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TECHNICAL PRINCIPLES OF GAS TURBINES

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TECHNICAL PRINCIPLES OF GAS TURBINES Powered By Docstoc
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TECHNICAL PRINCIPLES
  OF GAS TURBINES

  FIRST:
      WORKING PRINCIPLES OF GAS TURBINES

  SECOND:
     TURBINE OPERATION
                        FIRST
          WORKING PRINCIPLES OF GAS TURBINES

INTRODUCTION:
    Gas turbines are considered as a heat engine working according to
thermodynamic cycle of constant pressure. Air is used as the working medium in
the majority of practical applications, where it is compressed and heated up then
expanded. From thermodynamic point of view, it is possible to prove that the work
taken from air or exerted on it, at high temperature levels, is higher than that at low
temperature levels. Therefore, it is possible to obtain a quantity of work from the
turbine during expansion stage of the cycle higher than the work exerted during
compression. The difference is sufficient to overcome all different sources of losses
in addition to a huge quantity as output power used in different purposes.
    As it is possible to add heat to the cycle by burning the fuel internally in the
working medium (the air), or adding it from outside source, therefore the cycle may
be called of internal combustion or external combustion type.
    Air must be the working medium in case of internal combustion, as oxygen is
necessary for combustion. If the cycle is of external combustion, it is possible to use
any type of gases or vapors. If any other gas, rather than air is used, the cycle should
be closed cycle.
    Explaining the details of thermodynamics is out of this course, but some of
principle thermodynamics are shown as follows:

          P1 V1 P2 V2
     1-        =
           T1    T2
     2- P1V1γ = P2V2γ
     3- T . R = ( P. R) (γ −1) / γ
               Cp
     4- γ =
               Cv
     Where:
       P: The absolute pressure.
       V: The volume.
       T: Absolute temperature.
       γ: Specific heat ratio.
     The symbol (suffixes) are as follows:
       1: For compressor inlet
       2: For compressor outlet
                                                2
The simplest cycle:
In its simplest form, the gas turbine consists of three main components, working
according to the open internal combustion cycle. Figure (1/2) shows a diagrammatic
section for this turbine, while Fig. (2/2) shows the symbolic representation of it.
From this turbine, high-speed gases are obtained that cause jet thrust as in airplane
engines, or a shaft power as rotational speed and torque that can be used in driving
different engines. As mentioned previously, it has:
   • Compressor.
   • Combustion chamber.
   • Turbine wheel.




                                              3
   All thermodynamic functions occur at adiabatic compression, then heating at
constant pressure followed by adiabatic expansion. These processes occur at the
same sequence continuously. The working medium (the air) enters the cycle at the
                                            4
compressor inlet, where its pressure increases to several atmospheric pressure
values. The pressure ration used depends upon cycle design. The pressure ratio is
6:1 for middle size turbines. The air temperature increases due to the adiabatic
compression. The air temperature increases more by the effect of heat addition due
to fuel combustion to the maximum cycle temperature Tmax. The flame temperature
is much higher than this maximum temperature because the air coming from the
compressor at T2 cools the combustion products. The maximum temperature
depends on the thermal cycle design and the estimated life of the turbine. As the
maximum increases the turbine life decreases. In heavy turbines of long life and
uses mazout as a fuel, the maximum temperature reaches 650 oC. This temperature
increases to 900 oC in small turbines of high efficiency and long life.
     Finally, the working medium pressure reaches to atmospheric pressure due to
expansion through the turbine blades. The power generated is sufficient to drive the
compressor and the load. In normal case, two third of the generated work is
consumed in driving the compressor and the other resisting parts and the other third
is the useful load used in any external applications.
     Mechanically, the gas turbine is very simple, as the compressor is of the rotating
type either the axial-flow or the centrifugal type, which is directly connected to the
turbine forming the rotor. The rotor is carried by one or two couples of bearings,
either journal or roller bearings in condition that one of them is thrust bearing. The
output power can be taken from either sides of the rotor, but it is preferable to take
the power at the cold end to overcome the problems that may happen at the hot end.
     If we exclude the auxiliaries, as oil and fuel pumps the only movable part in the
turbine is the rotor.
     The burning system consists mainly of one or more highly developed blow-
lamps. They are designed specially to insure keeping suitable and continuous flame
at all operating conditions. This system also insure a mixing zone to mix the gases
coming from the flame zone with the relatively cold air coming from the
compressor, before entering to the turbine.
     The compressor, the combustion chamber and the turbine are connected by
carefully designed gas ducts. Depending on the type of application, the combustion
system may be separated. If the turbine is used in stationary application, the turbine
construction may include air filtration plant, inlet and exit exhaust ducts and
silencers.
     The idea of obtaining net work from the gas unit depends upon an important
physical characteristic. This characteristic is that the work required to or obtainable
on raising or lowering the pressure of a quantity of gases, increases as the gases
temperature increases. This is clear due to divergence of the constant pressure lines
as the temperature increases on the temperature-entropy diagram (T-S diagram). On
this diagram the vertical difference in temperature represents the work required to
increase or decrease the pressure of a quantity of gases. As the temperature of the
gases increases this vertical in temperature increases, as shown in Fig. (2/2m). This
figure represents the T-S diagram of a simple cycle having a single turbine. We find
                                                5
that the distance T2 – T1 represents the work required to raise the pressure of the air
quantity taken from atmospheric pressure P1 to the pressure P2.
In the combustion chamber a quantity of heat is added from point 2 to point 3 by
burning the fuel to raise the gas temperature from T2 to T3 at the constant pressure
P2. It may be considered that the quantity of the gases equals the quantity of the air
entering the compressor. On expanding through the turbine, the output work is
represented by the distance T3– T4. Due to the divergence in the pressure lines,
mentioned above, on the T-S diagram it is found that the output work of the turbine
due to gas expansion is larger than the work required to the compressor. The
difference is the net useful work. This work represents one third of the total work
obtained inside the turbine.

The simple cycle with free power turbine:




     Now it is suitable to attack the other thermal cycles that are used in gas
turbines to complete all the basics of operation. Comparing the previous Fig. (2/2)
and Fig. (3/2), the clear difference among them is that: the turbine in Fig. (3/2) is
divided to two parts. These parts are; the high-pressure turbine and the low-pressure
turbine. In this design the mechanically separated turbines have to provide power to
drive the compressor and to provide the output power. The high-pressure turbine is
connected directly by the compressor while the low-pressure turbine is responsible
of providing the output power. Gas ducts only connect the two turbines. The unit
containing the compressor, the combustion chamber and the high-pressure turbine is
                                                6
called charging set or gas generator, while the other turbine is called the power
turbine. The two sets are always connected aerodynamically and
thermodynamically, but they do not necessary have the same speed. This leads to
flexibility more better than the single-shaft turbine. For example, the power turbine
drives an alternator, then this turbine is adjusted to be coincide the necessary
frequency of the system. The speed of the charging set may be increased or
decreased according to the load demand. Adversely, the power set speed may be
changed according to load (other than alternators) and the charging set speed
remains constant as in case of driving cars or trains.
      There is only one problem occur as a result of separating the two turbines; this
problem is the lack of response of the engine on changing the load. For example, the
power set may have over-speed when the load decreases suddenly. Therefore, it is
necessary to have an advanced system or highly sophisticated control system.
      When the power set drives an electric generator it is necessary to take all
precautions to prevent the power-turbine from over-speed when the load decreases
suddenly.
      In this case, the charging set takes the work represented by T3-T4 that is needed
for the compressor and the power set takes the work represented by T4-T5 which is
the net work taken from the cycle as an output. Figure (3/2 m) shows a simple open
cycle for this unit on the T-S diagram.




                                                7
RECUPERATION:
     Usually, the exhaust gases leaves the turbine, in the simple cycle, at a
temperature much higher than that of the air entering the compressor. Therefore, it is
obvious, to improve the thermal efficiency of the cycle if we supply the turbine by
means to recuperate the maximum possible quantity of heat leaving with the exhaust
gases. This heat must be given to the air leaving the compressor before entering the
combustion chamber. Any temperature increase of air at this location safe a quantity
of fuel to be injected in the combustion chamber to reach the necessary maximum
temperature Tmax. It is possible using heat exchangers, and in this case, the
diagrammatic drawing of the unit should be as shown in Fig. (4/2). The efficiency
may increase by 20-27% approximately on using a reasonable-effectiveness heat
exchanger. The real advantage obtained from using a heat exchanger, which is
manifested as a reduction in the fuel consumption rate, depends on the general
considerations of the thermal cycle.
     Recuperation and adding heat exchangers increase the manufacturing cost of
the engine and the unit size increases. In addition, the engine reliability decreases
                                               8
due to the sever conditions caused by the heat exchanger. The sophisticated control
system, used in the separated turbine engine, may have more problems on using the
heat exchangers, due to the pressure needed in its passages. Then, the only
advantage of using the heat exchangers is increasing the engine efficiency, which in
some cases does not worth in front of the mentioned problems.




     This does not mean that the problems prevent using heat exchangers because
there are many engines use in their thermal cycles heat exchangers especially in
power plants. These cycles differ from the other cycles by making use of exhaust
gas temperature T6 to heat up the compressed air coming from the compressor.
Through the heat exchanger the temperature of the compressed air raise from T2 to
T3 that makes less fuel going to the combustion chamber to reach the maximum
temperature Tmax. Therefore, adding heat occur from point 3 to point 4 instead of
point 2 to point 4. Obviously the exhaust gas temperature T6 must be higher than air
temperature T2 to heat it up.
     Figure (4/2 m) represents on T-S diagram an open simple cycle with heat
recuperation.

                                              9
INTERCOOLING:
      There is a known thermodynamic fact, that is: the ratio of exerted work to
compress any gas to a certain pressure ratio reduces if the compression takes place
isothermally rather than adiabatic compression. Isothermal compression is not
feasible practically in gas turbines, but approaching a constant temperature during
compression is feasible by using two- or multi-stage compressor and adding inter-
coolers between stages. The inter-cooler cools the compressed air before
compression in the following stage. By this way, the work of compression reduces
to the necessary value as shown in Fig. (5/2). Intercooling may help in reducing the
size of the main gas turbine components. Its use does not improve the cycle
efficiency, as the heat extracted by the intercoolers can not be re-admitted to the air.
       As shown in Fig. (2/5 m) the distance T3-T4 is less than the distance Ta-T2
when using only one compressor without intercooling. Therefore, the work done to
reach the same pressure ratio reduces on using intercooler. In opposite, we need

                                                10
more fuel to reach the maximum temperature as combustion begins at point 4 to
poin 5 instead of beginning at point a to point 5.




                                          11
RE-HEAT:
      Re-heat is to add another stage of combustion after having partial expansion of
gases. This occurs when the engine has two separated turbines having a second
combustion chamber in between that raises the temperature of the partially
expanded gases to reach again to the maximum cycle temperature, Fig (6/2). Re-
heat, as recuperation does not improve the cycle efficiency. The idea of using re-
heat is to avoid reaching very high maximum temperature causes danger on the
turbine and its blades as shown in Fig. (6/2 m). in this figure the maximum
temperature becomes Tα which the turbine and its blades can not endure.




                                              12
COMPLEX CYCLES:
     Theoretically, gas turbines may be considered able to work according to
different cycles, as it contains multi stages of compression and expansion. Figure
                                            13
(7/2) shows a diagrammatic drawing of a gas turbine unit using a separate power
set. This type of units is considered practical in large prime mover over 20
megawatts or more. Figure (7/2) shows the T-S diagram of this unit that includes
intercooling and recuperation added to having three turbine stages to reduce the
maximum temperature. This is done to reduce the sizes of the turbines and
compressors as well as improving efficiency and reducing the cost.




                                           14
CLOSED CYCLE:
   These cycles are considered as external combustion cycles shown in Fig. (8/2).
The working medium that is air in many application (not necessary air in other
                                            15
applications) flows in a closed cycle where the exhaust gas exits from the turbine is
cooled before re-entering the compressor. The thermal efficiency of the closed
cycle, using conventional fuel, at part load, is higher as the mean pressure in the
cycle may mach the type of fuel keeping the same maximum temperature constant
during load range.




AIR-BLEED CYCLE:
     It is possible to use the charging set in the simple gas turbine unit in a very
useful way to produce moderate pressurized air to avoid using a separate
compressor driven by the power turbine. Figure (9/2) shows this arrangement that
has a large size compressor that supplies the cycle with the necessary air and the rest
that may be used in different applications.




                                               16
17
                              SECOND
                         TURBINE OPERATION
    From practical point of view, knowing how to operate a gas turbine is more
important than knowing all the design specifications. Every body deals with a gas
turbine must know how to operate it

ENGINE STARTING:




     When we would like to know the steps of operation of a gas turbine, we should
take into consideration the steps of operation of the plant that the gas turbine
represents one of its components. In power plats, we must realize the correct air
valves setting, the switches. In addition, leaving the unit to a long period of preheat,
and that the generator is in its correct phase that suits the local grid, and many other
aspects that that proceed the turbine starting. In big units some additional factors that
must be watched during turbine stating as control of cooling water circuits and
following the engine operation as its temperature increases. The increase in
temperature affects the thermal stresses of the thick metal components and their
expansions in different rates. In addition, the exhaust gas temperature must be
surveyed and the manual compressor control valves. These requirements differ from
unit to another, but they agree in the general rules.
     The changes in the net engine shaft torque due to change in rotational speed on
starting from rest until no load speed (acceleration from rest to idle) is shown in Fig.
                                                18
(10/2). The negative torque that begins from zero until reaching the self-sustaining
speed represents the need of the compressor during this period to external torque
greater than that produced by the turbine at the first stages of operation. Both the
turbine and the compressor are very inefficient at low speeds. Because airflow
angles and contraction or expansions happening in the passages do not match the
compression or expansion of gases and the low flow velocities limit the exerted
momentum and the work capacity.
     The starter supplies the turbine by the necessary torque to start operation. The
torque is equal to that needed for the compressor in addition to extra torque to
accelerate the shaft.
     In multi-shaft engines, the starter drives only the high-pressure turbine. That is
why, in multi-shaft engines, we need a starter smaller than that needed for an
equivalent engine having a single shaft. Usually the starter is an electric or hydraulic
motor or a compressed-air-driven turbine, connected to the engine shaft by system
of gears. It may also be a jet of air directed to the turbine blades. Starters separate
automatically when the turbine speed becomes little bit higher than the self-
sustaining speed ( usually it equals 40-50% of the shaft design speed of the high-
pressure turbine). The compressed air, needed for the jets, may come from a
separate small turbine or from compressed air bottles or cartridge.
     Starting procedure may greatly affect the turbine life, and ignition plugs must
start working before fuel injection to the combustion chamber to assure direct fuel
ignition rather than its accumulation in the combustion chamber. Accumulation of
the fuel inside the combustion chamber may cause explosion or excess heating of
the turbine and the passages.
     It is important that the turbine turns before letting fuel to the combustion
chamber in order not to burn the blades taking directly the gases from the initial
ignition zone. Letting the turbine to run fast before starting ignition is an important
and preferable action.
     There are standard checks should be done as measuring the temperature, the
time delay taken to begin ignition after beginning fuel injection. Also, after reaching
the self-sustaining speed, the starter should stops and the turbine continue
accelerating its speed to reaching the idle speed. The time needed to reach this speed
should be recorded.
     If the starter does not stop automatically at the suitable speed, dangerous
damages may happen to the engine if this was not noticed before reaching the
operating speed. Usually the automatic ignition system turns off after the
completion of the starting process, unless in bad weather cases when the humid air
turns off the flame.
     In some cases, starting the turbine fails due to some circumstances. For
example, if there is inadequate starter torque, or if fuel flow is insufficient or if the
ignition system fails starting at the suitable moment. In addition, if the turbine
reaches waving at low speeds may fail to reach self-sustaining speed, especially if it

                                                19
starts when it is cold. In this case, the cold engine metal absorbs heat from the flow
and then it over heats when starts while it is hot due to previous operation.
     The suitable solution of starting problems in using good starter with good
connections assure suitable pressure and volt continuation.




ENGINE TESTING AND CORRECTION OF DATA:
     Tests carried on engines to assure the good manufacturing and that the engine
realizes the specified performance. These tests are considered as a part of the
commissioning process, especially in heavy installation engines. The tests may be
the initial tests after the engine manufacturing, because heavy engines are not
assembled unless at the determined site. As for the small engines, they are tested in
the factory before delivery and in site after installation. In general, these tests carried
in this stage to be sure that all systems work efficiently and the turbine is
satisfactorily connected to fuel supply, control system, air and gas passages and
others. Tests may be repeated from time to time to prevent any mal-function and to
make the periodic control adjustments.
     In many cases, engines are manufactured for the development purposes, they
are not for sail. This accelerates of the fault finding process, as these engines are
subjected to sever conditions in testing as different frequencies and stresses to
determine blade life and blade reliability. Tests may also be carried on loading
similar customer load. Tests may be carried at site or in open area furnished by
special test beds.
     It is necessary to take the standard precautions before carrying tests which
include:
           • Safety and completion of all connections.
           • Calibrate instrumentation.
           • Inspection of air intake and exhaust passage to be free of tools or
               debris.
           • Inspection for any leakage.
           • Inspection of the environment and spaces in front of air intake to be
               free from tools, nails and small parts.
           • Be sure that exhaust exit is in free space outside the engine location.
      Figure (11/2) shows a schematically drawing of a standard unit used to test gas
turbines. In the unit, air is drawn through acoustically absorbent splitters. The
absorbers have crooked passages to prevent direct noise. The majority of air passes
through the turbine as a working medium, while the rest surrounds the turbine to it
then mix with the exhaust gases before passing on the quiet system. The gas engine
is bolted on a floating cradle in the test location and connected to the dynamometer
or to a test propeller according to the need.
                                                 20
      Two sets or more to fix the gas turbines on them may equip the test unit. Each
set is installed in separate room. The gas turbine is connected by air and fuel lines
and by the control equipment at the same time when another gas turbine is being
tested in the other room.
      Usually one control room equips the test unit containing tools and
instrumentation. On carrying a test the unit should be calibrated to know the
ambient conditions to determine the correction factors according which the
measured performance is adjusted. Two principle corrections exist:
      1- The correction to make up the cell depression as the air entering the test cell
          intake suffers depression equals ∆P1. This depression reaches (10-20) mm
          water gauge. This means that the engine works at pressure less than the
          normal atmospheric pressure and inlet pressure to the engine may be lower
          than the atmospheric pressure by ∆P1 if a mesh is installed on the entrance.
      2- The second correction concerns drag and thrust, this is related to jet engines
          used in airplanes, not with stationary units.




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