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- Steam Turbine Interview Questions- Part 03
- Gas Turbine for Power Generation: Introduction
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Siemens will deliver one of the world's most efficient combined cycle power plants to UK
A gas turbine , also called a combustion turbine , is a type of continuous and internal combustion engine. The main elements common to all gas turbine engines are:. A fourth component is often used to increase efficiency on turboprops and turbofans , to convert power into mechanical or electric form on turboshafts and electric generators , or to achieve greater thrust-to-weight ratio on afterburning engines. The basic operation of the gas turbine is a Brayton cycle with air as the working fluid : atmospheric air flows through the compressor that brings it to higher pressure; energy is then added by spraying fuel into the air and igniting it so that the combustion generates a high-temperature flow; this high-temperature pressurized gas enters a turbine, producing a shaft work output in the process, used to drive the compressor; the unused energy comes out in the exhaust gases that can be repurposed for external work, such as directly producing thrust in a turbojet engine , or rotating a second, independent turbine known as a power turbine that can be connected to a fan, propeller, or electrical generator.
The purpose of the gas turbine determines the design so that the most desirable split of energy between the thrust and the shaft work is achieved. The fourth step of the Brayton cycle cooling of the working fluid is omitted, as gas turbines are open systems that do not reuse the same air. Gas turbines are used to power aircraft, trains, ships, electrical generators, pumps, gas compressors, and tanks.
In an ideal gas turbine, gases undergo four thermodynamic processes: an isentropic compression, an isobaric constant pressure combustion, an isentropic expansion and heat rejection. Together, these make up the Brayton cycle.
In a real gas turbine, mechanical energy is changed irreversibly due to internal friction and turbulence into pressure and thermal energy when the gas is compressed in either a centrifugal or axial compressor. Heat is added in the combustion chamber and the specific volume of the gas increases, accompanied by a slight loss in pressure. During expansion through the stator and rotor passages in the turbine, irreversible energy transformation once again occurs.
Fresh air is taken in, in place of the heat rejection. If the engine has a power turbine added to drive an industrial generator or a helicopter rotor, the exit pressure will be as close to the entry pressure as possible with only enough energy left to overcome the pressure losses in the exhaust ducting and expel the exhaust. For a turboprop engine there will be a particular balance between propeller power and jet thrust which gives the most economical operation.
In a turbojet engine only enough pressure and energy is extracted from the flow to drive the compressor and other components. The remaining high-pressure gases are accelerated through a nozzle to provide a jet to propel an aircraft. The smaller the engine, the higher the rotation rate of the shaft must be to attain the required blade tip speed. Blade-tip speed determines the maximum pressure ratios that can be obtained by the turbine and the compressor.
This, in turn, limits the maximum power and efficiency that can be obtained by the engine. In order for tip speed to remain constant, if the diameter of a rotor is reduced by half, the rotational speed must double. For example, large jet engines operate around 10,, rpm, while micro turbines spin as fast as , rpm.
Mechanically, gas turbines can be considerably less complex than internal combustion piston engines. This, in turn, can translate into price. More advanced gas turbines such as those found in modern jet engines or combined cycle power plants may have 2 or 3 shafts spools , hundreds of compressor and turbine blades, movable stator blades, and extensive external tubing for fuel, oil and air systems; they use temperature resistant alloys, and are made with tight specifications requiring precision manufacture.
All this often makes the construction of a simple gas turbine more complicated than a piston engine. Moreover, to reach optimum performance in modern gas turbine power plants the gas needs to be prepared to exact fuel specifications.
Fuel gas conditioning systems treat the natural gas to reach the exact fuel specification prior to entering the turbine in terms of pressure, temperature, gas composition, and the related wobbe-index. The primary advantage of a gas turbine engine is its power to weight ratio.
Thrust bearings and journal bearings are a critical part of a design. They are hydrodynamic oil bearings or oil-cooled rolling-element bearings. A major challenge facing turbine design, especially turbine blades , is reducing the creep that is induced by the high temperatures and stresses that are experienced during operation. Higher operating temperatures are continuously sought in order to increase efficiency, but come at the cost of higher creep rates. Several methods have therefore been employed in an attempt to achieve optimal performance while limiting creep, with the most successful ones being high performance coatings and single crystal superalloys.
Protective coatings provide thermal insulation of the blade and offer oxidation and corrosion resistance. Using TBCs limits the temperature exposure of the superalloy substrate, thereby decreasing the diffusivity of the active species typically vacancies within the alloy and reducing dislocation and vacancy creep. The Al from the bond coats forms Al 2 O 3 on the TBC-bond coat interface which provides the oxidation resistance, but also results in the formation of an undesirable interdiffusion ID zone between itself and the substrate.
Nickel-based superalloys boast improved strength and creep resistance due to their composition and resultant microstructure. The addition of these elements reduces the diffusion of the gamma prime phase, thus preserving the fatigue resistance, strength, and creep resistance. Due to the lack of grain boundaries, single crystals eliminate Coble creep and consequently deform by fewer modes - decreasing the creep rate. Care needs to be taken in order to optimize the design parameters to limit high temperature creep while not decreasing low temperature yield strength.
Airbreathing jet engines are gas turbines optimized to produce thrust from the exhaust gases, or from ducted fans connected to the gas turbines. Gas turbines are also used in many liquid fuel rockets , where gas turbines are used to power a turbopump to permit the use of lightweight, low-pressure tanks, reducing the empty weight of the rocket.
A turboprop engine is a turbine engine that drives an aircraft propeller using a reduction gear. Turboprop engines are used on small aircraft such as the general-aviation Cessna Caravan and Embraer EMB Tucano military trainer, medium-sized commuter aircraft such as the Bombardier Dash 8 and large aircraft such as the Airbus AM transport and the year-old Tupolev Tu strategic bomber.
Aeroderivative gas turbines are generally based on existing aircraft gas turbine engines, and are smaller and lighter than industrial gas turbines. Aeroderivatives are used in electrical power generation due to their ability to be shut down and handle load changes more quickly than industrial machines.
In its most straightforward form, these are commercial turbines acquired through military surplus or scrapyard sales, then operated for display as part of the hobby of engine collecting.
The simplest form of self-constructed gas turbine employs an automotive turbocharger as the core component. A combustion chamber is fabricated and plumbed between the compressor and turbine sections. More sophisticated turbojets are also built, where their thrust and light weight are sufficient to power large model aircraft. Several small companies now manufacture small turbines and parts for the amateur. Most turbojet-powered model aircraft are now using these commercial and semi-commercial microturbines, rather than a Schreckling-like home-build.
Small gas turbines are used as auxiliary power units APUs to supply auxiliary power to larger, mobile, machines such as an aircraft. They supply:. Industrial gas turbines differ from aeronautical designs in that the frames, bearings, and blading are of heavier construction. They are also much more closely integrated with the devices they power— often an electric generator —and the secondary-energy equipment that is used to recover residual energy largely heat.
They range in size from portable mobile plants to large, complex systems weighing more than a hundred tonnes housed in purpose-built buildings. However, it may be cheaper to buy electricity than to generate it. Therefore, many engines are used in CHP Combined Heat and Power configurations that can be small enough to be integrated into portable container configurations.
Gas turbines can be particularly efficient when waste heat from the turbine is recovered by a heat recovery steam generator to power a conventional steam turbine in a combined cycle configuration. Aeroderivative gas turbines can also be used in combined cycles, leading to a higher efficiency, but it will not be as high as a specifically designed industrial gas turbine. They can also be run in a cogeneration configuration: the exhaust is used for space or water heating, or drives an absorption chiller for cooling the inlet air and increase the power output, technology known as turbine inlet air cooling.
Another significant advantage is their ability to be turned on and off within minutes, supplying power during peak, or unscheduled, demand. Since single cycle gas turbine only power plants are less efficient than combined cycle plants, they are usually used as peaking power plants , which operate anywhere from several hours per day to a few dozen hours per year—depending on the electricity demand and the generating capacity of the region.
In areas with a shortage of base-load and load following power plant capacity or with low fuel costs, a gas turbine powerplant may regularly operate most hours of the day. Industrial gas turbines that are used solely for mechanical drive or used in collaboration with a recovery steam generator differ from power generating sets in that they are often smaller and feature a dual shaft design as opposed to a single shaft.
The power range varies from 1 megawatt up to 50 megawatts. The majority of installations are used within the oil and gas industries. Oil and gas platforms require these engines to drive compressors to inject gas into the wells to force oil up via another bore, or to compress the gas for transportation. They are also often used to provide power for the platform. These platforms do not need to use the engine in collaboration with a CHP system due to getting the gas at an extremely reduced cost often free from burn off gas.
The same companies use pump sets to drive the fluids to land and across pipelines in various intervals. One modern development seeks to improve efficiency in another way, by separating the compressor and the turbine with a compressed air store. In a conventional turbine, up to half the generated power is used driving the compressor. In a compressed air energy storage configuration, power, perhaps from a wind farm or bought on the open market at a time of low demand and low price, is used to drive the compressor, and the compressed air released to operate the turbine when required.
Turboshaft engines are used to drive compressors in gas pumping stations and natural gas liquefaction plants. They are also used to power all but the smallest modern helicopters. A primary shaft carries the compressor and its turbine which, together with a combustor, is called a Gas Generator. A separately-spinning power-turbine is usually used to drive the rotor on helicopters.
Evolved from piston engine turbochargers , aircraft APUs or small jet engines , microturbines are 25 to kilowatt turbines the size of a refrigerator. Most gas turbines are internal combustion engines but it is also possible to manufacture an external combustion gas turbine which is, effectively, a turbine version of a hot air engine.
External combustion has been used for the purpose of using pulverized coal or finely ground biomass such as sawdust as a fuel. In the indirect system, a heat exchanger is used and only clean air with no combustion products travels through the power turbine. The thermal efficiency is lower in the indirect type of external combustion; however, the turbine blades are not subjected to combustion products and much lower quality and therefore cheaper fuels are able to be used.
When external combustion is used, it is possible to use exhaust air from the turbine as the primary combustion air. This effectively reduces global heat losses, although heat losses associated with the combustion exhaust remain inevitable. Closed-cycle gas turbines based on helium or supercritical carbon dioxide also hold promise for use with future high temperature solar and nuclear power generation.
Gas turbines are often used on ships , locomotives , helicopters , tanks , and to a lesser extent, on cars, buses, and motorcycles. A key advantage of jets and turboprops for airplane propulsion - their superior performance at high altitude compared to piston engines, particularly naturally aspirated ones - is irrelevant in most automobile applications. Their power-to-weight advantage, though less critical than for aircraft, is still important.
Gas turbines offer a high-powered engine in a very small and light package. However, they are not as responsive and efficient as small piston engines over the wide range of RPMs and powers needed in vehicle applications.
In series hybrid vehicles, as the driving electric motors are mechanically detached from the electricity generating engine, the responsiveness, poor performance at low speed and low efficiency at low output problems are much less important. The turbine can be run at optimum speed for its power output, and batteries and ultracapacitors can supply power as needed, with the engine cycled on and off to run it only at high efficiency.
The emergence of the continuously variable transmission may also alleviate the responsiveness problem. Turbines have historically been more expensive to produce than piston engines, though this is partly because piston engines have been mass-produced in huge quantities for decades, while small gas turbine engines are rarities; however, turbines are mass-produced in the closely related form of the turbocharger.
The turbocharger is basically a compact and simple free shaft radial gas turbine which is driven by the piston engine's exhaust gas. The centripetal turbine wheel drives a centrifugal compressor wheel through a common rotating shaft.
Steam Turbine Interview Questions- Part 03
Gas Turbine for Power Generation: Introduction
Today, gas turbines are one of the most widely-used power generating technologies. Gas turbines are a type of internal combustion IC engine in which burning of an air-fuel mixture produces hot gases that spin a turbine to produce power. It is the production of hot gas during fuel combustion, not the fuel itself that the gives gas turbines the name. Gas turbines can utilize a variety of fuels, including natural gas, fuel oils, and synthetic fuels.
A gas turbine , also called a combustion turbine , is a type of continuous and internal combustion engine. The main elements common to all gas turbine engines are:. A fourth component is often used to increase efficiency on turboprops and turbofans , to convert power into mechanical or electric form on turboshafts and electric generators , or to achieve greater thrust-to-weight ratio on afterburning engines.
nuclear power plant mcq questions and answers
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Post a comment. A turbo jet engine gives. The highest turbine bearing temperature takes place. On an EGT thermocouple system, the hot junction. Increasing ram effect with increased speed.
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