Gas Turbine Handbook
المشاركة الأصلية كتبت بواسطة ابو سيف العراقي
The comprehensive guide for the operation and maintenance of large turbo-generators
The first device that may be classified as a steam turbine was little more than a toy, the classic Aeolipile, created in the 1st century by Hero of Alexandria in Roman Egypt. Heron's steam engine was also used to open temple doors and so was the first mechanical use of steam power. A thousand years later, a steam turbine with practical applications was invented in 1551 by Taqi al-Din in Ottoman Egypt, who described it as a prime mover for rotating a spit. Yet another steam turbine device was created by Italian Giovanni Branca in 1629. These early devices, however, were very different from the modern steam turbine, invented in 1884 by English engineer, Charles A. Parsons, whose first model was connected to a dynamo that generated 7.5 kW of electricity. His patent was licensed and the turbine was scaled up shortly after by an American, George Westinghouse. The Parsons turbine turned out to be relatively easy to scale up. Within Parsons' lifetime the generating capacity of a unit was increased by a factor of about 10,000.
A number of other variations of turbines were developed that worked effectively with steam. The de Laval turbine (invented by Gustaf de Laval) places convergent-divergent nozzles in between stages in order to extract more energy from the steam.
The modern steam turbine has almost completely replaced the reciprocating pistonsteam engine (invented by Thomas Newcomen and greatly improved by James Watt), primarily because of its greater thermal efficiency and higher power-to-weight ratio. In addition, the turbine has only one moving part compared to a piston engine, which can have dozens or even hundreds.
Steam turbines are made in a variety of sizes ranging from rare 1 hp (0.75 kW) units used as mechanical drives for pumps, compressors and other shaft driven equipment, to 2,000,000 hp (1,500,000 kW) turbines used to generate electricity. There are several classifications for modern steam turbines. A turbine may be classified with several descriptors, for example: an impulse type turbine may be a noncondensing unit with two stages of reversing elements, cross-compounded with a low-pressure Reaction Turbine.
An impulse turbine has fixed nozzles that orient the steam flow into high speed jets. These jets contain significant kinetic energy, which the rotor blades, shaped like buckets, convert into shaft rotation as the steam jet changes direction. A pressure drop occurs in the nozzle. The pressure is the same when the steam enters the blade as it leaves the blade. As the steam flows through the nozzle, its pressure falls from steam chest pressure to condenser pressure (or atmosphere pressure). Due to this relatively higher ratio of expansion of steam in the nozzle, the steam leaves the nozzle with a very high velocity. At a specific temperature and pressure steam has certain physical properties. The certain amount of heat or thermal energy contained within the steam increases with an increase of temperature or pressure or vice versa. The flow of steam through a channel such as a nozzle reduces its thermal energy, however this decrease in thermal energy is equivalent to gain of kinetic energy. The thermal energy is converted from thermal to kinetic causing the steam to flow from high pressure, i.e. the steam chest, nozzle block, etc.. to an area of low pressure, i.e. the turbine casing. The steam leaving the moving blades still retains a large portion of the velocity it had after leaving the nozzle. The loss of energy due to this higher exit velocity is commonly called the "carry over velocity" or "leaving loss." In impulse turbines, steam expansion only happens at nozzles.
The types of impulse turbines are:
In a reaction turbine the rotor blades themselves are arranged to form convergent nozzles. This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the rotor. The steam then changes direction and increases its speed relative to the speed of the blades. A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor. These types of turbines create large amounts of axial thrust, therefore, anti-frictionthrust bearings are utilized.
The reaction turbines are :
 Steam path arrangements
Steam flow diagram of an extracting turbine
Steam flow diagram of a reversing turbine
Types of steam turbines include condensing, noncondensing, reheat, extraction and induction.
- Noncondensing or backpressure turbines are most widely used for process steam applications. 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 available.
Condensing turbines are most commonly found in electrical power plants, and marine propulsion plants. These turbines exhaust steam in a partially condensed state, typically of a quality near 90%, at a pressure well below atmospheric to a condenser. These turbines are the mainstay of the electric power generation industry. The moisture in the last turbine stages requires more expensive materials; otherwise erosion of the blades becomes a major problem. Condensing turbines are used for all coal fired generating stations, all oil and gas fired steam electric plants, all nuclear power plants, and all combined cycle power plants.
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 it is further superheated. The steam then goes back into an intermediate pressure section of the turbine and continues its expansion. Virtually all reheat turbines are also classed as condensing turbines.
Extraction turbines are common in many applications, particularly in certain manufacturing sectors such as papermaking which require steam at a certain pressure and temperature. In an extracting turbine, some of the steam is taken from a point of the turbine having the desired temperature and pressure, and used for industrial process needs or sent to boiler feedwater heaters. Extraction flows may be controlled with a valve, or left uncontrolled. A one-way valve is almost always located on the extraction piping. In the event of an emergency turbine shutdown, pressure from the extraction line could supply enough energy to overspeed the turbine if there is a loss of load on the machine. The check valve prevents this from occurring.
Reversing Turbines are equipped with one or more stages of blades that are faced in the opposite direction of the main blading. A valving arrangement allows for the main steam line to be closed to the forward blades and opened to the reversing blade elements. These reversing blades are mounted on the same shaft as the forward elements. Normally the reversing blades share the same condenser. During reversing operations, the forward blade elements are spinning backwards in hot steam. This incurrs a large efficiency loss known as windage loss. This steam is relatively stagnant and the forward blades may overheat during extended operation. Before the development of reversing turbines, steam turbine ships could not propel themselves in reverse. Reversing steam turbines were once common in the marine industry, although their use has declined with the rise of the diesel engine and electric drive.
Induction turbines introduce low pressure steam at an intermediate stage to produce additional power[citation