Unit Five
Steam Turbines Prof. Shashank S. Bhamble Mechanical Engineering Department
Shri Sant Gajanan Maharaj College of Engineering, Shegaon
Energy Conversion-I
STEAM TURBINES A Steam turbine is a prime mover in which rotary motion is obtained by gradual change of momentum of the steam. Classification of steam turbines: The steam turbines are classified into different categories according to following attributes: a) Based on the blade flow passage: In steam turbine thermal energy available with steam is converted into kinetic energy which in turn produces driving thrust on the shaft. Based upon the rotor blades the blade flow passage may be of (i) Constant cross section area type from blade inlet to exit (ii) Varying cross section area type from blade inlet to exit. Turbines having former type blading are called impulse turbines while later type are in reaction turbines. Figure 14.4 shows the impulse and reaction turbine blades.
b) Based on the cylinder flow arrangement: Steam turbines may be classified based upon the flow arrangement into following types. (i) Single flow single casing turbine (ii) Double flow single casing turbine (iii) Cross flow compound turbine with single flow (iv) Cross flow compound turbine with double flow (v) Triple cross flow compound turbine with double flow
c) Based on direction of flow: Steam turbines can be classified based on the direction of flow by which steam flows through turbine blading. Steam turbines can be: (i) Radial flow turbine (ii) Tangential flow turbine (iii) Axial flow turbine In radial flow turbines the steam is injected in middle near shaft and steam flows radially outwards through the successive moving blades placed concentrically. In radial flow turbines there are no stationary blades so pressure drop occurs in moving blade passage. Concentric moving blades rings are designed to move in opposite directions. 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. Steam Turbines
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Energy Conversion-I This is repeated several times as steam follows the helical path. Tangential flow turbines are very robust but less efficient. In axial flow turbines steam flows along the axis of turbine over blades. These axial flow turbines are well suited for large turbo generators and very commonly used presently. d) Based on number of stages: Steam turbines can also be classified based upon the number of stages in steam turbines i.e. depending upon the amount of heat drop. It can be: (i) Single stage turbine (ii) Multi stage turbine. Single stage turbines have the expansion occurring in single stage while in multi stage turbines the expansion occurs in more than one stages of turbine. When expansion occurs in two stages it is called double stage turbine and with expansion occurring in three stages it is called triple stage turbine. e) Based on the application of turbine: Depending upon application the steam turbine can be classified as below: (i) Condensing turbine (ii) Non-condensing turbine (iii) Back pressure turbine (iv) Pass out turbine Condensing steam turbines are those in which steam leaving turbine enters into condenser. Such type of steam turbines permit for recirculation of condensate leaving condenser. Also the pressure at the end of expansion can be lowered much below atmospheric pressure as the expanded steam is rejected into condenser where vacuum can be maintained. 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 the 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. Pressure control valve and control gear is required so as to keep the speeds of turbine and pressure of steam constant irrespective of variations of power and heating loads. f) Based on speed of turbine: Steam turbines can be classified based upon the steam turbine as low speed, normal speed and high speed turbines as given below. (i) Low speed steam turbine. (ii) Normal speed steam turbine. (iii) High speed steam turbine. Low speed turbines are those steam turbines which run at speed below 3000 rpm. Normal speed steam turbines are those turbines which run at speed of about 3000 rpm while high speed steam turbines are the one which run at more than 3000 rpm. g) Based on pressure in steam turbines: Steam turbines can also be classified based upon the inlet pressure of steam turbine as follows: (i) Low pressure steam turbine (ii) Medium pressure steam turbine (iii) High pressure steam turbine (iv) Super pressure steam turbine Low pressure steam turbines have pressure of inlet steam less than 20 kg/cm2 while medium pressure steam turbines have steam inlet pressure between 20 kg/cm2 to 40 kg/cm2. High pressure steam turbines have steam inlet pressure lying between 40 kg/cm2 to 170 kg/cm2 while turbines having inlet steam pressure more than 170 kg/cm2 are called super pressure steam turbines.
Steam Turbines
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Energy Conversion-I
IMPULSE TURBINE Schematic of impulse steam turbine is shown in Fig. 1. 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 here. Here pressure drop occurs only in the nozzle and ideally no pressure drop occurs in blades.
Fig.1 Schematic of simple impulse turbine stage
High pressure steam from boiler enters the nozzle through pipings and leaves nozzle at predefined angle so as to smoothly flow over the moving blades. Steam velocity gets increased during its flow through nozzle due to its expansion occurring in it. During the passage of steam over the moving blades steam undergoes change in its’ direction while losing the velocity and thus causing rotation of moving blade ring mounted on shaft. 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. Speed of operation of turbine can be regulated by ‘compounding’ of impulse turbine discussed ahead. Compounding of steam turbine is required as in case of simple impulse turbine, the single stage may offer speed of the order of 30,000 rpm which cannot be directly used for any engineering application and needs to be reduced. Also such a high speed shall induce large stresses in the blades. Compounding is a thermodynamic means for reducing the speed of turbine where speed reduction is realized without employing a gear box. Compounding can be of following three types: (i) Pressure compounded impulse turbine (ii) Velocity compounded impulse turbine (iii) Pressure-velocity compounded impulse turbine Detailed discussion upon the above three types of compoundings is given below: Pressure compounded impulse turbine: Pressure compounded impulse turbine is also called as ‘Rateau’ turbine. Here pressure staging is done to utilize high velocity steam at acceptable shaft speed. In this the entire pressure drop is realized in parts instead of taking it in single stroke. This segmentation of pressure drop results in moderate steam velocities and thus yielding acceptable rotational speed. In case of pressure compounding there is a ring of fixed nozzles followed by ring of moving blades and subsequently there is again a ring of nozzles followed by a ring of moving blades. Thus pressure compounded impulse turbine consists of a series of simple impulse stages or De Laval turbine stages. Discharge from each moving blade row is supplied to
Steam Turbines
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Energy Conversion-I stationary nozzle ring of the subsequent stage. In pressure compounding high pressure steam enters the first ring of stationary nozzles where part expansion occurs causing drop in pressure and increase in steam velocity. Steam subsequently enters moving blade ring where no pressure drop occurs due to symmetrical blading but velocity drops. Steam leaving moving blade ring enters the stationary nozzle ring where remaining part of expansion occurs and expanded steam subsequently enters the moving blade ring. Pressure and velocity variation in a pressure compounded impulse turbine stage are shown in Fig. 2 along with the schematic of such compounding.
Fig.2 Pressure compounded impulse turbine stage
In pressure compounding as the pressure drop occurs in parts so the steam velocities are not very large and hence the turbine velocity gets reduced to low value. Turbine velocity may be further lowered if number of stages is increased. Therefore, pressure compounded impulse turbine has large number of stages which make it most expensive. This type of compounding is of most efficient type as in this ratio of blade velocity to steam velocity remains constant. Pressure compounding is more prone to leakage of steam from one section to other section at the shaft and outer casing as all pressure drop occurs in the nozzles. (ii) Velocity compounded impulse turbine: Velocity compounded impulse turbine is called ‘Curtis’ turbine. Here velocity staging is employed in order to utilize the high velocity steam jet with acceptable rotational speed. In velocity compounded impulse turbine instead of absorbing all kinetic energy in a single moving blade ring it is divided into two or more moving blade rings with guide blades in between the rows. Schematic of velocity compounded impulse turbine stage with pressure and velocity distribution is shown in Fig. 3. In velocity compounded impulse turbine the high velocity steam from boiler enters the first ring of stationary nozzles and undergoes the complete pressure drop as desired in a stage along with increase in velocity. Low pressure and high velocity steam leaving nozzle enters the moving blade ring where a part of velocity drop takes place while pressure drop does not occur due to symmetrical blade profile. Steam leaves moving blade ring and enters the fixed blades
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Energy Conversion-I which act as guide blades. Steam is smoothly guided by fixed blade ring and passes on to ring of moving blades in which remaining part of velocity drop takes place. Thus in a stage of velocity compounded impulse turbine there is stationary nozzle ring followed by moving blade ring and subsequently a fixed blade ring and moving blade ring. Here pressure drop occurs only in nozzle and the velocity drop occurs in two parts in two moving blade rings respectively. For the smooth and symmetrical impulse turbine blades used as fixed guide blades there is no drop in velocity of steam passing through fixed blade ring. Velocity compounded impulse turbine offers advantages such as less number of stages compared to pressure compounding and so less cost. It also requires less space and is relatively more reliable and easy to start. In multi stage velocity compounded impulse turbine the first stage has large pressure drop and remaining turbine stages are subjected to lower pressure range, thus lesser number of stages. In velocity compounded impulse turbine since pressure drop occurs in nozzle itself so the rest of turbine and its’ casing need not be manufactured very strong. But the efficiency is low due to large frictional losses due to large initial velocity and ‘non optimum value of ratio of blade velocity to steam velocity for all blade rings’. Efficiency of velocity compounded impulse turbine goes on decreasing with increase in number of stages.
Fig.3 Velocity compounded impulse turbine stage
(iii) Pressure-velocity compounded impulse turbine: Pressure-velocity compounded impulse turbine is a combination of the two types of compoundings described earlier. In this, steam coming from boiler enters the stationary nozzle ring followed by moving blade ring and subsequently fixed blade ring followed by moving blade ring. Steam leaving moving blade ring enters the stationary nozzle ring followed by moving blade, fixed blade and moving blade ring respectively. Schematic of pressure-velocity compounded impulse turbine stage is shown in Fig. 4 along with pressure and velocity variation across the different sections. Here both pressure drop and velocity drop are divided into different sections as shown in Fig. 4. Thus here one or more ‘Curtis stage’ (velocity compound) followed by ‘Rateau stage’ (pressure compound) are provided. Curtis stages reduce pressure to a moderate level with high
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Energy Conversion-I proportion of work per stage and then the highly efficient ‘Rateau stages’ absorb the remaining energy available. Here it is possible to reduce over all-length of turbine and thus there is saving in initial cost which more than offsets the lower efficiency.
Fig.4 Pressure-velocity compounded impulse turbine
REACTION TURBINES In a reaction turbine the pressure drop occurs in both stationary and moving rows contrary to the impulse turbine where the total pressure drop occurs in stationary nozzles alone. The difference in blading of reaction and impulse has already been described earlier. In reaction turbine the passage between two consecutive blades is of converging type as compared to impulse turbine blading which has constant cross-sectional area passage between two consecutive blades. Figure 5 shows the schematic of a reaction turbine stage having fixed blades followed by moving blades row. Due to the varying cross section area for steam flow the pressure drop occurs in both stationary (fixed) blades row and moving blades row. The velocity increases in stationary blades which act as nozzles. Thus the passage formed in the stationary blades in reaction turbine are of nozzle type although they do not have conventional nozzle shape. Steam stream leaving stationary blades impinges upon the moving blades. This impinging stream exerts a force to the right as evident from the velocity diagrams of reaction blading. Velocity diagram of reaction turbine is similar in principle to the velocity diagram in impulse turbine. Steam entering moving blades is subjected to pass through converging area passage along with change in direction. Thus there is increase in velocity (V2 > V1) from inlet to exit in moving blade which results in a reaction force. Change in direction of velocity is accompanied by change in momentum thus an impulse force. It shows that the rotation of shaft is caused by the combination of impulse and reaction forces. The magnitude of impulse force depends upon the pressure drop in fixed blades. It may be noted that due to shaft rotation being caused by combination of impulse and reaction forces these reaction turbines are also termed as impulsereaction turbine. These are also called full admission turbines as the steam enters through fixed blade row over complete annulus. The enthalpy drop over the reaction turbine stage shows that heat drop occurs in both fixed blades and moving blades rows. If the total enthalpy drop in stage is equally divided between the stationary and moving blades then the stage is called 50% reaction stage. A mathematical parameter called ‘degree of reaction’ is used to quantify the proportion of enthalpy drops occurring in stationary and moving blades. The ‘degree of
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Energy Conversion-I reaction’ is defined as the ratio of enthalpy drop in moving blades row (rotor blades) to the total enthalpy drop in the stage. Mathematically it can be given as,
Fig.5 Reaction turbine stage
Degree of reaction, R= Enthalpy drop in moving blades rotor blades Total enthalpy drop in the stage = h1 – h2 h0 - h2 Such turbines having 50% degree of reaction are called ‘Parson’s turbine’. Parson’s turbine has symmetrical blades for moving and stationary blades i.e. inlet angles of stationary and moving blades are equal and also the exit angles of stationary and moving blades are equal. Term symmetrical blading in reaction turbine refers to the 50 per cent reaction stage. LOSSES IN STEAM TURBINES Steam turbine being work producing device running at quite high speed has number of losses occurring in it. These losses when put together result into substantial loss of energy. Therefore, while selecting a turbine due attention should be paid to the losses in turbine. Some of the losses occur within turbine stages while some are external to stage. These losses are described ahead. 1. Losses in nozzles: Steam turbine nozzle is designed for isentropic expansion so as to result in increase in velocity from inlet to exit. Practically in a nozzle the steam leaving nozzle may not have velocity equal to the designed velocity value. This deviation in operating state of nozzle may occur because of non-isentropic expansion. The reasons for non-isentropic expansion may be friction losses between the steam and nozzle wall, viscous friction resistance to flow in the steam particles, boundary layer formation and separation, heat loss during flow etc.
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Energy Conversion-I Mathematically, this shift from isentropic expansion to non-isentropic expansion is quantified using the parameter called ‘nozzle efficiency’. Nozzle efficiency as described earlier is defined by the ratio of ‘actual enthalpy drop’ to the ‘isentropic enthalpy drop’ between inlet and exit of nozzle. 2. Losses in moving blades: In steam turbine stage steam is supposed to glide smoothly over the moving blades after leaving nozzles or fixed blades. In actual turbine stage during flow of steam over moving blade, there may be number of factors causing loss of energy as given under: (i) Blade friction may incur frictional loss which is taken into account by the blade friction factor. This friction factor largely depends upon the Reynolds number, although it is earlier defined as the ratio of relative velocity leaving blades to the relative velocity of steam entering blades. This loss may be termed as “passage loss”. (ii) “Boundary layer separation” may occur due to sharp deflection of fluid within the blade passage. Deflection results in centrifugal force which causes compression near concave surface and the rarefaction near the convex surface of blade, thus resulting in separation of boundary layer. (iii) Loss of energy may be due to turbulence at outlet of preceding row of nozzles due to finite thickness of nozzle exit edge. There is mixing of steam jet leaving nozzles and entering moving blade. Due to this transition of flow from nozzle passage to blade passage there is formation of eddies and turbulence gets set in. This turbulence is generally in the form of trailing vortices which keep on disappearing at high velocities. These cause the reduction of kinetic energy delivered to blades and are called “wake losses”. Wake losses are visible at the trailing edge of fixed blades too due to thickness of trailing edge. (iv) Loss of energy is also there due to breakage of flow which occurs upon the impingement of steam upon the leading edge of moving blade. This is also termed as ‘impingement loss’. These losses are less if the flow is laminar as compared to the turbulent flow. (v) Loss of energy also occurs during passage of steam from one stage to other i.e. rows. This loss is also termed as ‘carry over loss’. This carry over loss is minimum if spacing between consecutive rows is kept small. The different losses as described above are accounted by taking the profile loss coefficient (kp), incidence loss coefficient (ki) and carry over loss coefficient (kc). Profile loss coefficient takes into account the losses due to turbulence, friction, fluid deflection in blade passage, curvature of blade and actual exit angle being different from blade exit angle. Incidence loss coefficient takes care of losses due to turbulence introduced by angle of incidence. The carry over loss coefficient takes care of losses due to kinetic energy loss during transition of flow between the rows. Therefore, the actual relative velocity leaving blade shall be, V2 = kp.ki.kc.V1 3. Disc friction loss: This is a kind of loss of energy visible whenever any object say disc is rotated in air or other medium. The disc would cut the atmosphere and impart motion to surrounding air. There shall always exist relative motion between solid wall of object and the air or surrounding fluid. Due to this relative motion surrounding medium always exerts a resistance to motion of moving object. This may result in loss of energy due to friction which may be felt by the increase in enthalpy of surrounding fluid. In case of steam turbines too the rotor is completely surrounded by the steam which offers resistance to the rotor motion. The loss of energy of rotor may go into the steam enveloping it. This loss of energy is termed as ‘disc friction loss’. The disc friction loss may cause heating of steam surrounding the rotor i.e. a portion of kinetic energy is transferred from the rotor disc to steam by heating of steam. Disc friction loss is substantial in case of impulse stages as compared to reaction stages where it is very small and can be neglected. 4. Windage loss: Windage loss occurs when the rotor blades come in contact with near stationary fluid (steam). In case of partial admission turbines i.e. generally impulse turbines there is churning of steam in the region having no active steam in steam turbine. When moving rotor blades come in contact with inactive steam then there is transfer of energy from blade to steam. This loss of energy from rotor to fluid is termed as ‘windage loss’. In case of full
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Energy Conversion-I admission turbine the region inside turbine having inactive steam is negligible and so the windage loss is nearly negligible. To minimize windage loss the turbine should be filled with moving steam (active steam). Windage losses are very small in case of low pressure stages. Reaction turbines have negligible windage losses as they are full admission turbines. 5. Loss due to leakage: Steam leakage may occur across the turbine shaft and between stages. Leakage of steam may result in availability of less work from stage as steam is not fully utilized for producing work. Leakage occurs during flow from one stage to other or from one row to other through the clearance space between diaphragm and shaft. Leakage also occurs across the blade tip. Leakage across diaphragm occurs in both impulse turbine and reaction turbine stages. Leakage across tip is not prominent in case of impulse turbine as the pressure difference is very small. Tip leakage is prominent in reaction turbine stages. Due to this diaphragm and tip leakage effective mass flow rate for doing work gets reduced and is consequently a loss of energy. Leakage is accompanied by the increase in entropy and so the decrease in availability of work due to throttling of steam which is an irreversible process. Leakage loss can be minimized by reducing the clearances as much as possible after providing for expansion of turbine parts so that the metal-to-metal rubbing is avoided. Different seals such as labyrinths, carbon rings, water, steam or air seals are used to prevent this leakage through clearance. Also in order to reduce leakage loss the drum type construction is preferred to diaphragm and wheel type construction in reaction turbines. Another type of leakage may be of balance-piston leakage which refers to leakage between balance piston and casing. Here fluid leaks out in high pressure region of turbine and atmospheric air bleeds into casing in low pressure sub atmospheric region (condensor side). Generally this is not a total loss as the leakage out and leakage in are not varying too much. This kind of loss can also be prevented by employing labyrinth packing. At low pressure/subsonic region (condenser side) of turbine the labyrinth packing is fed with low pressure steam so that steam leaks in instead of air in case of unavoidable leakage. 6. Losses in bearings: Turbine bearings are critical parts to support high speed rotation of shaft. Generally, a loss to the tune of 1% of turbine output occurs in bearings. Although this loss depends upon bearing load, oil viscosity, speed of shaft, bearing surface area and film thickness etc. 7. Losses at inlet and exit: Loss at inlet of steam turbine occurs at regulating valves at entry. At these valves which may be stop valve or governor valves the throttling loss generally occurs causing lowering of entering steam pressure. At the exit of steam turbine the steam becomes wet and the fluid now is mixture of water droplet and steam. Due to wetness of steam at exit end the water particles being heavier cause loss of kinetic energy. These water particles, if in excessive amount may also endanger the turbine blade. The kinetic energy of fluid at exit of steam turbine is a total loss and theoretically the fluid leaving turbine must have the lowest possible energy in it. This loss of energy may be of the order of 10 – 13 per cent. 8. Losses due to radiations: Radiation losses also occur in steam turbines, although they are very small compared to other losses and may be neglected. In case of steam turbines the high temperature steam is limited to small part of casing so losses are small. But the radiation losses are quite significant in gas turbines. In order to prevent radiation losses the pipings, turbine casing etc. carrying hot fluid should be well insulated. GOVERNING OF STEAM TURBINES Governing of steam turbine is required for controlling and regulating the output of a turbine according to variable demands in service. This regulation of turbine is done by maintaining accurate and positive control of speed, pressure and flow by employing governors. Governing of turbine maintains the speed of turbine at constant level irrespective of load. Governing of steam turbines can be of following types depending upon the mechanism of regulation. (i) Throttle governing Steam Turbines
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Energy Conversion-I (ii) Nozzle control governing (iii) By-pass governing (iv) Combined governing i.e., Combination of above as ‘throttle and nozzle control governing’ or ‘throttle and by-pass governing’. (i) Throttle Governing: Throttle governing of steam turbine bases upon the throttling of steam up to suitable pressure and regulate the mass flow of steam through the turbine so as to control the output of turbine. Here in throttle governing, aim is to alter the mass flow rate as for example by reducing it, the available energy gets reduced and hence lowering of output.
Fig.6 Throttle governing on h-s diagram
In throttle governing the steam entering is regulated by opening and closing of valve. As the valve is closed, the throttling or constant enthalpy process occurs across the valve with an increase in entropy and corresponding decrease in availability of energy per unit mass flow of steam. Also due to throttling the state of steam entering turbine stage gets modified and the modified expansion line for each load is obtained. It may be noted that even when the governor valve is full open the pressure drop does occur and thus it can be said that throttling is evident at all loads on turbine. Representation of throttle governing on h-s diagram shows that the steam is available at state ‘0’ at p0 pressure in the main steam line. At the inlet from main steam line when the governor valve is full open the throttling results in modified state 1 from where expansion occurs following path 1–3 under insentropic expansion and non-isentropic expansion occurs following path 1–3_. When mass flow rate is reduced for reduced load on turbine then this partial closing of valve causes throttling as shown by 1–2. Now as a result of this throttling from 1 to 2 the modified expansion paths are 2–4 and 2–4_ for isentropic and non-isentropic expansion in turbine. Thus it shows that as a result of throttling the available energy gets reduced from _h1-3 to _h2-4 and _h2-4 < _h1-3. Hence this lowering of available energy causes reduced output from turbine. Schematic of simple throttle governing in steam turbines is shown in Fig. 7. Here a centrifugal governor is used to sense the change in speed of shaft. The relay system has a pilot valve and servomotor. The displacement of servomotor piston either upward or downward decides the opening of throttle valve ‘C’. Servomotor piston is actuated by the high pressure oil entering from pilot valve to upper or lower half of servomotor piston D. Under normal operation the servomotor piston occupies middle position and pilot valves keep the inlet and exit ports in closed position. When oil enters the upper half of servomotor then servomotor piston lowers down and the throttle valve starts closing causing reduction of steam flow rate and so the output till the speed is maintained to normal running speed. Simultaneously, the oil from lower half of servomotor gets drained out through pilot valve port. When oil under pressure enters lower half of servomotor then servomotor piston gets lifted up causing lift of throttle valve.
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Energy Conversion-I
Fig.7 Schematic of simple throttle governing
For throttle governing of steam turbine the steam consumption rate may be plotted with load resulting into characteristic line called Willans line as shown in Fig. 8.
Fig.8 characteristics of throttle governing
During throttle governing the Willan’s line is straight line making an intercept on y-axis. Mathematically, it can be given as, M = K·L + M0 Where, M is steam consumption in kg/h at any load L. M0 is steam consumption in kg/h at no load i.e. L = 0 L is any load on turbine in kW. K is the constant and gives slope of Willan’s line. Here it shows that even at no load the steam consumption shall be M0 (kg/h) which is graphically given by intercept on y-axis. Above equation can also be written as, M/L= K +M0L Where, M/L is the specific steam consumption at any load, kg/kWh and M0/L is the specific steam consumption at no load, kg/ kWh. Throttle governing offers following disadvantages due to throttle action at inlet: (a) Throttling increases initial superheat at inlet and the greatest variations in steam velocity occur in the later stages.
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Energy Conversion-I (b) The wetness of steam gets reduced in later stages due to throttling. Due to this reduced wetness there occurs reduction in stage efficiency at part load operation of turbine. (ii) Nozzle Control Governing: Nozzle control governing is the one in which steam flowing through nozzles is regulated by valves. Nozzle control governing is generally employed at first stage of turbine due to practical limitations. The nozzle areas in remaining stages remain constant. If some how the nozzle governing is provided for all nozzles in each and every stage then an ideal condition of turbine flow passage areas conforming to mass flow rate at all loads shall exist. Under such ideal conditions the pressure, velocities and nozzle and blade efficiencies would be constant with load. For such ideal condition the Willan’s line would be straight line as indicated for throttle governing of turbine. In nozzle governing the nozzles of turbines are grouped in two, three or more groups upto six or eight groups. When nozzle governing is employed then the pressure and temperature of steam entering first stage nozzles are independent of load. Figure 9 shows the schematic of nozzle control governing.
Fig.9 Schematic of Nozzle control governing
As the valves are being regulated for actuating nozzle control governing so there occurs some throttling of steam at each valve. However, the amount of throttling is considerably lesser and the decrease in availability of energy to turbine is not too much. In order to avoid this occurrence of throttling very large number (infinite) of nozzle and governing valves may be put. (iii) By Pass Governing: In case of by-pass governing arrangement is made for by-passing surplus quantity of steam without allowing total steam quantity to contribute in turbine output when load reduces. Arrangement of by pass governing is shown in Fig. 10. Diagram shows that steam from main line enters the main valve which is controlled by speed governor. Steam from main valve enters the nozzle box or steam chest. By pass valve is also provided on the nozzle box. By pass valve is connected to a passage which delivers steam being by passed to later end of turbine. By pass valve is actuated when load varies, thus allowing only part of steam entering main valve to contribute in power output. By pass valve is controlled by speed governor for all loads within its’ range. In this kind of governing depending upon turbine and its’ application there may be more than one by pass valves. (iv) Combined governing: Some times when the governing requirements are not met by any one kind of arrangements of governing i.e. throttle, nozzle control and by pass governing, then the combination of two governing mechanisms may be employed. These popular combinations are ‘throttle and nozzle control combined governing’ and ‘throttle and by pass combined governing’. Steam Turbines
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Energy Conversion-I
Fig.10 Arrangement in by pass governing
DIFFERENCE BETWEEN IMPULSE AND REACTION TURBINES Impulse turbines
Reaction turbines
a) Impulse turbine has profile type blades
a) Reaction turbine has airfoil type blades
and has constant area between two consecutive blades. b) Impulse turbine stage has pressure drop occurring only in nozzles. No pressure drop occurs in moving blades row. c) Impulse turbines have incomplete admission of steam or steam being admitted at selected positions around the motor d) Impulse turbine diaphragm has nozzles mounted on it. Velocity of steam is quite large.
and has converging area between two consecutive blades. b) Reaction turbine stage has pressure drop occurring in both fixed as well as moving blades.
e) Impulse turbine has lesser efficiency
e)
and much power cannot be developed from it. f) These occupy less space for same power output. g) These are not very costly as the manufacturing of impulse turbine blades is much simpler h) Impulse turbines are suitable for small power requirement.
c) Reaction turbines have complete
d)
f) g)
h)
admission of steam or steam being admitted all round the rotor through fixed blade ring. Reaction turbine has fixed blade ring attached to casing to serve as nozzle and guide blades for entering steam. Velocity of steam is comparatively smaller. Reaction turbine has higher efficiency and is capable of producing large power output. These occupy large space for same power output. Reaction turbines are costly as their blades are very difficult to be manufactured. Reaction turbines are suited for medium and higher power requirements
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