steam turbine

A rotor of a modern steam turbine, used in a power plant. The steam path is from the smallest expanding through progressivly larger blade elements.

A Steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into useful mechanical work. It has almost completely replaced the reciprocating piston steam engine, primarily because of its greater thermal efficiency and higher power-to-weight ratio. Also, because the turbine generates rotary motion, rather than requiring a linkage mechanism to convert reciprocating to rotary motion, it is particularly suited for use driving an electrical generator — about 86% of all electric generation in the world is by use of steam turbines. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency from the use of multiple stages in the expansion of the steam, as opposed to the one stage in the Watt engine, which results in a closer approach to the ideal reversible process. Marine propulsion is perhaps the only industry to employ reversing steam turbines.

History

The Turbinia - the first steam turbine-powered ship

The first device that may be classified as a rudimentary steam turbine was little more than a toy, the classic Aeolipile, created in the 1st century by Hero of Alexandria in Roman Egypt.^[1][2][3] The first practical steam turbine was invented much later in 1551 by Taqi al-Din in Ottoman Egypt, who described it as a prime mover for rotating a spit.^[4] 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, which was 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 scaled 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) accelerated the steam to full speed before running it against a turbine blade. The turbine was simpler, less expensive, did not need to be pressure-proof, and could operate with any pressure of steam.

The modern steam turbine has almost completely replaced the reciprocating piston steam engine (invented by Thomas Newcomen and greatly improved by James Watt), primarily because of its greater thermal efficiency and higher power-to-weight ratio.

Parsons turbine from the Polish destroyer ORP Wicher in Hel, Poland.

Types

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.

Impulse turbines

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 with an increase of temperature or pressure the contained energy also increases 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:

• Banki turbine
• Girard turbine
• Pelton turbine
• Turgo turbine

Reaction turbines

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-friction thrust bearings are utilized.

The reaction turbines are :

• Fourneyron turbine
• Francis turbine
• Thompson turbine
• Kaplan turbine
• Propeller turbine

Steam path arrangements

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, especially nuclear 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 somewhat rare in the power industry because the condensing water in the last turbine stages requires more expensive materials; otherwise corrosion of the blades becomes a major problem. They are, however, very common in the nuclear power sector for various reasons.

• 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.

• Extracting 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, 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. Adding boiler feedwater heaters is done to improve overall cycle efficiency. Extraction flows may be controlled with a valve, or left uncontrolled. A one-way valve is always located on the extraction piping. In the event of an emergency turbine shutdown, pressure from the extraction line can spin the turbine backwards if not checked with such a one-way valve.

• Cruising turbines were used in US Navy designs of the 1950s and 60s. These turbines had staging which was designed for slow and medium speeds, with additional stages upstream which were only used for high speed operations. In normal cruising operation the upstream impulse stages were bypassed.

• 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 needed]}.

Casing or shaft arrangements

Some of the more common casing and shaft arrangements

These arrangements include single casing, tandem compound and cross compound turbines. Single casing is the most basic arrangement where a single casing and shaft are coupled to a load. Tandem compounding is used where two or more casings are directly coupled together to drive a single load on one shaft. A cross compounded turbine arrangement features two or more shafts driving two or more loads that may operate at different speeds. Gearboxes have also been developed to input the multiple shafts of the cross compound arrangements and output a single shaft.

Principle of operation and design

An ideal steam turbine is considered to be an isentropic process, or constant entropy process, in which the entropy of the steam entering the turbine is equal to the entropy of the steam leaving the turbine. No steam turbine is truly isentropic, however, with typical isentropic efficiencies ranging from 20%-95% based on the application of the turbine. The interior of a turbine comprises several sets of blades, commonly referred to as buckets. 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.

Operation and maintenance

When warming up a steam turbine for use, the main steam stop valves (after the boiler) have a bypass line to allow superheated steam to slowly bypass the valve and proceed to heat up the lines in the system along with the steam turbine. Also a turning gear is engaged when there is no steam to the turbine to slowly rotate the turbine to ensure even heating to prevent uneven expansion and rotor bowing. After first rotating the turbine by the turning gear, allowing time for the rotor to assume a straight plane (no bowing), then the turning gear is disengaged and steam is admitted to the turbine. For most utility and industrial steam turbines, a starting and loading chart is included in the unit instruction manual. The starting and loading chart is used to guide turbine operators in loading their units in such a way as to minimize rotor and shell thermal stresses, but also minimize the chances of the rotor heating faster than the shell, creating a rotor long condition. When starting a shipboard steam turbine (marine unit), steam is normally admitted to the astern blades located in the LP turbine, and then to the ahead blades slowly rotating the turbine at 10 to 15 revolutions per minute (RPM) to slowly warm the turbine.

Problems with turbines are rare and maintenance requirements are relatively small. Any imbalance of the rotor can lead to vibration, which in extreme cases can lead to a blade letting go and punching straight through the casing. Also, it is essential that the turbine be turned with dry steam. If water gets into the steam and is blasted onto the blades (moisture carryover) rapid impingement and erosion of the blades can occur, possibly leading to imbalance and catastrophic failure. Also, water entering the blades will likely result in the destruction of the thrust bearing for the turbine shaft. To prevent this, along with controls and baffles in the boilers to ensure high quality steam, condensate drains are installed in the steam piping leading to the turbine.

Speed regulation

The control of a turbine with a governor is essential, as turbines need to be run up slowly to prevent damage. Some applications, such as the generation of electricity, require precise speed control. Uncontrolled acceleration of the turbine rotor can lead to an overspeed trip, which causes the nozzle valves that control the flow of steam to the turbine to close. If this fails, the turbine may continue accelerating until it breaks apart, often spectacularly. Turbines are expensive to make, requiring precision manufacture and high quality materials.

Direct drive

Electrical power stations use large steam turbines driving electric generators to produce most of the world's electricity. These centralised stations include fossil fuel, nuclear, geothermal, solar thermal electric and biomass power plants. The turbines used for electric power generation are most often directly coupled to their generators. As the generators must rotate at constant synchronous speeds according to the frequency of the electric power system, the most common speeds are 3,000 revolutions per minute for 50 Hz systems, and 3,600 revolutions per minute for 60 Hz systems. Most large nuclear sets rotate at half those speeds, and have a 4-pole generator rather than the more common 2-pole one.

Efficiency

Schematic diagram outlining the difference between an impulse and a reaction turbine

To maximize turbine efficiency the steam is expanded in a number of stages. Work is generated from each steam expansion and pressure drop. These stages are characterized by how the energy is extracted from them and are known as either impulse or reaction turbines. Most modern steam turbines are a combination of the reaction and impulse blading stages. Typically in the United States and Canada, higher pressure sections are impulse type and lower pressure stages are reaction type.

Speed reduction

Another use of steam turbines is in ships; their small size, low maintenance, light weight, and low vibration are compelling advantages. Steam turbine locomotives were also tested, but with limited success. A steam turbine is efficient only when operating in the thousands of revolutions per minute (RPM) range while application of the power in propulsion applications may be only in the hundreds of RPM, which requires that expensive and precise reduction gears be used, although several ships, such as Turbinia, had direct drive from the steam turbine to the propeller shafts. These reduction gears are expensive because single reduction gears must have very large and heavy bull gears, which have a high material cost. Double or triple reduction gears are much more complex and require significantly more machining and labor, although they require less material and space.

The purchase cost of speed reduction gearing is offset by much lower fuel and maintenance requirements and the small size of a turbine when compared to a reciprocating engine having an equivalent power^{[citation needed]}. Most modern merchant vessels now use diesel engines, with a few high speed ferries operating gas turbines. Navy vessels may operate with gas turbines, diesel engines, or a combination of the two to increase maximum speed or low-speed efficiency. Nuclear powered vessels such as aircraft carriers and nuclear submarines use steam turbines driving the propeller shaft through a reduction gearbox as the main part of their propulsion systems.

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Steam turbine

References

External links

• Introduction to the steam turbine
• Photo tour of a 1,300 MW coal plant (The Zimmer Power Station, Moscow, Ohio, USA)

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