Turbine

 I  INTRODUCTION

 Turbines and Hydroelectric Power

Generators at the Bonneville Dam in Oregon produce electricity as water flows through large turbines and drives the axles of the generators. Water under high pressure behind the dam is released into a channel, where it strikes the wide blades of the turbines, causing them to spin rapidly. This mechanical energy is transferred to the generator by a drive shaft.

Turbine, rotary engine that converts the energy of a moving stream of water, steam, or gas into mechanical energy. The basic element in a turbine is a wheel or rotor with paddles, propellers, blades, or buckets arranged on its circumference in such a fashion that the moving fluid exerts a tangential force that turns the wheel and imparts energy to it. This mechanical energy is then transferred through a drive shaft to operate a machine, compressor, electric generator, or propeller. Turbines are classified as hydraulic, or water, turbines, steam turbines, or gas turbines. Today turbine-powered generators produce most of the world's electrical energy. Windmills that generate electricity are known as wind turbines.

II  HYDRAULIC TURBINES

Hydraulic Turbines

Hydraulic turbines are used to convert the power of flowing water into usable energy. The Kaplan turbine, designed by Viktor Kaplan in 1913, operates much like a boat propeller. Broad, swiveling blades on the turbine are spun by high-pressure water as it is released through a sluice, driving the axle of a generator. The Pelton turbine is a 19th-century model that operates more like a traditional water wheel. Developed by Lester Allen Pelton, the wheel is designed to revolve as water diverted from a reservoir through a penstock strikes its curved buckets. A nozzle converts the kinetic energy of high-pressure water into a powerful jet, and the buckets extract the momentum.

The oldest and simplest form of the hydraulic turbine was the waterwheel, first used in ancient Greece and subsequently adopted in most of ancient and medieval Europe for grinding grain. It consisted of a vertical shaft with a set of radial vanes or paddles positioned in a swiftly flowing stream or millrace. Its power output was about 0.5 horsepower. The horizontal-shaft waterwheel (that is, a horizontal shaft connected to a vertical paddle wheel), first described by the Roman architect and engineer Marcus Vitruvius Pollio during the 1st century bc, had the lower segment of the paddle wheel inserted into the stream, thus acting as a so-called undershot waterwheel. By about the 2nd century ad, the more efficient overshot wheel had come into use in hilly regions. Here the water was poured on the paddles from above, and additional energy was gained from the falling water. The maximum power of the waterwheel, which was constructed of wood, increased from about 3 horsepower to about 50 horsepower in the Middle Ages.

The transition from waterwheel to turbine is largely semantic. The first important attempt to formulate a theoretical basis for waterwheel design was in the 18th century by the British civil engineer John Smeaton, who proved that the overshot wheel was more efficient. The French military engineer Jean Victor Poncelet, however, devised an undershot wheel, the curved blades of which raised efficiency to nearly 70 percent; it quickly came into wide use. Another French military engineer, Claude Burdin, invented the term turbine, introduced as part of a theoretical discussion in which he stressed speed of rotation. Benoit Fourneyron, who studied under Burdin at the School of Mines at St. Étienne, designed and built wheels that achieved speeds of 60 or more rpm (revolutions per minute) and provided up to 50 horsepower for French ironworks. Ultimately Fourneyron built turbines that operated at 2300 rpm, developing 60 horsepower at an efficiency of more than 80 percent.

Despite its remarkable efficiency, the Fourneyron turbine had certain drawbacks as a result of the radial outward flow of water that passed through it. This created problems if water flow was reduced or load removed. The British-born American engineer James B. Francis designed a turbine in which the flow was inward, and the so-called reaction, or Francis, turbine, became the most widely used hydraulic turbine for water pressures, or heads, equivalent to a column of water 10 to 100 m (33 to 330 ft). This type of turbine operates by expanding the pressure energy in the water during the flow through the blade passages, resulting in a net force, or reaction, which has a tangential component that turns the wheel.

For installations where water heads of about 90 to 900 m (about 300 to 3000 ft) were available, the Pelton wheel, named after the American engineer Lester Allen Pelton, came into use during the second half of the 19th century. In this turbine, the water is piped from a high-level reservoir through a long duct, or penstock, to a nozzle where its energy is converted into the kinetic energy of a high-speed jet. This jet is then directed onto curved buckets, which turn the flow by nearly 180 degrees and extract the momentum. Because the action of the Pelton wheel depends on the impulse of the jet on the wheel, rather than on the reaction of the expanding water, this type of turbine is also known as an impulse turbine.

The increasing demand for hydroelectric power during the early 20th century led to the need for a turbine suitable for small water heads of 3 to 9 m (about 10 to 30 ft) that could be employed in many rivers where low dams could be built. In 1913 the Austrian engineer Viktor Kaplan first proposed his propeller (or Kaplan) turbine, which basically acts like a ship's propeller in reverse. Kaplan later improved his turbine by allowing the blades to swivel about their axis. These variable pitch propellers improved efficiency by optimally matching the blade angle to the head, or flow rate.

To maintain constant output voltage in a hydroelectric installation, the turbine speed must be kept constant regardless of variations in the water pressure acting on it. This requires extensive controls that, for both Francis and Kaplan turbines, act primarily to open or close the guide-vane passages to regulate the flow and, in the case of Kaplan turbines, to vary the pitch of the propellers. In a Pelton wheel installation, the water flow is adjusted by opening or closing the supply nozzles. Here a temporary spill bypass nozzle has to be provided since rapid flow changes in long penstocks would induce pressure surges, called water hammers, which can be highly destructive. During adjustments, the total water flow through both the supply and spill nozzles must be kept nearly constant with the eventual closing of the bypass nozzle, which must be carried out very slowly to avoid water hammer.

III  ADVANCES IN TURBINE DESIGN

The trend in modern hydraulic turbine installations has been toward higher heads and larger units. Depending on the size of the unit, Kaplan turbines are now used with heads up to about 60 m (about 200 ft), and Francis turbines up to 610 m (2000 ft). The world's highest head installation (about 1770 m/about 5800 ft), using a Pelton wheel, is at Reisseck, Austria, and the largest single units are installed in a plant at Itaipu, Brazil, where 18 Francis-type turbines sized at 700 megawatts (Mw) each have a total capacity of 12,600 Mw. The largest installation in North America is at La Grande on James Bay, in eastern Canada, where 22 units rated 333 Mw each have a total capacity of 7326 Mw. The largest installation in the United States is at the Grand Coulee Dam on the Columbia River, which has a total capacity of about 6500 Mw.

Many of the small dam hydroelectric systems built before 1930 were later abandoned because of high maintenance and labor costs. Increases in the cost of fossil fuels have focused renewed attention on these low head installations. With the development of standardized propeller turbines with nearly horizontal shafts, small installations have again become attractive.

Turbines can also be designed to run in reverse as pumps. This is done by inverting the electric generator to operate as a motor. Because electric power cannot be stored economically, the operation of the so-called pump-turbines with electricity generated from nuclear and fossil fuel power plants during off-peak hours enables additional water to be stored in a reservoir. It can then be reused to drive the turbine during peak periods. In recent years, pump-turbine technology has been developed to allow for heads up to about 600 m (about 2000 ft) of water and for unit capacities of more than 400 Mw.

IV  STEAM TURBINES

 

How a Steam Turbine Works

The success of the water turbine inevitably led to consideration of the turbine principle for extracting power from steam. Where the Watt-type reciprocating steam engine utilized the pressure of steam, the turbine could achieve higher efficiency by utilizing the kinetic energy of steam flow. The turbine can be made smaller, lighter, and cheaper than a reciprocating steam engine of comparable power. It can be made in far larger sizes than the conventional steam engine. Mechanically, it has the advantage of producing rotating motion directly without the necessity of using a crankshaft or other means of transforming reciprocal to rotary motion. As a result, the steam turbine has supplanted the reciprocating engine as a prime mover in large electricity-generating plants and is also used as a means of jet propulsion.

Steam turbines are used in the generation of nuclear power and in nuclear ship propulsion. They operate with fuel-fired boilers for power generation. In cogeneration applications requiring both process heat (heat used in an industrial process) and electricity, steam is raised at high pressure in the boiler and extracted from the turbine at the pressure and temperature required by the process. Steam turbines may be used in combined cycles with a steam generator which recovers heat that would otherwise be lost. Industrial units are used to drive machines, pumps, compressors, and electrical generators. Ratings range from a few horsepower to more than 1300 Mw.

The steam turbine was not invented by any one individual but was the result of work by a number of inventors in the latter part of the 19th century. Notable contributors to the development of the turbine were the British inventor Charles Algernon Parsons and the Swedish inventor Carl Gustaf Patrik de Laval. Parsons was responsible for the so-called principle of staging, whereby steam was permitted to expand in a number of stages, performing useful work at each stage. De Laval was the first to design suitable jets and blades for the efficient use of the expanding steam.

The action of the steam turbine is based on the thermodynamic principle that when a vapor is allowed to expand, its temperature drops, and its internal energy is thereby decreased. This reduction in internal energy is transformed into mechanical energy in the form of an acceleration of the particles of the vapor. This transformation makes a large amount of work energy directly available. In the case of expanding steam, a reduction of 100 Btu in internal energy through expansion can result in increasing the speed of the steam particles to a rate of almost 2900 km/h (almost 1800 mph). At such speeds the energy available is great, even though the particles are extremely light.

Although they are built according to two different principles, the essential parts of all steam turbines are similar. They consist of nozzles or jets through which steam flows and expands, dropping in temperature, and gaining kinetic energy, and blades against which the swiftly moving steam exerts pressure. The arrangement of jets and blades, whether fixed or stationary, depends upon the type of turbine. In addition to these two basic components, turbines are equipped with wheels or drums upon which the blades are mounted, a shaft for these wheels or drums, an outer casing that confines the steam to the area of the turbine proper, and various pieces of auxiliary equipment, including lubrication devices and governors.

The simplest form of steam turbine is the so-called impulse turbine, in which the turbine jets are fixed in place on the inside of the turbine casing, and the blades are set on the rims of revolving wheels mounted on a central shaft. Steam passing through a fixed nozzle passes over the curved blades; these absorb some of the kinetic energy of the expanded steam, turning the wheel and shaft on which they are mounted. The turbine is designed so that steam entering at one end of the turbine expands through a succession of nozzles until it has lost most of its internal energy.

 Steam Turbine in Power Station

The steam turbine has supplanted the steam engine as a prime mover in large electricity-generating plants. Steam turbines are also used in the generation of nuclear power and in jet and nuclear ship propulsion.

In the reaction turbine, mechanical energy is obtained to some degree by the impact of steam upon the blades, but primarily it is obtained by the acceleration of the steam as it expands. A turbine of this type consists of a set of fixed and a set of movable blades. The blades are arranged so that each pair acts as a nozzle through which the steam expands as its passes. The blades of a reaction turbine are usually mounted on a drum and not on a wheel. This drum acts as the shaft of the turbine.

In order to use the energy available in steam efficiently in a turbine of either type, it is necessary to employ a number of stages, in each of which a small amount of thermal energy is converted to kinetic energy. If the entire conversion of energy took place instead in a single expansion stage, the rotative speed of the turbine wheel would be excessive. In general, reaction turbines require more stages than impulse turbines. It can be shown that for the same diameter and energy range, a reaction turbine requires twice the number of stages for peak stage efficiency. Large turbines that are nominally of the impulse variety employ some reaction at the root of the steam path to assure efficient flow through the buckets. Many turbines that are nominally reactive have an impulse control stage first, which allows for a saving in the total number of stages.

Because of the increase in volume as the steam expands through the various stages of a turbine, the size of the openings through which the steam passes must increase from stage to stage. In the practical engineering design of turbines, this increase is accomplished by lengthening the blades from stage to stage and by increasing the diameter of the drum or wheel upon which the blades are mounted and by adding two or more turbine sections in parallel. As a result, a small industrial turbine may be more or less conical in shape, with its smallest diameter at the high-pressure, or inlet, end, and its largest at the low-pressure, or exhaust, end. A large unit for a nuclear power station may have four rotors consisting of one double-flow high-pressure section followed by three double-flow low-pressure sections.

Impulse turbines usually employ pressure or Rateau staging, named after the French engineer Auguste Rateau, in which the pressure ratio across each stage is nearly uniform. Impulse turbines built in the past have made use of velocity-compounded, or Curtis, staging named after its American inventor, Charles Gordon Curtis, which has two sets of moving buckets with an intermediate set of fixed blades following the nozzles. The staging of a reaction turbine may be called Parsons' staging, after its British inventor, Charles Parsons.

Steam turbines are comparatively simple machines, having only one major moving part, the rotor; however, auxiliary equipment is necessary for their operation. Journal bearings support the shaft. A thrust bearing positions the shaft axially. An oil system provides lubrication to the bearings. Seals minimize steam leakage within the steam path. A sealing system prevents steam leaking from the machine and air leaking from the outside into the machine. The speed of rotation is controlled by valves at the inlet(s) of the machine. In addition, reaction turbines develop considerable axial thrust owing to the pressure drop across the moving blades. This is usually compensated for by the use of a dummy piston, which creates a thrust in the opposite direction to that of the steam path.

The expansion efficiency of a modern multistage steam turbine is inherently high because of the state of development of the steam-path components and the ability to recover losses of one stage in those downstream through reheating. The efficiency with which a section of a turbine converts the theoretically available thermodynamic energy to mechanical work is commonly in excess of 90 percent. The thermodynamic efficiency of a steam-power installation is much less, owing to the energy lost in the exhaust steam from the turbine. 

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