressure transmitter will signal for the auxiliary pump to start running when the lube oil header pressure falls to the point at which pressure level alarm setting is reached.

Emergency Lube Pump

The emergency lube pump is a D.C., motor-driven pump, of the submerged centrifugal type. This pump supplies lube oil to the main bearing header during an emergency shutdown In the event the auxiliary pump has been forced out of service because of loss of A.C. power, or for other reasons. It operates as follows:

This pump is started automatically by the action of pressure transmitter 96 QA-2 whenever the lube pressure in the main bearing header falls below the pressure switch set ting.

Should the auxiliary pump fail during the shut-down sequence, because of an A.C. power failure or any other cause, the emergency lube pump will be started automatically by the action of low lube oil pressure transmitter 96 QA-2 and continue to run until the turbine shaft comes to rest.

Gas Turbine Heat Exchanger and Filters

Lube Fluid Heat Exchanger

The heat exchanger system is required to dissipate the heat absorbed by the lubricating fluid and to maintain the fluid at the proper bearing header temperature. This is accomplished by circulating cooling water through the cooling tubes of the heat exchanger as the lubricant flows over the tubes. Cooling water flow through the heat exchanger is controlled by temperature sensitive flow regulator valve VTR 1, that maintains the correct bearing temperature.

(See Cooling water system for information on this regulator valve). The lube fluid heat exchanger system uses a fluid-to-water cooler of the shell and tube bundle design. There is one heat exchanger, flange mounted in the lube reservoir in a horizontal position. A U-tube bundle extends into the center of the shell through which the cooling water is passed. The lube fluid flows in and out of the shell ; passing over the cooling tubes of the tube bundle. Cooling water connections are made at the external steel bonnet that bolts to the shell mounting flange through the tube sheet that supports the tubes of the tube bundle.

Filters

Main lube filtering system - Filtration of all lube oil is accomplished by a 5 micron, pleated paper filter installed in the lube system just after the lube oil heat exchanger.

One (duplex) filter is used with a transfer valve installed between the filters to direct oil flow through either filter and into the lube oil header.

The dual filters arranged side by side, are installed on the tank and connected into the pump discharge header through a manual transfer valve. Only one filter will be in service at a time, thus cleaning, inspection, and maintenance of the second one can be performed without interrupting oil flow or shutting the gas turbine down. By means of the manually operated, worm-driven transfer valve, one filter can be put into service as the second is taken out, without interrupting the oil flow to the main tube oil header. The transfer of operation from one filter to the other should be accomplished as follows:

1 -Open the filler valve and fill the standby filter until a solid oil flow can be seen in the flow sight in the filter vent pipe. This will indicate a "filled" condition.

2 -Operate the transfer valve with a wrench to bring the standby filter into service.

3 -Close the filler valve.

Filters should be changed when the differential pressure switch 63 QQ-1 indicates a differential pressure of 15 psig (i.e. about 1.03 bar). Refer to the "Gas turbine maintenance guide chapter" for inspection schedules. An alarm from 63 QQ-1 signals when the differential pressure exceeds 15 psig.

NOTE: For the detailed drawing of the lube oil system circuit and the settings, see "Gas turbine operation guide chapter”: Piping systems schematic.

Gas Turbine Cooling Water System

General

The cooling water system is a pressurized, closed system, designed to accommodate the heat dissipation requirements of the turbine, the lubrication system, the turbine support legs and the flame detector mounts. An aqueous solution of ethylene glycol is used in the system; therefore, it is capable of performing its function throughout the year if the ambient temperatures are not too high. During frost the cooling system must be filled with an aqueous solution of ethylene glycol. During high temperatures it is necessary to fill the system with a solution whose quality is specified in the "Gas turbine subcontractor’s literature chapter". In the text that follows, this solution is referred to as the cooling water.

Included in the cooling water system are the cooling cells, the pumps, miscellaneous valves and certain control and protection devices. The cooling system is connected to the customer's cooling water system.


Functional Description

The cooling water system circulates water as a cooling medium to maintain the lubricating oil at acceptable lubrication system temperature levels and to cool several turbine components. The system normally operates at a slightly positive pressure which results when the fuel in the system expands with the increase in temperature during operation.

During operation the coolant is supplied by the owner's cooling system and circulates through the chosen lube oil, the turbine support legs (in parallel with the system of heat exchanger) and the flame detector mounts. After absorbing the heat rejected by these items, the coolant flows through the owner's water cooling system where it is cooled.

The flame detector mounts are cooled to extend the life of the flame detectors. The coolant jackets on the flame detector mounts provide a thermal break in the heat conduction from the combustor can housing to the flame detector instrument.



Flow regulating valves

The coolant circuit for the lube oil has a temperature actuated 3-way valve (VTR-1) installed in the coolant inlet line to the heat exchanger.

This type of valve, which controls coolant flow to the heat exchanger, has a manually operated device which can override the thermal element. The manual override device should be used only when the valve's thermal element is inoperative but machine operation is required. Lube Oil feed header temperature is sensed by the bulb associated with the valve which controls the flow of coolant through the heat exchanger and maintains the lube oil temperatures at predetermined values. The valve automatically controls flow of the medium passing through it (coolant) to the heat exchanger by responding to temperature changes affecting the bulb.

The bulb contains a thermal-sensitive liquid which vaporizes when heated. Pressure thus generated in the bulb is transmitted through the capillary tube to the bellows, which positions the valve disc to control the flow of coolant through the heat exchanger. The valve is closed during turbine startup, and will start to open as the sensed fluid temperature approaches the control setting.

At the inlet of each cooling water circuit (lube oil heat exchanger circuit and turbine support legs circuit), an orifice allows water flow rate calibration to the circuit concerned.


Shut-off valves

Shut-off valves are provided in the piping so that the tube side of the lube oil heat exchanger may be isolated from the water system for maintenance.

Temperature, pressure measuring and/or protective devices Thermocouples, WT_TL-1,-2 at turbine support legs outlet located at GT cooling system outlet, give a GT cooling water temperature indication.

NOTE: For the recommendations about the various components of the circuits, refer to The "Gas turbine subcontractor’s literature chapter".

Gas Turbine Compressor Section

General Description
The axial-flow compressor section consists of the compressor rotor and the inclosing casing. Included within the compressor casing are the inlet guide vanes, the 17 stages of rotor and stator blading and the exit guide vanes.

In the compressor, air is confined to the space between the rotor and stator blading where it is compressed in stages by a series of alternate rotating (rotor) and stationary (stator) 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 for turbine cooling, for bearing sealing, and during startup 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.

 

Compressor Rotor

Description:
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 has slots broached around its periphery. The rotor blades and spacers are inserted into these slots and are held in axial position by staking at each end of the slot. The wheels and stub-shafts are assembled to each other with mating rabbets for concentricity control and are held together with tie bolts. Selective positioning of the wheels is made during assembly to reduce balance correction. After assembly, the rotor is dynamically balanced to a fine limit.

The forward stub-shaft is machined to provide the forward and aft thrust faces and the journal for the n° 1 bearing, as well as the sealing surfaces for the nø 1 bearing oil seals and the compressor low pressure air seals.

Compressor Stator
General:
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.

The casing bore is maintained to close tolerances with respect to the rotor blade tips for maximum efficiency.

Gas Turbine Combustion Section

General
The combustion system is of the reverse-flow type with 14 combustion chambers 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 combustion chambers, are used to drive the turbine. High pressure air from the compressor discharge is directed around the transition pieces and

into the combustion chambers liners. This air enters the combustion zone through metering holes for proper fuel combustion and through slots to cool the combustion liner. Fuel is supplied to each combustion chamber through a nozzle designed to disperse and mix the fuel with the proper amount of combustion air.

Orientation of the combustion chambers around the periphery of the compressor is shown on figure next page. Combustion chambers are numbered counter-clockwise when viewed looking down-stream and starting from the top of the machine. Spark plugs and flame detectors locations are also shown.

 

Combustion Chamber and Crossfire Tubes

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

Combustion chambers

Discharge air from the axial flow compressor flows into each combustion flow sleeve from the combustion wrapper (see figure). The air flows up-stream along the outside of the combustion liner toward the liner cap. This air enters the combustion chamber 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 the 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 fourteen combustion liners, flow sleeves and transition pieces are identical.

Crossfire tubes

All fourteen combustion chambers are interconnected by means of crossfire tubes. These tubes enable flame from the fired chambers to propagate to the unfired chambers.

Gas Turbine Spark Plugs and Flame Detectors

Spark plugs
Combustion is initiated by means of the discharge from two high-voltages, non-retractable spark plugs bolted to flanges on the combustion chambers and mounted in a primary zone cup in adjacent combustors (N° 11 and 12).

These spark plugs receive their energy from ignition transformers. At the time of firing, a spark at one or both of these plugs ignites the gases in the primary zone of the chamber; the remaining chambers are ignited by crossfire through the tubes that interconnect the reaction Zones of the remaining chambers.

Spark Plug

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 eight sensors, each pair installed on four combustion chambers (n° 4 and 5, 10 and 11 primary and secondary zone) 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 d.c. 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 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 reestablished 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 only one flame detector sensor, the control circuitry will cause an annunciation only of this condition. Flame detectors are water cooled.

For more information about the flame detectors, see "Gas turbine subcontractor’s literature chapter" for G.T. control and protection system).

Gas Turbine Fuel Nozzles (Gas)

Description
Each combustion chamber is equipped with a fuel nozzle that emits the metered amount of the required fuel into the combustion liner. Fuel nozzles are used in gas turbines burning gas. The fuel nozzle functions to distribute the gas fuel into the reaction zone of the combustion liner, in a manner which promotes uniform, rapid and complete combustion.

Gas fuel enters the fuel nozzle assembly through the fuel gas connection flange and is routed through nozzle internal passages to orifices located in the gas tip.

Gas Turbine Enclosures

Gas turbine enclosures, referred to as compartments, are those partitioned area 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 servicing. 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 packaged design for a power generating station, there is an in-line sequence of lagged compartments, the sequence being broken by the inlet plenum and the exhaust plenum. These compartments enclose the turbine control compartment, the turbine accessory compartment, the gas turbine unit, the load gear, the driven generator.

Gas Turbine Bearings

Introduction
The MS 9001 E gas turbine unit contains three main journal bearings used to support the gas turbine rotor. The unit also includes thrust bearings to maintain the rotor-to-stator axial position. These bearing assemblies are located in three housings: one at the inlet, one in the compressor discharge casing and one in the exhaust frame. 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.

Bearing N*. Class Type

1 -Journal Elliptical

2 -Journal Elliptical

3 -Journal Elliptical

1 -Loaded thrust Self-aligned (equalized)

1 -Unloaded thrust Tilting pad


Lubrication
The three main turbine bearings are pressure-lubricated with oil supplied by the 12540 liters 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. The 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 oil 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. Some of this air returns with the oil to the main lubricating oil reservoir and is vented through a lube oil vent.

 

Gas Turbine Bearing Assembly

 Gas Turbine Load Thrust Bearing

Gas Turbine Unload Thrust Bearing

Gas Turbine Exhaust Frame and Diffuser

The exhaust frame assembly (figure here after) 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. On the inner gas path surfaces of the two cylinders are attached the inner and outer diffusers. The no.3 bearing is supported from the inner cylinder.

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 flairs at the exit end at a right angle to the turbine centerline. 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 no.3 bearing in relation to the outer casing of the gas turbine. The struts must be maintained at a uniform temperature in order to control the center 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 into this space around the struts.

Turbine shell cooling air enters the space between the exhaust frame and the 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 wheelspace.

 

 

Exhaust frame assembly 

Gas Turbine Inlet and Exhaust Sections

General
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.


Air Inlet System
The air inlet system, down-stream of the air filtering installation, is not described in details in this paragraph as it is not a part of the gas turbine assembly itself. 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. More details are given in the "Auxiliary plants and systems" volume, especially about the filtering installation.


Exhaust system
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 when it is the case.

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, a circular arrangement of thermocouples permits exhaust gas temperature measurement. The thermocouples send their signals to the gas turbine temperature control and protection system. The exhaust plenum configuration is that of a box open at the top 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.

The exhaust system, down-stream of the exhaust plenum, is not described in detail here as it is not a part of the gas turbine assembly itself. More details are given in the "Auxiliary plants and systems".

Gas Turbine Turbine Section

General Description
The three stage turbine section is the area in which energy in the form of high energy pressured 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 the turbine rotor, turbine shell, nozzles, shrouds, exhaust frame and exhaust diffuser.

Turbine Section

Turbine Rotor
Structure
The turbine rotor assembly consists of two wheel shafts; the first, second, and third-stage turbine wheels with buckets ; and two turbine spacers. Concentricity control is achieved with mating rabbets on the turbine wheels, wheel shafts, and spacers. The wheels are held together with through bolts. Selective positioning of rotor members is performed to minimize balance corrections.

The forward wheel shaft extends from the first-stage turbine wheel to the aft flange of the compressor rotor assembly. The journal for the n° 2 bearing is a part of the wheel shaft. The aft wheel shaft connects from the third-stage turbine wheel to the load coupling. It includes the n° 3 bearing journal.

Spacers between the first and second, and between the second and third-stage turbine wheels determine the axial position of the individual wheels. These spacers carry the diaphragm sealing bands. The spacer forward face includes radial slots for cooling air passages. The 1-2 spacer also has radial slots for cooling air passages on the aft face.

 

Turbine Rotor Location (in black)

Buckets
The turbine buckets increase in size from the first to the third-stage. Because of the pressure reduction resulting from energy conversion in each stage, an increased annulus area is required to accommodate the gas flow; thus necessitating increasing the size of the buckets. 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. The shrouds Interlock from bucket to bucket to provide vibration damping.

Turbine buckets for each stage are attached to their wheels by straight, axial entry, multiple tang dovetails that fit into matching cutouts in the turbine wheel rims. Bucket vanes are connected to their dovetails by means of shanks. These shanks locate the bucket-to-wheel attachment at a significant distance from the hot gases, reducing the temperature at the dovetail. The turbine rotor assembly is arranged so that the buckets can be replaced without unstacking the wheels, spacers, and wheel shaft assemblies.

Turbine rotor cooling
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.

The turbine rotor is cooled by means of a positive flow of relatively cool (relative to hot gas path air) 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 first stage forward wheel space is cooled by air that passes through the high pressure packing seal at the aft end compressor rotor. The 1st stage aft and 2nd stage forward wheel spaces are cooled by compressor discharge air that passes through the stage 1 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.

 
Turbine stator
The turbine shell and the exhaust frame constitute the major portion of the gas turbine stator structure. The turbine nozzles, shrouds, n° 3 bearing and turbine exhaust diffuser are internally supported from these components.

 

Turbine Stator (in black)

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.

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 the shell design reduces the heat flow into the 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 an off base cooling fan.

Structurally, the shell forward flange is bolted to flanges at the aft end of the compressor discharge casing and combustion wrapper. The shell aft flange is bolted to the forward flange of the exhaust frame. Trunnions cast onto the sides of the shell are used with similar trunnions on the forward compressor casing to lift the gas turbine when it is separated from its base.




Turbine nozzles
In the turbine section, there are three stages of stationary nozzles which direct the high velocity flow of the expanded hot combustion gas against the turbine buckets, causing the rotor to rotate. Because of the high pressure drop across these nozzles, there are seals at both the inside diameters 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.


First stage nozzle
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, so minimizing leakage of compressor discharge air into the nozzles. The 18 cast nozzle segments, each with two partitions (or airfoils) are contained by a horizontally split retaining ring which is center-line supported to the turbine shell on lugs at the sides and guided by pins at the top and bottom vertical center-lines. This permits radial growth of the retaining ring, resulting from changes in temperature while the ring remains centered in the shell.

The Aft outer diameter of the retaining ring is loaded against the forward face of the first stage turbine shroud and acts as the air seal to prevent leakage of compressor discharge air between the nozzle and shell. On the inner sidewall, the nozzle is sealed by direct bearing of the nozzle inner load rail against the first-stage nozzle support ring bolted to the compressor discharge casing. The nozzle is prevented from moving forward by four lugs welded to the aft

outside diameter of the retaining ring at 45 degrees from vertical and horizontal centerlines. These lugs fit in a groove machined in the turbine shell just forward of the first stage shroud Thook. By moving the horizontal joint support block and the bottom centerline guide pine, the lower half of the nozzle can be rolled out with the turbine rotor in place.


Second stage nozzle
Combustion gas exiting from the first stage buckets is again expanded and redirected against the second stage turbine buckets by the second stage nozzle.

The second stage nozzle is made of 16 cast segments, each with three partitions (or airfoils).

The male hooks on the entrance and exit sides of the sidewall fit into 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 fitting tongue and groove fit between nozzle and shrouds acts as an outside diameter air seal.

The nozzle segments are held in a circumferential position by radial pins from the shell into axial slots in the nozzle outer sidewall.

The second stage nozzle partitions are cooled with compressor discharge air.


Third stage nozzle
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 (figure here after).

These diaphragms prevent air leakage past the inner sidewall of the nozzles and the turbine rotor. The high/low, labyrinth-type 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 (diaphragm and nozzles) and the moving rotor are essential for maintaining low interstage leakage; this results in higher turbine efficiency.

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 primary function of the shrouds is to provide a cylindrical surface for minimizing tip clearance leakage.

The 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.