236407565 gas-turbine-notes

Ministry of Electricity - Egypt Gas Turbine Notes

1 | P a g e M a h m o u d E l n a g g a r

Gas Turbine Notes

Prepared by:

Mahmoud Elsayed El naggar

Nubaria Power Station – Middle Delta Electricity Production Company

Ministry of Electricity & Energy – Egypt



Acknowledgement

This work is dedicated to all my friends and colleagues in Dubai Electricity and Water

Authority in all plants in Jebel Ali power station complex.

Special thanks to Eng. Ahmed Saeed Negm for his great effort in writing and incredible

assistance during this material preparation.

Any comments/questions please email me at:

[email protected]



Gas Turbine

The gas turbine is a rotary engine, and it's used in many applications like:

Power generation

Aviation

Transportation

Driving pumps and compressors for petrochemicals

The gas turbine is mainly consists of three main parts:

1. Air compressors

2. Combustion chamber(heat addition section)

3. Turbine

V94.3A (SGT5-4000F) Siemens Gas turbine



Working principle

Imagine that you are holding a small fan (like that one in child toys) and

blowing air towards its blades, what will happen?

The fan will rotate with a determined speed and torque proportional to the

amount and velocity of the blown air so, if we have another device stronger

than our lungs blowing air towards the fan, more power (torque\speed) will be

generated or in the other hand we can drive a larger fan.

Gas turbine working principle

It's clear from the above mentioned example that the fan is the turbine

itself and the air blowing device is the compressor so, what is the function of

the combustion chamber?

For a compressor to give an air flow it will consume a specific amount of

mechanical power (for rotation) this amount of energy could be divided into

two quantities:-

1. The driving quantity

2. The lost quantity (losses due to friction… etc.)

So, if we gave the compressor say 10 power units, assume the compressor

efficiency to be 90%, then the useful amount of driving power that will be

converted to air flow and pressure will be 9 units and 1 unit will be lost as

losses during energy conversion in the compressor now we have air flow

coming from the compressor carrying 9 power units, this power of air will be

the responsible for driving the turbine.



When the compressor discharge air flows over the turbine blades another

energy conversion process will take place converting the 9 power units 9 in air

into mechanical energy on the turbine rotor, but the turbine section also has its

own energy conversion efficiency so, we can say that the 9 power units of the

air will be converted to 8 power units.

Now the turbine is giving power less than that one required driving the

compressor so, we can conclude that the turbine engine is useless, but wait….

We can solve this problem, how?? If the losses during energy conversion in the

compressor section compensated by the same device and the compressor

discharge air energy level increased, the turbine will work and give net useful

work after giving the compressor the required power.

What about heating up the discharge air? By this way the air energy

level will increase due to the additional thermal energy, now the air is

pressurized and hot. The most efficient way to heat up the air is to add fuel and

burn it inside the air stream (direct heat exchange) so, the combustion chamber

will be added between the compressor and the turbine to manage the heat

addition process, after heat addition we can say the amount of power units in

the air will increase from 9 units to be 19.



The 19 power units will be converted to mechanical power again on the

turbine rotor, and due to losses this mechanical power will be less than 19 let's

say 17 according to the turbine section efficiency (about 90%), the compressor

will take the required 10 power units and the remaining 7 units will be the

useful work, and about 50% to 60% from the turbine power will be consumed

by the compressor, that's why the steam turbine has higher efficiency than the

gas turbine.

Gas turbine thermal cycle (Brayton cycle)

The gas turbine working principle is related to Brayton cycle. This thermal

cycle is consisting of four processes:-

1. Compression of atmospheric air by the compressor.

2. Heating up the compressor discharge air by combustion.

3. Expansion of high energy combustion gases on the turbine.

4. Heat rejection of the air after turbine (in closed cycle) or exhaust

rejection to atmosphere or HRSG (in open Brayton cycle).



Gas turbine compressor

The compressor is the device which draws air from the atmosphere and

compresses it to high pressure before entering combustion chamber.

The two types of the gas turbine compressor are:

1. Axial flow

2. Centrifugal

The axial compressor is giving high flow rates but relatively low pressure

ratios per stage. The centrifugal compressor gives lower flow rates and higher

pressure ratios per stage if compared with the axial type.

Axial compressor (Top) and axial – centrifugal compressor (bottom)

The commonly used type is the axial compressor especially in large frames,

because its efficiency in large turbines is high, in the other hand the centrifugal

type shows more efficiency in the small applications like vehicles'

turbochargers.



The axial compressor main components

The axial compressor consists of two rows of blading, rotating and

stationary one, the rotating row is a disc holding blades circumferentially in

axial slots at its periphery and is connecting to the driving mechanism (the

turbine in our case). The stationary row is a ring of blades fixed in the

compressor casing, the function of the rotating row is to draw air from outside

and accelerating it towards the stationary blades, and the stationary blades

(vanes) convert the kinetic energy of the air into potential energy in the form of

static pressure.



Working principle

When the compressor rotor starts rotation the blades draw air from

atmosphere according to their aerodynamic shape (airfoil), and then push the

air giving it kinetic energy, this energy source is the compressor driver

(turbine), after that the high velocity air enters the stationary row, the passage

between every two neighboring blades takes the shape of a diffuser and

according to the continuity equation and Bernoulli's principle, if the air entered

a diffuser with high velocity and low static pressure it will exist at low velocity

and high static pressure (kinetic energy compressed to potential energy), now

air pressure increased by a series of energy conversions (mechanical to kinetic

to potential) this is the single stage compressor.

Velocity/Pressure profile through axial compressor



But the single stage pressure ratio is limited and very low to drive a

turbine wheel so, to increase the pressure ratio of the compressor (discharge

pressure divided by suction pressure) multi-stages in series should be added so

that the pressure of the first stage will increase through the second one and so

on until the air exists the last stage of the compressor at the desired high

pressure ratio, it should be noticed that the multi-stage in series only increase

the pressure and the flow is kept constant like electrical batteries as voltage

increase and the current is constant.

Example describing the effect of multi-stage axial compressor effect on

pressure ratio

The stationary blades also help in directing air with a suitable angle to

the next rotating row of the moving blades to introduce air to the first stage of

compressor.

The compressor is equipped with a row of stationary blades its name is

the inlet guide vanes or ''IGV'', these blades have the property that they can

rotate around their axis to reduce or increase the cross-sectioned area between

every two adjacent blades to control air mass flow rates to the compressor, this

IGV is controlled by either electrical motor or hydraulic actuator, also the



compressor is equipped with an additional final/exit row that is a stationary

blades row but with a special design to make the air stream at the compressor

exist straight before entering the combustion chamber because the air leaves

the last stage of the compressor rotating due to exist angles of the last stage,

this row is called ''Air Straightener''.

Compressor Surge

When the compressor downstream pressure become higher than the

compressor designed discharge pressure, or the system downstream

compressor is stronger than the ability of the compressor to give air flow the

pressurized air downstream compressor will go back towards compressor

suction side then the pressure downstream compressor will fall due to air

relieving from both sides (combustor side and compressor side), this pressure

degradation will enable the compressor to push the air against the downstream

side again (recovery), after that the pressure downstream will start to increase

again up to a value higher than the compressor pressure ratio forcing the air to

flow back again, this flow reversal and forwarding is called ''Compressor

Surge''.



One interesting example analogous to compressor surge is that:

Imagine one man is pushing a mass towards a varying inclination hill (its

inclination is increasing gradually) like an inverted parabolic shape, the mass

has a mass acting downward all the time so, when the mass climbs the hill its

force will be described by two components one is acting perpendicular to the

hill surface and the other one will act against the man, as the hill inclination

increases the man will suffer more until he stops at some inclination, at this

point any more pushing from the man leads to further motion of the mass on

the hill will make the mass force to be more than the man ability to push, at this

moment the mass will roll back pushing the man downwards until the man

reaches a specified point at which the hill inclination angle makes a smaller

mass force against the man enabling him to recover the situation again and

starts to push forward up to that point of retardation and so on, this cyclic

action is analogous to the compressor surge, the man is like a compressor, the

mass is like the air flow, the mass force against the man is the compressor

downstream pressure and the hill inclination is the downstream system

resistance which is the reason of pressure rise (combustion process and\or

turbine).

A man pushing a mass over a variable inclination hill



Compressor Stall:

In normal operation condition the compressor delivers the design mass

flow rate (at max IGV and ISO conditions) and pressure ratio, if anything

happened during operation affecting the compressor discharge pressure (like

turbine overloading or excessive fuel injection) it will lead to discharge

pressure rise and decreasing the compressor flow rate (decelerating the flow)

so, the inlet flow velocity will be deformed in direction causing air separation

from the blade surface along with air wakes, this will force the air to stop

moving forward along the compressor at this particular point, and instead of

moving forward the air will rotate in vortices, this condition is called ''Stall''

and the air vortex is called '' Stall Cell'', this stall cell will induce local pressure

rise before its location causing the coming flow to divert in both sides instead

of going forward (chocking), when the coming air diverts in both sides it will

affect the inlet velocity vectors of the neighboring blades leading to stall at the

next blade in rotation and stabilizing the other side blade, when the stall cell

build up at that blade it will cause the same action (stabilizing the affected old

blade, and affecting the next blade inducing a new stall cell to build up) and so

on, this action\mechanism will lead to stall cell rotational action around the

blades disc in the compressor rotation plan, but counter to the compressor's

direction of rotation at a speed ranges from (20-80 %) of compressor rotational

speed, this condition is called ''Rotating Stall'', if the speed or rotating stall

approached the blades natural frequency it will lead to blade resonance due to

vortex shedding repetition causing blade and compressor catastrophic failure,

this condition is called ''flattering Stall''

Stall cell propagation due to local pressure rise at blade A

Direction of rotation



Compressor Surge Protection

The compressor inlet takes the shape of a bell (conical shape) so, it acts

as a nozzle, when the air goes through it its velocity will increase and the static

pressure will decrease so, if the air pressure draw measured it will give a good

indication of air flow rate (as the nozzle pressure drop is directly proportional

to flow rate), the compressor is equipped with three differential pressure

switches at its inlet for this purpose, if 2 out of 3 read differential pressure

lower than a specified value (e.g. 30 mbar) and the turbine speed is more than

47.5 r.p.s the gas turbine will trip immediately because that's indication of

compressor surge which is leading to inlet flow decrease within differential

pressure as well.

A start-up Problem

At startup condition the compressor rotational speed as well as inlet air

velocity, inlet air velocity is directly proportional to compressor speed so that

increasing compressor speed increases inlet air velocity and maintaining the air

velocity that's relative to the blade geometry, but at the same time increasing



compressor speed increases the discharge pressure as well due to air partial

accumulation and combustion back pressure so, the increased pressure at

compressor discharge will lead to both air flow rate and air inlet velocity

decrease, the situation now is that air inlet velocity is increasing with

compressor speed and decrease again with the same reason due to pressure rise

so, the final result is that the air relative velocity direction will deform and the

air will enter the compressor blades at positive incidence, this incidence will

increase gradually until a specified speed of rotation, at this speed air

separation due to critical angle of attack will take place causing stall and then

surge.

What's the solution?

Blow off valves (Bleed Valves) are used in gas turbines to relieve

compressor air during startup by bypassing it over the compressor discharge

and combustion system so, cancelling the effect of compressor discharge

pressure rise against the inlet air velocity incidence as mentioned above so, the

compressor air flow rate will be maintained by the blow off system, once the

system downstream compressor resists the discharge flow the air escapes from

the blow off system, finally the incidence of the inlet air velocity will be kept

constant at the desired value preventing air separation (stall and surge).



Parameters affecting compressor performance:

Deformed parts (blades) causing air dynamic losses.

Wear of seals and internal parts.

Lack of compressor washing (hot and cold).

Ambient conditions (temperature – pressure).

The environment in which the gas turbine operates (salty\dusty\ …..).

Gas turbine compressor tasks:

Supplying combustion system with combustion air.

Supplying turbine blades with cooling air.

Supplying fuel oil burners with seal air during NG operation.

Supplying the gas turbine with the necessary air for doing work.

Blow-off system

SGT5-4000F (V94.3A) gas turbine contains 3 blow off lines (sometimes

4), 2 lines are extracting air from the compressor's 5th stage and the 3

rd line is

extracting or actually bleeding air from the 9th

stage (the 4th line is connected to

the 13th stage), all these valves are pneumatically operated and always in open

condition during gas turbine shutdown times, during startup these valves are

kept open up to a specified speed range so that at 40 r.p.s the 9th

stage valve

starts to close slowly then at 49 r.p.s the 5th stage second valve closes followed

by the first one within 5 sec. this is the NG startup sequence.

During fuel oil startup the closing sequence will be as follows:

After 47.5 r.p.s by 60 sec. the first valve of the 5th stage starts to close followed

by the 2nd

one within 10 sec. followed by the 9th

stage valve within 10 more

seconds, during normal operation all blow off valves should be closed and they

open only and the condition of gas turbine trip and opens immediately, if GT

startup finished and turbine speed exceeded 47.5 r.p.s by 100 sec. and any blow

off valves still open the GT will shut down, all blow off valves can't control

manually except at a speed lower than 4 r.p.s, if the blow off system is closed



at the standstill condition it will open automatically if the rotor speed exceeded

4 r.p.s within 20 sec.

Fuel oil burners seal air system

During NG operation the fuel oil burners are idle so, it may leak some

fuel oil due to valve passing, this liquid fuel will burn at burner tip (cocking)

leading to burner clogging so, some air is extracted from the compressor

discharge for fuel oil burners sealing purpose, the sealing air is extracted from

compressor discharge at high temperature and it should be cooled to keep fuel

oil lines from losing (because they are fitted by shrink fit), the sealing air is

passed through air cooler this cooler consists of two VFD (Variable Frequency

Drive) fans and a heat exchanger, one fan will be in service and the other is in

reserve so that the seal air temperature will be maintained at 135o C, if the fan

in service reached 100% duty the second fan will start automatically and it will

stop at seal air temperature lower than 110o C, if the seal air temperature

increased to 180o C alarm announces, if reached 220

o C GT will trip, when

temperature decreases to 90o C for 5 mins. alarm announces.

The sealing air fans are changing over automatically every 99 hrs.

Compressor washing

The drawn air by GT compressor is full of fine dust and small particles,

although it's cleaned by filtered system the air goes inside the compressor with

some amount of small particles which deposit on the compressor blades, if the

compressor not properly washed the fouled blades will deteriorate the

compressor efficiency dramatically so, gas turbines are equipped with

compressor washing systems consist simply of detergent\water mixing tank,

pump and piping system for online and offline washing, the two cases of

compressor wash are:

Online (hot) washing during normal operation.

Offline (cold) washing during turning gear and SFC operation.

The washing solution is discharged by the pumps towards the compressor inlet

through the appropriate line (hot\cold) via water sprays to atomize the cleaning

solution to protect the compressor blades from pitting.



Online (hot) washing

IGV opening should be adjusting at about 95% to protect GT from high

temperature after the completion of compressor work procedure, after starting

the washing system the additional amount of washing solution will increase

inlet mass flow rate to the compressor so, the IGV will close to decrease air

mass flow by a value that is analogous to that amount of cleaning solution to

keep the GT output constant as load set point so, the GT operator should raise

the GT output to open IGV permitting cleaning solution to enter the

compressor efficiently.

The water detergent ratio should be 3: 1 i.e. 450 liter water with 150 liter

detergent.

Note: the online washing valve must be opened alone, the offline washing

valve must be closed during online washing because the spray type of offline

washing is jet type and this type is injecting heavy droplets that could be

harmful to compressor blades at the rated speed during hot washing.

Offline (cold) washing

The same steps will be carried out as in hot washing but additional

preparations should be taken into account as follow:

The GT should be on turning gear mode (not standstill)

Air intake flap should be opened and anti-condensate air heater should

be turned off

IGV controller should be in manual mode, IGV power supply should be

switched ON from local panel in PCC and from monitor in CCR

IGV openings should be 100% (no fear from overheating like hot

washing condition) to give water spray the chance to go inside the

compressor easily.

Open all drain valves of turbine body (16 valves), 14 valves inside the

enclosure, 1 valve under air intake and the last one is under the exhaust

diffuser downstream turbine.



Fuel oil false start drain line should be changed over from tank to sump

(the 3 way valve)

Switch the burner ignition transformers off from PCC.

Prepare the compressor washing skid as in hot washings.

Start washing procedure with 150 litre solution on turning gear mode.

After finishing, the SFC should be started in compressor wash mode,

when rotor speed reaches about 10 r.p.s start washing again with the

remaining 450 litre of washing solution, the washing procedure will

continue until rotor speed reaches 13 r.p.s the SFC will shut down

automatically during washing then the same steps of washing should be

carried out again for rinsing, during start-up drying of the GT will take

place due to HRSG purge and speed increase.

Note:

The compressor shouldn’t be washed after GT shutdown directly, at least after

6 hrs. to reach the cold condition first then compressor can be washed (thermal

stress protection), the compressor shouldn’t be washed as well at ambient

temperature less than 8 oC to prevent icing protecting the compressor blades

from pitting, and after washing all drains should be closed.

Compressor Measurements

Compressor inlet temperature (used in OTC calculations)

Compressor outlet temperature (compressor efficiency prediction)

Compressor suction and discharge pressure (used in compressor pressure

ratio calculations)

Differential pressure at compressor suction bellmouth to predict surge as

mentioned before

IGV position measurements



Turbine

The turbine is the converter of the hot gas energy into mechanical energy on

the shaft, the turbine are divided into two types:

1. Axial type

2. Radial type

The commonly used one is the axial type like in power generation and oil and

gas industry

The radial type is used in the small applications like vehicle turbo chargers

Turbine components

The simple form of turbine is one fixed blade row (nozzles) followed by

one moving blade row

The fixed row acts as a nozzle set the converts the hot gases energy into kinetic

energy by expanding and accelerating gases after that the high K.E gases enter

the turbine moving blades row and drive it by one of two techniques:

1. Impulse

2. Reaction



Impulse turbines

The impulse turbine contain moving blades in a bucket shape and the

passage between every 2 adjacent blades takes the shape of crescent with

constant spacing so that the pressure is kept constant through the blades

passage (no pressure drop in the impulse blades) but the hot gases velocity will

drop due to energy conversion in the rotor, so the hot gases are expanded only

in the fixed blades row but no expansion in the rotor.

Reaction turbine blades and velocity/pressure profiles

Reaction turbines

This type of turbines depends on a nozzle shape moving blades (the

passage between blades looks like a nozzle) so the gases will expand first in the

fixed blades then it will drive the moving blades by impulse effect and just

before it exist the blades passage (nozzle) its velocity will increase due to

narrow passage at trailing age so that a reaction will take place enhancing the

hot gases force against the moving blades increasing the torque of rotation, so

every reaction blade is an impulse blade but not vice versa.

Reaction turbine blades and velocity/pressure profiles



According to the above mentioned topics, the turbine theory of operation is to

convert the hot gases energy to kinetic energy used in driving the rotor via

moving blades.

Moving blades fixation

The moving blade is fixed in the disc by engagement between its fire

tree root and the slot on the disc.

The blade root should be designed so that it can sustain the huge centrifugal

force due to rotation

Example:- one blade mass is 2kg rotates at 1m from the center of rotation with

3000rom , so the centrifugal force which tends to take off the blade from the

disc will be:

Fc = m. r. w2 = 2kg * 1m * ((2pi * 3000)/60)

2 = 197.192kn approximately

equals (20tons)

Also turbine blades are made of special materials basically contains nickel and

chrome to sustain high temperature and corrosion due to(oxidation) thermal

barrier coating is provided for blades as well to keep the blades from high

temperature.

Turbine blade fixation to disc



Turbine blades cooling

The turbine blades (moving and fixed) are cooled by air, this air is taken

from the compressor via pipes or through hollow shaft for fixed and moving

blades respectively

The fixed blades of the 1st stage are cooled by air from the compressor's

last stage (discharge),these blades use the film cooling technique

The moving blades of the 1st stage are cooled by the same air (discharge

air) and the same technique (film cooling)

The fixed blades of the 2nd

stage are cooled by air extracted from the 13th

stage of compressor via long pipes equipped with control valves

(motorized), the air is flowing through the pipes outside the turbine

casing and enters again the casing but at turbine casing section to be

disturbed on the blades through holes in the fixed blades carrier. These

blades are cooled by impingement method that depends on an perforated

insert inside the blade, the air goes inside this insert and exist from many

bores to impinge on the inner wall of the blade and this technique of

cooling is the highest in cooling efficiency after the film cooling type.

The moving blades of stage 2 are cooled by air from the 12th

stage of

compressor but from inside the rotor to go to the turbine rotor directly

and through holes in the blades disc it will enter the blades via its roots

and go directly to blade body and exit from holes at blade top tailing

edge, this cooling technique is called convection cooling and it is the

lowest in cooling efficiency between the three types.

The 3rd

stage fixed blades are cooled by the same method of 2nd

stage

blades but bya air extracted from the compressor's 9th

stage.

The 3rd

stage moving blades are cooled by the same method of 2nd

stage

moving blades but by air from compressor's 10th

stage.

The 4th

stage is cooled by air from the compressor's 5th

stage and its

cooling method is convection



The4th stage moving blades are cooled at its roots only by the air of

compressor's 10th

stage that is used in the 3rd

stage moving blades

cooling.





Turbine cooling valves

These valves open automatically when the turbine speed exceeds 4 r.p.s,

the valve opens for protection if the compressor discharge pressure

measurement faulted or the pressure of blades cooling air changed according to

the following equation:

For stage 2 fixed blades (GV2) (0.69 – (GV2P/CDP)) * 100

If the result is higher than 2 alarm will be announced [GV2 cooling air pressure

low]

If higher than 3 so [GV2 cooling air pressure too low] will be announced and

GT will shutdown

If the result is lower than -3 [GV2 cooling air pressure high] will be announced

If valves openings differ by 5% [diff < max] will be announced

The same actions will be taken also in GV3 but the equation will be (0.4 –

(GV2P/CDP)) * 100

If GT is starting-up and the signal of valves opening have come but the valves

did not open by 100% within 60 sec. the GT will shut down

The cooling valves could be closed manually only at speed lower than 4 r.p.s



Turbine exhaust protection

The exhaust temperature of V94.3A GT is monitored by 24

thermocouples, every one involves three channels A, B and C, and they

are used in exhaust protection and monitoring system as shown in the

figure:



Turbine rotor

The turbine rotor is the shaft, discs and moving blades together, and it has

three design or shapes

1. (Monoblock) this design combines multi-stages in one block instead of

separate discs

2. (Circumferential tie bolts) this design depends on mounting all discs on

many tie bolts through holes on the disc circumference

3. (center tie rod) this design depends on one long rod all discs have rods in

its centers and are mounted on that rod one after one then spacer

between the last disc of the compressor and the first disc of turbine will

be added to make a space for combustor, and at the end of the rod some

locking nut is used to complete the assembly, hirth coupling/serrations is

used to prevent relative motion between discs



Turbine rotor is held by two journal bearings at its ends (compressor side and

turbine side) and prevented from axial motion by a thrust bearing combined

with that journal bearing at compressor side

These bearings are lubricated by a separate lubrication system. Journal

bearings are actually oil film bearings and rolling element bearings, the used

type here is the oil film type and it has many types and designs depend on the

load of the rotor and its speed.

1 front hollow shaft 2 15 compressor wheel disks 3 Torque Disk 4 4 turbine wheel disks 5 rear hollow shaft 6 tie bolt nut 7 central tie bolt

8 truncated conical springs Detail Z

The compressor bearing type is cylindrical type its load carrying

capacity is high but its oil film stability is low, in the other hand the turbine

bearing is a tilting pad type, its load carrying capacity is very low but its oil

film stability is very high and its function is used for one direction of rotation,

inverse rotation during turning gear manual operation could be harmful for this

bearing type – so be careful –

The rotor contains the long pipes; these pipes are used to separate air streams

from compressors 10th

and 12th stages, and to deliver cooling air to turbine

moving blades. The outer shell of the GT carries the fixed blades and covers

the combustion chamber and form the compressor discharge air plenum.

This shell is divided into two halves [upper and lower].



Protections

The protections that are concerned with rotor and casing are vibration

[for both] and speed [for rotor only]

The rotor vibration is measured relative to bearings and its unit is µm (0.001

mm) the measuring device is proximity probes, if the vibration level reached

195 µm at generator bearings only during HRSG purge or compressor wash

then GT shutdown, and that is because the speed at these conditions

approaches the critical speed of generator rotor only.

If the value is just 125 µm alarm is announced another device is used along

with proximity probe it is key phasor or ''one pulse per revolution device'' its

function is to make a reference for vibration analyst to know the position of

vibration peaks and for rotor balancing.

For casing vibration, velocity meters are used to measure the casing vibration

in mm/s units.

If the value reached 9.3 mm/s alarm will announced if 14.7mm/s GT trip.

Bearing Protection

The bearing is protected from high temperature due to oil starvation or

oil pressure drop, if the bearing metal temperature reached 110 oC an alarm

will be announced, if reached 120 oC GT trip.

Turbine speed measurement

The rotor speed is measured by 6 sensors (magnetic pickup) 3 of them

are called software, the others are called hardware, the software group is

connected to the fuel valves through the protection system software but the

hardware group is connected directly to the fuel valves for safety.

Turbine speed protection

This protection philosophy is staged as follows:-

1. If rotor speed reached 47.5 r.p.s alarm annunciate and a timer will start,

if it takes 20 sec without increasing again ''load rejection'' will take

place, another more 20 r.p.s GT trip, and the same thing if speed reached

51.5 r.p.s



2. If turbine speed reached 47 r.p.s the load is rejected immediately and

after 10sec if the seed didn’t group again GT trip, the same thing at 52

r.p.s

3. If GT speed reached 54r.p.s GT will trip immediately to protect turbine

blades and generator coils from take-off away from the rotor.

Combustion chamber

It is that place inside which fuel is burned after mixing with air to release high

thermal energy inside compressor discharge air to increase its energy before

entering the turbine stages.

The fuel is injected into the combustion chamber via fuel burners and tis mixed

with air by a certain ratio (fuel to air ratio) to ensure complete combustion

without too much excess air the flame is started by a separate ignition system

(electrical) and then it continue by itself .

Combustion chamber components

1. Combustor body

2. Fuel burners

The combustor body/structure is that place which holds the fuel burners at its

inlet and delivers the hot gases to the turbine via 1st stage nozzles which may

be fixed at combustor outlet.

Cannular type Annular type



There are three types of combustors:

1. Annular: which takes the form of two cylinders mounted at the same

center axis forming annular space for combustion

2. Can: which is a cylinder involves a smaller perforated (for cooling)

cylinder inside it (liner), and between them some space takes compressor

air and deliver it to the inlet of the interior cylinder through the fuel

burners and continue to the turbine via transition piece.

3. Cannular : this type is a multi-can design with connections between

each can via ross fire tubes, so it's can type and annular because of

connections at the same type this design takes the advantage of annular

type that is the even distribution of pressure and cancels its disadvantage

which is the additional length of the GT due to combustion chamber

space because it depends on reversing discharge air back to fuel burners

so that no space is required for cans between compressor and turbine as

shown in figures at the same time the disadvantage of this design is it's

volume its bulk volume is very big, so it may lead to more thermal losses

due to longer surface area.

The second part of combustion chamber is the fuel burners or the fuel injection.

It is the responsible of the fuel injection, mixing with air and burning inside the

combustor, fuel burners may burn fuel oil or NG or both (dual burner).



Combustion theory

Consider a combustion system burns NG (CH4), so to get a flame air

should be available along with heat, so that the heat ignites air/fuel mixture to

make a flame. The process has the rule that the air/fuel ratio should be certain

value (the stoichiometric value) this value is the theoretical value for complete

combustion at high flame temperature. If air/fuel ratio is lower than the

theoretical value the combustion air and the flame temperature will be too high

as well so that NOx will increase due to high combustion temperature and in

the other hand if the air/fuel ratio is too high the flame will blow-out due to

cooling and if the flame stabilized the combustion will be incomplete also due

to low flame temperature and it will lead to carbon monoxide increase.

There are two types of flames:

1- Diffusion flame. 2- Premix flame



Diffusion flame is produced when the fuel comes out the burner and starts

to mix with air freely by the difference of concentration between air and fuel,

this process is called diffusion because the air/fuel is diffused inside each other,

when the diffusion process reaches some value of air/fuel ratio (lower than the

theoretical one) the flame starts but earlier than required so that the combustion

will be incomplete and its temperature will be too high providing a good

environment for NOx production, so the problem is that the flame starts earlier

than required then the solution will be:

Mixing the air with the fuel at the required air/fuel ratio before entering the

combustion zone so that the mixture will burn directly without diffusion delay

and at the same time it will burn completely with moderate flame temperature

and flame length, this flame is called ''Premix'' to decrease NOx production

more. The premix combustion is provided with additional amount of air

(excess air) this excess air enters the combustion reaction as air and exits as air

as well but the difference is that the air enters cold and exists hot due to

combustion, so it takes some heat from the combustion such heat rejection

inside excess air leads to flame temperature lowering and so NOx production

will be lower than diffusion flame condition.

The premix flame has a serious disadvantage which is the instability

The flame stabilities are two categories:

1- Static 2- Dynamic

The static stability of the flame is its ability to stay on without quenching the

premix flame is weak due to lean combustion, so it could easily extinguished

and move away from its attach point at combustion zone.

The dynamic stability of the flame is its ability to overcome extinguishing and

reigniting near lean blow-out limit (LBO) or to stay stable at fuel flow or air

flow oscillation (combustion dynamic or humming)

To increase the stability of premix flame some additional amount of fuel is

added and burned by the diffusion mechanism (more stable flame), so this

small diffusion flame stabilizes the main premix flame statically and

dynamically and this flame is called ''pilot flame''.



Due to burning fuel by two types of combustion (diffusion as pilot and premix

as main flame), so the burner name will be hybrid burner.

Using premix mode in combustion reduces NOx emission from 300 ppm in

diffusion to 25 ppm in hybrid operator.

Flame problems

Flame off

It happens when the speed off flame propagation is lower than that of the

incoming air/fuel mixture so that the incoming mixture purges the flame

from its attaching point away and cut the continuous combustion process

extinguishing the flame.

The flame is observed inside V94.3A annular combustor by two flame

detectors (left and right) both are observing 11 burner together from the 24

burners the signals from these flame detector is conducted to two processing

units because the setting of flame intensity of NG differs from that of fuel

oil, at start up condition the GT will trip after opening NG ESV if the flame

signal did not come during 12 sec, if the GT is in normal operation and no

flame signal came from both detectors, the GT will trip, if only one detector

, so just alarm will be announced.



Flash back

It happens when the speed of flame propagation is higher than that of the

incoming air/fuel mixture so that the flame will move back until it hits the

burner body increasing its temperature, sometimes flash back destroys the

burner body due to high flame temperatures.

The gas turbine designer takes in his account that the main fuel of GT is

NG, so he designs the compressor exit velocity to match the NG flame

velocity to provide flame stability but sometimes GT operators need to start

and operate GT with liquid fuels at emergency conditions. The liquid fuel

flame velocity depends on the hydrogen content in the fuel, increasing the

hydrogen content increase the flame velocity increase as well.

Due to high hydrogen content of the liquid fuel is higher than it in NG, so

the flame velocity is matched with that of NG flame, so during operation

with liquid fuel care should be taken from the flash back.



A protection system containing two thermocouples per burner is used to

protect burners from flash back depending on the compressor discharge

temperature, the burner body temperature is measured by the thermocouples

and compared with that of compressor discharge, if the burner body

temperature is higher than discharge air (the normal value of burner body

temperature) by 100 degree alarm will be announced and will be negotiated

if the difference drop to 80 degree, if the difference increased to 150 oC and

G is working on diffusion mode a block on premix mode operation is

ensured, if the GT was operating on PM automatic C/O from PM to DM

will take place, if the difference did not drop from 150 oC for 5 min. the GT

will shut down and will not accept start-up again unless burner inspection

carried out. See fig. above.

Combustion chamber (C.C) ΔP

Due to the complicated path of discharge air and burners the compressor

discharge pressure will drop through combustion chamber to the same

value, this value should be observed and compared with the compressor

discharge pressure as follows:

(RPD) relative pressure dissipation = ΔPc.c/Pcd * 100

If RPD is lower than 1.8% this means that the C.C. ΔP is low due to wears

of C.C. body or C.C. cooling passages, this will increase the secondary air

flow for cooling and affect the combustion air flow, in PM operation any

changes in the combustion air may lead to big troubles, so the protection

system will change over from PM to DM and if the GT is already working

with DM alarm will be announced.

Combustion dynamics (humming)

The combustion process specially PM combustion is affected easily by

any disturbances in air or fuel flow, this disturbances may take the form of

oscillations and when the air/fuel mixture reaches the combustion zone

these oscillations lead to unsteady heat release rate (UHRR) from the flame,

so this UHRR will affect the flame temperature and combustion chamber to

oscillate as well with the same frequency, if the oscillation frequency

consider the combustor volume acoustic frequency the oscillations will be

amplified, these air oscillations will make a humming sound, so the name of



humming is a good description for this phenomena, these oscillations

propagates until it reach the C.C. boundaries and then reflected back

towards the flame point, if there is an appropriate phase lock between these

oscillations and the current flame oscillations, a feedback loop will exist

making the oscillations amplitude to grow up until it reaches a limit cycle,

at this point the humming will be too severe, so that it will induce

mechanical vibrations in the C.C. body (acceleration) leading to failure.

To protect GT from compressor dynamics, the dynamic pressure in the C.C.

is observed by piezo pressure transducer (humming sensors) to measure the

amplitude and frequency of the humming waves.

Also, C.C. body vibration is measured by piezo sensors to measure the

amount of acceleration and the frequency of these vibrations to protect the

C.C. from failure.

For the protections of C.C. humming and acceleration please refer to O&M

manual for V94.3A for more details.

To reduce these phenomena, manufacturers are tending to use special

technique during burner manufacturing process and there are two types of

measures that used to attenuate the combustion dynamics:

1- Active measures 2- Passive measures

The passive measures are working properly at certain loads (base load) and

conditions but at other conditions they are useless such measure are like

modifications of burner design, flame velocity, equivalence ratio, fuel

composition and/or Helmholtz resonators.

The active measures are working and covering the entire load range along with

start-up condition, such measures observes the combustion condition by

monitoring systems and take the appropriate action immediately to suppress the

compressor dynamics.

Siemens is using its own invention AIC (active instability control) system, this

system monitors the humming inside the C.C. and then modulates the pilot gas

fuel flow rate by giving it the oscillatory behavior of the humming wave but at

different phase angle so that the induced humming wave by the fuel

modulation cancels that original humming wave of the flame.



Annular plenum

Burners without CBO

Rotating

oscillation damper

1

Rotating

oscillation damper

2

Rotating

oscillation damper

3

Siemens V94.3A2 Combustion System Configuration for passive controls of combustion

oscillation

For burners 7, 10, 15 they are fitted with Piezo pressure transducer to measure sound

pressure fluctuations (Humming), the humming values of burners 7, 10, 15 equals that

of burners 19, 22, 3 which are in the opposite direction to them, but the values are

inverted.

For burners from 1 to 20 they are fitted with CBO (Cylindrical Burner Outlet) to help

for humming suppression.

The burners 21, 22, 23, 24 are without CBO, this helps too for humming suppression.

The rotating oscillation dampers are welded to the outer casing on which the diffusion

burners are installed, and they help for damping the rotating sound waves.

The distance between every tow neighboring dampers must not be equal for best work

thus: the distance between damper 1, 2 clockwise is 3.5 m and 2, 3 is 3 m and 3, 1 is 5.5

m the whole circumference of the ring is 12 m.

The circumference of the premix burners' holder is 10 m.

The premix burner inlet provided with a metallic grid to break the large eddies in the

combustion air flow to the premix burner.

With CBO Without CBO

Eng.M.Elnaggar



Starting ignition inside combustion chamber

The ignition is initiated inside C.C. by means of electrical spark igniters,

these igniters are divided into two electrodes (PV+ , NG

-) and connected to

ignition transformers, when the high voltage is charged at the electrodes the

electrical spark starts in the air gap between the electrodes and ignites the

air/fuel mixture at the burner tip, in liquid fuel operation mode the igniters start

the flame by ignition gas first until the C.C. warms up then the liquid fuel is

injected and burn by the flame of the ignition gas every burner is equipped with

its own igniter.

Turbine casing drain system

After compressor washing procedure the accumulated water inside the

casing should be drained otherwise this water could lead to compressor/turbine

blades failure during start-up, so the turbine casing is drained by 14 drain lines

plus 1 drain line for the intake housing and another 1 drain at exhaust diffuser,

all these lines valves should be opened during offline (cold) washing of the

compressor.

During fuel oil start-up if the start-up failed after fuel injection some liquid fuel

may accumulate inside the combustor, so the drain line of this area is common

for washing water and liquid fuel of false start but the line is separated at its

end to two lines one is the false start drain line and the other is the normal drain

line.

Turbine supports

The turbine body is supported at compressor end by (I) beam structure

holding the turbine casing at the bottom of compressor bearing, the other side

(turbine) support is two steel legs holding the turbine bearing side from both

sides.

236407565 gas-turbine-notes

Engineering

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