236407565 gas-turbine-notes

Ministry of Electricity - EgyptGas 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:
Hybrid_burner@yahoo.com

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 (torquespeed) 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 andor
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 actionmechanism 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 (saltydusty …..).
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 3rd
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 220o
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 detergentwater 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 (hotcold) 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 o
C 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 o
C an alarm
will be announced, if reached 120 o
C 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 o
C 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 o
C 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 CBOWithout 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

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236407565 gas-turbine-notes