The working substance differs for different types of turbines. ✓ Steam The pressure of working medium used in steam turbines is very high, whereas the. Based on the nature of blade profile, steam perform work on the blades. 13 the blades differently. • Accordingly there are two types of turbine. – Impulse. In book: Power Plant Lecture Notes, Edition: 1, Chapter: CHAPTER-4 STEAM TURBINE, Publisher: Mishaal A AbdulKareem, Editors: Mishaal A Abdulkareem, pp Pages. Classification of Steam Turbines: Impulse turbines, Reaction turbines, The impulse. Reheat Factor, Overall Efficiency.
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of Steam Turbines pdf| Classifications of Steam Turbines ppt| Since this type of turbine may be warmed and started quickly, so it is very fit. A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to . In an extracting type turbine, steam is released from various stages of the turbine, and Archived (PDF) from the original on 18 November Steam turbines are being started based on operating instructions developed by between the above types of start-up consists in the fact that in the former case.
Blade Fastening After turbine blades are machined through the milling process. Then the blades are inserted in the rotor groove. Depend upon the application the blade root section varies Blade roots are subject to take four types of stress Tensile stress due to the centrifugal forces Bending stress due to fluid forces act on the blade in tangential direction Stress due to vibration forces. Thermal stress also due to the uneven heating of the blade root and the rim.
Twisted Blades This type of blades is used in the last stage of a large multistage steam turbine. Due to larger in size, these types of blades are subjected to high centrifugal and bending forces. To overcome these forces twisted construction is used. Shrouds Shrouds are used to reinforce the turbine blades free ends to reduce vibration and leakage. This is done by reverting a flat end over the blades refer figure. In some cases especially at the early stages, the shroud may be integral with the blade.
When the blades are very long as in the case of the last stage of LP turbine. The rotor blades are further reinforced by using lacing wires caulking wire which circumferentially connects all the blades at a desired radius and shrouding is eliminated.
Steam Turbines Types
Turbine Barring device When a turbine is left cold and at standstill, the weight of the rotor will tend to bend the rotor slightly. In both cases, the turbine would be difficult if not impossible to start up.
To overcome the problem the manufacturer supplies the larger turbines with a turning or barring gear consisting of an electric motor which through several sets of reducing gears turns the turbine shaft at low speed.
Some turning gears, electric or hydraulic, turn the shaft 1 at set times over a period of 24 hours. Before a cold turbine is started up it should be on the barring gear for approximately three hours. When a turbine is shut down, it should be barring for the next 24 hours. If a hydrogen-cooled generator is involved the turbine should be kept on barring gear to prevent excessive loss of hydrogen, All barring gears are interlocked with a lubricating oil pressure switch and an engagement limit switch operated by the engagement handle.
Turbine Bearings One of the steam turbine basic part is bearing. They are two types of bearings used based on the type of load act on them Radial Bearing Thrust Bearing Radial Bearings For small turbines mostly equipped with anti-friction type bearings. Widely used anti-friction bearings are the self-aligning spherical ball or roller bearing with flooded type lubrication is used. In the case of medium turbines used plain journal bearing.
They may be ring lubricated sleeve bearings with bronze or Babbitt lining. Both flooded and force types are employed.
For larger turbines, the radial bearing will be a tilting pad type. The number of pad per bearing will be selected based on the weight of the rotor. For these types of bearing forced lubrication is used. Thrust Bearings The main two purposes of the thrust bearing are: To keep the rotor in an exact position in the casing. To absorb axial thrust on the rotor due to steam flow. The thrust bearing is located on the free end of the rotor or we can say at the steam inlet of the turbine. The axial thrust force is very small for impulse turbines.
This is due to the presence of pressure equalizing holes in the rotor discs to balance the thrust force generated across the disc.
A simple thrust bearing such as a ball bearing for small turbines and radial babbitt facing on journal bearings are commonly used in small and medium-size turbines. Tilling pad type thrust bearings are used in the large steam turbines. In the case of reaction turbine, the pressure drop across the moving blades creates a heavy axial thrust force in the direction of steam flow through the turbine.
Due to greater thrust force, the heavy duty thrust bearing such as tilting pad type thrust bearings are used. The axial thrust in reaction turbines can be nearly reduced by the using off balance or dummy pistons. Since ships are rarely operated in reverse, efficiency is not a priority in astern turbines, so only a few stages are used to save cost.
Blade design challenges[ edit ] A major challenge facing turbine design was reducing the creep experienced by the blades. Because of the high temperatures and high stresses of operation, steam turbine materials become damaged through these mechanisms.
As temperatures are increased in an effort to improve turbine efficiency, creep becomes significant. To limit creep, thermal coatings and superalloys with solid-solution strengthening and grain boundary strengthening are used in blade designs. Protective coatings are used to reduce the thermal damage and to limit oxidation. These coatings are often stabilized zirconium dioxide -based ceramics. Using a thermal protective coating limits the temperature exposure of the nickel superalloy.
This reduces the creep mechanisms experienced in the blade.
Steam Turbine - Working Principle and Types of Steam Turbine
Oxidation coatings limit efficiency losses caused by a buildup on the outside of the blades, which is especially important in the high-temperature environment. The microstructure of these alloys is composed of different regions of composition.
A uniform dispersion of the gamma-prime phase — a combination of nickel, aluminum, and titanium — promotes the strength and creep resistance of the blade due to the microstructure. The addition of these elements reduces the diffusion of the gamma prime phase, thus preserving the fatigue resistance, strength, and creep resistance. These turbines exhaust steam at a pressure below atmospheric.
These types include condensing, non-condensing, reheat, extraction and induction. Condensing turbines are most commonly found in electrical power plants.
These turbines receive steam from a boiler and exhaust it to a condenser. Non-condensing or back pressure turbines are most widely used for process steam applications, in which the steam will be used for additional purposes after being exhausted from the turbine. The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure.
These are commonly found at refineries, district heating units, pulp and paper plants, and desalination facilities where large amounts of low pressure process steam are needed. Reheat turbines are also used almost exclusively in electrical power plants. In a reheat turbine, steam flow exits from a high-pressure section of the turbine and is returned to the boiler where additional superheat is added. The steam then goes back into an intermediate pressure section of the turbine and continues its expansion.
Using reheat in a cycle increases the work output from the turbine and also the expansion reaches conclusion before the steam condenses, thereby minimizing the erosion of the blades in last rows. In most of the cases, maximum number of reheats employed in a cycle is 2 as the cost of super-heating the steam negates the increase in the work output from turbine.
Extracting type turbines are common in all applications. In an extracting type turbine, steam is released from various stages of the turbine, and used for industrial process needs or sent to boiler feedwater heaters to improve overall cycle efficiency.
Extraction flows may be controlled with a valve, or left uncontrolled. Extracted steam results in a loss of power in the downstream stages of the turbine.
Induction turbines introduce low pressure steam at an intermediate stage to produce additional power. Casing or shaft arrangements[ edit ] These arrangements include single casing, tandem compound and cross compound turbines.
Single casing units are the most basic style where a single casing and shaft are coupled to a generator. Tandem compound are used where two or more casings are directly coupled together to drive a single generator. A cross compound turbine arrangement features two or more shafts not in line driving two or more generators that often operate at different speeds. A cross compound turbine is typically used for many large applications. A typical ss naval installation is illustrated below; this shows high- and low-pressure turbines driving a common reduction gear, with a geared cruising turbine on one high-pressure turbine.
Starboard steam turbine machinery arrangement of Japanese Furutaka- and Aoba-class cruisers. Two-flow rotors[ edit ] A two-flow turbine rotor. The steam enters in the middle of the shaft, and exits at each end, balancing the axial force. The moving steam imparts both a tangential and axial thrust on the turbine shaft, but the axial thrust in a simple turbine is unopposed. To maintain the correct rotor position and balancing, this force must be counteracted by an opposing force.
Thrust bearings can be used for the shaft bearings, the rotor can use dummy pistons, it can be double flow- the steam enters in the middle of the shaft and exits at both ends, or a combination of any of these. In a double flow rotor, the blades in each half face opposite ways, so that the axial forces negate each other but the tangential forces act together.
This design of rotor is also called two-flow, double-axial-flow, or double-exhaust. This arrangement is common in low-pressure casings of a compound turbine. The interior of a turbine comprises several sets of blades or buckets.
03_Steam turbines types.pdf
One set of stationary blades is connected to the casing and one set of rotating blades is connected to the shaft. The sets intermesh with certain minimum clearances, with the size and configuration of sets varying to efficiently exploit the expansion of steam at each stage.This prevents the rapid impingement and erosion of the blades which occurs when condensed water is blasted onto the blades moisture carry over.
Because of the high pressures used in the steam circuits and the materials used, steam turbines and their casings have high thermal inertia. Pressure compounding occurs in set of nozzles while velocity compounding occurs in moving and fixed blades.
One of the founders of the modern theory of steam and gas turbines was Aurel Stodola , a Slovak physicist and engineer and professor at the Swiss Polytechnical Institute now ETH in Zurich.
This jet of steam impinges on the moving vanes or blades, mounted on a shaft. When a turbine is shut down, it should be barring for the next 24 hours.
Because the rotor blade passages do not cause any acceleration of fluid, therefore chances of flow separation are greater which results in lower stage efficiency. Knox -class frigates and two steam-powered former U. When all stages of turbine are housed in only one casing, then it is known as single cylinder type of steam turbine.
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