gas turbine in cairo north power station

09/05/2012 Gas Turbine In Cairo North Power Station

1/38dc220.4shared.com/doc/u2BZFNqQ/preview.html

Chapter 4Gas Turbine Unit In Cairo North Power Station

4. GAS TURBINE UNIT IN CAIRO NORTH POWER STATION.

4.1 Introduction

In this chapter we are going to discuss how gas turbine works, the classification of gas

turbine, the component of the gas turbine and the using of fuel and air to produce power. Air

is the working fluid and the fuel is heating source in combustion process to heat the air.

Also in this chapter starting up will be discussed.

4.2 Description of Gas Turbine in Site

1. Single shaft gas turbine.

2. Open cycle gas turbine.

3. Horizontal gas turbine.

4.2.1 Single Shaft Gas Turbine

With single shaft turbine, the compressor, turbine and generator are mounted on a single

shaft. In this arrangement, the compressor and turbine rotate at the same speed. This is

shown in figure (4.1) which shows the way of connecting all the devices together on a single

shaft.



Fig. 4.1 The compressor, turbine and generator are mounted on a single shaft

4.2.2 Open Cycle Gas Turbine

A simple open cycle gas turbine consists of a compressor, combustion chamber and

a turbine, the compressor takes in ambient air and raises its pressure. Heat is added to the

air in combustion chamber by burning the fuel and raises its temperature. The heated gases

coming out of combustion chamber are then passed to the turbine where it expands doing

mechanical work. This is shown in figure (4.2).



Fig. 4.2 Simple open cycle gas turbine

4.3 Components of gas turbine

1. Starting motor.

2. Air inlet and filter.

3. Compressor.

4. Combustion chamber.

5. Turbine.

6. Exhaust.

4.3.1 Starting motor

The solenoid also closes high-current contacts for the starter motor, which begins to turn.

Once the engine starts, the key-operated switch is opened; a spring in the solenoid assembly

pulls the pinion gear away from the ring gear, and the starter motor stops.

The starter's pinion is clutched to its driveshaft through an overrunning sprag clutch which

permits the pinion to transmit drive in only one direction. In this manner, drive is

transmitted through the pinion to the flywheel ring gear, but if the pinion remains engaged

the pinion will spin independently of its driveshaft. This prevents the engine driving the

starter, for such back drive would cause the starter to spin so fast as to fly apart. Figure

(4.3) shows the starter installed underneath gas turbine.



Fig. 4.3 Starting motor

Figure (4.4) shows the components of the starting engine.

1. Main Housing.

2. Overrunning clutch, and Pinion gear.

3. Armature.

4. Field coils with Brushes attached.

5. Brush-carrier.

6. Solenoid.



Fig .4.4 Components of the starting motor

4.3.2 Air Inlet System

Air quality can have an enormous impact on gas turbine performance and reliability and is

heavily influenced by the surrounding environment in which the unit is installed.

Furthermore, within any given location, the quality of air can change dramatically over a

year‘s time or, in some situations, within hours. Poor air quality leads to compressor

fouling. The next figure (4.5) shows the air filter in the power plant.

Fig. 4.5 Air filter tower



4.3.2.1 Air Inlet System Description

The air inlet system consists of the following: A filter house with weather hoods, an

automatic self cleaning filtration system using high efficiency filter elements, and an inlet

ducting system.

Air enters the filter house and continues through the transition piece, the acoustical

silencer, the inlet heating module, the trash screen, and then to the turbine compressor

through the inlet plenum. The elevated filter house arrangement provides a compact system

that minimizes the pickup of dust in the filter house.

The inlet system makes use of materials and coatings in its construction, which are designed

to require minimal maintenance.

All external and internal surface areas (exposed to airflow) of the filter house are coated

with a protective corrosion- preventive inorganic-zinc primer and epoxy overcoat.

4.3.2.2 Components of Filter Towers

a) Diffuser: Is used to pull air into the housing.

b) Hoods: Work to guide the air into the filter housing.

c) Socks filter: Used to capture dust and particulates from the air.

d)Filters canister : Any tower Consists of number of filters in 5 rows each filter divided

into two parts the first having cylindrical shape and the second having cone shape, each row

having 160 filter with total number 800 filter per tower. The cylindrical shape filters is

shown in figure (4.6).

e) Pulsation nozzle: make a back flow to clean the filters taking the air from air processing

unit. This is shown in figure (4.7).



Fig. 4.6 Cylindrical shape filters



Fig. 4.7 Pulsation nozzle

f) Evaporative cooling media: used to cool the air by remove water vapor from the air.

g) Mist eliminator: separated water droplet from the air.

h) Silencer: reduce the noise.

4.3.2.3 Inlet Compartment

Internal lighting is provided for the filter change out areas. Details for the operation and

maintenance of the inlet filtration system are contained in the maintenance manual in this

section. Access for maintenance and inspection of the filter elements are by use of stairs

and outside platform in conjunction with lower level exterior doors and access platforms at

each level in the filter house as shown in Figure (4.8).

Fig. 4.8 Inlet compartment

4.3.2.4 Inlet Ducting and Silencing

The inlet silencer consists of an acoustically lined duct, which contains silencing baffles

constructed of mineral wool insulation wrapped with fiberglass cloth and encapsulated by



perforated, stainless steel sheet metal.

The inlet bleed heat system consists of a series of stainless steel pipes mounted onto the

section of duct work immediately following the silencer. A manifold external to the duct

distributes air to these vertical pipes extending into the duct. Within the duct, the bleed air

is dispersed to the inlet airflow through a series of holes integral to the distribution pipes.

The elbow houses the single piece, stationary, stainless steel trash screen. The purpose of

the trash screen is to protect the compressor from loose pieces of hardware from the filter

house, ductwork, or through maintenance error. A removable access panel is positioned

upstream of the trash screen for cleaning and inspection purposes.

The inlet duct system also contains provisions for the dew point humidity sensor used in

monitoring the airflow downstream of the inlet bleed heat module. The sensor minimizes

the performance degradation associated with the inlet bleed heat system through

communication with the Mark V to keep all parts of the inlet system at arelative humidity

below the frost point. The location of the humidity sensor is shown on the Inlet and Exhaust

Flow Diagram.

4.3.3 Compressor

4.3.3.1 General

The axial-flow compressor section consists of 17 stages of the compressor rotor and the

compressor casing. Within The compressor casings are the variable inlet guide vanes, the

various stages of rotor and stator balding, and the exit guide vanes.

In the compressor, air is confined to the space between the rotor and stator 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 and for pulsation

control during startup.

Air may also be extracted from the compressor when the combustion turbine is operating

for use in the plant compressed air system. The next figure (4.9) shows the stages of



compressor.

Fig. 4.9 The axial-flow compressor section

4.3.3.2 Compressor components

4.3.3.2.1 The Rotor of Compressor

The compressor portion of the gas turbine rotor is an assembly of wheels; a speed ring, tie

bolts, the compressor rotor blades, and a forward stub shaft. Each wheel has slots broached

around its periphery. The rotor blades and spacers are inserted into these slots and held in

axial position by staking at each end of the slot. The wheels 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.

The forward stub shaft is machined to provide the thrust collar, which carries the forward

and aft thrust loads. The stub shaft also provides the journal for the No. 1 bearing, the

sealing surface for the No. 1 bearing oil seals and the compressor low-pressure air seal. The



stage 17 wheel carries the rotor blades and also provides the sealing surface for the high-

pressure air seal and the compressor-to-turbine marriage flange. The next figure (4.10)

shows the Compressor Rotor Assembly.

Fig. 4.10 Compressor rotor assembly

4.3.3.2.2 The Stator of Compressor

The casing area of the compressor section is composed of three major sections. Which are?

1. Inlet casing.

2. Compressor casing.

3. Compressor discharge casing.

These casings, in conjunction with the turbine casing, form the primary structure of the gas

turbine. They support the rotor at the bearing points and constitute the outer wall of the gas-



path Annulus. All of these casings are split horizontally to facilitate servicing.

1. Inlet Casing

Inlet guide vanes used at compressor inlet to ensure the air enters the first stage rotor as

desired angle, Variable inlet guide vanes are mechanically positioned, by a controlling and

pinion gear arrangement connected to a hydraulic actuator drive. This is shown in figure

(4.11).

Fig. 4.11 Inlet guide vanes

2. Compressor Casing

The forward compressor casing contains the stage 0 through stage 4 compressor stator

stages. The compressor casing lower half is equipped with two large integrally cast

trunnions which are used to lift the gas turbine when it is separated from its base. The aft



compressor casing contains stage 5 through stage 12 compressor stator stages. Extraction

ports in casing permit removal of 13th-stage compressor air. This air is used for cooling

functions and is also used for pulsation control during startup and shutdown. Figure (4.12)

show casing.

Fig. 4.12 Compressor casing

3. Compressor Discharge Casing

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

longest single casting, is situated at midpoint - between the forward and supports keystone

of the gas turbine structure. The compressor discharge casing contains the final compressor

stages, forms both the inner and outer walls of the compressor diffuser, and joins the

compressor and turbine casings. The discharge casing also provides support for the

combustion outer casings and the inner support of the first-stage turbine nozzle. The

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

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

rotor. The two Cylinders are concentrically positioned by fourteen radial struts.

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

static pressure for the combustion air supply. This is shown in figure (4.13).



Fig. 4.13 Compressor discharge casing

4.3.3.2.3 The Bleeding Process

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

efficiently at high blade tip velocities. The blades are attached to the compressor wheels by

dovetail arrangements. The dovetail is very precise in size and position to maintain each

blade in the desired position and location on the wheel.

The compressor stators blades are airfoil shaped and are mounted by similar dovetails into

ring segments in the first five stages. The ring segments are inserted into circumferential

grooves in the casing and are held in place with locking keys. The stator blades of the

remaining stages have a square base dovetail and are inserted directly into circumferential

grooves in the casing. Locking keys hold them in place. The bleeding process is shown in

figure (4.14).



Fig. 4.14 The bleeding process in gas turbine

4.3.3.3 Background information

Compressor stall or surge is not peculiar to any one particular brand or type of engine. It

may occur on any turbine engine if conditions are right. Stall has been uncountered on two-

stage or turbo-supercharged piston engines, so there is no need to look upon stall as some

mysterious product of gas turbine engines.

Any number of mechanical defects, such as bad spark plugs, lean arburetion, poor timing, or

sticking valves, can result in reciprocating engines backfiring. Similarly, for gas turbine

engines, maintenance or flight conditions can influence the compressor stall or surge

appreciably. The condition and operation of the bleed valve and fuel system components are

of vital importance in maintaining surge-free operation.

Why are engines at risk of surge? As engines are designed to meet demands for higher

power or lower specific fuel consumption, the engines must accommodate.

1. Increased mass airflow.

2. Increased pressure (compression) ratio.

3. Increased maximum allowable turbine inlet and outlet temperatures.

4. Improved efficiency of the compressor and turbine sections.



Quick engine starts and rapid accelerations are also desirable. To provide higher power with

low specific fuel consumption and acceptable starting and acceleration characteristics, it is

necessary to operate as close to the surge region as possible.

To prevent compressor stall or surge, fuel flow must be properly metered during the start

and acceleration cycle of any gas turbine engine. To accomplish this, the example engine

incorporates 5th and 10th stage acceleration bleed valves.

In general, there are fewer surge problems on centrifugal compressors than on axial flow

compressors. There are several reasons for the difference; the primary reason is that

centrifugal flow compressors operate at somewhat lower pressure ratios than axial flow

compressors. A surge from a turbine engine is the result of instability of the engine's

operating cycle. As discussed earlier, the operating cycle of the turbine engine consists of

intake, compression, combustion, and exhaust, which occur simultaneously in different

places in the engine. The part of the cycle susceptible to instability is the compression

phase. Compressor surge may be caused by engine deterioration, it may be the result of

ingestion of birds or ice, or it may be the final sound from a “severe engine damage” type of

failure.

In a turbine engine, compression is accomplished aerodynamically as the air passes through

the stages of the compressor, rather than by confinement, as is the case in a piston engine.

The air flowing over the compressor airfoils can stall just as the air over the wing of an

airplane can. When this airfoil stall occurs, the passage of air through the compressor

becomes unstable and the compressor can no longer compress the incoming air. The high-

pressure air behind the stall further back in the engine escapes forward through the

compressor and out the inlet.

This escape is sudden, rapid and often quite audible as a loud bang. Engine surge can be

accompanied by visible flames forward out the inlet and rearward out the tailpipe.

Instruments may show high EGT and EPR or rotor speed changes; but, in many stalls, the

event is over so quickly that the instruments do not have time to respond.

Once the air from within the engine escapes, the reason (reasons) for the instability may

self-correct and the compression process may re-establish itself. A single surge and

recovery will occur quite rapidly, usually within fractions of a second.

Depending on the reason for the cause of the compressor instability, an engine might

experience:



1. A single self-recovering surge.

2. Multiple surges prior to self-recovery.

3. Multiple surges requiring pilot action in order to recover.

4. A non-recoverable surge.

For complete, detailed procedures, flight crews must follow the appropriate checklists and

emergency procedures detailed in their specific Airplane Flight Manual. In general,

however, during a single self-recovering surge, the cockpit engine indications may fluctuate

slightly and briefly. The flight crew may not notice the fluctuation. (Some of the more

recent engines may even have fuel-flow logic that helps the engine self-recover from a

surge without crew intervention. The stall may go completely unnoticed, or it may be

annunciated to the crew – for information only – via EICAS messages.) Alternatively, the

engine may surge two or three times before full self recovery.

When this happens, there is likely to be cockpit engine instrumentation Shifts of sufficient

magnitude and duration to be noticed by the flight crew. If the engine does not recover

automatically from the surge, it may surge continually until the pilot takes action to stop the

process. The desired pilot action is to retard the power lever until the engine recovers. The

flight crew should then slowly re-advance the power lever. Occasionally, an engine may

surge only once but still not self-recover.

The actual cause for the compressor surge is often complex and may or may not result from

severe engine damage. Rarely does a single compressor surge cause severe engine damage,

but sustained surging will eventually over-heat the turbine, as too much fuel is being

provided for the volume of air that is reaching the combustor.

Compressor blades may also be damaged and fail as a result of repeated violent surges; this

will rapidly result in an engine which cannot run at any power setting. As shown in figure

(4.15).



Fig. 4.15 Surge effect on gas turbine air compressor

4.3.4 Combustion system

The combustion system is of the reverse-flow type with the 18 combustion chambers

arranged around the periphery of the compressor discharge casing as. Combustion chambers

are numbered counterclockwise when viewed looking downstream and starting from the top

left of the machine. This system also includes the fuel nozzles, a spark plug ignition system,

flame detectors, and crossfire tubes. Hot gases, generated from burning fuel in the

combustion chambers, flow through the impingement cooled transition pieces to the

turbine.

High pressure air from the compressor discharge is directed around the transition pieces.

Some of the air enters the holes in the impingement sleeve to cool the transition pieces and

flows into the flow sleeve. The rest enters the annulus between the flow sleeve and the

combustion liner through holes in the downstream end of the flow sleeve. This air enters the

combustion zone through the cap assembly for proper fuel combustion. Fuel is supplied to

each combustion chamber through five nozzles designed to disperse and mix the fuel with

the proper amount of combustion air.

4.3.4.1 Classification of fuel



Dual fuel-The DLN-2 combustion system shown in Figure (3.16) is a single stage, dual

mode combustor capable of operation on both gaseous and liquid fuel. On gas, the

combustor operates in a diffusion mode at low loads (<50% load), and a pre-mixed mode at

high loads (>50% load). While the combustor is capable of operating in the diffusion mode

across the load range, diluents injection would be required for NOx abatement. Oil

operation on this combustor is in the diffusion mode across the entire load range, with

diluents injection used for NOx. Figure (4.17) shows combustion chamber arrangement.

Fig. 4.16 the component of a single combustion chamber



Fig. 4.17 Typical DLN-2 combustion system arrangement

4.3.4.2 Outer Combustion Chambers and Flow Sleeves

The outer combustion chambers act as the pressure shells for the combustors. They also

provide flanges for the fuel nozzle-end cover assemblies, crossfire tube flanges, spark

plugs, flame detectors and false start drains. The flow sleeves form an annular space around

the cap and liner assemblies that directs the combustion and cooling air flows into the

reaction region. To maintain the impingement sleeve pressure drop, the openings for

crossfire tubes, spark plugs, and flame detectors are sealed with sliding grommets. Figure

(4.18) the flow sleeve assembly.

Fig. 4.18 Flow Sleeve Assembly

4.3.4.3 Cross fire Tubes

All combustion chambers are interconnected by means of crossfire tubes. The outer

chambers are connected with an outer crossfire tube and the combustion liner primary

zones are connected by the inner crossfire tubes. Figure (4.19) shows the crossfire tube.



Fig. 4.19 the cross fire Tubes

4.3.4.4 Fuel Nozzle End Covers

There are five fuel nozzle assemblies in each combustor. The nozzle is for the dual fuel

option and shows the passages for diffusion gas, premixed gas, oil, and water. When

mounted on the end cover, the diffusion passages of four of the fuel nozzles are fed from a

common manifold, called the primary that is built into the end cover. The premixed passage

of the same four nozzles is fed from another internal manifold called the secondary. The

premixed passages of the remaining nozzle are supplied by the tertiary fuel system; the

diffusion passage of that nozzle is always purged with compressor discharge air and passes

no fuel. Figure (4.20) shows fuel nozzle assembly. Figure (4.21) shows the nozzle passage.



Fig. 4.20 Fuel nozzle assembly

Fig. 4.21 Nozzle passage

4.3.4.5 Cap and Liner Assemblies

The combustion liners Figure (4.22) use external ridges and conventional cooling slots for

cooling. Interior surfaces of the liner and the cap are thermal barrier coated to reduce metal

temperatures and thermal gradients. The cap has five premiers’ tubes that engage each of the

five fuel nozzle. It is cooled by a combination of film cooling and impingement cooling and

has thermal barrier coating on the inner surfaces.



Fig. 4.22 Combustion liner assembly

4.3.4.6 Spark Plugs

Combustion is initiated by means of the discharge from spark plugs which are bolted to

flanges on the combustion cans and centered within the liner and flow sleeve in adjacent

combustion chambers. A spark plug that is used in the power plant plug is shown in Figure

(3.23). These plugs receive their energy from high energy-capacitor discharge power

supplies. At the time of firing, a spark at one or more 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.



Fig. 4.23 Spark Plugs

4.3.4.7 Ultraviolet 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 multiple flame detectors. The flame detectors figures (4.24) have water

cooled jackets to maintain acceptable temperatures.

The ultraviolet flame sensor contains a gas filled detector. The gas within this 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 in the circuit which

activates the electronics to give an output indicating flame. Conversely, the absence of

flame will generate an output indicating no flame. The signals from the four flame detectors

are sent to the control system which uses an internal logic system to determine whether a

flame or loss of flame condition exists. For detailed operating and maintenance information

covering this equipment, refer to the vendor publications.

Fig. 4.24 The flame detectors with water cooled jackets



The cap has five pre-mixer tubes that engage each of the five fuel nozzle. It is cooled by a

combination of film cooling and impingement cooling and has thermal barrier coating on

the inner surfaces. This is shown in figure (4.25).

Fig. 4.25 Cap assembly-view from downstream

4.3.5 The Gas Turbine

The three-stage turbine section is the area in which energy in the form of high temperature

pressurized gas, produced by the compressor and combustion sections, is converted to

mechanical energy. Gas turbine hardware includes the turbine rotor, turbine casing, exhaust

frame, exhaust diffuser, nozzles, and shrouds.

Table of main specifications of the station turbine:

Table 4.1 of main specifications of the station turbine

Type Axial flow(reaction)

Number of stage 3 stage

Speed 3000 rpm



4.3.5.1 Main Components of the Turbines

4.3.5.1.1 Turbine base and support

The base that supports the gas turbine is structural steel fabrication of welded steel beams.

And plate its prime function is to provide a support up on which to mount the gas turbine is

mounted to its base by vertical supports at three location. The forward support at the lower

half-vertical flange of the forward compressor casing and the two on either side of the

turbine exhaust frame.

2. Turbine Rotor

1. Structure

The turbine rotor assembly consists of the forward and turbine wheel shafts and the first,

second and third stage turbine wheel assemblies with spacers and turbine buckets.

Concentricity control is achieved with mating rabbets on the turbine wheels, wheel shafts,

and spacers. The wheels are held together with through bolts mating up with bolting flanges

on the wheel shafts and spacers. Selective positioning of rotor members is performed to

minimize balance corrections.

2. Wheel Shafts

The turbine rotor distance piece extends from the first-stage turbine wheel to the flange of

the compressor rotor assembly. The turbine rotor shaft includes the No. 2 bearing journal.

3. Wheel Assemblies



Spacers between the first and second, and between the second and third-stage turbine

wheels determine the axial position of the individual wheels. These spacers carry the

diaphragm sealing lands. The 1-2 spacer forward and faces include radial slots for cooling

air passages.

Turbine buckets are assembled in the wheels with fir-tree-shaped dovetails that fit into

matching cut-outs in the turbine wheel rims. All three turbine stages have precision

investment-cast, long shank buckets. The long-shank bucket design effectively shields the

wheel rims and bucket root fastenings from the high temperatures in the hot gas path while

providing mechanical damping of bucket vibrations. As a further aid in vibration damping,

the stage-two and stage-three buckets have interlocking shrouds at the bucket tips. These

shrouds also increase the turbine efficiency by minimizing tip leakage. Radial teeth on the

bucket shrouds combine with stepped surfaces on the stator to provide a labyrinth seal

against gas leakage past the bucket tips. Figure (4.27) shows typical first-, second-, and

third-stage turbine buckets for the MS9001FA. The increase in the size of the buckets from

the first to the third stage is necessitated by the pressure Reduction resulting from energy

conversion in each stage, requiring an increased annulus area to accommodate the gas flow.

Fig. 4.26 Turbine rotor assembly

Figure (4.27) shows the stages of the turbine in the power plant.



Fig. 4.27 MS6001B First, Second and Third-Stage Turbine Elements

4. Cooling of Turbine

The turbine rotor is 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 extracted from the compressor and discharged radialy outward

through a space between the turbine wheel and the stator, into the main gas stream. This area

is called the wheel space. Figure (4.28) shows the turbine cooling air flows.

Fig. 4.28 The turbine cooling air flows

Figure (4.29) shows blade cooling.



Fig. 4.29 Convection and film cooling

5. First-Stage Wheel spaces

The first-stage forward wheel space is cooled by compressor discharge air. A labyrinth seal

is installed at the end of the compressor rotor between the rotor and inner barrel of the

compressor discharge casing. The leakage through this labyrinth furnishes the air flow

through the first-stage forward wheel space. This cooling air flow discharges into the main

gas stream of the first-stage nozzle. The first-stage wheel space is cooled by 13th stage

extraction air ported through the 2nd stage nozzle. This air returns to the gas path forward of

the 2nd stage nozzle.

6. Second-Stage Wheel spaces

The second-stage forward wheel space is cooled by leakage from the first-stage wheel

space through the interstate labyrinth. This air returns to the gas path at the entrance of

the second-stage buckets. The second-stage wheel space is cooled by 13th stage

extraction air ported through the 3rd Stage nozzle.

7. Third-Stage Wheel spaces



The third-stage forward wheel space is cooled by leakage from the second-stage wheel

space through the interstate labyrinth. This air reenters the gas path at the third-stage bucket

entrance. The third-stage wheel space obtains its cooling air from the discharge of the

exhaust frame cooling air annulus. This air flows through the third-stage wheel space and

into the gas pat at the entrance to the exhaust diffuser.

8. Buckets

Air is introduced into each first-stage bucket through a plenum at the base of the bucket

dovetail as shown in Figure (4.30). It flows through serpentine cooling holes extending the

length of the bucket and exits at the trailing edge and the bucket tip. The holes are spaced

and sized to obtain optimum cooling of the airfoil with minimum compressor extraction air

Figure (4.30) shows the MS9001FA first-stage bucket design unlike the first-stage buckets,

the second-stage buckets are cooled by span wise air passages the length of the airfoil. Air

is introduced like the first-stage, with a plenum at the base of the bucket dovetail. 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.

Fig. 4.30 First-stage bucket cooling passages



Fig. 4.31 Second and third-stage bucket details

4.3.5.1.3 Turbine Stator

1. Structure

The turbine casing and the exhaust frame constitute the major portion of the turbine gas

turbine stator structure. The turbine nozzles, shrouds, and turbine exhaust diffuser are

internally supported from these components.

2. Turbine Casing

The turbine casing 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 casing are a source of heat flow into the casing. To control the casing diameter, it is

important to reduce the heat flow into the casing and to limit its temperature. Heat flow

limitations incorporate insulation, cooling, and multi-layered structures. 13th stage

extraction air is piped into the turbine casing annular spaces around the 2nd and 3rd stage

nozzles. From there the air is ported through the nozzle partitions and into the wheel spaces.

Structurally, the turbine casing forward flange is bolted to the bulkhead flange at the end of

the compressor discharge casing. The turbine casing flange is bolted to the forward flange



of the exhaust frame.

3. 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

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.

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; this minimizes leakage of compressor discharge air into

the nozzles. The Model 6001B gas turbine first-stage nozzle contains a forward and cavity

in the vane and is cooled by a combination of film, impingement and convection techniques

in both the vane and sidewall regions. The nozzle segments, each with two partitions or

airfoils, are contained by a horizontally split retaining ring which is centerline supported to

the turbine casing on lugs at the sides and guided by pins at the top and bottom vertical

centerlines. This permits radial growth of the retaining ring, resulting from changes in

temperature, while the ring remains centered in the casing. The 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 turbine

casing.

On the inner sidewall, the nozzle is sealed by a flange cast on the inner diameter of the

sidewall that rests against a mating face on the first-stage nozzle support ring.

Circumferential rotation of the segment inner sidewall is prevented by an eccentric bushing

and a locating dowel that engages a lug on the inner sidewall. The nozzle is prevented from

moving forward by the lugs welded to the outside diameter of the retaining ring at 45

degrees from vertical and horizontal centerlines. These lugs fit in groove machined in the

turbine shell just forward of the first-stage shroud T hook. By moving the horizontal joint

support block and the bottom centerline guide pin and then removing the inner sidewall

locating dowels, the lower half of the nozzle can be rolled out with the turbine rotor in



place.

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 cast

segments, each with two partitions or airfoils. The male hooks on the entrance and exit

sides of the outer sidewall fit into female grooves on the 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 is

cooled with 13th stage extraction air.

4. Diaphragms

Attached to the inside diameters of both the second and third-stage nozzle segments are the

nozzle diaphragms. These diaphragms prevent air leakage past the inner sidewall of the

nozzle 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 (diaphragm and nozzles) and the moving

rotor are essential for maintaining low inter stage leakage, these results in higher turbine

efficiency.

5. Shrouds

Unlike the compressor bleeding, the turbine bucket tips 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.

4.3.6 Exhaust frame assembly



The exhaust frame is bolted to the flange of the turbine casing. Structurally, the frame

consists of an outer cylinder and an inner cylinder interconnected by the radial struts. The

No. 2 bearing is supported from the inner cylinder. The exhaust diffuser located at the after

end of the turbine is bolted to the exhaust frame. 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, the gases are directed into the exhaust plenum. Exhaust frame radial

struts cross the exhaust gas stream.

These struts position the inner cylinder and No. 2 bearing in relation to the outer casing of

the gas turbine. The struts must be maintained at a constant 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 fro fairing that forms an air space around each strut

and provides a rotated, combined airfoil shape. Off through the space between the struts and

the wrapper to maintain uniform temperature of the struts. This air is then directed to the

third wheel space. Trunnions on the sides of the exhaust frame are used with similar

trunnions on the forward compressor casing to lift the gas turbine when it is separated from

its base Figure (4.32) shows the exhaust duct which included silencers and a flange of the

turbine casing.

Fig. 4.32 Exhaust duct

4.3.6.1 Silencers

The disclosed silencer has noise gas duct in parallel with but with as to define gas passage



means and noise duct. The noise-absorbing space means is closed by noise at inlet and

outlet ends of the duct. A plurality of gas chambers aligned along the line of gas flow

through the duct are formed in said noise absorbing space means by disposing noise therein

as shown in Figure (4.33) Silencers noise-absorbing partitions disposed in a spacing from

outer walls of the duct, so noise-absorbing space means in the noising noise-shielding

sectional walls Figure (4.33) -shielding plates noise-shielding.

Fig. 4.33 The silencers

4.4 Start–Up

Start–up control brings the gas turbine from zero speed up to operating speed safely by

providing proper fuel to establish flame, accelerate the turbine, and to do it in such a manner

as to minimize the low cycle fatigue of the hot gas path parts during the sequence. This

involves proper sequencing of command signals to the accessories, starting device and fuel

control system. Since a safe and successful start–up depends on proper functioning of the

gas turbine equipment, it is important to verify the state of selected devices in the sequence.

Much of the control logic circuitry is associated not only with actuating control devices,

but enabling protective circuits and obtaining permissive conditions before proceeding. The

gas turbine uses a static start system whereby the generator serves as a starting motor. A

turning gear is used for rotor breakaway. General values for control settings are given in

this description to help in the understanding of the operating system. Actual values for

control settings are given in the Control Specifications for a particular machine.

4.4.1 Speed Detectors



An important part of the start–up/shutdown sequence control of the gas turbine is proper

speed sensing. Turbine speed is measured by magnetic pickups and will be discussed under

speed control.

The following speed detectors and speed relays are typically used:

1. L14HR Zero–Speed (approx. 0% speed).

2. L14HM Minimum Speed (approx. 16% speeds).

3. L14HA Accelerating Speed (approx. 50% speeds).

4. L14HS Operating Speed (approx. 95% speeds).

The zero–speed detector, L14HR, provides the signal when the turbine shaft starts or stops

rotating. When the shaft speed is blown 14HR, or at zero–speed, L14HR picks–up (fail

safe) and the permissive logic initiates turning gear or slow–roll operation during the

automatic start–up sequence of the turbine. The minimum speed detector L14HM indicates

that the turbine has reached the minimum firing speed and initiates the purge cycle prior to

the introduction of fuel and ignition. The dropout of the L14HM minimum speed relay

provides several permissive functions in the restarting of the gas turbine after shutdown.

The accelerating speed relay L14HA pickup indicates when the turbine has reached

approximately 50 percent speeds; this indicates that turbine start–up is progressing and keys

certain protective features. The high–speed sensor L14HS pickup indicates when the turbine

is at speed and that the accelerating sequence is almost complete. This signal provides the

logic for various control sequences such as stopping auxiliary lube oil pumps and starting

turbine shell/exhaust frame blowers.

The turbine and generator should slow during an under-frequency situation; L14HS

will drop out at the under–frequency speed setting. After L14HS drops out the generator

breaker will trip open and the Turbine Speed Reference (TNR) will be reset to 100.3%. As

the turbine accelerates, L14HS will again pick up; the turbine will then require another start

signal before the generator will attempt to auto–synchronize to the system again. The actual

settings of the speed relays are listed in the Control Specification and are programmed in

the <RST> processors as EEPROM control constants.

4.4.2 Start–Up Control



The start–up control operates as an open loop control using preset levels of the fuel

command signal FSR. The levels are: “ZERO”, “FIRE”, “WARM–UP”, “ACCELERATE” and

“MAX”. The Control Specifications provide proper settings calculated for the fuel

anticipated at the site. The FSR levels are set as Control Constants in the SPEEDTRONIC

Mark VI start–up control. Start–up control FSR signals operate through the minimum value

gate to ensure that other control functions can limit FSR as required. The fuel command

signals are generated by the SPEEDTRONIC control start–up software. In addition to the

three active start–up levels, the software sets maximum and minimum FSR and provides for

manual control of FSR. Clicking on the targets for “MAN FSR CONTROL” and “FSR GAG

RAISE OR LOWER” allows manual adjustment of FSR setting between FSRMIN and

FSRMAX.

While the turbine is at rest, electronic checks are made of the fuel system stop and control

valves, the accessories, and the voltage supplies

Fig. 4.34 Mark VI Start-up Curves

When the turbine ‘breaks away’ the turning gear .it will rotate the turbine rotor from 5 to 7

rpm. As the static starter begins its sequence, and accelerates the rotor the starting clutch

will automatically disengage the turning gear from the turbine rotor. The turbine speed relay

L14HM indicates that the turbine is turning at the speed required for proper purging and

ignition in the combustors. Gas fired units that have exhaust configurations which can trap

gas leakage (i.e., boilers) have a purge timer, L2TV, which is initiated with the L14HM

signal. The purge time is set to allow three to four changes of air through the unit to ensure

that any combustible mixture has been purged from the system. The starting means will hold



speed until L2TV has completed its cycle. Units which do not have extensive exhaust

systems may not have a purge timer, but rely on the starting cycle and natural draft to purge

the system.

49

gas turbine in cairo north power station

Documents