GAS Turbine Functional Decription

GENERAL, FUNCTIONAL DESCRIPTION

A Gas turbine unit is a mechanical power engine installed for Power Generation Plant

The gas turbine power engine consists of  axial airflow compressor, a multi chamber

combustion system and a three stages turbine. Main components of the gas turbine are listed here below.

1.    The axial airflow compressor is a 17 stages compressor with :

2.    Adjustable inlet guide vanes (IGV) to control the airflow during starting and loading sequences.

3.     Bleed valves to bypass part of the air flow for starting and shut down to escape from surging.

The combustion system comprises :

1.    Fuel nozzles fitted on the combustion chamber’s cover.

2.    Ten combustion chambers where the fuel burns permanently from firing speed to full load.

3.     Ten cross fire tubes connecting the combustion chamber.

4.     Ten transition pieces downstream the combustion chamber connected to the first turbine stage nozzle.

5.     Two spark plugs for the fuel ignition.

6.    A set of flame detectors.

The three stages turbine includes first, second and third stage nozzle and first, second and third wheel.

The turbine and the axial flow compressor belong  to the same shaft connected to :

1.    The auxiliary gear box and the starting means at the front end.

2.    The generator at the rear end through a load gear box.

The gas turbine components and function are detailed in the text here after.

GAS TURBINE FUNCTIONAL DESCRIPTION

Refer to gas turbine simplified flow diagram here below

Functional description at nominal speed :

While the gas turbine is running, filtered ambient air is drawn through the inlet plenum assembly, then compressed in the 17th-stage axial flow compressor. Compressed air from the compressor flows into the annular space surrounding the ten combustion chambers, from which it flows into the spaces between the outer combustion casings and the combustion liners, and enters the combustion zone through metering holes in each of the combustion liners.

The fuel nozzles introduce the fuel into each of the ten combustion chambers where it mixes with the combustion air and burns. The hot gases from the combustion chambers expand into the ten separate transition pieces attached to the downstream end of the combustion chamber liners and flows from there to the three-stage turbine section of the machine. Each stage consists of a row of fixed nozzles followed by a row of 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  casing and diffuser which contains a series of turning vanes to turn the gases from an axial direction to a radial direction, thereby minimizing exhaust hood losses. Then, the gases pass into the exhaust plenum and are introduced to atmosphere through the exhaust stack. Resultant shaft rotation turns the generator rotor to generate electrical power or to drive a centrifugal compressor in industrial power applications and drives the auxiliaries through the accessory gearbox.

Starting sequence :

The gas turbine cannot run itself from zero speed. A starting means bring the shaft line up to the self-sustaining speed. When the starting means is actuated, the IGV are in the closed shut down position and the compressor bleed valves are open. The cranking torque from the ratchet system breaks away the turbine shaft through the mechanical clutch, the cranking motor brings the gas turbine to firing speed. Fuel is injected in the combustion chamber, spark plug provide ignition in two combustion chambers and the flame spreads to the other combustion chambers through the crossfire tubes. Flame detectors confirm full ignition to the control panel. Starting means remain actuated to accelerate the unit to self-sustaining speed. When the turbine torque is higher than the cranking torque at clutch level, the clutch opens and stops the starting means function. The gas turbine reaches nominal speed, the IGV move to full speed no load (FSNL) operating position and the bleed valve closes.      Main shaft driven lube oil pump provides lubricating oil for the shaft line bearings. During starting sequence the auxiliary lube oil pump feeds the header.

Cool down sequence :

Due to the high temperature within the gas path, the gas turbine must follow a 24 hours ratchet sequence, after shut down, to provide cool down cycle to the shaft line by turning a eighth turn every three minutes.

Compressor Section :

GENERAL

Description :

The axial-flow compressor section consists of the compressor 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 2 exit guide vanes rows. 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 (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 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 are made and assembled very accurately.

COMPRESSOR ROTOR

Description :

The compressor rotor is an assembly of 15 wheels, 2 stubshafts and wheels assemblies,

through bolts, and the rotor blades. Each wheel and the wheel portion of each stubshaft has slots broached around its periphery. The rotor blades are inserted into these slots and they are held in axial position by staking at each end of the slot. The wheels and stubshafts are assembled to each other with mating rabbets for concentricity control and are held together with tie bolts.

The forward stubshaft is machined to provide the active and inactive thrust faces and the

journal for the n° 1 bearing, as well as the sealing surfaces for the n° 1 bearing oil seals and

the compressor low pressure air seal.

After assembly, the rotor is dynamically balanced to a fine limit.

COMPRESSOR STATOR

General :

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

• Inlet casing.

• 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.

Variable inlet guide vanes :

Variable inlet guide vanes are located at the aft end of the inlet casing.

The position of these vanes has an effect on the quantity of compressor air flow. Movement of the inlet guide vanes is actuated by a hydraulic cylinder connected to the inlet guide vane control ring that turns the individual pinion gears mounted on the end of each vane. The gears, the ring and the vanes are shown next page.

Compressor casing :

The forward compressor casing contains the first- through tenth- compressor stages. It also

transfers the structural loads from the adjoining casing to the forward support which is bolted

and doweled to this compressor casing’s forward flange. Extraction ports in the casing permit removal of fifth- eleventh- and thirteenth-stage  compressor air. Air from fifth and eleventh-stages is used for cooling and sealing functions and is also used for starting and shutdown pulsation control. Air from therteenth-stage is used for the cooling of the second-stage nozzle

Discharge casing :

The compressor discharge casing is the final portion of the compressor section.

It is the longest single casting. It is situated at the midpoint between the forward and aft

supports and is, in effect, the keystone of the gas turbine structure.

The functions of the compressor discharge casing are to contain the final seven compressor

stages, to form both the inner and outer walls of the compressor diffuser, provide inner

support for the first-stage nozzle and join the compressor and turbine stators, and support

the outer combustion cans.

The compressor discharge casing consists of two cylinders, one being a continuation of the

compressor casings and the other being an inner cylinder that surrounds the compressor

rotor. The two cylinders are concentrically positioned by ten radial struts. These struts ext

from the inner cylinder outward to a vertical bulkhead.

The bulkhead has ten circular openings permitting air flow to enter the combustion system.

This bulkhead also provides support to the ten combustion chamber assemblies.

A diffuser is formed by the tapered annulus between the outer cylinder and inner cylinder of

the discharge casing. The diffuser converts some of the compressor exit velocity into added

pressure.

Blading:

The compressor rotor and stator blades are airfoil shaped and were designed to compress

air efficiently at high blade tip velocities. The blades are attached to their wheels by dovetails

arrangements. The dovetail is very precise in size and position so as to maintain each blade in the desiredposition and location on the wheel. The stator blades for stages 1 through 4 are mounted by similar dovetails into ring segments. The ring segments are inserted into circumferential grooves in the casing and are held in place with locking keys. In stages 5 through 17, the stator blades and exit guide vanes 1 and 2 have a square base dovetail and are inserted directly into circumferential grooves in the casing. Locking keys are used as with the blade ring design.

COMBUSTION SECTION

1.4.1. GENERAL

The combustion system is the reverse flow type which includes 10 combustion chambers

having the following components :

• Liners.

• flow sleeves.

• Transition pieces.

• Crossfire tubes.

Flame detectors, crossfire tubes, fuel nozzles and spark plugs igniters are also part of the

total system.

Hot gases, generated from burning fuel in the combustion chambers, are used to drive the

turbine.

In the reverse flow system high pressure air from the compressor discharge is directed

around the transition pieces and into the annular spaces that surround each of the 10

combustion chamber liners.

Compressor discharge air which surrounds the liner, flows radially inward through small

holes in the liner wall and impinges against rings that are brazed to the liner wall. This air

then flows right toward the liner discharge end and forms a film of air that shields the liner

wall from the hot combustion gases. Fuel is supplied to each combustion chamber through a

nozzle.

Combustion chambers are numbered counterclockwise when viewed looking down stream

and starting from the top of the machine.

The ten combustion chambers are interconnected by means of crossfire tubes. These tubes

enable flame from the fired chambers containing spark plugs to propagate to the unfired

chambers.

TRANSITION PIECES

Description :

Transition pieces direct the hot gases from the liners to the turbine first-stage nozzle. Thus,

the first nozzle area is divided into ten 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 nozzle

FALSE START DRAIN

In liquid fuel units, for safety reasons a false start drain manifold, with piping to chambers 3

through 7, protects the turbine from an accumulation of unburned liquid fuel in the

combustion system. At turbine start-up, the drain is open. After firing, combustion pressure

increases to a value which activates a valve in the false start drain.

Should the turbine fail to fire, the valve remains open to drain the liquid fuel. There is an

additional false start drain at the lower vertical center-line of the turbine shell to drain any

liquid fuel from that area, should a false start occur.

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 GENERAL, FUNCTIONAL DESCRIPTION

A Gas turbine unit is a mechanical power engine installed for Power Generation Plant

The gas turbine power engine consists of  axial airflow compressor, a multi chamber

combustion system and a three stages turbine. Main components of the gas turbine are listed here below.

1.    The axial airflow compressor is a 17 stages compressor with :

2.    Adjustable inlet guide vanes (IGV) to control the airflow during starting and loading sequences.

3.     Bleed valves to bypass part of the air flow for starting and shut down to escape from surging.

The combustion system comprises :

1.    Fuel nozzles fitted on the combustion chamber’s cover.

2.    Ten combustion chambers where the fuel burns permanently from firing speed to full load.

3.     Ten cross fire tubes connecting the combustion chamber.

4.     Ten transition pieces downstream the combustion chamber connected to the first turbine stage nozzle.

5.     Two spark plugs for the fuel ignition.

6.    A set of flame detectors.

The three stages turbine includes first, second and third stage nozzle and first, second and third wheel.

The turbine and the axial flow compressor belong  to the same shaft connected to :

1.    The auxiliary gear box and the starting means at the front end.

2.    The generator at the rear end through a load gear box.

The gas turbine components and function are detailed in the text here after.

GAS TURBINE FUNCTIONAL DESCRIPTION

Refer to gas turbine simplified flow diagram here below

Functional description at nominal speed :

While the gas turbine is running, filtered ambient air is drawn through the inlet plenum assembly, then compressed in the 17th-stage axial flow compressor. Compressed air from the compressor flows into the annular space surrounding the ten combustion chambers, from which it flows into the spaces between the outer combustion casings and the combustion liners, and enters the combustion zone through metering holes in each of the combustion liners.

The fuel nozzles introduce the fuel into each of the ten combustion chambers where it mixes with the combustion air and burns. The hot gases from the combustion chambers expand into the ten separate transition pieces attached to the downstream end of the combustion chamber liners and flows from there to the three-stage turbine section of the machine. Each stage consists of a row of fixed nozzles followed by a row of 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  casing and diffuser which contains a series of turning vanes to turn the gases from an axial direction to a radial direction, thereby minimizing exhaust hood losses. Then, the gases pass into the exhaust plenum and are introduced to atmosphere through the exhaust stack. Resultant shaft rotation turns the generator rotor to generate electrical power or to drive a centrifugal compressor in industrial power applications and drives the auxiliaries through the accessory gearbox.

Starting sequence :

The gas turbine cannot run itself from zero speed. A starting means bring the shaft line up to the self-sustaining speed. When the starting means is actuated, the IGV are in the closed shut down position and the compressor bleed valves are open. The cranking torque from the ratchet system breaks away the turbine shaft through the mechanical clutch, the cranking motor brings the gas turbine to firing speed. Fuel is injected in the combustion chamber, spark plug provide ignition in two combustion chambers and the flame spreads to the other combustion chambers through the crossfire tubes. Flame detectors confirm full ignition to the control panel. Starting means remain actuated to accelerate the unit to self-sustaining speed. When the turbine torque is higher than the cranking torque at clutch level, the clutch opens and stops the starting means function. The gas turbine reaches nominal speed, the IGV move to full speed no load (FSNL) operating position and the bleed valve closes.      Main shaft driven lube oil pump provides lubricating oil for the shaft line bearings. During starting sequence the auxiliary lube oil pump feeds the header.

Cool down sequence :

Due to the high temperature within the gas path, the gas turbine must follow a 24 hours ratchet sequence, after shut down, to provide cool down cycle to the shaft line by turning a eighth turn every three minutes.

Compressor Section :

GENERAL

Description :

The axial-flow compressor section consists of the compressor 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 2 exit guide vanes rows. 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 (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 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 are made and assembled very accurately.

COMPRESSOR ROTOR

Description :

The compressor rotor is an assembly of 15 wheels, 2 stubshafts and wheels assemblies,

through bolts, and the rotor blades. Each wheel and the wheel portion of each stubshaft has slots broached around its periphery. The rotor blades are inserted into these slots and they are held in axial position by staking at each end of the slot. The wheels and stubshafts are assembled to each other with mating rabbets for concentricity control and are held together with tie bolts.

The forward stubshaft is machined to provide the active and inactive thrust faces and the

journal for the n° 1 bearing, as well as the sealing surfaces for the n° 1 bearing oil seals and

the compressor low pressure air seal.

After assembly, the rotor is dynamically balanced to a fine limit.

COMPRESSOR STATOR

General :

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

• Inlet casing.

• 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.

Variable inlet guide vanes :

Variable inlet guide vanes are located at the aft end of the inlet casing.

The position of these vanes has an effect on the quantity of compressor air flow. Movement of the inlet guide vanes is actuated by a hydraulic cylinder connected to the inlet guide vane control ring that turns the individual pinion gears mounted on the end of each vane. The gears, the ring and the vanes are shown next page.

Compressor casing :

The forward compressor casing contains the first- through tenth- compressor stages. It also

transfers the structural loads from the adjoining casing to the forward support which is bolted

and doweled to this compressor casing’s forward flange. Extraction ports in the casing permit removal of fifth- eleventh- and thirteenth-stage  compressor air. Air from fifth and eleventh-stages is used for cooling and sealing functions and is also used for starting and shutdown pulsation control. Air from therteenth-stage is used for the cooling of the second-stage nozzle

Discharge casing :

The compressor discharge casing is the final portion of the compressor section.

It is the longest single casting. It is situated at the midpoint between the forward and aft

supports and is, in effect, the keystone of the gas turbine structure.

The functions of the compressor discharge casing are to contain the final seven compressor

stages, to form both the inner and outer walls of the compressor diffuser, provide inner

support for the first-stage nozzle and join the compressor and turbine stators, and support

the outer combustion cans.

The compressor discharge casing consists of two cylinders, one being a continuation of the

compressor casings and the other being an inner cylinder that surrounds the compressor

rotor. The two cylinders are concentrically positioned by ten radial struts. These struts ext

from the inner cylinder outward to a vertical bulkhead.

The bulkhead has ten circular openings permitting air flow to enter the combustion system.

This bulkhead also provides support to the ten combustion chamber assemblies.

A diffuser is formed by the tapered annulus between the outer cylinder and inner cylinder of

the discharge casing. The diffuser converts some of the compressor exit velocity into added

pressure.

Blading:

The compressor rotor and stator blades are airfoil shaped and were designed to compress

air efficiently at high blade tip velocities. The blades are attached to their wheels by dovetails

arrangements. The dovetail is very precise in size and position so as to maintain each blade in the desiredposition and location on the wheel. The stator blades for stages 1 through 4 are mounted by similar dovetails into ring segments. The ring segments are inserted into circumferential grooves in the casing and are held in place with locking keys. In stages 5 through 17, the stator blades and exit guide vanes 1 and 2 have a square base dovetail and are inserted directly into circumferential grooves in the casing. Locking keys are used as with the blade ring design.

COMBUSTION SECTION

1.4.1. GENERAL

The combustion system is the reverse flow type which includes 10 combustion chambers

having the following components :

• Liners.

• flow sleeves.

• Transition pieces.

• Crossfire tubes.

Flame detectors, crossfire tubes, fuel nozzles and spark plugs igniters are also part of the

total system.

Hot gases, generated from burning fuel in the combustion chambers, are used to drive the

turbine.

In the reverse flow system high pressure air from the compressor discharge is directed

around the transition pieces and into the annular spaces that surround each of the 10

combustion chamber liners.

Compressor discharge air which surrounds the liner, flows radially inward through small

holes in the liner wall and impinges against rings that are brazed to the liner wall. This air

then flows right toward the liner discharge end and forms a film of air that shields the liner

wall from the hot combustion gases. Fuel is supplied to each combustion chamber through a

nozzle.

Combustion chambers are numbered counterclockwise when viewed looking down stream

and starting from the top of the machine.

The ten combustion chambers are interconnected by means of crossfire tubes. These tubes

enable flame from the fired chambers containing spark plugs to propagate to the unfired

chambers.

TRANSITION PIECES

Description :

Transition pieces direct the hot gases from the liners to the turbine first-stage nozzle. Thus,

the first nozzle area is divided into ten 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 nozzle

FALSE START DRAIN

In liquid fuel units, for safety reasons a false start drain manifold, with piping to chambers 3

through 7, protects the turbine from an accumulation of unburned liquid fuel in the

combustion system. At turbine start-up, the drain is open. After firing, combustion pressure

increases to a value which activates a valve in the false start drain.

Should the turbine fail to fire, the valve remains open to drain the liquid fuel. There is an

additional false start drain at the lower vertical center-line of the turbine shell to drain any

liquid fuel from that area, should a false start occur.