Learning Center
Plans & pricing Sign in
Sign Out



									         UNIT (3)






Three types of compressors are used in gas turbine unit; they are:
    1- Axial-flow or turbo compressor.
    2- Radial flow or centrifugal compressor.
    3- Positive displacement compressor.

The last type is not being used nowadays in gas turbines. It was firstly used in
Lysholm engine that was used in Elliott marine gas turbine. The axial-flow
compressor is almost the only one being used with gas turbines.

      In discussing the axial-flow compressor, it is suitable to begin by showing a
diagrammatic sketch representing a section of an axial-flow compressor as in Fig.
(1/3). The figure shows that the compressor is a turbine of reversed performance.
When the compressor rotor is rotated by any prime mover, the rotor blades push the
air (or any gas) as a flow passing through these blades and the fixed blades of the
stator. This forces the air to continue flowing to the outlet even if its pressure is
many time the intake pressure. The rotor blades and the stator blades may be
repeated many times in multi-stage compressors. This increases the pressure by each
stage, and the final pressure may be many times higher than the pressure produced
by one-stage compressor. Figure (2/3) shows the rotor and the upper half of the
stator containing thee stages.

     The other type is the centrifugal compressor that resembles the known
centrifugal pump, except the diffuser of the compressor transfers much quantity of
kinetic energy to pressure energy. Figure (3/3) shows a centrifugal compressor
having more than one diffuser.
     In spite of the big competition between the two types in the earlier days of
inventing the gas turbines, yet nowadays the axial-flow compressor is almost the
only one that is used. They are of lower size and weight and of higher efficiency in
addition, they can be designed at small diameters and very high speeds. In small size
gas turbines (up to 500 HP) centrifugal compressor are used. Some companies used
the two types in a single turbine as Centrax CS 600, Boeing 550 and lycoming TF
20 designs, as shown in Fig. (4/3).

    These vanes or blades orient the air on entering the compressor in direction to
avoid shocks and losses. These blades mat be fixed or adjustable to match all
rotational speed of the compressor rotor. Figures (5/3) and (6/3) shows the
compressor characteristics in case of prewhirl.

    Air entrance to the compressor causes high degree of unacceptable noise that
causes environmental pollution. This noise has a higher pitch tone, which can be

attenuated by using splitters or air filters made of fabric types. Any one of these
methods attenuates the noise to an acceptable level.

     When no filters are used, it is recommended to clean the turbine blades,
following the manufacturer manual to know the cleaning period and the method of
cleaning. In case of using filters, the filter packing or components should be cleaned
according to its design. If the filter becomes dirty, this affect the compressor
performance due to the power lost in overcoming the resistance of the dust and dirt.
This leads to lowering the unit efficiency. Care should be taken when the weather is
full of clouds and humidity because this with the dust and dirt causes accumulation
and plug the filter porous.

    Usually the relation between the pressure ratio and mass flow rate indicates
compressor performance. To compare easily the compressor performance at
different working condition of barometric pressure and ambient temperature, it is
preferable to plot the curves in dimensionless equivalents of pressure and mass flow
and speed.
The most important equations are:
          • Pr essure ratio =

                               m•        T1
          • Mass flow =
          • Speed (rev / min) =
          m•        T1         N
Neither                  nor             are, strictly speaking, dimensionless numbers. They
               P1              T1
need the addition of a characteristic dimension of the compressor.

     From Fig. (5/3) we notice that the performance curves are limited by a surge
line, to the lift of which there are no data. The reason for that is the inefficient and
unstable compressor operation to the lift of the surge line. At any speed, if the
delivery valve is controlled to reduce the compressor discharge, the delivery
pressure increases systematically until reaching the surge line. At this point or near
to it, the pressure becomes unstable and high noise occurs. The reason of this is
discussed as follows: at low flow rate the air velocity entering the rotor blades
becomes small and wrong relative angle causes shocks and turbulence around the
blades that weakens the compressor performance. If this happens during the turbine
operation, the power developed reduces and the engine may stop running. This may
also happen if excess fuel enters the combustion chamber when the turbine is at low
speed at idle. The high temperature produced in the gases entering the combustion
chamber increases its specific volume so that the turbine can not swallow it and
surge occurs.
     When surge occurs in a turbine equipped with centrifugal compressor the
turbine may recuperate its power again and returns to operation. This is not the case
if an axial-flow compressor exists, even if the fuel flow rate is reduced and the
turbine needs initiating a fresh start.
     Industrial gas turbines are equipped by governor unit to limit the rate of
acceleration to operate at safe limits.

     The diffuser is a device used to transfer kinetic energy in air to pressure energy
by decelerating its velocity through divergent passages designed specially to avoid
formation of eddies. Since the diffuser depends in its operation on its geometric
shape, therefore it should be clean of the deposits of dirt and dust.
     Two effects occur in the diffuser; they are velocity reduction and density
increase. The two effects have opposite effects. Reducing velocity needs large cross
sectional area to pass the same mass flow, while increasing the density needs the
opposite. The kinetic energy is proportional with the square of the velocity. The
ratio between the two effects does not stay constant. When the value of the speed is
low the important change is due to velocity change, while the density change is
important when the velocity is very high. The two effects cancel each other when
the velocity equals sonic speed. That is why the subsonic flow diffusers are of
diverging passages and the supersonic flow diffusers are of converging passages.

The design details of any type of compressors are out of this course, but some
design characteristic of the axial-flow compressor are shown as follows:
    a- Construction:

          It is constructed of number of successive stages; each consists of a rotating
      disc carrying a group of blades, followed directly by a stationary blade ring. It
      is possible to have a stationary blade ring or guide blades at entrance, and the
      last a stationary blade ring is called outlet guide vanes to force the flow in the
      axial direction. Figure (7/3) shows the ideal arrangement of an eight-stage
      axial-flow compressor of constant mean radius, indicating the method of
      blades fixation to the rotor. The fixation may be:
           • Dovetail root
           • Fixing pin
      • Fir tree
      • Straddle “T” root
    The figure shows that fixation, at the fifth stage, is by integral cast of the
 blades with the rotor, while fixation in the stages from 6-8 by dovetail root.
 The latter method is suitable for airplane gas turbines, while the root fixing as
 T diverged from the two sides suits the big gas turbines used in industrial
 applications. We will explain, in details, the fixing methods on showing
 turbine construction, since fixing methods are the same for compressors and
 turbines. Slight difference is found because the rotor blades of the compressor
 do not have protection shield as those of the turbine. The figure shows the
 design components of the rotor, which are:
      - Stage 1 on a disc
      - Stages 2 and 3 on a drum
      - Stages from 4 to 6 on (through-bolted discs)
      - Stages 7 and 8 on clamped discs splinted to a shaft
   In general at low speed, the drum construction and light blades are the most
   convenient. It consists of group of rings in welded assembly. When the
   blades are heavy it is important to have inner hub as stages (3-8) in the
   previous figure.

b- Operation and performance:
    The performance of an axial-flow compressor is simply due to scooping
action of rotating blades to the air and throwing it (hurl) to the following
stationary blades. These blades decelerate the flow to raise its pressure and
orient the flow to enter the following rotating blades in the suitable direction
and so on. Finally, all the stages produce the required pressure ratio, which is
greater than unity. Decelerating the flow is a diffusing process that produces
increase in the pressure (inspite of friction that causes some pressure drop). It is
known that the energy is added through the rotating blades, but diffusion occurs
in both rotating and stationary blades. The ratio between the diffusion occurs in
the rotating part to the total stage diffusion by the degree of reaction (D). The
diffusion happens because the blades diverge on approaching the trailing edge,
opposite to that of turbine. Figure (8/3) declares the changes in the magnitude
and direction of air speed.

c- Compressor material and manufacture:
    The axial-flow compressor blades are made of different materials as fibrous
composites, aluminum, titanium, steel and nickel alloys. These materials are
arranged according to the hardness, which increases from fibrous composites to
nickel alloys. Although the blades made of fibrous composites are well in
strength-to-weight ratio but they can not support shocks due to drops of rain or
hail, if the turbine works at rainy weather. Aluminum blades are suitable for
airplane engine for its lightweight and its softness and the resistance to
corrosion. Titanium is suitable for compressor first stages due to its durability,
stiffness and lightweight, but it suffers, usually, from rapid fatigue failure due to
vibrations. As for steel, it is safer but of heavy weight. Nickel alloys are used at
very high temperature (700 K).
     The rotor is usually made of steel either the shaft or the discs. In airplane
engine, titanium is used in primary stages and nickel alloys in the back stages.
Stator vanes are made of the same material, nevertheless steel is the most
usable, because the stress upon these vanes differ than those of the rotor.
The compressor casing is made of cast magnesium, aluminum, or steel.
Soldering of titanium or steel, depending on weight and cost and application
may make it.

d- Operational problems:
Some problems concerning the compressor operation that affect its design and
use. The most important of these problems are:
  - Performance defect.
      Compressor may fail to produce the required pressure ratio at any value
  of the operating speed range and its efficiency may drastically decrease.
  There is no mean to overcome this problem unless using the available data
  obtained from cascade testing and experience. These tests are used to study
  the flow and blade twist angles to have the best flow through the blades.
  Despite determining the angles and profiles of blades at design stage, yet they
  change during operation because of:
  1- Dirt and dust accumulation due to weather.
2- Salts accumulation at marine environments.
3- Dust and sand accumulation at desert environments that blunt the blades.
   Overcoming performance defect is done by periodic cleaning of the blades,
either by water jet, water mixed by alcohol or by spraying some detergents in
the flow passages at idle speed or driven by the starter. The difficult
accumulation are cleaned by blasting the blades by a stream of air carrying
walnut shells, granulated coke or rice grains. Orifices and air intakes should
be covered to avoid their blockage or blockage of heat exchangers of small
passages. The eroded blades may be corrected if they are not badly damaged,
and filing and polishing the blunted leading edges.
   Filters, of 99.99% efficiency, are used to protect blades from erosion and
dirt accumulation in dusty weather. Another reason for performance defect is
the increase of blade tip clearance. This can be overcome using abradable
lining for casing.

- Damage due to abnormal conditions.
This damage occur due to entrance of solid objects with air as pieces of
stones, birds, animals, head covers, mechanical parts (nuts, bolts, sleeves,
washers etc…) or tools (spanners, pliers, wrenches, etc…). These things
enter due to strong suction of air that causes eddied and pre-rotation carry
them to the compressor, Fig (9/3)

- Operational errors
     Operational errors mean, in general, reduction in flow and pressure ratio
at the rotational speed. These defects appear in both centrifugal or axial- flow
compressors, either they work in compressor plant, turbo-chargers or in gas
turbine. This error is accompanied by stopping airflow or surge that causes
forward and backward airflow. Another error is the deep stall which is a case
in which both the flow and pressure become so very low that causes
dangerous turbine overheat. Sometimes, continuous operation in this case
causes blade breaking that damage both the compressor and the turbine.
     Sometimes operational errors occur due to internal causes as blade
erosion or dirt accumulation without cleaning it quickly, or due to external
causes as upstream or downstream surge in the flow or passage distortion.

                            THE TURBINE
    Complete gas-turbines power plant contains of a group of units; each consists of
a compressor, combustion chamber, heat exchanger and a turbine. The most
important part is the turbine, which is the producing power component that needs
periodic maintenance. It receives the highest temperature gases. Therefore, it suffers
of thermal stress, and must be of special design characteristics and material.
Thermal expansion is the cause of all problems that face the turbine and needs care
to overcome it. Thermal expansion problems are severe at starting, due to the
differences in expansion of different parts.

Three different designs of turbines are found depending on shaft support.
• Overhung power turbine:          Figure (10/3) shows a section of this kind of
   turbines used in small units.
• Two-bearing turbines: This design is used in bigger size turbines and is
   shown in Fig. (11/3).
• Three-bearing turbines: This design is used in the turbines having only one
   driving shaft with the compressor. This design increases the value of the first
   critical speed, but it needs great care in assembly, Fig. (12/3).
The small axial-flow turbines need very small clearance between the blade tips and
casing. Since this needs very fine machining, therefore they are centrifugal type
turbines. The centrifugal turbine is in reality a centrifugal compressor with reversed
flow, Fig.(13/3).

    Turbine parts are made of austenitic steel or of nickel-chrome alloys that are able
to sustain stresses at very high temperatures. Therefore, they are used in
manufacturing the blades and vanes as well as rotors of small turbines. Because they
are hard in forging, the rotor of big turbines can not be made one unit but in separate
discs that bolted together or welded. Many trials were made to make the blades from
ceramic materials, but all are too brittle and can not support tension and suffer of
cracks due to thermal shocks.

    Turbine reliability increases if the rotor and blades are cooled in addition of
getting long periods between overhauls. Cooling also permits using cheap materials.
Reducing the temperature of the rotating parts reduces the casing. Naturally air is
the suitable coolant due to easy use, inspite of using water in cooling the turbine
built by Solar company of U.S. Navy, Fig (14/3).
    The simplest method of cooling a turbine is to direct part of the compressor air at
any intermediate stage in the radial direction towards the rotor center to pass over
blades. This air will mix with the main stream of gas to exit the turbine with the
exhaust. This is called boundary layer cooling. It has an excellent effect as the
maximum temperature did not exceed 350 oC when the gas temperature was 727 oC

in one application. Boundary layer cooling method is not used in multi-stage
turbines, which are used in industrial applications. In these application the cooling
system shown in Fig. (15/3) is used, where the blade roots are made in such a way
that a passage is left to let the cooling air to pass. This cools the rotor and the blade
roots that suffer of high stresses. Turbine casing is cooled in the same way or as
shown in Fig. (16/3), where vanes are mounted on separate carrier rings made of
austenitic materials inside the casing as well as ventilated annular air space between
them. Air is taken from the main circuit before passing in the heat exchanger or to
the combustion chamber using small auxiliary compressor. Air may be cooled and
filtered before being admitted to cooling passages. After cooling, this air is mixed
with the main gas stream. Special materials are used in uncooled gas turbines, as
austenitic steel bolts.

    There was no difference between impulse and reaction bladed old turbines.
 Nowadays, turbine blade should be of the two types together. The reaction
 increases from top to root. In pure impulse turbine, the fixed vanes work as
 nozzles and the rotating blades as buckets (see Pelton wheel shown in Fig. (17/3)).
 In the other hand, in reaction turbine the fixed and rotating blades are similar
 doing as nozzles, but the jet exiting from the fixed blades are much less powerful.
 The jet speed is higher than the rotating-blade linear velocity. The effect that
 generates power is the reaction of the reward-pointing jets caused by the fluid
 leaving at the back of the rotating blades. In impulse turbine, the jets leaving the
 fixed blades are of speed much higher than double the rotating blades. Blade tip
 has higher linear speed than its root. Then, at tip reaction effect occurs and at root
 impulse effect is found.

Tip clearance is one of the conventional problems in turbo-machinery. Expansion
due to heating and contraction due to cooling in rotor blades must mach that of the
casing or stator diameter to keep constant clearance. In addition, the casing should
be circular all the time for the same reason. This clearance should be small to avoid
leakage and not too small to avoid friction between blades and casing.

   Figure (18/3) shows blades fixing methods. Fir tree root is suitable to mount the
blades through axial (side-entry blading) or circumferential grooves in the root.
Side-entry blading is better, but it is difficult to be used in multi-stage engines.
Obviously, the blades are heated more than the rotor itself; therefore, blade
expansion is greater than the slots in which it is mounted. So, clearance should be
found to avoid slot cracks due to blade root expansion. Due to the clearance the
blades move slightly during maintenance, this is normal.
    The blades may be mounted to the rotor by other methods, among of them:
   - Welding: where the blade root is made like a tooth mounted in a groove in
      the rotor the it is welded.
   - Accurate casting: the wheel with all discs and blades are totally made as a
      unit by fine casting from suitable steel, which is thermally treated to increase
      hardness and toughness.
   - Integral forging: as in casting, the disc and the blades are totally made by
      forging. After machining, they are mounted on turbine shaft, Fig. (19/3).

    When the turbine is properly balanced no vibration occurs. The blades are
designed and tested to determine the natural frequency. In long blades, locking wire
are used to reinforce them and to increase the natural frequency to safe values,
despite the loss the wires cause. Whirling effect is the vibration occurs when the
rotor shaft rotates at double the natural frequency. The shaft surges as the it surges
when it falls on bearings during assembly. It is said that the turbine has elastic shaft
when it surges before reaching its normal running speed. When it surges at speed
higher than its normal running speed, it is called stiff shaft turbine. If it is elastic
shaft, its speed should be accelerated quickly during passing by the critical speed,
where whirling occurs. No one can predict the type of shaft during manufacturing,
but after testing it.

    Simplicity should be considered on designing the turbine casing. Lack of its
symmetry leads to unequal expansion on heating up and losses the actual circular
form. This may lead, by consequence, to dirt accumulation or difference in blade tip
clearance. Experience, shows that casing should be one unit to avoid distortion and
the rotor is inserted inside it. This is impractical if we have more than three stages.
Gas admission should be regular around casing to have uniform gas velocity.

    Some form of seal is required to prevent working gas leakage. Labyrinth type
sealing system, which is a group of projections on the turbine shaft matching others
on the inner surrounding stationary casing sleeve. To form complicated passages
preventing leakage. Figure (20/3) shows three types of these labyrinths, which is
composed of number of throttling points in series. Despite the effectiveness of the
labyrinth in reducing leakage yet, this small leakage affects the bearing due to
heating them in abnormal way. Therefore, cooling air should be at pressure higher
than gas pressure inside the turbine to return the leakage to inside.

    Usually the bearings are conventional in design, made of plain white metal and
using forced lubrication. No problems are mentioned due to temperature, as the big
turbines have cooling systems. In uncooled turbines, the shaft is made of austenitic
steel, which is a bad conductor. In the charging set (compressor and turbine
assemblies) that has overhung rotor, the shaft is shortened , therefore sealing system,
which is near to bearings, is cancelled. The bearing will be subjected to pressure
equals the compressor delivery pressure. By this way, gas seal through the bearings
is found. In small turbines ball bearings and angular contact bearings (to compensate
axial thrust) are used. Oiling the tilting pad thrust bearings is done by injecting oil at
bearing from down , letting flow at bearing top.

    The axial thrust is acting towards turbine delivery, which tends to push the rotor.
Any movement of the rotor may damage labyrinth glands or blades. Therefore,
thrust-block setting protects the turbine. It is important that the axial flow from the
compressor must be in the same direction the flow in the turbine to compensate each
other and the difference will be carried by the thrust-block.

11-     LAGGING
    Until now the best substance, to cover and insulate the turbine casing is not
known. Some substances that include silica-based have good insulating properties
and endure high temperatures, but it is found in somewhat coarse granular form. It is
also difficult to have good means for fixing it to stay in place during operation. The
best way found to fix a lagging or an insulating layer is preparing a thin layer (5 cm
approximately) claded by an aluminum or steel sheets. A clearance should be left
between the clading and the insulating layer to let a stream of air passing to cool the
the casing surface.

     Burning liquid fuel in gas turbines is smokeless, regular and continuous
combustion, because there is excess air and needs ignition starter. Figure (21/3)
shows a diagrammatic sketch of a typical combustion chamber that has the
following items:

     Air enters the combustion chamber through a ring that separates the air stream
into two parts. One takes central cone flow and the other as an outer annulus. The
first is used in combustion and is called primary air and its volume is 20% more
than the quantity needed for complete combustion of the injected fuel at full load.
The outer annulus air is to cool the flame tube and diluting products of primary
combustion, in order not to exceed the maximum temperature Tmax. The combustion
temperature may be controlled at (1500 – 2000 oC at flame core) by using small air
quantity at combustion zone. This high temperature assures that combustion is
complete. Using good combustion chambers lead to 99% combustion efficiency.
     Combustion efficiency is measured by dividing the quantity of heat given to the
air entering the turbine by the calorific value of the fuel. The loss in efficiency (1%)
is due to unburned or partially burned residual of fuel. It may be in the form of
carbon or carbon monoxide that leaves the unit with the exhaust.
    Completeness and temperature of combustion depend on the mixing level
between air and fuel. Complete mixing reduces greatly the quantity the excess air
entering the primary zone that leads to condense combustion heat and short flame
does not reach the secondary mixing zone. It is important to know that long flame
means that combustion is still on. Continuation of flame till reaching the secondary
mixing zone may be stopped burning of the rest of fuel by the action of relatively
cool secondary air that joins combustion gases there.
    Air entering in eddies flow improve mixing that results complete combustion.

     Stability of combustion, in the primary zone, is achieved by the swirler system
that makes air flowing as miniature free vortex that has less pressure at primary zone
center than at periphery. Getting far from the swirler, the whirling effect weakens
and air pressure becomes more regular and returns in recirculation way, Fig. (22/3).
This forces the flame tongue towards entrance preventing flame from getting off.

 There are many types of combustion chambers in addition to the straight-through
flow, shown in Fig. (23/3). Among these types are the annular, reverse-flow and
elbow type. While the combustion chambers of the straight-through flow are
situated on regular circumferential distance around the turbine, as in airplane, those
of reverse flow are situated on or beside the turbine, as freestanding units. The
annular type is as the straight-through flow. Sometimes the reverse flow combustion
chambers are situated parallel to the axis or in conical form around the turbine as
those of Westinghouse. Small turbines have the reverse flow type situated vertically,
angularly, or transversely.

    Increasing of the turbine, twin reverse-flow chambers are more suitable. In very
large industrial turbines (more than 20 megawatts), single or twin chambers are used
situated far from the turbine.
    The essential criterion on which the success of combustion system is determined
are obtaining uniform gas outlet temperature without fuel impingement on the walls,
small in size and sustaining long life. Manufacturers prefer cylindrical chambers
having swirl-type air director.
    Vortex type chamber is not famous because of its geometrically complicated
internal configuration that leads to cooling problems. In annular type, continuous
exiting of gases exerts regular moment on blades that leads to smooth rotation.
    The inlet of air, in reverse flow type, is near to discharge end, then reverse flow
along the outside of the flame tube before passing into the primary zone. This make
useful preheat of the primary air, Fig. (24/3).
    Figure (25/3) shows an elbow-type chamber in which big eddies are generated
without need to a swirler.

     Lining of refractory material as aluminum oxide firebricks internally covers
flame tube, Fig. (26/3). This lagging material prevents heat loss from combustion
zone and protects the flame tube from high temperatures. The radiant heat from the
liner helps in entering fuel ignition to continue combustion and make it stable,
especially in heavy fuels that do not volatilize so readily.
    The difficulty is focused in finding liner that last long life, because usual
materials either oxidize very fast or gets mosaic cracks due to thermal shocks
happening from frequent operation and stop. The liner, then smashes to pieces those
are carried and damage the turbine blades.
    Finally, manufacturers succeeded to have liner composed of huge number of
small bricks containing voids between them for air-cooling.

    It is the hottest part in any gas turbine either with or without refractory liner.
Therefore, it erodes by the effect of combustion around it or by distortion. The life
of the flame tube differs from type to another. Those in airplanes have life less than
1500 hrs, while in heavy industrial gas turbines flame tube life is ten times. The life
depends by the efficiency of cooling system.
    English National Gas Turbine Establishment found that when cooling air is
oriented so that an internal cooling film exists, the flame tube life is much less than
if the air layer is outside that takes heat away.

   Pattern of holes and openings in the secondary zone is designed on theoretical
basis, and then it is usually modified after testing to reach best mixing between cold
and hot gases, determined by the temperature distribution inside combustion
chamber. If the temperature distribution over cross-section of the duct is not
reasonably regular the turbine heats up in asymmetrically manner and distorts.

    Burner or fuel nozzle secures a consistent form of spray. Beginning by no load
and ending by full load, fuel consumption changes ten times or more. Some burners
have two separate characteristics, one for high flow and the other for the low flow.
Mechanical means can switch from one to the other depending on fuel delivery
pressure. Swirl pressure-jet atomizers are used in European industrial turbines, but
simplex type is largely used in single shaft turbines that used distillated fuel. The
flow rate of this fuel is low that can be obtained without high pressure.
    Fuel sprayer, having two nozzles, is used in airplanes, where one is a pilot and
the other the main discharge orifice. It is also used in stationary applications and is
called duplex. The two nozzles at concentric and arranged so that pilot spray cone
angle is less than principle cone angle. The main nozzle discharges when the
pressure in the pilot nozzle reaches 40 kg/cm2. This is externally controlled.
    It is not preferable to use moving parts in the sprayer assembly for fear of solid
particles that mat jam.
    Spill atomizers suit units that need high level of combustion at a wide range of
loading. Operation of this type depends mainly on admitting fuel far from atomizing
nozzle by a valve at the end of system that lets fuel passing completely outside
atomizing nozzle zone, or passing only part of it, Fig. (27/3).

   In complex cycle, the rate of airflow depends on load and fuel flow. In some
cycles (simple cycle with heat exchanger) airflow rate stays constant at all loads,
that makes the air/fuel ratio very low at low loads that needs special care in burner
design and its maintenance to avoid accidental blow-outs. In complex cycles,
fuel/air ratio decreases at transient conditions, such as sudden decrease in load, to a
low value. This occurs on running at full power.

 Alarm systems may be mounted to the unit to shut down the plant by closing fuel
line when accident extinction of flame in any combustion chambers. This occurs to
avoid explosion due to fuel accumulation. It works by photo-electric cell that
receives light through mica window inside chamber, or by the ions of the gas in the
flame that play as conductor.

10-     HEAVY FUEL
   There is no difficulty in burning heavy fuel oil (mazout) in combustion
chambers. The oil is preheated the fuel to reduce its viscosity and to be easy
ignition. Fuel should be controlled in order not to touch the tube liner as this type of
fuel has very high erosion effect. Heavy fuel is so slower in burning that it needs
longer chamber so that every particle finds time to burn inside the chamber.
Sometimes fuel is injected with air to help atomizing it. Down-stream of the
combustion chamber, bad effects appear in the ducts, turbine blades and heat
exchangers due to condensation and solidification of the products of combustion.
Care should be taken to avoid sodium and vanadium salts.
   Vanadium pentoxide is soft and sticky and analyzes into oxide sticks to turbine
blades, forming rough surface, which increases erosion rate. Efforts are made to
reduce combustion product effect on the unit by reducing fuel droplet size or
reducing maximum temperature of the cycle.
   Another method to avoid this bad effect by additives, like silica, magnesia, zinc
oxide and kaolin, that raise melting point or softening the ashes of combustion to
stay at its solid case at the operating temperatures.
    In Swiss, practical experience showed that in a 20-megawatt power station, they
get rid completely of turbine deposits, by adding additives of perlon 602 to mazout.

    All gaseous fuels are suitable for gas turbines and the majority is considered as
ideal fuel. Natural gas, for example, is a clean fuel presenting no problems in
condition to get rid of any suspended fluids. Usually it is found at sufficient
pressures to be used directly through regulators and control equipment without
pumping to avoid fuel pump problems. Sometimes, natural gas is found at high
pressure to be used to obtain extra power by expanding it in a simple expansion
turbine before admitting to combustion chamber. Even, if the pressure is low a
turbine like this can be used as a starting motor. Combustion chambers using
gaseous fuel are similar to those using distillated fuels, the difference is in the burner
or sprayer that will have larger passages on using gaseous fuel. Gaseous fuel burns
easier and faster and has luminous flame, and has no burner problems.
     There is a general problem on using gaseous fuel, which is the reduction of fuel
air ratio, at light load, even at primary zone, to an extend that there is no burning.
Therefore, it is necessary to overcome it by controlling fuel flow to the limit of
continuation of combustion. Figure (28/3) shows a sketch of a gaseous fuel system
used in small gas turbine (250HP).

Solid fuel as cock can be used in gas turbines, but the problem is concentrated in
blade heavy erosion if fuel ashes pass to the turbine. In addition, some other
difficulties appear due to dust accumulation in ducts, nozzles and blade passages.
Pulverizing the cock is important and causing eddies is also important to have
continuous combustion. The combustion chamber should be longer. Figure (29/3)
shows a section of a combustion chamber in Ruston and Hornsby turbine.

There are many systems of combustion chambers used in gas turbines, they are:
   1- Budwarth Buzzard systems.
   2- Orenda OT-5
   3- Orenda OT-C-5
   4- Dowty fuel system, Figs. (30/3 –31/3)
   5- Lucas system, Fig. (32/3)
   6- Austin system, Fig. (33/3)
   7- Rover system, Fig. (34/3)
   8- Sulzer system
   9- Brown Boveri

                      AIR INTAKE SYSTEM
     Accumulation of dirt and dust on compressor blades in gas turbines is a big
problem facing the designers and the operators especially in polluted air sites that
carries dust and sand. Axial-flow compressors are sensitive and the performance is
badly affected due to that accumulation and tends to surge. This leads to low
produced power and in the overall thermal efficiency. Huge accumulation may
affect unit starting and turbine operation at low loads. Therefore, it is necessary to
have efficient filtration system before compressor entrance.
     Since air/fuel ratio, in gas turbines, is large, therefore air intake installation is
bigger in size than the turbine itself. In industrial environment, the filter packing is
throwaway elements of static or moving screen. Figures (35/3 and 36/3) show the
types of replaceable packing, while Fig. (37/3) is the rotating filter.

     In desert sites, it is important to have screens to trap sand from air before
entering the main filter, which may be rotating and oil-wetted. Figure (38/3) shows a
filter equipped with sand trap screen, while Gig. (39/3) shows a section in the
rotating, which is cleaned automatically by oil bath. In rainy weather, water drops
accumulate in the oil tank of the filter; therefore, it is important to notice oil level
and to get rid of water.
     Electrostatic filters prove high efficiency, inspite its huge size and cost. They
need to clean electrodes regularly and to prevent humidity from entering to avoid
short circuits.
     Noise caused by the compressor represent turbulence and annoyance source
especially in inhabitant areas near to power stations. Wide range frequency hurt
human ears, therefore tuned splitters are situated in intake duct to attenuate sounds.

                           EXHAUST SYSTEM
     Portable or mobile units have stub duct for exhaust facing upward, but
stationary units, especially those near to inhabitant areas, should have advanced
exhaust system to suppress noise and to discharge gases at elevated location in
upward direction.
     Chimneys are made from steel sheets up to 300 feet high, and may have
silencing equipment as concentric splitter, situated inside them. We must not
exaggerate in the degree of attenuation, as this affect turbine efficiency due to
friction. Figure (40/3) shows the standard model of noise and the degree of
attenuation in power stations. Fig. (41/3) shows exhaust gas duts of an electric
power station using gas turbines.


To top