GAS TURBINES
COURSE OBJECTIVES
At the end of the course, delegates will be able to:
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Explai Explain n the funct function ion of a gas gas turbin turbine. e. List the the most common common types types of gas turbines turbines in use. Understand Understand the thermody thermodynamic namic princip principles les of gas turbine turbine system systems. s. Understanding Understanding some some of the criteria criteria for selecting selecting a gas turbine turbine for a given given set of design conditions. 5- Explain Explain the main main aspects aspects of gas turbine turbine protection protection and and control. control. 6- Appreciate Appreciate how the design design of an overall-plant overall-plant control control scheme scheme relates relates to the gas turbine system. 7- Explore the the safe and efficient efficient operatio operation n of a gas turbine system, system, using using dynamic dynamic simulation model. 8Expl Explor oree co common mmon ope operati rating ng prob proble lems ms..
CONTENTS 2
1- INTRODUCTION - Fluid mechanics properties and laws - Thermodynamics principles - Definition of a gas turbine - Historical background about gas turbine - Brief idea about types of gas turbine - Brief idea about advantages of gas turbine - Brief idea about applications of gas turbine 2- TECHNICAL PRINCIPLES OF GAS TURBINES - Working principles of gas turbine - Turbine operation 3- MAIN COMPONENTS OF GAS TURBINES - Compressor - Combustion - Turbine - Air intake system - Exhaust System 4- AUXILIARY SYSTEMS FOR GAS TURBINES - Lube oil system - Fuel oil system - Fuel gas system - Hydraulic trip oil system 5- OPERATION AND MAINTENANCE OF GAS TURBINES - Introduction - Turbine operation - Maintenance
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UNIT (1) INTRODUCTION
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Definition of a gas turbine 1- Definitions: Since long time, the theory and method by which gas turbine operates was known even before knowing the material from which the gas turbine should be built and before knowing the fluid mechanics theories and fluid flow. Engineers waited the development of material science very long to obtain some materials that endure high temperatures that happen in the turbine system. The gas turbine is defined as a type of prime movers that can transform the fuel energy to useful mechanical energy developed at its shaft at high rotational speeds. The gas turbine consists, in its simplest form, of two main parts, they are: 1- Gas generator section. 2- Power conversion section. The gas generator section by itself consists of: 1- The compressor. 2- The combustion chamber, 3- The turbine, the turns the compressor. There are different types of gas turbines depending on the nature of inlet and exit of the gases. Figures (1/1, 2/1, 3/1, 4/1) show the different types of these turbines. Figure (1/1) is a section of a gas turbine that operates according to the basic cycle, while Fig. (2/1) is a section in a turbo-jet that is equipped with afterburner. As for Fig. (3/1) is a section in non-mixed turbo-fan engine.
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PRINCIPLES OF FLUID MECHANICS & THERMODYNAMICS
FLUID MECHANICS PROPERTIES OF FLUIDS
The equations of fluid mechanics allow us to predict the behavior of fluids in various flow situations. To use the equations, however, there must be information regarding properties. The properties, which are discussed in this 9
chapter, include viscosity, pressure, density, kinematic viscosity, surface tension, specific heat, internal energy, enthalpy, and compressibility. Viscosity: A fluid has many properties. One important property is viscosity, which is a measure of the resistance the fluid has to an external applied shear. Because this property arises from the definition of a fluid, it is examined in that regard. Consider again a fluid-filled space formed by two horizontal parallel plates shown in figure. The upper plate has an area A in contact with the fluid and is pulled to the right with a force F/ at a velocity Vi. If the velocity at each point within the fluid could be measured, a velocity distribution like that illustrated in the figure might result. The fluid velocity at the moving plate is Vi because the fluid adheres to that surface. At the bottom, the velocity is zero with respect to the boundary, owing to the non-slip condition. The slope of the velocity distribution is: dV 1 /dy.
If this experiment is repeated with F2 as the force, a different slope or strain rate results: dV 1 /dy. In general, to each applied force there corresponds only one shear stress and only one strain rate. If data from a series of these experiments were plotted as T versus dV 1 /dy., the shown figure would result for a fluid such as water. The points lie on a straight line that passes through the origin. The slope of the resulting line in the figure is the viscosity of the fluid because it is a measure of the fluid's resistance to shear. In other words, viscosity indicates how a fluid will react ( dV/ d y) under the action of an external shear stress (τ ). The plot of that figure is a straight line that passes through the origin. This result is characteristic of a Newtonian fluid, but there are other types of fluids called non-Newtonian fluids. A graph of T versus dV/ dy, called a rheological diagram, is shown in figure for several types of fluids. 10
Newtonian fluids follow Newton's law of viscosity and are represented by the equation
τ µ ( dV =
/ dy )
Where:
τ = the applied shear stress in dimensions of F/L2{ Ibf/ft2 or N/m2) µ = the absolute or dynamic viscosity of the fluid in dimensions of F. T/L2 (Ibf.s/ft2 or N.s/m2 ) dV/ dy = the strain rate in dimensions of 1/T (rad/s).
In the cgs system of units, the unit for viscosity is poise, corresponding to 1 g/cm .s. The centipoise is 1/100 of a poise. The SI system, unit for viscosity is 1 kg/m.s. It has no particular name. It is 10 times the size of the poise, as it is clear from the basic units. Examples of Newtonian fluids are water, oil, and air. If a fluid cannot be described by Equation 1.3, it is called a non-Newtonian fluid. On the basis of their behavior, these fluids are divided into three categories: timeindependent, time-dependent, and viscoelastic as in figure. 11
Density: The density of a fluid is its mass per unit volume, represented by the letter p. If the mass of 1 ft3 of water is 1.94 slug, its density is p = 1.94 slug/ft3. If the mass of 1 m3 of liquid is 820 kg, its density is p = 820 kg/m3. Density has dimensions of M/L3. The density of various substances is given in the property tables in the appendices of any Fluid Mechanics HandBook.
One quantity of importance related to density is specific weight. Whereas density is mass per unit volume, Specific weight is weight per unit volume. Specific weight is related to density by: with dimension F/L3 (Ibf/ft3 or N/m3)
SW = ρ g ,
Another useful quantity is specific gravity, which is also related to density of a substance. The specific gravity of a substance is the ratio of its density to the density of water at 4 °C: SG= ρ /ρ For usual ρ
w
w
(ρ
w
density of water)
is taken to be 1.94 slug/ft 3 or 1000 kg/m3.
Specific heat: The specific heat of a substance is the heat required to raise a unit mass of | the substance by 1° . The dimension of a specific heat is energy/ (mass. temperature): F. L /(M. t). The process by which the heat is added also makes a difference, particularly for gases. The specific heat for a gas that undergoes a process occurring at constant pressure involves a different specific heat than that for a constant volume process. For example, the specific heat at constant pressure C p for air is 0.24 Btu/(Ibm . °R), or 1005 J/ (kg . K), and the specific heat at constant volume Cy is 0.17 Btu/(Ibm . °R), or 717 J/(kg . K). Also of importance when dealing with these properties is the ratio of specific heats, defined as
γ = C p /Cv,
(For air, the ratio of specific heats y is 1.4)
The Btu (British thermal unit) is the unit of energy measurement in the English engineering System. One Btu is defined as the energy required to raise the temperature of 1 Ibm of water by 1 °F. However, because we are 12
using the British gravitational system, the units encountered are Btu/(slug. °R). Both specific heats vary with temperature for real substances, but to simplify calculations they are assumed to be constant. Internal Energy: Internal energy is the energy associated with the motion of molecules of a substance. Consider a quantity of gas. The gas can have three types of energy: energy of position (potential energy), energy of translation (kinetic energy), and energy of molecular motion (internal energy). Adding heat to a quantity of gas at constant volume affects only the motion of the molecules and does not increase the potential or kinetic energies of the gas. This effect is manifested as an increase in temperature. In fact, for a perfect gas with constant specific heats, it can be shown that: ∆
U = C v ∆ T
where ∆ U is a change in the internal energy per unit mass of the gas with dimensions of energy /mass ( F. L / M ).
Enthalpy: A quantity that appears often in equations is (u + p/r), this quantity is given the special name enthalpy, h. adding heat at constant pressure goes into increasing the internal energy of the gas and raising the position. Again for a perfect gas with constant specific heats, it can be shown that: ∆ h = C p ∆ T
BERNOULLI'S THEOREM: Bernoulli's Theorem is a special application of the first law of thermodynamics for flowing fluids. The energy content of a flowing fluid can be split into three components:
Elevation Head, Velocity Head, Pressure Head Bernoulli's Theorem states that the sum of these three energy terms at one set of conditions is equal to their sum at another set of conditions. Accelerating fluid
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Bernoulli's equation shows that we can convert the Kinetic energy of a gas to pressure (potential) energy by decreasing it's velocity. Velocity Head + Pressure Head + Elevation Head = Constant Decrease in velocity head causes an increase in pressure head. Bernoulli's Equation U 2 2 g
+
p
g ρ
+ z =cons
U = fluid velocity,
ρ = density, U 2 2 g
velocity
=
g = gravitational constant,
p = pressure
z = elevation head head
,
p g ρ
=
pressure
head
,
z = Elevation Head
NB: Each term has dimensions of length and can be regarded as representing a contribution to the total fluid head. Incompressible, frictionless flow is assumed.
THERMODYNAMICS
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PRESSURE: The pressure is the normal force per unit area, and its dimension is (N/m2). The pressure measured by any instrument, that reads zero when it is open to the atmosphere, is called gauge pressure. The absolute pressure is the sum of gauge pressure plus the atmospheric pressure. The standard atmospheric pressure, (at sea level and 15 oC), is:
1 bar (105 Pa or 105 N/m2) or 76 cm Hg or 10.3 m of H 2O or 14.7 Psi Then: Absolute pressure = gauge pressure + atmospheric pressure
TEMPERATURE: Two types of temperature scales are found, namely centigrade and Fahrenheit. The absolute temperature is the local temperature plus 273 for centigrade scale or 460 for Fahrenheit scale. To convert from one scale to another the following two relation are used: C= (F-32) x (5/9) F= C x (9/5) +32 First Law Of Thermodynamics (Conservation Of Energy): Energy cannot be created nor destroyed during a process (e.g. compression), although it may change from one form to another. Potential Energy: Energy due to a body's elevation (pressure). Kinetic Energy: Energy due to a body's motion (velocity).
From the first law of thermodynamics, it is possible to convert energy from one form to another. To understand the principles of compression we need to consider thermodynamics. Gas Laws: Boyle's Law: At constant temperature, the volume of an ideal gas is inversely proportional to the pressure. 15
1 V α p
Therefore:
PV = Constant, Where:
V = Volume
and P = Pressure
Charles' Law At constant pressure, the volume of an ideal gas is proportional absolute temperature. V αT
Therefore: V/T = Constant, Absolute temperature
Where:
V = Volume
and T
=
Ideal Gas Equation From Boyle's Law and Charles' Law: PV = RT Where: R = Universal Gas Constant This is true for all ideal gases, no gases are ideal and this equation is corrected by using compressibility factors, which are determined experimentally. Several forms of ideal gas equation are as follows:
Pv = RT PV = mRT PV = nMRT PV = nℜT (Where: P is the gas absolute pressure, v is the gas specific volume, R is the gas constant, T is the gas absolute temperature, m is the mass of the gas, n is number of moles of the gas, M is the gas molecular weight, is the universal gas constant and ℜ=MR) Decreasing Volume From Boyle's Law (PV = Constant), it can be seen that if you reduce the volume of a gas then it's pressure will increase. PV = Constant, as V reduces, P increases (assuming constant temperature).
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Compressibility All gases deviate from the ideal gas law and these deviations are accounted for by the compressibility factor (Z). The ideal gas equation is modified to:
PV = ZRT,
Rearranging:
pv Z = RT
Z is experimentally derived from actual
gas data and is often generated from equations of state.
The Second Law of Thermodynamics This is more abstract and can be stated several ways. ♣ Heat cannot, of itself, pass from a colder to a hotter body. ♣ Heat can be made to go from a body at lower temperature to one at higher temperature only if external work is done. ♣The available energy of the isolated system decreases in all real processes. ♣ Heat or energy (or water), of itself, will flow only downhill.
Basically, these statements say that energy exists at various levels and is available for use only if it can move from a higher to a lower level. In Thermodynamics a measure of the unavailability of energy has been devised and is known as entropy. It is defined by the differential equation: dS
dQ
=
T
, Where: S = Entropy,
Q = Heat,
T = Temperature
Note that: entropy (as a measure of unavailability) increases as a system loses heat, but remains constant when there is no gain or loss of heat (as in an adiabatic process).
DEFINITIONS: ♥ISOTHERMAL PROCESS: constant temperature process ♥POLYTROPIC PROCESS: a reversible process ♥ADIABATIC PROCESS: no heat gain to or lost from the system ♥ISENTROPIC PROCESS:reversible and adiabatic process 17
Compressor Cycles Compressors are compared to theoretical compression cycles as a basis for calculations and comparisons. Isothermal Compression: This occurs when the temperature is kept constant as the pressure increases. This requires continuous heat removal. p1 V 1 = p2 V 2 =const
It is not commercially possible to remove all heat although compressors are usually designed for as much heat removal as possible. Adiabatic Reversible (Isentropic) Compression: This occurs when no heat is added or removed during compression. p1 V 1k
p2 V 2k
=
const ,
=
Where k = ratio of specific heat
Adiabatic compression is never obtained because there is always some heat removed or added. Polytropic Compression: This is the cycle along which actual compression takes place. p1 V 1n
p2 V 2n
=
const
=
The exponent n is determined experimentally for a given type of machine and may be lower or higher than the adiabatic exponent k. In positive displacement compressors n is usually less than k. n or
n
−1 n
can be calculated from test data if the suction and
discharge n pressures and temperatures are known.
T 2 T 1
p2 = p 1
n −1
n (can be derivation)
2- Historical background about gas turbines: 1- Early model of gas turbine: Utilization of gas turbines, driven by combustion gases flowing from fire, began since long time. It was during the era of the Hero of Alexandria at year 150 BC. In addition, at the same time Chinese used windmills (that may be considered one type of gas turbines). 18
2- Gas turbine models between 1791-1930: 1- John Barbar turbine (Fig. 4/1): It is to be sure that the real gas turbines era had begun in 1791 when John Barbar get a patent for his pioneer invention concerning gas turbines. That invention consists of reciprocating compressor and a combustion chamber and an impulse turbine. He was able to recognize the need of cooling the turbine blades and suggested the water injection. 2- Stolze turbine: In year 1872, Stolze was able to get a patent for his invention concerning gas turbines. The design of that invention consisted of the following parts: • Multi-stage axial flow compressor, which perhaps was the first type at all. • Multi-stage reaction turbine whose shaft is connected to the shaft of the compressor. • Heat exchanger. • Combustion chamber. The turbine was ten stages, while the compressor was only nine stages. 3- Charles G. Curtiss turbine: In June 1884, Charles G. Curtiss presented a complete gas turbine that was the first gas turbine designed in USA. 4- Stolze second turbine: The second gas turbine built by Stolze in France in year 1900, was the first real turbine capable to produce work. However, the tests carried on it were not encouraging because its very low unsatisfactorily efficiency. 5- Armanganed Brother’s turbine: Armanganed Brother carried series of effective trials to build a large gas turbine in Paris at the same time during which Stolze was building his turbine. They began their trial on a 25-HP De Laval turbine using an air compressor that work by the compressed air from the principal Paris compressed air network. After this, they produced another turbine using a centrifugal compressor of four bars designed by Rateau in year 1905. In addition, they used Curtiss turbine wheel whose diameter was 37.4 inches. The turbine was running at 4250 rpm, while the turbine exit gas temperature was 1040 oF. That turbine was able to produce compressed air instead of mechanical power and its thermal efficiency was 3%. 6- Holzwarth turbine: 19
The efforts of Holzwarth in the field of gas turbines were the most important efforts appeared after this, and to him was the grace in building the first practical and economical gas turbine. Holzwarth turbine was operating by explosion cycle without pre-compression. It was a rotating engine of intermittent combustion. The first turbine of this type was built and tested in Hanover. Who is interested of more details about Holzwarth turbine should look to Stodola book. 7- Sanford Moss turbine: Sanford Moss was able to operate the first turbine in USA in year 1919, and at the same year, the first civil airplane could fly using a gas turbine.
3- Developments between 1930-1940: Many developments happened during the thirties especially Velox boilers and the first gas turbine that worked successfully in power generation was built by the efforts of Brown Boveri. The British and German governments did great efforts to develop gas turbines which are used in airplanes propulsion. Among the efforts exerted during the thirties are: 1- Brown Boveri efforts: Thanks to Brown Boveri, that building of gas turbines to generate electrical energy had increased either in power stations or in other industrial applications. The first gas turbine built by Brown Boveri was used to operate combustion air compressors used in Velox boilers. The first turbine built to be used in Velox boilers in the year 1932. That turbine used auxiliary power, as the power needed by the compressor was higher than that developed by the turbine. After while it was able to increase the developed power from the turbine to be higher than that needed by the compressor. This was realized by using charging sets added to the turbine. In November 1936, new utilization of the charging sets appeared when Sun Oil Company in Philadelphia in USA declared the discovery of the supercharger. It was to burn the carbon residuals of petroleum product distillation to produce the maximum power of the combustion process. In year 1939, Brown Boveri had built the first power station using large gas turbines to produce 4000 kW. That unit worked on a simple cycle, as it was stand-by unit. In year 1940 that unit was used in an underground-power station in Newchatel. The total hours use of this unit did not increase more than 1200 hrs, till year 1953. Brown Boveri invited Stodola to carry standard tests on that unit. The results of these tests are shown in table (1/1). In year 1939, the first gas turbine-using mazoot to produce 2 megawatt. It was used during 6000 hrs. since that year till 1977. 20
Table (1/1) test results carried by Stodola on Brown Boveri gas turbine Type of test results 1st test 2nd test 3rdtest Load (kW) light 4021 3057 Fuel mazout mazout Mazout Compressor pressure ratio 3.82 4.38 4.38 Compressor efficiency (%) 86.4 86.4 86.4 Compressor speed (rpm) 3020 3020 3030 Compressor air flow (Lb/hr) 499620 498176 498049 o Turbine inlet temperature ( F) 705.2 1067 987.8 Turbine efficiency (%) 85.4 88.4 88.4 Fuel consumption (Lb/kW) 1.078 1.193 Thermal efficiency (%) 18.04 16.37
2- British efforts: With the beginning of the thirties two separate groups began in building and testing gas turbines utilized in power stations and airplanes in England. One of these groups worked under the supervision of Whittle in jet engines using centrifugal compressors. The other group worked under the supervision of Griffith and Constant in building and testing axial flow compressors. In 1930 Whittle had registered his first patent in this field but he failed to get financial support from British aviation ministry or special association that is why he oriented his efforts to theoretical studies since 1930 till 1936. Power Jet Ltd. Company was formed and made the commitment to transform his theoretical studies to simple jet turbine. It was a single-stage centrifugal compressor of bilateral intakes and a single-stage turbine connected directly to the compressor having single combustion chamber. The test of the first turbine of this type was in 12 April 1937, during 11 days. The combustion chamber represented the principal problem and the compressor performance was less than all expectations. Redesigning this turbine was in 16 April 1938 till 6 May when big damage happened due to the failure of wheel blades. The third trial was under test in October 1938. In summer 1939 British aviation ministry signed a contract with Power Jet Ltd. Company for building an airplane turbine knew by W1 which was tested in 1941. 21
The other group under the supervision of Griffith and Constant began building and testing the axial flow compressors in the Royal Aircraft Establishment. 3- German efforts: Hans Von Ohain paid attention to gas turbines used in propulsion at the beginning of the year 1935. He was able to register a patent in turbo-jet engine supplied with centrifugal compressor and Ernst Heinkel appointed him as a general manager to Ernst Heinkel company in 1936. In 1938, he was able to test the first turbine used for airplanes and after redesigning this turbine to be the model He 5-36, that had the following specifications: Compressor Weight of unit Static thrust for unit Specific fuel consumption for thrust
Centrifugal 795 Lb 1100 Lb 1.6 Lb/(Lb-hr)
4- USA efforts: The efforts were concentrated on raising the gas turbine efficiency. The efficiency was raised to be between 60-65% for compressor and 6570 for the turbine 4- Developments between 1940-1945: 1- British efforts: Power Jet Ltd., under supervision of Whittle, continued developing the turbine W1 and the turbine wheel was cooled by water. The first Gloucester airplane trip powered by gas turbine was in May 1941. In June 1945, Rolls-Royce produced a more powerful engine known as Derwent (V) of repulsion 3500 Lb.
2- German efforts: Sinc 1939 to 1942, German began developing the Von Ohain engine using centrifugal compressor after this at the end of 1941 they replaced it by Heinkel engine of axial-flow compressor. In November 1942, Junkers 004 (known as Jumo 004) was tested and installed in airplane Me-262. 3- USA efforts: • Turbo dyne (2500 HP) was suggested to the army in 1940 by Northup Aircraft Inc. 22
• Lockhead Aircraft suggested engine L1000 • Westinghouse Electric Corporation built (The 19) as a turbo-jet using axial flow compressor. • Wright Aeronautical Corporation failed to produce Whittle engine. • General Electric began in 1941, after getting the design and drawings of W2B from Power Jet, Ltd. And produced engine I installed in sirplane Bell P-59 A in 1942. In 1945 they began building engine G35. 4- Other efforts: A side of using the gas turbine in airplanes, Swiss Federal Railway installed an engine of 2200 HP in a train.
5- Developments between 1945-1950: During this period, old companies returned their efforts because the governmental support was good in this field. Therefore, gas turbine development continued in a fast rate to compete other prime mover either diesel engines or steam turbines.
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BRIEF IDEA ABOUT TYPES OF GAS TURBINE There are two types of gas turbines, they are: 1- Constant volume gas turbine. 2- Constant pressure gas turbine.
Constant volume gas turbine. The constant volume gas turbine, Fig. (5/1) consists of an air compressor that sucks atmospheric air at point (1). It compresses the air and delivers it to the combustion chamber through charging valve (a). At the combustion chamber a quantity of fuel is injected through nozzle (c) by the fuel injection pump. Therefore the fuel-air mixture burns by the electric spark (d), due to which the pressure increases suddenly follows by opening the valve (b) to discharge the combustion products to the turbine. There, the pressure energy is converted to mechanical energy on the turbine shaft that may be used to drive an electric generator to produce electric power. Finally, the exhaust gases escapes to the atmosphere.
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THE HEAT CYCLE The constant volume gas turbine follows a cycle called constant volume cycle, as the combustion of the fuel-air mixture takes place at constant volume. The only effect due to heat addition in the combustion chamber is to increase the pressure only. Figure (6/1) shows the heat cycle curve, where air of volume (1-5) enters at atmospheric pressure at point (1). Air is compressed to point (2) where its pressure is increased and its volume becomes (2-6). At this point air enters the combustion chamber and its temperature increases due to burning of the fuel injected. Therefore, the pressure increases from point (2) to point (3) at which delivery valve (b) opens to permit gases to pass and expand through the turbine. The gas pressure decreases and its volume increases, which is represented by curve (3-4). The exhaust gases leaves the turbine at point (4).
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In the constant volume gas turbine, usually multi-combustion chambers are used that are charged by the compressed air one after another using the same compressor. An automatic timer for the valves regulates the charging periods. That is, after charging a combustion chamber by air the charging valve (a ) is closed, fuel is injected inside this combustion chamber, the fuel-air mixture is burned that an increase in pressure occurs after which delivery valve (b) opens to exit the exhaust gases as a fast stream facing the turbine blades. As the pressure inside the combustion chamber decreases the gas speed decreases. When the pressure reaches to almost atmospheric pressure the delivery valve (b) closes and charging valve (a) opens to recharge the combustion chamber by air. The most important disadvantage of this type of turbines is the reduced of thermal capability, that is why it is not widely used and direction to the other type (constant pressure gas turbine) happened.
Constant pressure gas turbine. The constant pressure gas turbine, Fig. (7/1) consists of an air compressor that sucks atmospheric air at point (1). It compresses the air and delivers it to the combustion chamber, where it burns at constant pressure. The air volume and its temperature increase. Then, the gases pass to the turbine, where it expands and its 26
pressure decreases to atmospheric pressure. The pressure energy changes to mechanical energy on the turbine shaft.
THE HEAT CYCLE The constant-pressure gas turbine follows Brayton cycle called constant pressure cycle, as the combustion of the fuel-air mixture takes place at constant pressure. The only effect due to heat addition in the combustion chamber is to increase the air volume only. Figure (8/1) shows the heat cycle curve, where air of volume (1-5) enters at atmospheric pressure at point (1). Air is compressed to point (2) where its pressure is increased and its volume becomes (2-6). At this point air enters the combustion chamber and its temperature increases due to burning of the fuel injected. Therefore, the volume increases from point (2) to point (3) without no pressure increase. The delivery valve (b) opens to permit gases to pass and expand through the turbine. The gas pressure decreases and its volume increases, which is represented by curve (34). The gas volume increases from (3-6) to (4-5) and the pressure decreases from point (3) to point (4). The exhaust gases leaves the turbine at point (4).
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From the cycle, it is clear that exhaust and combustion take place at constant pressure while compression and expansion take place continuously without
intermittence. Therefore, output power is generated at constant rate.
BRIEF IDEA ABOUT ADVANTAGES OF GAS TURBINE: Gas turbines have the following advantages: a- Supply and installation of a gas turbine can be done in short time not exceeding few months. b- It does not need water source for operation. c- Operation and loading of the unit takes short time not exceeding few minutes. d- It is possible to use either liquid or gaseous fuel or their mixture for gas turbine operation. e- It is possible to use exhaust gases in heating processes or to help in operation of steam turbine in combined systems. f- It can be installed in deserts and remote areas, as it does not need water for cooling. g- It needs few operators and can be remote-operated. BRIEF IDEA ABOUT APPLICATIONS OF GAS TURBINE:
It is well known that airplanes use gas turbines but they are also used in surface transportation and many other stationary applications. Scientifically, the field of 28
stationary applications includes many purposes of high efficiency. This field is always increasing while that of aviation has no innovation. Here, in a scientific view the stationary applications are discussed as follows: • In electricity generation • In oil and gas industry • In combined cycles or heating purposes • In chemical and process industry applications
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