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Steam Turbine
steam turbine
[′stēm ¦tər·bən]Steam Turbine
a steam prime mover with rotary motion of the driving element, or rotor, and continuous operation. It converts the thermal energy of steam into mechanical work. The steam flow proceeds through directing devices and impinges on curved blades mounted along the periphery of the rotor. By exerting a force on the blades, the steam flow causes the rotor to rotate. Unlike the reciprocating steam engine, the steam turbine makes use of the kinetic rather than the potential energy of steam.
The first attempts at building a steam turbine were made long ago. A description exists of a primitive steam turbine constructed by Hero of Alexandria in the first century B.C. Not until the late 19th century, however, when thermodynamics, mechanical engineering, and metallurgy had attained a sufficiently high level, did there appear steam turbines suitable for industry. They were built independently by C. G. P. de Laval of Sweden and C. A. Parsons of Great Britain between 1884 and 1889. Laval made use of the expansion of steam in conical fixed nozzles in a single step from the initial to the final pressure and directed the resulting jet, which had a supersonic exit velocity, onto a single row of moving blades mounted on a disk. Steam turbines based on this principle came to be called impulse turbines. Parsons constructed a multistage reaction turbine, in which the expansion of the steam occurred in a large number of sequential stages both in the passages of the stationary blades, or nozzles, and between the moving rotor blades.
The steam turbine proved to be a very convenient engine for driving rotative mechanisms, such as electric generators, pumps, blowers, and ship propellers. It was operable at higher speeds and was more compact, lighter, better balanced, and more economical than the reciprocating steam engine. The development of steam turbines was extremely rapid—efficiency was improved, output capacity was increased, and specialized turbines were designed for various uses.
Single-stage turbines of the Laval type cannot achieve high power outputs, and they have very high rotative speeds—up to 30,000 rpm in the first prototypes. As a result, they have continued to be important only as drives for auxiliary mechanisms. Impulse steam turbines developed in the direction of multistage designs, in which the expansion of steam was performed in a row of sequentially arranged stages. Such staging permitted a considerable increase in the power output of steam turbines, while preserving the moderate rotative speed required for the direct coupling of the turbine shaft to the mechanism driven by the turbine.
Parsons’ reaction turbine was used for some time, particularly in naval vessels, but it gradually was supplanted by more compact combination impulse-reaction turbines, in which the high-pressure reaction part was replaced by a one- or two-stage impulse disk. As a result, losses owing to steam leakage through the gaps in the blade apparatus were decreased, and the turbine became simpler and more efficient.
Classification. Steam turbines are usually divided into three basic groups, depending on the character of the thermal process: once-through condensing turbines, combined heat- and power-supply turbines, and special-purpose turbines.
ONCE-THROUGH CONDENSING TURBINES. Once-through condensing turbines are used for converting the greatest possible portion of the heat of steam into mechanical work. The turbines’ exhaust steam is discharged to a condenser, where a vacuum is maintained. Such turbines can be used for industrial, central-station, or transportation purposes.
When connected to generators of alternating electric current in turbogenerators, once-through condensing turbines constitute the basic equipment of condensation electric power plants. The greater the output capacity of a turbogenerator, the more economical it is and the lower is the cost of 1 kilowatt (kW) of installed capacity. For this reason, the power output of steam generators has increased yearly. By 1974 the aggregate capacity had reached 1,200 megawatts (MW) with live-steam pressures of up to 35 meganewtons (MN)/m2 (1 newton/m2 = 10–5 kilogram-force/cm2) and temperatures of up to 650°C. The accepted frequency of electric current in the USSR is 50 hertz; accordingly, the rotative speed of a steam turbine directly connected to a two-pole generator must be equal to 3,000 rpm. Turbines are classified into three types, depending on their intended use in power plants: base-load turbines designed for carrying a constant base load; peak-load turbines, which operate for short periods of time and carry peak loads; and house turbines, which supply the plant’s own electrical energy needs. Base-load turbines must provide a high degree of efficiency under nearly full loads (about 80 percent). Peak-load turbines must be capable of fast start-up and rapid attainment of normal operating characteristics. Special reliability of operation is required of house turbines. All steam turbines for power stations are designed to operate for 100,000 hours before a general overhaul is carried out.
Transportation steam turbines are used as main and auxiliary engines on ships and other vessels. Numerous attempts have been made to use steam turbines in locomotives, but such locomotives have not received wide acceptance. Reduction gearing is used to couple high-speed steam turbines to ship propellers, which require moderate (from 100 to 500 rpm) rotative speeds. Unlike industrial and central-station turbines (with the exception of turboblowers), ship turbines operate with a variable rotative speed, which is determined by the speed of the vessel.
COMBINED HEAT- AND POWER-SUPPLY TURBINES. Combined heat- and power-supply turbines are used for the simultaneous generation of electrical and thermal energy. They include such types as back-pressure turbines, automatic extraction turbines, and back-pressure extraction turbines.
In back-pressure turbines, all the exhaust steam is used for industrial purposes, such as boiling, drying, or heating. The electric power generated by a turbounit with such a turbine depends on the need of the plant or heating system for heating steam and varies with that need. A back-pressure turbounit, therefore, usually operates in parallel with a condensing steam turbine or power-supply network that covers any power shortages. In automatic extraction turbines, part of the steam is extracted from one or two intermediate stages, and the remaining steam proceeds to the condenser. The pressure of the extracted steam is maintained within predetermined limits by a control system. The extraction points are chosen in accordance with the required parameters of the steam. In back-pressure extraction turbines, part of the steam is extracted from one or two intermediate stages, and all of the exhaust steam is piped from the outlet into the heating system.
The steam pressure in turbines for heating purposes is usually 0.12 MN/m2. The steam pressure for industrial needs—for example, in sugar refineries, wood-processing plants, and food-processing plants—ranges from 0.5 to 1.5 MN/m2.
SPECIAL-PURPOSE TURBINES. Special-purpose turbines usually operate on the waste heat of metallurgical, machine-building, and chemical plants. They include exhaust-steam turbines, double-pressure turbines, and superposition turbines. Exhaust-steam turbines use exhaust steam from reciprocating machines, steam hammers, and presses, which is at a pressure slightly above atmospheric. Double-pressure turbines use both live steam and the exhaust steam of steam mechanisms; the steam is fed into one of the intermediate stages. Superposition turbines are steam turbines with high initial pressures and high back-pressures; all of the exhaust steam from such turbines is piped to other steam turbines with lower initial steam pressure. Superposition turbines are required in the modernization of power plants, when steam boilers are installed for operation at high pressures incompatible with the existing steam turbines in a plant.
Unlike condensing and combined heat- and power-supply turbines, special-purpose turbines are not batch produced. In most cases, they are custom built.
All industrial and central-station turbines are equipped with devices for unregulated bleeding of steam from two to five pressure stages for the regenerative heating of the feedwater. Four stages of initial steam parameters have been established in the USSR: (1) a pressure of 3.5 MN/m2 and a temperature of 435°C for steam turbines with a power output of up to 12 MW, (2) 9 MN/m2 and 535°C for turbines up to 50 MW, (3) 13 MN/m2 and 565°C for turbines up to 100 MW, and (4) 24 MN/m2 and 565°C for turbines with a power output of 200 and 300 MW. The exhaust-steam pressure ranges from 3.5 to 5 kilonewtons/m2. The heat duty varies from 7.6 kilojoules (kj) per watt-hour (W-hr) for the most powerful steam turbines to 13 kJ/W-hr for small condensing turbines.
Thermal process. The kinetic energy acquired by steam when it expands is equivalent to the decrease in its enthalpy during the expansion process. The work of the steam in kilograms-force-meters (1 kgf-m = 10 joules) is equal to
W = 427(i0 – i1)
The spouting velocity in meters per second is
Here, i0 is the initial enthalpy and i1 the final enthalpy of the steam. The power in kW obtainable from a turbine when D kg/hr of steam is consumed is equal to
The steam consumption consequently is
If i0 – i1 is understood as an adiabatic enthalpy change, then the above is valid only for an ideal steam turbine, which operates without losses. The actual power output on the shaft of a real turbine is equal to
where ηe is the relative effective efficiency, which is the ratio of the actual power output obtained on the turbine shaft to the power output of the ideal turbine:
Here de is the steam consumption in kg/(kW-hr). For existing steam turbines, the steam rate is determined experimentally, and i0 — i1 is found from an enthalpy-entropy, or Mollier, chart.
In an impulse turbine, live steam with the pressure p0 and velocity c0 proceeds to the nozzle, where it expands to the pressure p1. In the process, the velocity of the steam flow increases to c1, which is the velocity at which the steam impinges on the rotor blades. As a result of its change in direction in the curved passages between the blades, the steam flow exerts pressure on the blades and causes the disk and shaft to rotate. On leaving the blades, the steam flow has the velocity c2, which is less than c1 because a considerable part of the kinetic energy has been transformed into the mechanical energy of the rotation of the shaft. The pressure p1 at the entrance to a passage is equal to the pressure p2 at the exit, since the passages between the blades have the same cross-sectional area over their entire lengths and expansion of steam does not occur in them. In actual impulse turbines, however, the cross-sectional areas of the passages between the blades increase slightly in the direction of the steam flow in order to preserve the equality of pressures at the inlets and outlets of the passages. This increase is necessary because the steam enthalpy increases as the steam passes between the blades owing to friction and impacts against the blade edges. Within the curved passages, the pressures are different in different places: it is precisely the difference between the pressures on the concave and the convex side of each blade that causes the rotor to rotate. Thus, in an impulse turbine the steam pressure drop occurs in the nozzle or nozzles; the steam pressure at the inlet to the blades and at the outlet is the same.
The kinetic energy will be fully utilized if the absolute steam velocity c2 at the outlet from the blades is equal to zero. This condition is met if c1 = 2 u, where u is the peripheral velocity. The peripheral velocity, measured in m/sec, is equal to
u = πdn/60
where d is the mean diameter of the blade row in m and n is the rotative velocity in rpm. Consequently, the optimum peripheral velocity of the blades must be u = c1/2.
It is evident that in a real turbine C2 cannot be equal to zero, since the steam must flow from the blades into the condenser. The exit velocity, however, should be minimal because the kinetic energy of the exiting steam flow constitutes a loss of useful work. Deviation from the optimum value of the ratio u/c1 leads to a sharp drop in the efficiency of the turbine. Thus, the construction of single-stage turbines with high values of the initial steam parameters is not yet possible, since, as of the early 1970’s, there do not exist materials capable of withstanding the stresses resulting from the centrifugal forces at peripheral velocities that exceed 400 m/sec. Single-stage impulse turbines are, therefore, used only for driving high-speed auxiliary mechanisms whose efficiency is not of paramount importance. High efficiency of steam turbines that operate at moderate peripheral velocities and with a large temperature drop is achieved by pressure staging.
If the pressure drop is divided into several stages with equal temperature drops, then the exit velocity, measured in m/sec, in the stages is equal to
where z is the number of stages. Consequently, the velocity in each stage will be 
of the velocity in a single-stage turbine. The optimum peripheral velocity u, that is, the rotative speed of the rotor, will be correspondingly lower.
A steam turbine casing with several pressure stages is divided into separate chambers by diaphragms; each chamber contains one of the disks with rotor blades (Figure 1). The steam can penetrate from one chamber into the next only through nozzles located along the periphery of the diaphragms. The steam pressure is lowered after each stage, and the steam exit velocities C1 remain about the same—a result achieved by the selection of appropriate nozzle dimensions. The number of pressure stages in high-power turbines with high initial steam parameters can be as high as 30 or 40. Since the steam volume increases as the steam expands, the cross-sectional areas of the nozzles and the heights of the blades increase from the first stage to the last. The last stages of high-power turbines are usually arranged for double flow. In very large turbines the last stages may be arranged for triple or even quadruple flow because of the unacceptably large blade size that would be required in the last stages were the entire volume of the steam to pass through a single stage.
In a pressure stage, the kinetic energy can be utilized in several —rather than one—rows of blades by means of velocity-compounded staging. For this purpose, two, rarely three, rows of rotor blades are mounted on the rim of the disk, and a row of stationary directing blades is mounted between them. Steam at the pressure p0 is supplied to the nozzles (Figure 2) and, with the velocity c2, impinges on the first row of rotor blades, where its dynamic pressure is partly converted into work and its direction of flow changes. After emerging with the velocity C2 from the first row of rotor blades, the steam passes through the stationary blades and, having again changed direction, enters the second row of blades with the velocity c1, which is somewhat lower than C2 owing to losses in the stationary blades. The steam leaves the second row of blades with the insignificant velocity C1’.
Theoretically, for a two-row velocity-compounded stage, the peripheral velocity u will be half that for a single-row stage using the same enthalpy drop. For Z velocity stages, the optimum ratio u/c = 1/2z. In practice, however, large numbers of velocity stages are not used owing to large losses in the blades. The most widespread type of turbine is the impulse turbine with a single two-row disk in the first pressure stage and with single-row disks in the remaining stages. The importance of the two-row disk is that by using a considerable portion of the available enthalpy drop in the first pressure stage it permits a lowering of the temperature and pressure in the steam turbine casing and, at the same time, a reduction in the number of pressure stages required. In other words, it makes possible a shortening and decrease in the cost of the turbine.
The distinctive characteristic of the reaction steam turbine is that the steam expansion occurs in it within the passages of both the stationary and moving rows of blades—that is, within both the nozzles and the rotor blades. The degree of reaction ρ is the ratio of the part of the available adiabatic enthalpy drop h2 that is transferred to the rotor blades to the total adiabatic drop of the stage h0 = h1 + h2, where h1 is the temperature drop in the stationary blades: ρ = h2/(h1 + h2).
If ρ ≥ 1/2, the turbine is called a reaction turbine. In a pure impulse turbine, ρ should be equal to zero. In practice, however, impulse turbines always operate with some degree of reaction, which is greater in the later stages. There results a certain increase of efficiency, particularly under operating conditions that are different from the design operating conditions.
The rows of moving blades in reaction turbines are mounted in grooves of drum-type rotors. The rows of stationary blades are located in the spaces between the rows of rotor blades and are mounted on the turbine casing; they form the nozzle passages. The profiles of the moving and stationary blades are usually the same. Live steam enters a ring-shaped chamber (Figure 3), from which it proceeds into the first row of stationary blades. In the interblade passages of the row, the steam expands, its pressure decreases somewhat, and its velocity increases from c0 to c1. The steam then impinges on the first row of rotor blades. The steam also expands between the rotor blades, and its relative velocity increases. The absolute velocity c2 at the exit from the rotor blades, however, will be lower than c1, since mechanical work was obtained at the expense of kinetic energy. The process is repeated in the subsequent stages. To reduce steam leakage through the spaces between the turbine’s blades, rotor, and casing, the available pressure drop is divided into a large number —up to 100—of stages. As a result, the pressure difference between adjacent stages is small.
Industrial and central-station reaction steam turbines are not built in the USSR, but a few foreign firms have continued to produce steam turbines with a high-pressure action part followed by reaction stages.
Structure. Steam turbines are classified according to the direction of steam flow as axial and radial. In axial turbines, the steam flows along the axis of the turbine. In radial turbines, the direction of steam flow is perpendicular to the axis of rotation, and the moving blades are positioned parallel to the axis. Only axial steam turbines are built in the USSR.
According to the number of casings, or cylinders, steam turbines are divided into single-, double-, triple-, and quadruple-casing turbines. Quadruple-casing turbines are rarely built. Multiple-casing construction permits the use of large available enthalpy drops by incorporating a large number of pressure stages, the use of high-quality metals in the high-pressure part, and the splitting of the steam flow in the low-pressure part. Such a turbine, however, is more expensive, heavy, and complicated.
Steam turbines are classified by shaft arrangement as single-shaft turbines and cross-compound turbines. In single-shaft turbines, the shafts of all casings are located on the same axis. Cross-compound turbines have two, rarely three, shafts arranged in parallel and linked by the common thermal process; in marine turbines, the shafts are also connected by a common reduction gear.
The stationary part of a steam turbine—the casing—is designed for disassembly in the horizontal plane to permit installation of the rotor. The casing contains grooves for the placing of the diaphragms, which are separable in the same plane as the casing. The nozzle passages are located along the periphery of the diaphragms. The passages are formed by curved blades cast into the bodies of the diaphragms or welded to the diaphragms. In the places where the shaft passes through the casing walls, labyrinth packings are used to prevent steam leakage to the outside on the high-pressure end and the sucking-in of air on the low-pressure end. Labyrinth packings are used in the places where the rotor passes through the diaphragms to prevent steam from flowing from one stage to the next without passing through the nozzles. An overspeed governor is mounted on the front end of the shaft.
It automatically stops the turbine if the rotative speed reaches 110–112 percent of the rated speed. The back end of the rotor is equipped with an electrically powered turning gear for slow (4–6 rpm) rotation of the rotor after stoppage of the turbine. Such rotation is necessary for the uniform cooling of the turbine.
REFERENCES
Losev, S. M. Parovye turbiny i kondensatsionnye ustroistva: Teoriia konstruktsii i ekspluatatsiia, 10th ed. Moscow-Leningrad, 1964.Shchegliaev, A. V. Parovye turbiny: Teoriia teplovogo protsessa i konstruktsii turbin, 4th ed. Moscow-Leningrad, 1967.
S. M. LOSEV
Steam turbine
A machine for generating mechanical power in rotary motion from the energy of steam at temperature and pressure above that of an available sink. By far the most widely used and most powerful turbines are those driven by steam. Until the 1960s essentially all steam used in turbine cycles was raised in boilers burning fossil fuels (coal, oil, and gas) or, in minor quantities, certain waste products. However, modern turbine technology includes nuclear steam plants as well as production of steam supplies from other sources. See Nuclear reactor
The illustration shows a small, simple mechanical-drive turbine of a few horsepower. It illustrates the essential parts for all steam turbines regardless of rating or complexity: (1) a casing, or shell, usually divided at the horizontal center line, with the halves bolted together for ease of assembly and disassembly; it contains the stationary blade system; (2) a rotor carrying the moving buckets (blades or vanes) either on wheels or drums, with bearing journals on the ends of the rotor; (3) a set of bearings attached to the casing to support the shaft; (4) a governor and valve system for regulating the speed and power of the turbine by controlling the steam flow, and an oil system for lubrication of the bearings and, on all but the smallest machines, for operating the control valves by a relay system connected with the governor; (5) a coupling to connect with the driven machine; and (6) pipe connections to the steam supply at the inlet and to an exhaust system at the outlet of the casing or shell.
Steam turbines are ideal prime movers for driving machines requiring rotational mechanical input power. They can deliver constant or variable speed and are capable of close speed control. Drive applications include centrifugal pumps, compressors, ship propellers, and, most important, electric generators.
Steam turbines are classified (1) by mechanical arrangement, as single-casing, cross-compound (more than one shaft side by side), or tandem-compound (more than one casing with a single shaft); (2) by steam flow direction (axial for most, but radial for a few); (3) by steam cycle, whether condensing, noncon-densing, automatic extraction, reheat, fossil fuel, or nuclear; and (4) by number of exhaust flows of a condensing unit, as single, double, triple flow, and so on. Units with as many as eight exhaust flows are in use. See Turbine
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