Siemens gas turbine

Gas Turbine Notes

Ministry of Electricity - Egypt

Gas Turbine Notes

Prepared by: Mahmoud Elsayed El naggar Nubaria Power Station – Middle Delta Electricity Production Company Ministry of Electricity & Energy – Egypt

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - 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

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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.

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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.

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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.

2.

3.

4.

Compression of atmospheric air by the compressor.

Heating up the compressor discharge air by combustion.

Expansion of high energy combustion gases on the turbine.

Heat rejection of the air after turbine (in closed cycle) or exhaust rejection to atmosphere or HRSG (in open Brayton cycle).

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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.

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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.

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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 10 | P a g e

Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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''.

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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''

Direction of rotation

Stall cell propagation due to local pressure rise at blade A

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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 14 | P a g e

Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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).

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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 5 th 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 9 th 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 5 th 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

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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 135 o 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 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.

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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.

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

 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

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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 21 | P a g e

Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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 22 | P a g e

Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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 12 th 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 2 nd 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

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

 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.

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Mahmoud Elnaggar

Gas Turbine Notes

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Ministry of Electricity - Egypt

Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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 â&#x20AC;&#x201C; (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

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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:

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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 â&#x20AC;&#x201C; so be careful â&#x20AC;&#x201C; 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].

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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 Âľm (0.001 mm) the measuring device is proximity probes, if the vibration level reached 195 Âľm 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 Âľm 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 30 | P a g e

Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

2. If turbine speed reached 47 r.p.s the load is rejected immediately and after 10sec if the seed didnâ&#x20AC;&#x2122;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 1 st stage nozzles which may be fixed at combustor outlet.

Cannular type

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Annular type

Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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).

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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.

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2- Premix flame

Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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''.

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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 ď&#x201A;ˇ 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.

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

ď&#x201A;ˇ 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.

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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) Î&#x201D;P 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 = Î&#x201D;Pc.c/Pcd * 100 If RPD is lower than 1.8% this means that the C.C. Î&#x201D;P 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

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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.

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Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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

Burners without CBO

Annular plenum Rotating oscillation damper 1

Rotating oscillation damper 3

Rotating oscillation damper 2

Eng.M.Elnaggar

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.

Without CBO

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With CBO

Mahmoud Elnaggar

Gas Turbine Notes

Ministry of Electricity - Egypt

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.

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Mahmoud Elnaggar