Lever Safety Valve It is suitable for stationary boilers. It can be contrasted with a piston engine which uses a piston instead of a turbine to extract energy. Steam turbine II. Water turbine III. Wind turbine IV. Gas turbine A rotor of a modern steam turbine, used in a power plant Engr. In a steam turbine the force exerted on the blades is due to the velocity of steam. The resulting impulse as described by Newton's second law of motion spins the turbine and removes kinetic energy from the fluid flow.
Torque is produced by the momentum changes in the rotor and by reaction from fluid accelerating out of the rotor Engr. Phase during part of the Cycle, and in liquid phase during remaining part. The ideal Rankine cycle does not involve any internal irreversibilities. Irteza Hossain The simple ideal Rankine cycle. The thermal efficiency can be interpreted as the ratio of the area enclosed by the cycle on a T-s diagram to the area under Engr.
Irteza Hossain process. Fluid friction and heat loss to the surroundings are the two common sources of irreversibilities. Isentropic efficiencies a Deviation of actual vapor power cycle from the ideal Rankine cycle. The basic idea behind all the modifications to increase the thermal efficiency of a power cycle is the same: Increase the average temperature at which heat is transferred to the working fluid in the boiler, or decrease the average temperature at which heat is rejected from the working fluid in the condenser.
Lowering the Condenser Pressure Lowers Tlow,avg To take advantage of the increased efficiencies at low pressures, the condensers of steam power plants usually operate well below the atmospheric pressure. There is a lower limit to this pressure depending on the temperature of the cooling medium Side effect: Lowering the condenser pressure increases the moisture content of the steam at the final stages of the turbine.
The effect of lowering the condenser Engr. Irteza Hossain pressure on the ideal Rankine cycle. The overall effect is an increase in thermal efficiency since the average temperature at which heat is added increases.
Superheating to higher temperatures decreases the moisture content of the steam at the turbine exit, which is desirable. The temperature is limited by metallurgical considerations. The effect of increasing the boiler A supercritical Rankine cycle. Irteza Hossain cycle. Superheat the steam to very high temperatures.
It is limited metallurgically. Expand the steam in the turbine in two stages, and reheat it in between reheat The ideal reheat Rankine cycle. The average temperature during the reheat process can be increased by increasing the number of expansion and reheat stages. As the number of stages is increased, the expansion and reheat processes approach an isothermal process at the maximum temperature.
The use of more than two reheat stages is not practical. The theoretical improvement in The average temperature at which efficiency from the second reheat is heat is transferred during about half of that which results from a reheating increases as the number single reheat. The reheat temperatures are very close or equal to the turbine inlet temperature. This lowers the average heat-addition temperature and thus the cycle efficiency. In steam power plants, steam is extracted from the turbine at various points.
This steam, which could have produced more work by expanding further in the turbine, is used to heat the feedwater instead. The device where the feedwater is heated by regeneration is called a regenerator, or a The first part of the heat-addition feedwater heater FWH. A feedwater heater is basically a heat exchanger where heat is transferred from the steam to the feedwater either by mixing the two fluid streams open feedwater Engr.
Open Feedwater Heaters An open or direct-contact feedwater heater is basically a mixing chamber, where the steam extracted from the turbine mixes with the feedwater exiting the pump. Ideally, the mixture leaves the heater as a saturated liquid at the heater pressure. The ideal regenerative Rankine Engr. Irteza an open feedwater heater.
The two streams now can be at different pressures, since they do not mix. The ideal regenerative Rankine cycle Engr. Irteza a closed feedwater heater. The result is a combined gas—steam cycle.
Consequently, many new power plants operate on combined cycles, and many more existing steam- or gas-turbine plants are being converted to combined-cycle power plants. The vapors lose their latent heat of vaporization; hence, vapors transfer their heat into liquid and the liquid becomes hot.
In this type of condensation, the vapor and liquid are of same type of substance. In another type of direct contact condenser, cold water is sprayed into the vapors to be condensed Engr. These condensers are heat exchangers which convert steam from its gaseous to its liquid state at a pressure below atmospheric pressure. In thermal power plants, the primary purpose of a surface condenser is to condense the exhaust steam from a steam turbine to obtain maximum efficiency and also to convert the turbine exhaust steam into pure water referred to as steam condensate so that it may be reused in the steam generator or boiler as boiler feed water.
The water may be freshly supplied or returning condensate produced as a result of the condensation of the steam produced by the boiler. These pumps are normally high pressure units that take suction from a condensate return system.
Centrifugal pump 2. Reciprocating pump Engr. Related Papers. Therefore most of nuclear power plants operates a single-shaft turbine-generator that consists of one multi-stage HP turbine and three parallel multi-stage LP turbines , a main generator and an exciter. LP turbines are usually double-flow reaction turbines with about stages with shrouded blades and with free-standing blades of last 3 stages.
Each turbine rotor is mounted on two bearings, i. See also: HP Turbine. See also: LP Turbine. The steam must be reheated in order to avoid damages that could be caused to blades of steam turbine by low quality steam. The reheater heats the steam point D and then the steam is directed to the low-pressure stage of steam turbine, where expands point E to F. The exhausted steam then condenses in the condenser and it is at a pressure well below atmospheric absolute pressure of 0.
Steam turbines may be classified into different categories depending on their construction, working pressures, size and many other parameters. But there are two basic types of steam turbines:. The main distinction is the manner in which the steam is expanded as it passes through the turbine. Steam turbine types based on blade geometry and energy conversion process are impulse turbine and reaction turbine. The impulse turbine is composed of moving blades alternating with fixed nozzles.
In the impulse turbine, the steam is expanded in fixed nozzles and remains at constant pressure when passing over the blades. Curtis turbine , Rateau turbine , or Brown-Curtis turbine are impulse type turbines. The original steam turbine, the De Laval, was an impulse turbine having a single-blade wheel. The entire pressure drop of steam take place in stationary nozzles only. Though the theoretical impulse blades have zero pressure drop in the moving blades, practically, for the flow to take place across the moving blades, there must be a small pressure drop across the moving blades also.
In impulse turbines, the steam expands through the nozzle, where most of the pressure potential energy is converted to kinetic energy. The high-velocity steam from fixed nozzles impacts the blades , changes its direction , which in turn applies a force.
The resulting impulse drives the blades forward, causing the rotor to turn. The main feature of these turbines is that the pressure drop per single stage can be quite large, allowing for large blades and a smaller number of stages. Except for low-power applications, turbine blades are arranged in multiple stages in series, called compounding, which greatly improves efficiency at low speeds. Modern steam turbines frequently employ both reaction and impulse in the same unit, typically varying the degree of reaction and impulse from the blade root to its periphery.
The rotor blades are usually designed like an impulse blade at the rot and like a reaction blade at the tip. Since the Curtis stages reduce significantly the pressure and temperature of the fluid to a moderate level with a high proportion of work per stage. An usual arrangement is to provide on the high pressure side one or more Curtis stages, followed by Rateau or reaction staging.
In general, when friction is taken into account reaction stages the reaction stage is found to be the most efficient, followed by Rateau and Curtis in that order. Frictional losses are significant for Curtis stages, since these are proportional to steam velocity squared.
The reason that frictional losses are less significant in the reaction stage lies in the fact that the steam expands continuously and therefore flow velocities are lower. A compounded steam turbine has multiple stages i. For example, large HP Turbine used in nuclear power plants can be double-flow reaction turbine with about 10 stages with shrouded blades. Large LP turbines used in nuclear power plants are usually double-flow reaction turbines with about stages with shrouded blades and with free-standing blades of last 3 stages.
A velocity-compounded impulse stage consist of a row of fixed nozzles followed by two or more rows of moving blades and fixed blades without expansion. This divides the velocity drop across the stage into several smaller drops. In this type, the total pressure drop expansion of the steam take place only in the first nozzle ring. This produces very high velocity steam , which flows through multiple stages of fixed and moving blades.
At each stage, only a portion of the high velocity is absorbed, the remainder is exhausted on to the next ring of fixed blades. The function of the fixed blades is to redirect the steam without appreciably altering the velocity leaving from the first ring of moving blades to the second ring of moving blades. The jet then passes on to the next ring of moving blades, the process repeating itself until practically all the velocity of the jet has been absorbed.
This method of velocity compounding is used to solve the problem of single stage impulse turbine for use of high pressure steam i. A pressure-compounded impulse stage is a row of fixed nozzles followed by a row of moving blades , with multiple stages for compounding. In this type, the total pressure drop of the steam does not take place in the first nozzle ring, but is divided up between all the nozzle rings.
The effect of absorbing the pressure drop in stages is to reduce the velocity of the steam entering the moving blades. The steam from the boiler is passed through the first nozzle ring in which it is only partially expanded. It then passes over the first moving blade ring where nearly all of its velocity momentum is absorbed. From this ring it exhausts into the next nozzle ring and is again partially expanded. This method of pressure compounding is used in Rateau and Zoelly turbines, but such turbines are bigger and bulkier in size.
Impulse stages may be either pressure-compounded, velocity-compounded, or pressure-velocity compounded. The pressure-velocity compounding is a combination of the above two types of compounding. In fact, a series of velocity-compounded impulse stages is called a pressure-velocity compounded turbine.
Each stage consists of rings of fixed and moving blades. Each set of rings of moving blades is separated by a single ring of fixed nozzles. In each stage there is one ring of fixed nozzles and rings of moving blades with fixed blades between them. Each stage acts as a velocity compounded impulse turbine.
The steam coming from the steam generator is passed to the first ring of fixed nozzles, where it gets partially expanded. The pressure partially decreases and the velocity rises correspondingly. It then passes over the rings of moving blades with fixed blades between them where nearly all of its velocity is absorbed. From the last ring of the stage it exhausts into the next nozzle ring and is again partially expanded.
This has the advantage of allowing a bigger pressure drop in each stage and, consequently, less stages are necessary, resulting in a shorter turbine for a given pressure drop. It may be seen that the pressure is constant during each stage; the turbine is, therefore, an impulse turbine. The method of pressure-velocity compounding is used in the Curtis turbine. The reaction turbine is composed of moving blades nozzles alternating with fixed nozzles.
In the reaction turbine, the steam is expanded in fixed nozzles and also in the moving nozzles. In other words, the steam is continually expanding as it flows over the blades. There is pressure and velocity loss in the moving blades. The moving blades have a converging steam nozzle.
Hence when the steam passes over the fixed blades, it expands with decrease in steam pressure and increase in kinetic energy. In reaction turbines, the steam expands through the fixed nozzle , where the pressure potential energy is converted to kinetic energy. The high-velocity steam from fixed nozzles impacts the blades nozzles , changes its direction and undergo further expansion. The change in its direction and the s team acceleration applies a force.
Ther is 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 this type of turbine the pressure drops take place in a number of stages, because the pressure drop in a single stage is limited. The main feature of this type of turbine is that in contrast to the impulse turbine, the pressure drop per stage is lower , so the blades become smaller and the number of stages increases.
On the other hand, reaction turbines are usually more efficient, i. The reaction turbine was invented by Sir Charles Parsons and is known as the Parsons turbine. In the case of steam turbines, such as would be used for electricity generation, a reaction turbine would require approximately double the number of blade rows as an impulse turbine, for the same degree of thermal energy conversion.
Whilst this makes the reaction turbine much longer and heavier, the overall efficiency of a reaction turbine is slightly higher than the equivalent impulse turbine for the same thermal energy conversion.
In a reaction steam turbine compounding can be achieved only in the pressure compounding. In fact, it is not exactly the same as what it was discussed in impulse turbines. Note that, there is steam expansion in both the fixed and moving blades. Steam turbines may be classified into different categories depending on their purpose and working pressures. The industrial usage of a turbine influences the initial and final conditions of steam.
For any steam turbine to operate, a pressure difference must exist between the steam supply and the exhaust. Condensing steam turbines are most commonly found in thermal power plants. In a condensing steam turbine , the maximum amount of energy is extracted from the steam, because there is very high enthalpy difference between the initial e. This is achieved by passing the exhaust steam into a condenser called a surface condenser , which condenses the exhaust steam from the low-pressure stages of the main turbine decreases the temperature and pressure of exhausted steam.
The exhausted steam is condensed by passing over tubes containing water from the cooling system. The goal of maintaining the lowest practical turbine exhaust pressure is a primary reason for including the condenser in a thermal power plant. The condenser provides a vacuum that maximizes the energy extracted from the steam, resulting in a significant increase in net work and thermal efficiency. But also this parameter condenser pressure has its engineering limits:.
In a typical condensing steam turbine , the exhausted steam condenses in the condenser and it is at a pressure well below atmospheric absolute pressure of 0.
Note that, the pressure inside the condenser is also dependent on the ambient atmospheric conditions:. In other words, the electrical output of a power plant may vary with ambient conditions , while the thermal power remains constant. The pressure inside condenser is given by the ambient air temperature i. The lowest feasible condenser pressure is the saturation pressure corresponding to the ambient temperature e. Back-pressure steam turbines or non-condensing turbines are most widely used for process steam applications.
Steam is a principle energy source for many industrial processes. The popularity of process steam as an energy source stems from its many advantages, which include:. The process steam can be produced by back-pressure steam turbines , which also generates mechanical work or electrical energy.
Back-pressure turbines expand the live steam supplied by the boiler to the pressure at which the steam is required for the process. The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure.
Back-pressure turbines 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.
The electric power generated by the back-pressure turbine is directly proportional to the amount of process steam required. Reheat turbines are also used almost exclusively in thermal power plants.
All turbines, that have high-pressure turbine and low-pressure turbines use a steam reheat between these stages. Reheat allows to deliver more of the heat at a temperature close to the peak of the cycle i. This requires the addition of another type of heat exchanger called a reheater.
The use of the reheater involves splitting the turbine, i. It was observed that more than two stages of reheating are unnecessary, since the next stage increases the cycle efficiency only half as much as the preceding stage. High pressure and low pressure stages of the turbine are usually on the same shaft to drive a common generator, but they have separate cases. With a reheater , the flow is extracted after a partial expansion point D , run back through the heat exchanger to heat it back up to the peak temperature point E , and then passed to the low-pressure turbine.
The expansion is then completed in the low-pressure turbine from point E to point F. The steam must be reheated or superheated in order to avoid damages that could be caused to blades of steam turbine by low quality steam. High content of water droplets can cause the rapid impingement and erosion of the blades which occurs when condensed water is blasted onto the blades.
To prevent this, condensate drains are installed in the steam piping leading to the turbine. Accordingly, superheating also tends to alleviate the problem of low vapor quality at the turbine exhaust.
Almost all large steam turbines use the heat regeneration i. The reduction in the heat added can be done by transferring heat partially expanded steam from certain sections of the steam turbine, which is normally well above the ambient temperature, to the feedwater.
Note that, most of energy contained in the steam is in the form of latent heat of vaporization. Extraction flows may be controlled with a valve, or left uncontrolled.
For example, most of nuclear power plants operates a single-shaft turbine-generator that consists of one multi-stage HP turbine with 3 or 4 self-regulating extraction lines and three parallel multi-stage LP turbines with 3 or 4 self-regulating extraction lines.
The high pressure feedwater heaters are usually heated by extraction steam from the high pressure turbine, HP, whereas the low-pressure feedwater heaters are usually heated by extraction steam from the low pressure turbine, LP. Both are usually self-regulating. It means that the greater the flow of feedwater the greater the rate of heat absorption from the steam and the greater the flow of extraction steam.
The most important turbine elements are the turbine blades. They are the principal elements that convert pressure energy of working fluid into kinetic energy. Turbine blades are of two basic types:. In steam turbines , the steam expands through the fixed blade nozzle , where the pressure potential energy is converted to kinetic energy. The high-velocity steam from fixed nozzles impacts the moving blades, changes its direction and also expands in case of reaction type blades.
The change in its direction and the steam acceleration in case of reaction type blades applies a force. Steam turbine types based on blade geometry and energy conversion process are:. The efficiency and reliability of a turbine depend on the proper design of the blades.
It is therefore necessary for all engineers involved in the turbines engineering to have an overview of the importance and the basic design aspects of the steam turbine blades. Engineering of turbine blades is a multi-disciplinary task. It involves the thermodynamics , aerodynamics, mechanical and material engineering.
For gas turbines , the turbine blades are often the limiting component. The highest temperature in the cycle occurs at the end of the combustion process, and it is limited by the maximum temperature that the turbine blades can withstand.
As usual, metallurgical considerations about K place an upper limits on thermal efficiency. Therefore turbine blades often use exotic materials like superalloys and many different methods of cooling, such as internal air channels, boundary layer cooling, and thermal barrier coatings.
The development of superalloys in the s and new processing methods such as vacuum induction melting in the s greatly increased the temperature capability of turbine blades. Modern turbine blades often use nickel-based superalloys that incorporate chromium, cobalt, and rhenium. Steam turbine blades are not exposed to such high temperatures, but they must withstand an operation with two-phase fluid.
To prevent this, for example, condensate drains are installed in the steam piping leading to the turbine. Another challenge for engineers is the design of blades of the last stage of LP turbine. These blades must be due to high specific volume of steam very long, which induces enormous centrifugal forces during operation. Therefore, turbine blades are subjected to stress from centrifugal force turbine stages can rotate at tens of thousands of revolutions per minute RPM , but usually at RPM and fluid forces that can cause fracture, yielding, or creep failures.
The steam turbine is not a perfect heat engine. Energy losses tend to decrease the efficiency and work output of a turbine. This inefficiency can be attributed to the following causes. Governing of steam turbine is the procedure of controlling the flow rate of steam to a steam turbine so as to maintain the speed of the turbine fairly constant irrespective of load on the turbine.
The typical main turbine in nuclear power plants, in which steam expands from pressures about 6 MPa to pressures about 0. The variation in load power output during the operation of a steam turbine can have a significant impact on its performance and its efficiency.
Traditionally, nuclear power plants NPPs have been considered as baseload sources of electricity as they rely on a technology with high fixed costs and low variable costs. However, this simple state of affairs no longer applies in all countries. The share of nuclear power in the national electricity mix of some countries has become so large that the utilities have had to implement or to improve the manoeuvrability capabilities of their power plants in order to be able to adapt electricity supply to daily, seasonal or other variations in power demand.
The primary objective in the steam turbine operation is to maintain a constant speed of rotation irrespective of the varying load. This can be achieved by means of governing in a steam turbine.
The principal methods of governing which are used in steam turbines are:. Throttle governing. The main parts of a simple throttle governing system are the throttle-stop valves and especially control valves between steam generators and main turbine. The primary aim of control valves is to reduce the steam flow rate. Incidental to reducing the mass rate of flow, the steam experiences an increasing pressure drop across the control valve, which is in fact an isenthalpic process.
Although throttling is an isenthalpic process, the enthalpy drop available for work in the turbine is reduced, because this causes an increase in vapor quality of outlet steam. Every steam turbine is also provided with emergency governors which come into action under specific conditions. The turbine trip signal initiates fast closure of all steam inlet valves e.
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