water turbine - wikipedia, the free encyclopedia
TRANSCRIPT
8/15/13 Water turbine - Wikipedia, the free encyclopedia
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Kaplan turbine and electrical generator
cut-away view.
The runner of the small water turbine
Water turbineFrom Wikipedia, the free encyclopedia
A water turbine is a rotary engine that takes energy from movingwater.
Water turbines were developed in the 19th century and werewidely used for industrial power prior to electrical grids. Now theyare mostly used for electric power generation.
Contents
1 History
1.1 Swirl
1.2 Time line
1.3 New concept
2 Theory of operation
2.1 Reaction turbines2.2 Impulse turbines
2.3 Power
2.4 Pumped storage
2.5 Efficiency
3 Types of water turbines4 Design and application
4.1 Typical range of heads
4.2 Specific speed
4.3 Affinity laws
4.4 Runaway speed
5 Maintenance
6 Environmental impact
7 See also8 References
9 Notes
10 Sources
11 External links
History
Water wheels have been used for thousands of years for industrial power. Their main shortcoming is size, whichlimits the flow rate and head that can be harnessed. The migration from water wheels to modern turbines tookabout one hundred years. Development occurred during the Industrial revolution, using scientific principles andmethods. They also made extensive use of new materials and manufacturing methods developed at the time.
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Roman turbine mill at Chemtou,
Tunisia. The tangential water inflow
of the millrace made the submerged
horizontal wheel in the shaft turn like
a true turbine.[1]
Swirl
The word turbine was introduced by the French engineer Claude Burdin in the early 19th century and is derivedfrom the Latin word for "whirling" or a "vortex". The main difference between early water turbines and water wheelsis a swirl component of the water which passes energy to a spinning rotor. This additional component of motionallowed the turbine to be smaller than a water wheel of the same power. They could process more water byspinning faster and could harness much greater heads. (Later, impulse turbines were developed which didn't useswirl).
Time line
The earliest known water turbines date to the Roman Empire. Two helix-turbine mill sites of almost identical design were found at Chemtou andTestour, modern-day Tunisia, dating to the late 3rd or early 4th centuryAD. The horizontal water wheel with angled blades was installed at thebottom of a water-filled, circular shaft. The water from the mill-raceentered the pit tangentially, creating a swirling water column which made
the fully submerged wheel act like a true turbine.[1]
Ján Andrej Segner developed a reactive water turbine (Segner wheel) inthe mid-18th century. It had a horizontal axis and was a precursor tomodern water turbines. It is a very simple machine that is still producedtoday for use in small hydro sites. Segner worked with Euler on some ofthe early mathematical theories of turbine design. In the 18th century, aDr. Barker invented a similar reaction hydraulic turbine that becamepopular as a lecture-hall demonstration. The only known survivingexample of this type of engine used in power production, dating from
1851, is found at Hacienda Buena Vista in Ponce, Puerto Rico. [2][3]
In 1820, Jean-Victor Poncelet developed an inward-flow turbine.
In 1826, Benoit Fourneyron developed an outward-flow turbine. Thiswas an efficient machine (~80%) that sent water through a runner withblades curved in one dimension. The stationary outlet also had curvedguides.
In 1844, Uriah A. Boyden developed an outward flow turbine that improved on the performance of the Fourneyronturbine. Its runner shape was similar to that of a Francis turbine.
In 1849, James B. Francis improved the inward flow reaction turbine to over 90% efficiency. He also conductedsophisticated tests and developed engineering methods for water turbine design. The Francis turbine, named forhim, is the first modern water turbine. It is still the most widely used water turbine in the world today. The Francisturbine is also called a radial flow turbine, since water flows from the outer circumference towards the centre ofrunner.
Inward flow water turbines have a better mechanical arrangement and all modern reaction water turbines are of thisdesign. As the water swirls inward, it accelerates, and transfers energy to the runner. Water pressure decreases toatmospheric, or in some cases subatmospheric, as the water passes through the turbine blades and loses energy.
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A Francis turbine runner, rated at
nearly one million hp (750 MW),
being installed at the Grand Coulee
Dam, United States.
A propeller-type runner rated 28,000 hp (21
MW)
Around 1890, the modern fluid bearing was invented, now universallyused to support heavy water turbine spindles. As of 2002, fluid bearingsappear to have a mean time between failures of more than 1300 years.
Around 1913, Viktor Kaplan created the Kaplan turbine, a propeller-type machine. It was an evolution of the Francis turbine butrevolutionized the ability to develop low-head hydro sites.
New concept
Main article: Pelton wheel
All common water machines until the late 19th century (including waterwheels) were basically reaction machines; water pressure head acted onthe machine and produced work. A reaction turbine needs to fullycontain the water during energy transfer.
In 1866, California millwright Samuel Knight invented a machine
that took the impulse system to a new level.[4][5] Inspired by thehigh pressure jet systems used in hydraulic mining in the gold fields,Knight developed a bucketed wheel which captured the energy ofa free jet, which had converted a high head (hundreds of verticalfeet in a pipe or penstock) of water to kinetic energy. This is calledan impulse or tangential turbine. The water's velocity, roughlytwice the velocity of the bucket periphery, does a u-turn in thebucket and drops out of the runner at low velocity.
In 1879, Lester Pelton (1829-1908), experimenting with a KnightWheel, developed a double bucket design, which exhausted thewater to the side, eliminating some energy loss of the Knight wheelwhich exhausted some water back against the center of the wheel.In about 1895, William Doble improved on Pelton's half-cylindrical bucket form with an elliptical bucket thatincluded a cut in it to allow the jet a cleaner bucket entry. This is the modern form of the Pelton turbine which todayachieves up to 92% efficiency. Pelton had been quite an effective promoter of his design and although Doble tookover the Pelton company he did not change the name to Doble because it had brand name recognition.
Turgo and Crossflow turbines were later impulse designs.
The turbine pictured to the right is located at the Manitoba Electrical Museum in Winnipeg, Manitoba, Canada.
Theory of operation
Flowing water is directed on to the blades of a turbine runner, creating a force on the blades. Since the runner isspinning, the force acts through a distance (force acting through a distance is the definition of work). In this way,energy is transferred from the water flow to the turbine
Water turbines are divided into two groups; reaction turbines and impulse turbines.
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Figure from Pelton's original patent (October
1880)
The precise shape of water turbine blades is a function of thesupply pressure of water, and the type of impeller selected.
Reaction turbines
Reaction turbines are acted on by water, which changes pressureas it moves through the turbine and gives up its energy. They mustbe encased to contain the water pressure (or suction), or theymust be fully submerged in the water flow.
Newton's third law describes the transfer of energy for reactionturbines.
Most water turbines in use are reaction turbines and are used inlow (<30m/98 ft) and medium (30-300m/98–984 ft) headapplications. In reaction turbine pressure drop occurs in both fixedand moving blades.
Impulse turbines
Impulse turbines change the velocity of a water jet. The jet pusheson the turbine's curved blades which changes the direction of the flow. The resulting change in momentum (impulse)causes a force on the turbine blades. Since the turbine is spinning, the force acts through a distance (work) and thediverted water flow is left with diminished energy.An impulse turbine is one which the pressure of the fluid flowingover the rotor blades is constant and all the work output is due to the change in kinetic energy of the fluid.
Prior to hitting the turbine blades, the water's pressure (potential energy) is converted to kinetic energy by a nozzleand focused on the turbine. No pressure change occurs at the turbine blades, and the turbine doesn't require ahousing for operation.
Newton's second law describes the transfer of energy for impulse turbines.
Impulse turbines are often used in very high (>300m/984 ft) head applications .
Power
The power available in a stream of water is;
where:
power (J/s or watts)
turbine efficiency
density of water (kg/m³)
acceleration of gravity (9.81 m/s²) head (m). For still water, this is the difference in height between the inlet and outlet surfaces. Moving
water has an additional component added to account for the kinetic energy of the flow. The total head equals
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Various types of water turbine
runners. From left to right: Pelton
Wheel, two types of Francis Turbine
and Kaplan Turbine
the pressure head plus velocity head.
= flow rate (m³/s)
Pumped storage
Some water turbines are designed for pumped storage hydroelectricity. They can reverse flow and operate as apump to fill a high reservoir during off-peak electrical hours, and then revert to a turbine for power generationduring peak electrical demand. This type of turbine is usually a Deriaz or Francis in design.
Efficiency
Large modern water turbines operate at mechanical efficiencies greater than 90%.
Types of water turbines
Reaction turbines:
Francis
Kaplan, Propeller, Bulb, Tube, Straflo
TysonGorlov
Impulse turbine
Waterwheel
PeltonTurgo
Crossflow (also known as the Michell-Banki or Ossberger turbine)
Jonval turbine
Reverse overshot water-wheelArchimedes' screw turbine
Barkh Turbine
Design and application
Turbine selection is based mostly on the available water head, and less so on the available flow rate. In general,impulse turbines are used for high head sites, and reaction turbines are used for low head sites. Kaplan turbines withadjustable blade pitch are well-adapted to wide ranges of flow or head conditions, since their peak efficiency canbe achieved over a wide range of flow conditions.
Small turbines (mostly under 10 MW) may have horizontal shafts, and even fairly large bulb-type turbines up to 100MW or so may be horizontal. Very large Francis and Kaplan machines usually have vertical shafts because thismakes best use of the available head, and makes installation of a generator more economical. Pelton wheels may beeither vertical or horizontal shaft machines because the size of the machine is so much less than the available head.Some impulse turbines use multiple water jets per runner to increase specific speed and balance shaft thrust.
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Typical range of heads
• Hydraulic wheel turbine• Archimedes' screw turbine• Kaplan• Francis• Pelton• Turgo
0.2 < H < 4 (H = head in m)1 < H < 1020 < H < 4010 < H < 35050 < H < 130050 < H < 250
Specific speed
Main article: Specific speed
The specific speed of a turbine characterizes the turbine's shape in a way that is not related to its size. Thisallows a new turbine design to be scaled from an existing design of known performance. The specific speed is alsothe main criteria for matching a specific hydro site with the correct turbine type. The specific speed is the speed withwhich the turbine turns for a particular discharge Q, with unit head and thereby is able to produce unit power.
Affinity laws
Affinity Laws allow the output of a turbine to be predicted based on model tests. A miniature replica of a proposeddesign, about one foot (0.3 m) in diameter, can be tested and the laboratory measurements applied to the finalapplication with high confidence. Affinity laws are derived by requiring similitude between the test model and theapplication.
Flow through the turbine is controlled either by a large valve or by wicket gates arranged around the outside of theturbine runner. Differential head and flow can be plotted for a number of different values of gate opening, producinga hill diagram used to show the efficiency of the turbine at varying conditions.
Runaway speed
The runaway speed of a water turbine is its speed at full flow, and no shaft load. The turbine will be designed tosurvive the mechanical forces of this speed. The manufacturer will supply the runaway speed rating.
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A Francis turbine at the end of its life
showing cavitation pitting, fatigue cracking
and a catastrophic failure. Earlier repair
jobs that used stainless steel weld rods are
visible.
Maintenance
Turbines are designed to run for decades with very littlemaintenance of the main elements; overhaul intervals are on theorder of several years. Maintenance of the runners and partsexposed to water include removal, inspection, and repair of wornparts.
Normal wear and tear includes pitting from cavitation, fatiguecracking, and abrasion from suspended solids in the water. Steelelements are repaired by welding, usually with stainless steel rods.Damaged areas are cut or ground out, then welded back up to theiroriginal or an improved profile. Old turbine runners may have asignificant amount of stainless steel added this way by the end oftheir lifetime. Elaborate welding procedures may be used to achieve
the highest quality repairs.[6]
Other elements requiring inspection and repair during overhaulsinclude bearings, packing box and shaft sleeves, servomotors,cooling systems for the bearings and generator coils, seal rings,
wicket gate linkage elements and all surfaces.[7]
Environmental impact
Main article: Environmental impacts of reservoirs
Water turbines are generally considered a clean power producer, as the turbine causes essentially no change to thewater. They use a renewable energy source and are designed to operate for decades. They produce significantamounts of the world's electrical supply.
Historically there have also been negative consequences, mostly associated with the dams normally required forpower production. Dams alter the natural ecology of rivers, potentially killing fish, stopping migrations, anddisrupting peoples' livelihoods. For example, American Indian tribes in the Pacific Northwest had livelihoods builtaround salmon fishing, but aggressive dam-building destroyed their way of life. Dams also cause less obvious, butpotentially serious consequences, including increased evaporation of water (especially in arid regions), build up ofsilt behind the dam, and changes to water temperature and flow patterns. In the United States, it is now illegal toblock the migration of fish, for example the endangered great white sturgeon in North America, so fish ladders mustbe provided by dam builders.
See also
Archimedes' screw
Banki turbine
Gorlov helical turbine
Hydroelectricity
Hydropower
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Water wheelHacienda Buena Vista
References
1. ̂a b Wilson 1995, pp. 507f.; Wikander 2000, p. 377; Donners, Waelkens & Deckers 2002, p. 13
2. ^ R. Sackett, p. 16.
3. ^ Barker Turbine/Hacienda Buena Vista (1853) Nomination. American Society of Mechanical Engineers.Nomination Number 177. (http://www.asme.org/about-asme/history/landmarks/topics-m-z/mechanical-power-production-steam/-177-barker-turbine-hacienda-buena-vista-(1853))
4. ^ W. A. Doble, The Tangential Water Wheel, Transactions of the American Institute of Mining Engineers, Vol.XXIX, 1899.
5. ^ W. F. Durrand, The Pelton Water Wheel, Stanford University, Mechanical Engineering, 1939.
6. ^ Cline, Roger:Mechanical Overhaul Procedures for Hydroelectric Units (Facilities Instructions, Standards, andTechniques, Volume 2-7) (http://www.usbr.gov/power/data/fist/fist2_7/fist2-7.pdf); United States Department ofthe Interior Bureau of Reclamation, Denver, Colorado, July 1994 (800KB pdf).
7. ^ United States Department of the Interior Bureau of Reclamation; Duncan, William (revised April 1989): TurbineRepair (Facilities Instructions, Standards & Techniques, Volume 2-5)(http://www.usbr.gov/power/data/fist/fist2_5/vol2-5.pdf) (1.5 MB pdf).
Notes
Robert Sackett, Preservationist, PRSHPO (Original 1990 draft). Arleen Pabon, Certifying Official and State
Historic Preservation Officer, State Historic Preservation Office, San Juan, Puerto Rico. September 9,1994. In National Register of Historic Places Registration Form—Hacienda Buena Vista. United States
Department of the Interior. National Park Service. (Washington, D.C.)
Sources
Donners, K.; Waelkens, M.; Deckers, J. (2002), "Water Mills in the Area of Sagalassos: A Disappearing
Ancient Technology", Anatolian Studies 52: 1–17
Wikander, Örjan (2000), "The Water-Mill", in Wikander, Örjan, Handbook of Ancient Water
Technology, Technology and Change in History 2, Leiden: Brill, pp. 371–400, ISBN 90-04-11123-9
Wilson, Andrew (1995), "Water-Power in North Africa and the Development of the Horizontal Water-
Wheel", Journal of Roman Archaeology 8: 499–510
External links
Introductory turbine math (http://www.du.edu/~jcalvert/tech/fluids/turbine.htm)
European Union publication, Layman's hydropower handbook,12 MB pdf
(http://ec.europa.eu/comm/energy/library/hydro/layman2.pdf)
"Selecting Hydraulic Reaction Turbines", US Bureau of Reclamation publication, 48 MB pdf(http://www.usbr.gov/pmts/hydraulics_lab/pubs/em/EM20.pdf)
"Laboratory for hydraulic machines", Lausanne (Switzerland) (http://lmh.epfl.ch/)
DoradoVista, Small Hydro Power Information (http://www.doradovista.com/DVPower2.html)
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