A gas turbine is a rotating machine in which the chemical energy of a fuel is converted into mechanical or kinetic energy in terms of power axis. In other words, it is a mechanical device or a driving force. It uses a gaseous working fluid for this purpose. The generated mechanical power can be used by industrial devices. There is a continuous flow of working fluid in the gas turbine. Gas turbines generate forces that produce shaft power. Gas turbines are used to propel aircraft, which converts fuel energy into kinetic energy to generate propulsion. The figure below shows a typical view of a gas turbine.
Gas turbine application
The main applications of gas turbines are found in the following cases:
As a direct and mechanical actuator for various industries
- making electricity
- Oil and gas industry
- Marine propulsion
- Turbo generators
- Turbo compressor
- Car section
The working principle of the gas turbine
The operation of a gas turbine is based on the Brighton thermodynamic cycle. The Brighton cycle consists of two adiabatic work transfers and two constant pressure heat transfer processes (Figure below). The gas is subjected to isotropic and adiabatic pressure in state 1 to 2. This process increases the temperature, pressure and density of the gas. Then, heat with constant pressure is added to state 3 in state 2. For a gas turbine, it adds a process of heat combustion. During mode 3 to mode 4, the gas passes through an adiabatic isentropic turbine, which reduces the temperature and pressure of the gas. In the case of the Brighton closed gas turbine cycle, heat is released from the gas through mode 4 and mode 1 through a heat exchanger.
Let us understand the principle of operation of a gas turbine with the following example:
Imagine a rocket that burns fuel, producing high-pressure exhaust gas. According to the Energy Conservation Act, in high-pressure exhaust gases, the chemical energy of the fuel is converted into mechanical energy. Exhaust gas drift tries to move the missile forward when firing the missile. The question now is whether the body fixes the missile with a mechanical structure to prevent it from moving. what will happen?
In this case, the exhaust gas is released at high pressure but in the reverse direction. Now another case is what if we add a set of turbine blades to this return exhaust gas?
The released mechanical energy, which is backwards in a linear direction, is converted into the rotational motion of the turbine shaft, which is a great success. This means that the chemical energy of the fuel gas is converted to the rotational mechanical energy of the turbine shaft, as shown in the figure below.
Simply put, in a gas turbine, hot gases travel through a multi-stage gas turbine. It has fixed and movable blades just like steam turbines. The fixed blades adjust their speed and direct the moving gases towards the rotor blades. The turbine shaft is connected to a generator.
The working principle of gas turbine in the power plant is as follows:
In a gas turbine power plant, there is a generator known as an electric car, and this generator needs a main engine, which is a gas turbine, to generate electricity, as shown in the figure below.
This chemical energy converts fuel into mechanical energy, or in other words, natural gas into mechanical energy. The mechanical energy generated is then transmitted to the generator shaft via the gearbox. The turbine can now generate electrical energy as shown in the figure below.
This initial form of electrical energy is usually low or medium voltage. In order to manage power losses in transmission lines, step-up transformers are used to increase this voltage, and the increased voltage is supplied to electrical energy, which in turn is transmitted through transmission lines and delivered to the grid, as Shown in the figure below.
The principle of operation of gas turbines in the oil and gas industry
The following points should be considered, such as:
For the oil and gas production process, the turbine is connected to a compressor or pump instead of being coupled to the turbine.
When a gas turbine is used to drive a compressor, a similar arrangement to a steam turbine is considered.
Header tanks and lubricating oil are required in the auxiliary piping system.
The exhaust system must be considered to have several heat recovery systems, ie a process heater or a steam generating plant.
Maintenance and operation of all machines must be provided.
Outside the compressor house, combustion air must be transferred from a safe place to the turbine burner. Most likely needed are input shutter and filter.
Gas turbine components
In the figure below, gas turbines have three main parts:
1. Air compressor
2. Combustion chamber
With the combustion chamber between the air compressor and the turbine, both the air compressor and the turbine are mounted on a common shaft on both sides. Gas turbines need a starter motor because they do not start on their own. It is the use of an air compressor to suck in air and compress it, thus increasing its pressure. Axial (multi-stage) compressors are preferred for the most advanced and largest gas turbines.
Here, compressed air combines with fuel to form a mixture of fuel and air, delivering combustion products to a gas turbine. With high air pressure, the fuel mixture burns completely. Today, liquefied petroleum gas, natural gas or natural gas is used in gas turbines. Three types of combustion chambers are generally used:
- Cyclic combustion chambers
- Can have (multiple) combustion chambers
- Combustion chambers annular chamber
The fuel is injected at the upper end of the burner in the form of a highly atomic spray. Fuel nozzles may be simplex or dual-burner. Some gas turbines are “dual fuel”, meaning that they can burn a mixture of gas and liquid fuel.
There is a multi-stage gas turbine from which hot gases move and kinetic energy is converted into horsepower. A gas turbine, just like a steam turbine, has fixed and moving blades. The purpose of fixed blades is to direct moving gases toward the rotor blades and then to adjust their speed. The turbine shaft is connected to a generator.
The hot gases from the gas turbine come out of the exhaust. The exhaust chamber consists of an internal and external chamber.
Other parts are gas turbines
- cooling system
- Bearings and lubrication system
- Fuel system and so on
Types of gas turbines
The following types of gas turbines are listed:
- Open cycle gas turbine
- Closed cycle gas turbine
- Air-derived gas turbine
- Jet Engine Scale
- Auxiliary power unit
- Jet engine
Open cycle gas turbine
The open cycle gas turbine consists of three parts: the combustion chamber, the turbine and the compressor. The compressor increases the pressure by absorbing ambient air. The fuel burns to add heat to the air in the combustion chamber, resulting in a rise in temperature. The heated gases are then transferred from the combustion chamber to the turbine, where they perform their mechanical work while expanding. The figure below shows an image of an open cycle gas turbine.
Schematic of open cycle gas turbine
Closed cycle gas turbine:
The working fluid used in a closed air cycle gas turbine or any other suitable medium leaving the compressor is heated in the heater by an external source at a relatively constant pressure. The air is then transferred to the turbine with high pressure and high temperature. The turbine fluid is then cooled to its initial temperature by some external cooling agents and then transferred to the compressor. In this way, the working fluid is continuously used in the system and the required heat is given to the fluid by the heat exchanger without significant change in its phase. The following figure shows a picture of a closed cycle gas turbine.
Air-derived gas turbine:
These types of gas turbines are used in power generation due to their ability to control load changes faster and their ability to shut down better than industrial machines. These are used in the marine industry to reduce weight.
Scaled jet engines:
These turbines are also known as miniature gas turbines.
Scale jet engines are capable of producing up to 22 Nm of thrust and are easily built by most thoughtful mechanical engineers with basic engineering tools such as metal turning.
Auxiliary gas turbine:
These are smaller types of gas turbines that are used to power aircraft. Auxiliary gas turbines are used to provide air conditioning and ventilation. They provide compressed air power for jet engines. They also provide the gearbox mechanical power to run larger jet engines or drive axle accessories.
Gas turbine design
Factors that limit the size and efficiency of a gas turbine include: cooking temperature, compression ratio, mass flow, and centrifugal stresses.
The most important areas in the design of a gas turbine that determine the engine. The efficiency and life of the gas path is hot, ie the combustion chambers and fixed nozzles of the first stage of the turbine and rotating buckets. The components in these areas make up about 2% of the total cost of the gas turbine, but control the output and efficiency of the gas turbine. Nickel superalloys are commonly used for gas turbine nozzles and buckets. They are vacuum coated with special metals (platinum-chromium-aluminide) to protect against hot corrosion that occurs at high temperatures in the presence of contaminants such as sodium, vanadium and potassium.
The continuous performance of gas turbines can be increased by continuously improving the firing temperature and compression ratios. Nozzles and air-cooled buckets Using air from the compressor is a major improvement over the firing temperature. This limits the metal temperature of the nozzles and buckets to resist hot corrosion and creep.
To provide additional turbine output by increasing the final compressor pressure, additional compressor steps can be added to the compressor rotor assembly to achieve a higher compression ratio. Natural gas, diesel oil, residues or crude oil can be used as gas turbine fuel. As the ambient temperature and altitude increase, the air density decreases. This significantly reduces the output power and efficiency of the gas turbine. Ambient temperature and altitude changes do not affect steam and diesel plants.
Codes and standards
Common codes and standards used in the design, construction, testing, etc. are gas turbines
ASME PTC 22
Gas turbine performance
Factors that affect the performance of gas turbines are:
Inlet air density
Ambient air temperature
Height and ambient pressure
Inlet and outlet pressure drop
Number of shafts
Advantages of gas turbines
- Fuel storage requires less space and is easy to move.
- Lower maintenance costs.
- Construction is very simple.
- Compared to steam power plants, it does not require condensers, boilers and other accessories.
- Fuels such as kerosene, benzene, paraffin and coal can be used, which are cheaper than gasoline and diesel.
- Gas turbines can be used in water-scarce areas.
- Creates less pollution.
- It needs less water.
Disadvantages of gas turbines
- Most of the developed power is used to drive the compressor. For this reason, gas turbines have low thermal efficiency.
- High-frequency noise comes from the compressor, which is still questionable.
- Special metals and alloys are used for different parts of the turbine because the working speed of the turbine is 40,000 to 100,000 rpm and its operating temperature is 1100 to 1260 degrees Celsius.
Gas turbine manufacturers
The major share of gas turbine production is controlled by the following organizations:
– General Electric (United States)
– Siemens (Germany)
– Mitsubishi Hitachi Power Systems (Japan)
– Ansaldo STS (Italy)
– Solar Turbines (United States)
– Kawasaki Heavy Industries Co., Ltd. (Japan)
– Doosan Heavy Industries (South Korea)
– Bharat Heavy Electrical Limited (India)
– OPRA turbines (Netherlands)
– Vericor Power Systems LLC (United States)
– Rolls-Royce (UK)
The top three companies in the list above together have more than 80% of the gas turbine market share.
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