Editing Steam turbine (section)

==Types==
Steam turbines are made in a variety of sizes ranging from small <0.75&nbsp;kW (<1&nbsp;hp) units (rare) used as mechanical drives for pumps, compressors and other shaft driven equipment, to {{convert|1500|MW|hp|abbr=on}} turbines used to generate electricity. There are several classifications for modern steam turbines.

===Blade and stage design===
[[File:Turbines impulse v reaction.svg|thumb|upright=1.25|Schematic diagram outlining the difference between an impulse and a 50% reaction turbine]]

Turbine blades are of two basic types, blades and [[nozzles]]. Blades move entirely due to the impact of steam on them and their profiles do not converge. This results in a steam velocity drop and essentially no pressure drop as steam moves through the blades. A turbine composed of blades alternating with fixed nozzles is called an [[Impulse turbines|impulse turbine]], {{visible anchor|Curtis turbine}}, [[Pressure compounding in turbines|Rateau turbine]], or [[#History|Brown-Curtis turbine]]. Nozzles appear similar to blades, but their profiles converge near the exit. This results in a steam pressure drop and velocity increase as steam moves through the nozzles. Nozzles move due to both the impact of steam on them and the reaction due to the high-velocity steam at the exit. A turbine composed of moving nozzles alternating with fixed nozzles is called a reaction turbine or [[Parsons Marine Steam Turbine Company|Parsons turbine]].

Except for low-power applications, turbine blades are arranged in multiple stages in series, called [[Compounding of steam turbines|compounding]], which greatly improves [[#Turbine efficiency|efficiency]] at low speeds.{{sfn|Parsons|1911|pp=7-8}} A reaction stage is a row of fixed nozzles followed by a row of moving nozzles. Multiple reaction stages divide the pressure drop between the steam inlet and exhaust into numerous small drops, resulting in a '''pressure-compounded''' turbine. Impulse stages may be either pressure-compounded, velocity-compounded, or pressure-velocity compounded. A pressure-compounded impulse stage is a row of fixed nozzles followed by a row of moving blades, with multiple stages for compounding. This is also known as a Rateau turbine, after its inventor. A '''velocity-compounded''' impulse stage (invented by Curtis and also called a "Curtis wheel") is a row of fixed nozzles followed by two or more rows of moving blades alternating with rows of fixed blades. This divides the velocity drop across the stage into several smaller drops.{{sfn|Parsons|1911|pp=20–22}} A series of velocity-compounded impulse stages is called a '''pressure-velocity compounded''' turbine.

[[File:AEG marine steam turbine (Rankin Kennedy, Modern Engines, Vol VI).jpg|thumb|left|upright=1.2|Diagram of an AEG marine steam turbine circa 1905]]

By 1905, when steam turbines were coming into use on fast ships (such as {{HMS|Dreadnought|1906|6}}) and in land-based power applications, it had been determined that it was desirable to use one or more Curtis wheels at the beginning of a multi-stage turbine (where the steam pressure is highest), followed by reaction stages. This was more efficient with high-pressure steam due to reduced leakage between the turbine rotor and the casing.{{sfn|Parsons|1911|pp=23–25}} This is illustrated in the drawing of the German 1905 [[AEG (German company)|AEG]] marine steam turbine. The steam from the [[boiler]]s enters from the right at high pressure through a [[throttle]], controlled manually by an operator (in this case a [[sailor]] known as the throttleman). It passes through five Curtis wheels and numerous reaction stages (the small blades at the edges of the two large rotors in the middle) before exiting at low pressure, almost certainly to a [[surface condenser|condenser]]. The condenser provides a vacuum that maximizes the energy extracted from the steam, and condenses the steam into [[boiler feedwater|feedwater]] to be returned to the boilers. On the left are several additional reaction stages (on two large rotors) that rotate the turbine in reverse for astern operation, with steam admitted by a separate throttle. 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===
A major challenge facing turbine design was reducing the [[creep (deformation)|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 [[superalloy]]s 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. Oxidation coatings limit efficiency losses caused by a buildup on the outside of the blades, which is especially important in the high-temperature environment.{{sfn|Tamarin|2002|p=5–}}

The nickel-based blades are alloyed with aluminum and titanium to improve strength and creep resistance. 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.{{sfn|Bhadeshia|2003}}

[[Refractory]] elements such as [[rhenium]] and [[ruthenium]] can be added to the alloy to improve creep strength. The addition of these elements reduces the diffusion of the gamma prime phase, thus preserving the [[Fatigue (material)|fatigue]] resistance, strength, and creep resistance.{{sfn|Latief|Kakehi|2013}}

===Steam supply and exhaust conditions===
[[File:BalNPP m st2.jpg|thumb|A low-pressure steam turbine in a nuclear power plant. These turbines exhaust steam at a pressure below atmospheric.]]

Turbine types include condensing, non-condensing, reheat, extracting and induction.

====Condensing turbines====
Condensing turbines are most commonly found in electrical power plants. These turbines receive steam from a [[boiler]] and exhaust it to a [[surface condenser|condenser]]. The exhausted steam is at a pressure well below atmospheric, and is in a partially condensed state, typically of a [[steam quality|quality]] near 90%.

====Non-condensing turbines====
Non-condensing 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====
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 turbines====
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 heater]]s to improve overall cycle efficiency. Extraction flows may be controlled with a valve, or left uncontrolled. Extracted steam results in a [[power loss factor|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===
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 1930s-1960s 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.

[[File:Starboard turbine sets of Furutaka and Aoba class cruisers.svg|thumb|upright=1.3|Starboard steam turbine machinery arrangement of Japanese ''Furutaka''- and ''Aoba''-class cruisers]]

===Two-flow rotors===
[[File:Turbine Philippsburg-1.jpg|thumb|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.<ref name="phdengineer">{{cite web |title=Steam Turbines (Course No. M-3006) |url=http://www.pdhengineer.com/Course%20Files/Completed%20Course%20PDF%20Files/Mechanical/Steam%20Turbines.pdf |access-date=22 September 2011 |publisher=PhD Engineer |url-status=live |archive-url= https://web.archive.org/web/20120402185518/http://www.pdhengineer.com/Course%20Files/Completed%20Course%20PDF%20Files/Mechanical/Steam%20Turbines.pdf |archive-date=2 April 2012}}</ref>