Steam Turbine and Power Generation
Steam Turbine and Power Generation
Steam turbine is one of the most versatile and the oldest prime mover technology which is still in normal production. The steam turbine is used to drive a generator or mechanical machinery. In a power plant, it is attached to an electrical generator. Steam turbine is driven by steam which acts as an intermediate energy carrier. Majority of the electricity produced around the world today is generated by conventional steam turbine power plants. In a steam turbine, rotor is the spinning component which has wheels and blades attached to it. The blade is the component which extracts energy from the steam.
Steam has three components of energy namely (i) kinetic energy by virtue of its velocity, (ii) pressure energy by virtue of its pressure, and (iii) internal energy by virtue of its temperature. Last two components of energy together are known as enthalpy. Total energy of steam can be represented as sum of kinetic energy and enthalpy. Energy generation using steam turbine involves three energy conversions namely (i) extracting thermal energy from the fuel and using it to raise steam, (ii) converting the thermal energy of the steam into kinetic energy in the turbine, and (iii) using a rotary generator to convert the mechanical energy of the turbine into electrical energy.
Steam turbine is the heart of a power plant. It has a high thermodynamic efficiency. It is a form of heat engine which derives much of its improvement in thermodynamic efficiency through the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible process. It is a mechanical device which extracts thermal energy from pressurized steam, and converts it into rotary motion. Since the turbine generates rotary motion, it is particularly suited for used to drive an electrical generator.
The modern manifestation of steam turbine was invented by Charles Parsons in 1884, whose first model was connected to a dynamo which generated 7.5 kW (kilowatt) of electricity. Power generation using steam turbines has replaced reciprocating steam engines because of their higher thermal efficiencies, lower costs, and higher power-to-weight ratio. Steam turbines are made in a variety of sizes ranging from less than 0.75 kW units (rare) used as mechanical drives for pumps, compressors, and other shaft driven equipment, to 1,500 MW (megawatt) turbines which are used to generate electricity. Steam turbines are widely used for CHP (combined heat and power) applications.
The steam turbine ranks with the internal-combustion engine as one of the major achievements of mechanical engineering in the 19th century. It has steadily increased in size, reliability and efficiency. It is safe to say that the steam turbine is a known, well understood, and proven technology.
Steam turbine is a thermodynamic device which converts the energy available in the high-pressure, high-temperature steam into shaft power which can in turn be used to turn a generator and produce electric power. Unlike gas turbine and reciprocating engine CHP systems where heat is a by-product of the power generation, steam turbine CHP systems normally generate electricity as a by-product of heat (steam) generation.
Steam turbine needs a separate heat source and does not directly convert fuel to electric energy. The energy contained in the fuel is transferred from the boiler to the turbine through high-pressure steam, which in turn powers the turbine and generator. This separation of functions enables steam turbines to operate with an enormous variety of fuels, from natural gas to solid waste, including all types of coal, wood, wood waste, and agricultural by-products (sugar cane bagasse, fruit pits, and rice husk etc.). In CHP applications, steam at lower pressure is extracted from the steam turbine and is used directly or is converted to other forms of thermal energy. Fig 1 gives schematic diagram of a steam power plant.
Fig 1 Schematic diagram of a steam power plant
Steam turbines are slightly different than other CHP prime movers in that they need a separate boiler or a heat recovery steam generator (HRSG) also known as waste heat boiler (WHB) to create its working fluid (steam). In CHP applications, a boiler or HRSG generate steam which is put through a steam turbine. The steam turbine produces electricity and the remaining exhaust steam can be used for hot water for heating / cooling.
A power plant steam turbine is normally connected to an electrical generator. The term ‘turbo-set’ or ‘turbo-generator’ is used for turbine cum electrical-generator. A power plant can have (i) simple turbo-generator, (ii) single reheat turbo-generator, or (iii) double reheat turbo-generator.
Simple turbo-generator is used where power demand is light or fuel is cheap. In this turbo-generator steam passes through a single turbine. Steam pressure is limited to around 10 MPa, and electric power output to 100 megawatts. Maximum thermal efficiency of simple turbo-generator is around 37 %. Single reheat turbo-generator is normally preferred when the demand for power exceeds 100 megawatts and fuel costs make a thermal efficiency above 40 % desirable. After steam leaves the high-pressure-turbine it is reheated and returned to the double-flow intermediate-pressure turbine. From there it passes to the double-flow low-pressure turbine. The double reheat turbo-generator provides the highest power output and the highest efficiency of around 47 %. In all three systems, the feed-water is heated by steam bled from the turbines. Cooling water for the condensers frequently comes from a nearby river, lake, or ocean. Fig 2 gives the three types of turbo-generators.
Fig 2 Types of turbo-generators.
The critical factors in the design of a steam turbine are temperature, pressure, and specific volume. The curves in Fig 3 show how these three factors vary throughout a typical turbine system. Pressure is the highest at the exit of the feed-water pump leading to the boiler. At the entrance to the high-pressure turbine the pressure drops. Afterwards, it falls rapidly as it passes through the turbine cascade. The steam temperature is raised in the super-heater and again in the reheater, finally falls sharply, as it leaves the low-pressure turbine. The specific volume of the steam varies over the greatest range and is hence plotted on a logarithmic scale. At the inlet to the high-pressure turbine, the steam occupies around 1/1,000 of the volume occupied as compared to the steam which leaves the turbine cascade. Fig 3 shows the critical factors in steam turbine design.
Fig 3 Critical factors in steam turbine 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 % to 90 % based on the application of the turbine.
Steam turbines operate as per the thermo-dynamic cycle, called the Rankine cycle. In this thermo-dynamic cycle, water is pumped to high pressure and then heated to generate high pressure steam. The high-pressure steam is then expanded through a steam turbine where steam energy is converted to mechanical power which drives an electrical generator. For CHP configurations, low pressure steam which exits the steam turbine is then available to satisfy on-site thermal needs. A condenser and pump are used to collect the steam exiting the turbine, feeding it into the boiler and completing the cycle. Fig 4 shows the Rankine cycle and the T-S diagram.
Fig 4 Rankine cycle and T-S diagram
In the thermodynamic cycle in Fig 4, liquid water is converted to high-pressure steam in the boiler and is fed into the steam turbine. The steam causes the turbine blades to rotate, creating power which is turned into electricity with a generator.
Types of turbines – Based on steam supply and exhaust conditions, steam turbines are classified in five types. These types of turbines include (i) condensing, (ii) non-condensing, (iii) reheat, (iv) extraction, and (v) induction turbines.
Condensing steam turbines are normally used 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. These turbines expand the pressurized steam to low pressure at which point a steam / liquid water mixture is exhausted to a condenser at vacuum conditions.
Non-condensing are back-pressure turbines which exhaust the entire flow of steam to the process or the facility at the needed pressure of the steam. Hence, the back-pressure 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 normally used in those plants where there is requirement for large quantities of low-pressure steam for meeting the requirements of the industrial processes.
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. Th turbines have openings in their casings for extraction of a portion of the steam. Steam is released from various stages of the turbine at some intermediate pressure for use in industrial processes, or sent to boiler feedwater heaters to improve overall cycle efficiency. Extraction flows can be controlled with a valve, or left uncontrolled.
In the case of the induction turbines, low pressure steam is introduced at an intermediate stage for the production of additional power.
Based on casing or shaft arrangements, the types of turbines include (i) single casing, (ii) tandem compound, and (iii) cross compound turbines. Single casing turbines are the most basic type where a single casing and shaft are coupled to a generator. Tandem compound type turbines 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 which frequently operate at different speeds. A cross compound turbine is typically used for several large applications.
Principle of operation and design – Steam turbine basically consists of a rotor from which project several rows of closely spaced buckets, or blades. Between each row of moving blades, there is a row of fixed blades which project inward from a circumferential housing. The fixed blades are carefully shaped to direct the flow of steam against the moving blades at an angle and a velocity which maximizes the conversion of the steam’s heat energy into the kinetic energy of rotary motion. Since the temperature, pressure, and volume of steam change continuously as it progresses through the turbine, each row of blades has a slightly different length, and in certain parts of the turbine the twist of the blade is normally varied along the length of the blade, from root to tip. At the inlet of the turbine, the blades are stubby, with little or no twist, while at the outlet, the blades are much longer and the twist is pronounced.
In the first 50 years of the steam turbine’s history (roughly 1890 to 1940), the design of the turbine design was guided mainly by the intuition and ingenuity of engineers who were never far from the shop floor. Today, with the sharp increase in power density and the use of steam at higher pressures and temperatures, turbine progress depends increasingly on scientific understanding and the skillful application of new problem-solving tools such as the electronic computer. The design of a modern turbine needs the solution of difficult problems in aerodynamics, applied mathematics, metallurgy, vibration, and the physical behaviour of steam, together with attention to several manufacturing issues (such as the production of complex blade contours at reasonable cost, the setting of permissible tolerances, and size limitations imposed by transportation).
Early in the history of turbine, two concepts of blade arrangement namely (i) impulse turbine, and (ii) reaction turbine, have been developed, each with its admirers. Parsons favoured what became known as reaction blading. Some of the manufacturers have adopted impulse blading. Fig 5 shows the difference between an impulse and a reaction turbine.
Fig 5 Difference between an impulse and a reaction turbine
Impulse turbine – An impulse turbine has fixed nozzles which orient the steam flow into high-speed jets. These jets contain considerable kinetic energy, which the rotor blades, shaped like buckets, convert into shaft rotation as the steam jet changes direction. A pressure drop occurs across only the stationary blades, with a net increase in steam velocity across the stage. As the steam flows through the nozzle, its pressure falls from the inlet pressure to the exit pressure (atmospheric pressure, or more normally, the condenser vacuum). Because of this higher ratio of expansion of steam in the nozzle, the steam leaves the nozzle with a very high velocity. The steam leaving the moving blades has a large portion of the maximum velocity of the steam when leaving the nozzle. The loss of energy because of this higher exit velocity is normally called the ‘carry over velocity’ or ‘leaving loss’.
In the impulse turbine, the fixed blades are quite different in shape from the moving ones since their job is to accelerate the steam until its velocity in the direction of rotation is around twice that of the moving blades. The moving blades are designed to absorb this impulse and to transfer it to the rotor in the form of kinetic energy. In this arrangement, majority of the pressure drop in each complete stage takes place in the fixed blades; the pressure drop through the moving blades is only sufficient to maintain the forward flow of steam. The quantity of energy transferred to the rotor in each stage is proportional to the change in absolute steam velocity in the direction of rotation. This is the value labeled ‘delta Cu’ in the figure. Fig 6 gives the impulse blading design of a steam turbine.
Fig 6 Impulse blading design of a steam turbine
Impulse blading is one of two normal methods for extracting kinetic energy from steam in a turbine. In the stationary blades steam is accelerated to a velocity (C1) around twice that of the moving blades (U). Velocity is achieved at the expense of pressure (curves at right). The moving blades extract kinetic energy from the fast-moving steam, so that it leaves with essentially no tangential component of velocity (C2). In passing through one row of fixed blades and one row of moving blades, called a stage, the quantity of energy transferred to the rotor is proportional to the change in absolute steam velocity, delta Cu (vector diagram).
Reaction turbine – 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 which 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.
In the reaction turbine, the fixed blades and the moving blades which constitute one stage are practically identical in design and function, each accounting for about half of the pressure-drop which is converted to kinetic energy in the entire stage. In the fixed blades, the pressure is harnessed to increase the velocity of the steam so that it slightly exceeds the velocity of the moving blades in the direction of rotation. In the moving blades, the pressure drop is again used to accelerate the steam but at the same time to turn it around (with respect to the blades), so that its absolute tangential velocity is almost zero as it enters the next bank of stationary blades. Hence, thrust is imparted to the moving blades as the absolute tangential velocity of the steam is reduced from slightly above blade speed to around zero. An imaginary observer moving with the steam cannot tell whether he is passing through the fixed blades or the moving ones. As he approaches either type of blade, it appears to be nearly motionless, but as he travels in the channel between blades, his velocity increases steadily until he reaches their trailing edges, which then seems to be receding rapidly. Fig 7 gives the reaction blading design of a steam turbine.
Fig 7 Reaction blading design of a steam turbine
Reaction blading concept is used in the design of steam turbine. Here the pressure drop per stage is equally divided between fixed and moving blades (curves at right). In the fixed blades steam is accelerated to a velocity (C1) only slightly higher than that of the moving blades (U). Continued expansion of the steam in the moving blades provides thrust and gives the steam a relative velocity (W2) equal and opposite to its former absolute velocity (C1). In reaction blading the energy, ‘delta Cu’ transferred to the rotor in a single stage (vector diagram) is only around half which is transferred by impulse blading. Efficiencies, however, are comparable.
It turns out that the velocity value is around twice as high for impulse blading as it is for reaction blading. This means, in turn, that an impulse turbine needs fewer stages for the same power output than a reaction turbine. However, the efficiency is about the same for both the types. This being the case, one expects impulse blading to be more popular and have carried the day. But it not so. As frequently happens in engineering, a design which seems clearly superior can cause secondary issues of such magnitude that the choice between the alternatives becomes very nearly equal.
In turbine design, one of the major secondary issues is providing seals to keep the steam from leaking through the narrow spaces between the rotor and the stator. In impulse blading the complete expansion in each stage takes place in the fixed blades. It is hence desirable to place the seals on as small a diameter as possible. This has led to a turbine design known as the diaphragm type. Since the pressure differential is large, the diaphragm needs considerable space in the axial direction. Hence, the width of the fixed blade is to be made larger than it is otherwise have to be. A circumferential shroud is frequently placed around each ring of moving blades.
In reaction blading, the pressure drop per stage is less than it is in impulse blading. Also, it is divided equally between fixed and moving blades. Hence, both blades can be fitted with similar seals, and the seals need not be as effective as those needed on the fixed blades in impulse blading. The result is a drum turbine. Another advantage of the reaction turbine is that the stationary and moving blades in each stage can have the same shape, which simplifies design and yields manufacturing economies.
For more than 50 years these two kinds of turbines, i.e., the diaphragm turbine and the drum turbine, have been in competition without demonstrating a distinctive advantage of either type. Along the way, the advocates of the two designs have moved somewhat away from pure reaction or pure impulse blading to adopt different compromise arrangements. The efficiency, which can be attained at each stage in a large modern turbine, is quite remarkable, more than 90 %. For large units with reheat systems, an overall turbine efficiency of around 88 % can be achieved. However, this is not the net thermal efficiency of the steam turbine as a heat engine.
For maximizing 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 either impulse or reaction turbines. Majority of the steam turbines use a mixture of the reaction and impulse designs i.e., each stage behaves as either one or the other, but the overall turbine uses both. Typically, higher pressure sections are impulse type and lower pressure stages are reaction type.
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, first to the astern blades then to the ahead blades slowly rotating the turbine at 10 rpm (revolutions per minute) to 15 rpm to slowly warm the turbine.
Problems with turbines are now rare and maintenance requirements are relatively lesser. 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. However, it is necessary that the turbine be turned with dry steam, i.e., superheated steam with a minimal liquid water content. 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. Further, water entering the blades is also likely to result in the destruction of the thrust bearing for the turbine shaft. For preventing this, condensate drains are installed in the steam piping leading to the turbine along with controls and baffles in the boilers to ensure high quality steam.
Speed regulation – The control of a turbine with a governor is necessary, as turbines need to be run up slowly, to prevent damage while some applications (such as the generation of alternating current electricity) need precise speed control. Uncontrolled acceleration of the turbine rotor can lead to an overspeed trip, which causes the nozzle valves which control the flow of steam to the turbine to close. If this fails then the turbine can continue accelerating until it breaks apart, frequently spectacularly. Modern turbines have an electronic governor which uses a sensor to monitor the turbine speed by ‘looking’ at the rotor teeth.
Turbines are expensive to make, needing precision manufacture and special quality materials. During normal operation in synchronization with the electricity network, power plants are governed with a 5 % droop speed control. This means the full load speed is 100 % and the no-load speed is 105 %. This is needed for the stable operation of the network without hunting and drop-outs of power plants.
Normally the changes in speed are minor. Adjustments in power output are made by slowly raising the droop curve by increasing the spring pressure on a centrifugal governor. Normally, this is a basic system requirement for all power plants since the older and newer plants have to be compatible in response to the instantaneous changes in frequency without depending on outside communication.
Thermodynamics of steam turbines – The steam turbine operates on basic principles of thermodynamics using the part of the Rankine cycle. Super-heated vapour (or dry saturated vapour, depending on application) enters the turbine, after it having exited the boiler, at high temperature and high pressure. The high heat / pressure steam is converted into kinetic energy using a nozzle (a fixed nozzle in an impulse type turbine or the fixed blades in a reaction type turbine). Once the steam has exited the nozzle it is moving at high velocity and is sent to the blades of the turbine. A force is created on the blades because of the pressure of the vapour on the blades causing them to move. Fig 8 gives the Rankine cycle of a steam turbine with superheat.
Fig 8 Rankine cycle with superheat
A generator or other such device can be placed on the shaft and the energy which has been in the vapour can now be stored and used. The gas exits the turbine as a saturated vapour (or liquid-vapour mixture depending on application) at a lower temperature and pressure than it enters with and is sent to the condenser to be cooled.
As per the first law, an equation can be found comparing the rate at which work is developed per unit mass. Assuming there is no heat transfer to the surrounding environment and that the change in kinetic and potential energy is negligible when compared to the change in specific entropy, then the equation Wt/m = h1 – h2 holds good. In the equation ‘Wt’ is the rate at which work is developed per unit mass, ‘m’ is the rate of mass flow through the turbine, ‘h1’ is the specific entropy at state one, and ‘h2’ is the specific entropy at state two for an actual process.
Isentropic turbine efficiency – For measuring how well a turbine is performing one can look at the isentropic efficiency. Isentropic efficiencies involve a comparison between the actual performance of a device and the performance which is to be achieved under idealized conditions. When calculating the isentropic efficiency, heat to the surroundings is assumed to be zero. The starting pressure and temperature are the same for both the isentropic and actual efficiency. Since state 1 is the same for both efficiencies, the specific enthalpy ‘h1’ is known.
The specific entropy for the isentropic process is higher than the specific entropy for the actual process because of the irreversibility in the process. The specific entropy is evaluated at the same pressure for the actual and isentropic processes in order to give a good comparison between the two. The isentropic efficiency is given by the actual work divided by the maximum work which can be achieved if there are no irreversibly in the process. Hence. Efficiency Et = (Wt/m)/(Wt1/m1) = (h1- h2)/(h1 – h2s) where ‘h1’ is the specific entropy at state one, ‘h2’ is the specific entropy at state two for an actual process, and ‘h2s’ is the specific entropy at state two for an isentropic process.
Calculating turbine efficiency – The efficiency ‘E’ of the steam turbine can be calculated by using the Kelvin statement of the Second law of Thermodynamics. E = Wcycle/Qh, where Wcycle is the work done during one cycle, and ‘Qh’ is the heat transfer received from the heat source.
If one look at the Carnot cycle, the maximum efficiency ‘Emax’ of a steam turbine can be calculated as Emax = 1- Tl/Th, where ‘Tl’ is the absolute temperature of the vapour moving out of the turbine, and ‘Th’ is the absolute temperature of the vapour coming from the boiler. This efficiency can never be achieved in the real world because of the irreversibility during the process, but it does give a good measure as to how a particular turbine is performing.
Direct drive – Electrical power stations use large steam turbines driving electric generators to produce electricity. Majority of these centralized power stations are of two types, fossil fuel power plants and nuclear power plants, but some countries are using concentrating solar power (CSP) to create the steam. Steam turbines can also be used directly to drive large centrifugal pumps, such as feed-water pumps at a thermal power plant, or a blower for supplying blast air to blast furnace stoves. The turbines used for electric power generation are most frequently directly coupled to their generators. As the generators are required to rotate at constant synchronous speeds according to the frequency of the electric power system, the most common speeds are 3,000 rpm for 50 Hz (hertz) systems, and 3,600 rpm for 60 Hz systems. In installations with high steam output, as can be found in nuclear power stations, the generator sets can be arranged to operate at half these speeds, but with four-pole generators. Half-speed turbines (1,500 rpm instead of 3,000 rpm or 1,800 rpm instead of 3,600 rpm) have been introduced to provide large output in a single turbine set, but their weight and cost, megawatt for megawatt, are higher than those for full-speed turbines.