Steam Turbine is a type of turbo machine. Turbo machine are those devices in which energy is transferred either to or from, a continuously flowing fluid by the dynamic action of one or more moving blade rows. In steam turbine energy is transferred from fluid to blade rows and is decreasing along the flow directions. It is power producing thermodynamics device. Steam turbine converts heat energy of steam (at high pressure and temperature) into mechanical energy.
The so utilized can be used in various filed of industry such as electricity generation, transport, in driving of umps, fan and compressor etc. The basic cycle on which steam turbine works is Ranking Cycle. The reciprocating steam engine was still inefficient, cumbersome, had a very low power to weight ratio, and was a high maintenance piece of machinery. The development of the steam turbine was a vast improvement in all of these respects. A turbine consist of one set of stationary blades or nozzles and an adjacent set of moving blades or buckets.
These stationary and rotating elements act together to allow the steam flow to do work on the rotor. The Ark is transmitted to the load through the shaft or shafts. Figure 1. 1 Block diagram off Turbine . Steam turbines date back to 120 B. C. When the first steam turbine was developed by Hero of Alexandria. Subsequently number of steam turbines came up but the practically successful steam turbine appeared at the end of nineteenth century when August De Lava designed a high speed turbine built on the principle of reaction turbine in 1883.
Before this in 1629 G. Branch developed the first impulse turbine. Branch’s impulse turbine and Hero’s reaction turbine are shown in Fig. 1. 1 . Figure 1. 2 Hero and Branch’s turbine. In nineteenth century some more steam turbines were developed by Sir Charles A. Parsons and C. G. Curtis which gave a fillip to the development to the modern steam turbine. Over the period of time the modern steam turbines evolved with capacity from few kilowatts to 350,000 k and in speed from 1000 RPM to 40,000 RPM.
Steam turbines offer the advantages over other prime movers in terms of simplicity, reliability and low maintenance costs. Reciprocating steam engines use pressure energy of steam while steam turbines use dynamic action of the steam. Steam turbines require less space as compared to diesel engine or steam engine and also the absence of reciprocating parts ; reciprocating motion in steam turbine exults in lesser vibrations and lighter foundation. In steam turbine the expanding steam does not come into contact Witt lubricant and so exhaust steam leaves uncontaminated.
The basic principle on which steam turbine works is Newton’s Second law of motion. The motive power of a high velocity Jet impinging on a curved blade. The steam from boiler is expanded in a nozzle where due to fall in pressure of steam, thermal energy of steam is converted into kinetic energy of steam, resulting in the emission of a high velocity Jet of steam which impinges on the moving vanes or blades, mounted on a haft; here it undergoes a change in direction of motion which give rise to a change in momentum and therefore, a force.
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. Steam turbines are mostly ‘axial flow’ types; the steam flows over the blades in a direction Parallel to the axis of the wheel. ‘Radial flow’ types are rarely used. Let should be noted that the blade obtains no motive force from the static pressure of the steam or from NY impact of the Jet, because the blade is designed such that the steam Jet will glide on and off the blade without and tendency to strike it.
If the flow of steam through the nozzles and moving blades of a turbine takes place in such a manner that “the steam is expanded only in nozzles, and pressure at the outlet side of blade is equal to that at the inlet side”, I. Drop in pressure of steam takes place only in nozzles and not in moving blades; such a turbine is termed as impulse turbine because it works on the principle of impulse. This is obtained by making the blade passage of constant cross-section area. In impulse turbine, the energy transformation takes place in nozzles while energy transfer takes place in moving blades. Simple impulse turbine is used where small output at very high speed is required or only a small pressure drop is available.
These are not suited for applications requiring conversion of large thermal energy into work. The expansion of steam takes place in nozzle (fixed blades) as well as in moving blades. If the pressure of steam at the outlet from the moving blades of a turbine is less than that at the inlet side of blades; this pressure drop suffered steam while passing through the moving blades, giving rise to reaction and adds on the propelling force which is applied through the rotor to the turbine shaft. Such turbine is termed as impulse-and reaction both.
Figure 1. 4 Difference between the two turbines Working of Impulse and Steam turbine achieved by varying the blade passage cross-section This is (converging type). Here energy transformation takes place in nozzles (fixed blade) while both energy transfer and transformation takes place in moving blades. Steam turbines can be classified based on the direction of flow by which steam flows through turbine blaming.
These axial flow turbines are well suited for large turbo generators and very commonly used presently. Radial flow turbine incorporates two shafts end to end and can be of suitably small sizes. Radial flow turbines can be started quickly and so well suited for peak load and used as stand by turbine or peak load turbines. These are also termed as Lustrous turbines. In tangential flow turbines the nozzle directs steam tangentially into buckets at the periphery of single wheel and steam reverses back and re-enters other bucket at its’ periphery.
Condensing turbines are frequently used in thermal power plants. Non- condensing steam turbines are those in which steam leaving turbine is rejected to atmosphere and not to condenser as in case of condensing turbine. Back pressure turbines reject steam at a pressure much above he atmospheric pressure and steam leaving turbine with substantially high pressure can be used for some other purposes such as heating or running small condensing turbines.Pass out turbines are those in which certain quantity of steam is continuously extracted for the purpose of heating and allowing remaining steam to pass through pressure control valve into the low pressure section of turbine.
The Ranking cycle is a steam cycle for a steam plant operating under The best theoretical conditions for most efficient operation. This is an ideal imaginary cycle against which all other real steam working cycles can be compared. The theoretic cycle can be considered with reference to the figure below. There will no losses of energy by radiation, leakage of steam, or frictional losses in the mechanical components. The condenser cooling will condense the steam to water with only sensible heat (saturated water). The feed pump will add no energy to the water. The chimney gases would be at the same pressure as the atmosphere.
Within the turbine the work done would be equal to the energy entering the turbine as steam (Hal) minus the energy leaving the turbine as steam after perfect expansion (h2o) this being isentropic (reversible adiabatic) I. E. (Hal- h2o). Figure 1. 5 Basic ranking cycle. The energy supplied by the steam by heat transfer from the combustion and flue gases in the furnace to the water and steam in the boiler will be the difference in the enthalpy of the steam leaving the boiler and the water entering the boiler = (Hal – ha). The various energy streams flowing in a simple steam turbine system are as indicated in the diagram below.
It is clear that the working fluid is in a closed circuit apart from the free surface of the hot well. Every time the working fluid flows at a uniform rate around the circuit it experiences a series of processes making up a thermodynamic cycle. The complete plant is enclosed in an outer boundary and the working fluid crosses inner boundaries (control surfaces). The inner boundaries defines a flow process. This type of turbine works on the principle of impulse. It consist of a nozzles, a rotor mounted on the shaft, one set of moving blades attached to the rotor and a casting, etc.
A set row of nozzles and moving blades constitutes a stage. The uppermost portion of the diagram (Fig. 2. 1) shows a longitudinal section through the upper half of turbine. The middle portion shows the development of the nozzles and blaming, I. E. The actual shape of nozzle and blaming, and the bottom portion shows the variation tot absolute pressure during tool tot stream through passage tot nozzles and blades. Figurer. 1 working of simple Impulse turbine An example of this type of turbine is the De-Level turbine. It has single-stage having a nozzle fitted in the casing followed by ring of moving blades mounted on the shaft.
Variation of velocity and pressure along the axis of turbine is also shown in the figure. It can be seen from the figure that the complete expansion of steam from steam chest pressure to the exhaust pressure of the condenser pressure takes place only in one set of nozzle I. E. The pressure drop takes place only in nozzles. It is assumed that the pressure in the recess between nozzles and blades remain the same. The steam at the condenser pressure or exhaust pressure enters the blades and comes out at the pressure I. E. The pressure of steam in the blade passages remain approximately instant and equal to the condenser pressure.
Generally, converging-diverging nozzle are used due to the relative large ratio of expansion of steam in the nozzles, the steam leaves the nozzles at very high velocity (supersonic) of about mm/s. It is assumed that the velocity remains constant in the recess between the nozzles and the blades. The steam at such high velocity enters the blades and comes out with a velocity that is appreciable.
Velocity diagrams for single stage of simple impulse turbine is shown in figure 2. 1 . Velocity diagram gives an account of velocity of fluid entering and leaving the urbane. Figure 2. Schematic diagram of an Impulse Turbine Figure 2. 3 Velocity diagram of an Impulse Turbine Figure 2. 1 gives the inlet and outlet velocity diagrams at inlet edge and outlet edge of moving blade along with the combined inlet and outlet velocity diagram for a stage of simple impulse turbine. The notations used for denoting velocity angles and other parameters during calculations are explained as under, (SSL system of units is used here). IS=Linear velocity of blade.
VI and VI= Inlet and outlet absolute velocity. IVR and IVR= Inlet and outlet relative velocity (Velocity relative to the trot blades. = Nozzle angle, = absolute fluid angle at outlet (It is to be mentioned that all angles are with respect to the tangential velocity the direction of U. ) and = Inlet and outlet blade angles. In = Tangential or whirl component tot absolute velocity at Inlet and outlet.
And = Axial component of velocity at inlet and outlet. This turbine utilizes the principle of impulse and reaction both and is shown in the figures. Turbine Figurer. 4 Diagrammatic Arrangement of Impulse-reaction Figurer. 5 In a Steam turbine, steam strikes a set of movable blades and exerts force against them.
The steam is then deflected by a set of fixed blades to another set of movable blades, and so on. There are a number of rows of fixed blades attached to the casting with an equal number of moving blades attached to the rotor. In this turbine, the fixed blades are set in a reversed manner compared to the moving blades, and correspond to nozzles mentioned in connection with impulse turbine. Due to the position of the fixed row of blades at the entrance, in place of the nozzles, steam is admitted for the whole circumference and hence there is an all-round or complete admission.
In passing wrought the first row of fixed blades, the steam undergoes a small drop in pressure and hence in velocity increases slightly. It then enters the first row of moving blades just as in the impulse turbine, and suffers a change in direction and, therefore results in momentum. This momentum gives rise to an impulse on the blades. But in this turbine, the passage of the moving blades is so designed that there is a drop in pressure of steam in the moving blades which result in an Figure 2. 6 Pressure velocity variation of Reaction Turbine increase in the kinetic energy of steam.
This kinetic energy gives rise to a reaction in he direction opposite to that of added velocity thus, the gross propelling force or driving force is the vector sum of impulse and reaction forces. Commonly, this type of turbine is known as Reaction Turbine. The pressure and velocity variations are shown in figure 2. 5. It can be seen in . The figure that there is a gradual drop in pressure in both the moving blades and the fixed blades. In this turbine as the pressure falls, the specific volume increases and hence the height of blades is increased in steps I. . , Upton 4 stages it may remain constant, then it may increase and remain constant for the next two stages.
In this turbine, the steam velocities are comparatively moderate and its maximum value is nearly equal to blade velocity. In general practice, to reduce the number of stages, the steam velocity is arranged greater than the blade velocity. The leaving velocity loss is about 1 to 2 percent of the total initial available energy. This turbine is popular in steam power plants. An example of this type of turbine is the Parsons- Reaction Turbine. ) In Impulse Turbine, steam completely expands in the nozzle itself. Hence its pressure remains constant on both ends of the moving blades, while, in Reaction Turbine, Fixed blades act as nozzles.
Hence steam expands both in fixed and moving blades continuously as it passes over them. Thus the pressure drop occurs gradually and continuously over both the fixed and moving blades. In Impulse Turbine, Blades Passage is of constant cross section area, as there is no expansion of steam, while, in Reaction Turbine, Blade passage is of variable cross-sectional area (converging type) due to expansion of steam. In Impulse Turbine, As pressure remains constant in moving blades, the relative velocity of steam passing over the moving blades remains constant, while, in Reaction Turbine, Continuous expansion of team meaner relative velocity in the moving blades increases.
In Impulse Turbine, Blades are of symmetrical profile types: hence, manufacturing of blade is simple, while, in Reaction Turbine, The blade shapes are of aerofoil and non- symmetrical type; hence manufacturing is difficult. In Impulse Turbine, Because of large pressure drop, the steam speed and the running speed are high, while, in Reaction Turbine, Due to small pressure drop, the steam speed and the running speed are low.
In Impulse Turbine, Because of large pressure drop in the nozzles, the number of stages are less. The size of an impulse turbine for power output is comparatively small, while, in Reaction Turbine, Because of small pressure drop in each stage, the number of stages are more for the same pressure drop. Hence the size of the reaction turbine for the same power output is large.
The maximum force is develops when the blades is locked while the Jet enters and leave with equal velocity. Since the blade velocity is zero, no mechanical work is done. As the blades is allowed to speed up, the velocity of Jet from the blade reduces, which reduces the force. Due to blade velocity work is done and maximum work is done when the blade velocity is Just half the steam velocity. Force and work done become zero when blade velocity is equal to the steam velocity. In this case, steam velocity from the blade is near about zero I. . The trail of inert steam since all the kinetic energy of steam is converted into work.
We know that for economy or maximum work, the blade velocity should be one half of the steam velocity, blade velocity of about 500 m/s is deemed very high. This type of turbine is generally employed where relatively small power is required and where the rotor diameter is fairly small. The small rotor gives a very high rotational speed, reaching 30,000 RPM. Such high rotational speed can only be utilized to drive generators with large reduction gearing arrangements.
In this turbine, the leaving velocity of steam is quite appreciable, resulting in an energy loss, called “carry over loss” or “leaving velocity loss”. This leaving loss is so high that it may be as much as 11 percent of the initial kinetic energy. Figure 3. 1 Carry over loss in impulse turbine. In this turbine, the leaving velocity of steam is quite appreciable, resulting in an energy loss, called “carry over loss” or “leaving velocity loss”. This leaving loss is so gig that it may be as much as 11 percent of the initial in kinetic energy. The diagram shows carry over loss or lost velocity that occurs the simple impulse turbine.
This loss very high which result in the lower efficiency of the turbine result in the loss of the useful work. In order to prevent this velocity loss and to reduce the maximum speed of rotor under permissible limit compounding is employed. Compounding is employed for reducing the rotational speed of the impulse turbine to practical limits. We know that when high velocity of steam is allowed to low through one row of the moving blades, it produces a rotor speed of about 30,000 RPM which is too high for practical use. Not only this, the leaving velocity loss is very high.
In this type of turbine, the compounding is done for pressure of steam only I. E. To reduce the high rotational speed of the turbine the whole expansion of steam is arranged in a number of steps by employing a number of simple impulse turbine in a series on the same shaft. Each of the simple impulse turbine consist of one set (row) of nozzles and one row of moving blades; known as a stage of the turbine, and hush, this turbine consist so several stages. The exhaust from each row of moving blades enters the succeeding set of nozzles.
Figure 3. Diagrammatic arrangement tot Pressure Compounding Thus, we can say that this arrangement is nothing but splitting up of the whole pressure drop from the steam chest pressure to the condenser pressure into a series of smaller pressure drops across several stages of impulse turbine, and hence, this turbine is called pressure-compounded impulse turbine. The pressure and velocity variation in pressure compounded impulse turbine is shown in figure (Fig. 3. 1). The nozzles are fitted in the diaphragm which is locked in the casting. This diaphragm separates one wheel chamber from another.
All rotors are mounted on the same shaft and the blades are attached on the rotor. The expansion of steam only takes place in the nozzles while pressure remains constant in the moving blades because each stage is simple impulse turbine. It can be seen from the pressure curve that the space between any two consecutive diaphragms is filled with steam at constant pressure, the pressure on either side of diaphragm is different. Since the diaphragm is a stationary part, there must be learned between the rotating shaft and the diaphragm. The steam tends to leak through this clearance for which devices like labyrinth packing, etc. Re used.
Since drop in pressure of steam per stage is reduced, the steam velocity leaving the nozzles and entering the moving blades is reduced which in turn reduces the blade velocity. Hence for economy and maximum work shaft speed is significantly reduced to suit practical purpose. Thus, rotational speed may be reduced to suit practical purposes. Thus rotational speed may be reduced by increasing the number of stages according to one’s need. The leaving velocity of the last stage is much less compared to De-Level turbine and, the leaving loss is not more than 1 to 2 percent of the initial total available energy.
It consist of a nozzle or a set of nozzles and rows of moving blades attached to the rotor or wheel and, rows of fixed blades attached to the casting. The fixed blades are guide blades that guide the steam to the succeeding rows of moving blades suitably arranged between the moving blades but set in a reverse manner. In this turbine, three rows or rings of moving blades are fixed on a single wheel or rotor and this wheel is termed as the three row wheel. The arrangement insist of two rows of guide blades or fixed blades placed between the first and the second and the second and the third rows of moving blades.
The expansion of steam from the steam chest-pressure down to the exhaust pressure takes down in the nozzles only. There is no drop in pressure either in moving blades or the fixed blades I. E. The pressure remains constant in the blades as in the simple impulse turbine. The steam velocity from the exit of the nozzle is very high, similar to the simple impulse turbine. The steam with high velocity enters the first row of moving blades and, on passing through these blades, the velocity reduces lightly I. E. The steam gives up a part of kinetic energy and reissues from this row of blades.
It then enters the first row of guide blades which directs it to the second row of moving blades. A slight drop in velocity takes place in the fixed or guide blades due to friction. On passing through the second row of moving blades, there is a slight drop in velocity again I. E. , steam gives up some more of its kinetic energy to the rotor. After this, it is again directed by the second row of guide blades to the third row of moving blades, again drop in velocity occurs and finally steam leaves the wheel with a much educed velocity in a more or less axial direction.
Compared to the simple impulse turbine, the leaving velocity is small being about 2 percent of initial total available energy of steam. So we can say that this arrangement is nothing more than the splitting up of the velocity gained from the exit of one set of nozzles into many small drops through several rows of fixed and moving blades. This type of turbine is also termed as Curtis Turbine. Because of its low efficiency a three row wheel is used for driving small machines. It may be noted that a two row wheel is more efficient than the three-row- wheel. Figure 3.
Re-entry Velocity compounding Velocity compounding is also possible with only one row of moving blades as in the Re-entry Turbine (shown in fig). The whole pressure drop takes place in the nozzles and the high velocity steam passes through the moving blades into a reversing chamber where the direction of the steam is changed and, the same steam is directed to pass through the moving blades of the same rotor so that instead of using two or three rows of moving blades, only one row is required to pass the steam again and again and velocity as stated earlier.