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About Steam Turbines

General

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 (invented by Thomas Newcomen and greatly improved by James Watt and Sir Charles Parsons) 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.

History

The first steam engine was little more than a toy, the classic Aeolipile made by Heron of Alexandria. Another steam turbine device was created by Italian Giovanni Branca in year 1629. The modern steam turbine 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 scaled up shortly after by an American, George Westinghouse. A number of other variations of turbines have been developed that work 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. This was good, because the turbine is simpler, less expensive and does not need to be pressure-proof. It can operate with any pressure of steam. The Parsons turbine also turned out to be relatively easy to scale up. Within Parsons' lifetime the generating capacity of a unit was scaled up by about 10,000 times.

Types


Steam turbines are made in a variety of sizes ranging from small 1 hp (0.75 kW) units (rare) 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.


Team Supply and Exhaust Conditions
These types 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. These turbines exhaust steam in a partially condensed state, typically of a quality near 90%, at a pressure well below atmospheric to a condenser.

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 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 feed water heaters to improve overall cycle efficiency. Extraction flows may be controlled with a valve, or left uncontrolled. Induction turbines introduce low pressure steam at an intermediate stage to produce additional power.

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%-90% based on the application of the turbine. The interior of a turbine is comprised of several sets of blades, or buckets as they are more commonly referred to. 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.



Turbine Efficiency
To maximize turbine efficiency, the steam is expanded, generating work, in a number of stages. These stages are characterized by how the energy is extracted from them and are known as impulse or reaction turbines. Most modern steam turbines are a combination of the reaction and impulse design. Typically, higher pressure sections are impulse type and lower pressure stages are reaction type.



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 is a large portion of the maximum velocity of the steam when 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.

Reaction Turbines

In the 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.


Casing or Shaft Arrangements
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.


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. 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 admited to the astern blades located in the LP turbine, and then to the ahead blades slowly rotating the turbine at 10 to 15 RPM to slowly warm the turbine.

Problems with turbines are now 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. It is, however, 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 while some applications (such as the generation of alternating current 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 then the turbine may continue accelerating until it breaks apart, often spectacularly. Turbines are expensive to make, requiring precision manufacture and special quality materials


Direct drive

Electrical power stations use large steam turbines driving electric generators to produce most ( about 86%) of the world's electricity. These centralised stations are of two types: fossil fuel power plants and nuclear 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 3000 r/min for 50 Hz systems, and 3600 r/min 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.


Speed reduction

The Turbinia - the first steam turbine-powered ship
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 RPM range while application of the power in propulsion applications may be only in the hundreds of RPM and so requiring that expensive and precise reduction gears must be used, although several ships, such as Turbinia, had direct drive from the steam turbine to the propeller shafts. This purchase cost 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.

Most modern vessels now use either gas turbines or diesel engines, however, nuclear powered vessels such as some aircraft carriers and nuclear submarines still use steam turbines as part of their propulsion systems.