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Detail study Of Steam Turbine

- 1. Prepared By : Ravi Chhugani (121040119005) Submit to : Assistant Prof. Piyush Mistri.. (G.P.E.R.I. Mehsana)
- 3. Introduction • A Turbine is a device which converts the heat energy of steam into the kinetic energy & then to rotational energy. • The Power in a steam turbine is obtained by the rate of change in momentum of a high velocity jet of steam impinging on a curved blade which is free to rotate. • The basic cycle for the steam turbine power plant is the Rankine cycle. The modern Power plant uses the rankine cycle modified to include superheating, regenerative feed water heating & reheating.
- 4. STEAM TURBINE is a prime mover in which pressure energy of steam is converted into mechanical energy & further electrical energy.
- 5. A steam turbine is a prime mover in which potential energy is converted into kinetic energy and then to Mechanical energy. Potential Energy Kinetic energy Mechanical Energy
- 6. Principle of operation • The power in a steam turbine is obtained by the rate of change in momentum of a high velocity jet of steam impinging on a curved blade which is free to rotate. • The steam from the boiler is expanded in a nozzle, resulting in the emission of a high velocity jet. This jet of steam impinges on the moving vanes or blades, mounted on a shaft. Here it undergoes a change of direction of motion which gives rise to a change in momentum and therefore a force. • 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.
- 7. The steam energy is converted mechanical work by expansion through the turbine. Expansion takes place through a series of fixed blades(nozzles) and moving blades. In each row fixed blade and moving blade are called stage.
- 8. Classification of steam turbine a) According to the method of steam expansion in the turbine. Impulse turbine Reaction Turbine Combination of impulse and reaction turbine. b) According to the direction of flow of steam in the turbine Axial flow turbine Radial flow Turbine Mixed flow Turbine
- 9. c) According to the final delivery pressure (or) Exhaust condition of steam turbine. Condensing Turbine Back Pressure Turbine. Extraction Turbine. d) According to the number of stages of turbines. Single stage turbine. Multi stage turbine. e) According to the pressure of steam turbines. Low pressure turbine. Medium pressure turbine. High pressure turbine.
- 10. . Compounding in Steam Turbine The compounding is the way of reducing the wheel or rotor speed of the turbine to optimum value. Different methods of compounding are: 1.Velocity Compounding 2.Pressure Compounding 3.Pressure Velocity Compounding. In a Reaction turbine compounding can be achieved only by Pressure compounding.
- 11. Velocity Compounding : There are number of moving blades separated by rings of fixed blades as shown in the figure. All the moving blades are keyed on a common shaft. When the steam passed through the nozzles where it is expanded to condenser pressure. It's Velocity becomes very high. This high velocity steam then passes through a series of moving and fixed blades. When the steam passes over the moving blades it's velocity decreases. The function of the fixed blades is to re-direct the steam flow without altering it's velocity to the following next row moving blades where a work is done on them and steam leaves the turbine with allow velocity as shown in diagram.
- 13. Pressure Compounding : These are the rings of moving blades which are keyed on a same shaft in series, are separated by the rings of fixed nozzles. The steam at boiler pressure enters the first set of nozzles and expanded partially. The kinetic energy of the steam thus obtained is absorbed by moving blades. The steam is then expanded partially in second set of nozzles where it's pressure again falls and the velocity increase the kinetic energy so obtained is absorbed by second ring of moving blades.
- 15. Pressure velocity compounding : This method of compounding is the combination of two previously discussed methods. The total drop in steam pressure is divided into stages and the velocity obtained in each stage is also compounded. The rings of nozzles are fixed at the beginning of each stage and pressure remains constant during each stage as shown in figure. The turbine employing this method of compounding may be said to combine many of the advantages of both pressure and velocity staging By allowing a bigger pressure drop in each stage, less number stages are necessary and hence a shorter turbine will be obtained for a given pressure drop.
- 17. Impulse turbine : In the impulse turbine, the steam expands in the nozzles and it's pressure does not alter as it moves over the blades. In the impulse turbine, the steam expanded within the nozzle and there is no any change in the steam pressure as it passes over the blades.
- 18. In this type, the drop in pressure takes place in fixed nozzles as well as moving blades. The pressure drops suffered by steam while passing through the moving blades causes a further generation of kinetic energy within these blades, giving rise to reaction and add to the propelling force, which is applied through the rotor to the turbine shaft. The blade passage cross-sectional area is varied (converging type).
- 19. Velocity diagram : •V1 and V2 are the absolute velocities at the inlet and outlet respectively. •Vf1 and Vf2 are the flow velocities at the inlet and outlet respectively. •Vw1+U and Vw2 are the whirl velocities at the inlet and outlet respectively. • Vr1 and Vr2 are the relative velocities at the inlet and outlet respectively. •U1 and U2 are the velocities of the blade at the inlet and outlet respectively. •Alpha is the guide vane angle and Beta is the blade angle.
- 21. Then by the law of moment of momentum, the torque on the fluid is given by : For an impulse steam turbine : Therefore, the tangential force on the blades is : The work done per unit time or power developed : When ω is the angular velocity of the turbine, then the blade speed is : The power developed is then :
- 22. Blade efficiency : Blade efficiency can be defined as the ratio of the work done on the blades to kinetic energy supplied to the fluid, and is given by : Stage efficiency : A stage of an impulse turbine consists of a nozzle set and a moving wheel. The stage efficiency defines a relationship between enthalpy drop in the nozzle and work done in the stage. Where, is the specific enthalpy drop of steam in the nozzle.
- 23. By the first law of thermodynamic : Assuming that V1 is appreciably less than V2, we get ≈ Furthermore, stage efficiency is the product of blade efficiency and nozzle efficiency, or Nozzle efficiency is given by = where the enthalpy (in J/Kg) of steam at the entrance of the nozzle is and the enthalpy of steam at the exit of the nozzle is . .
- 24. The ratio of the cosines of the blade angles at the outlet and inlet can be taken and denoted The ratio of steam velocities relative to the rotor speed at the outlet to the inlet of the blade is defined by the friction coefficient K<1 and depicts the loss in the relative velocity due to friction as the steam flows around the blades ( K=1for smooth blades) The ratio of the blade speed to the absolute steam velocity at the inlet is termed the blade speed ratio = is maximum when or That implies and therefore Now (for a single stage impulse turbine)
- 25. Therefore, the maximum value of stage efficiency is obtained by putting the value of in the expression of / We get: . For equiangular blades, therefore C=1 , and we get If the friction due to the blade surface is neglected then 1. For a given steam velocity work done per kg of steam would be maximum when or . 2. As increases, the work done on the blades reduces, but at the same time surface area of the blade reduces, therefore there are less frictional losses.
- 26. Reaction turbine : In the reaction turbine the steam expanded continuously as it passes over the blades and thus there is gradually fall in the pressure during expansion below the atmospheric pressure.
- 27. In the reaction turbine high pressure steam from the boiler, passes through the nozzle. When the steam comes out through these nozzles the velocity of steam increases & then strike on fixed blade. In this type of turbine there is a gradually pressure drop takes place over fixed & moving blade.
- 29. Difference between impulse and reaction turbine :
- 31. Degree of reaction : Degree of reaction or reaction ratio (R) is defined as the ratio of static pressure drop in the rotor to the static pressure drop in the stage or as the ratio of static enthalpy drop in the rotor to the static enthalpy drop in the stage. It is given as;
- 32. Parson’s turbine : Parson’s turbine is a particular case of reaction turbine in which the degree of reaction is half. The section of blades of this turbine is the same in both fixed and moving rows of blades. In the parson’s turbine, the blade section and the mean diameter of fixed as well as the moving blades are the same. The blade height is progressively so increased such that the velocity of steam at exit from each row of blades is uniform throughout the stage.
- 33. Thus, the velocity triangle at the inlet and outlet of moving blades will be similar. The parson’s turbine is designed for 50% reaction, then the equation of degree of reaction can be written as R=1/2=Vf(cot -cot )/2Vb therefore, Vb=Vf(cot -- cot ) Also , Vb can be written as Vb=Vf(cot -cot α) Vb=Vf(cot α -cot β) Comparing the equations, α= and = β
- 34. Velocity diagram for Parson’s reaction turbine :
- 35. Condition for maximum efficiency of parson’s reaction turbine : Work done per kg of steam is given by W=Vb(Vw1+Vw2)=Vb[V1cosα+(Vr2cos -Vb) For parson’s turbine , =α and Vr2=V1 Therefore, W=Vb[2V1 cosα- Vb] =V1 2[2VbV1 cosα/v1 2-Vb 2/V1 2] = V1 2 [2 cosα-2] Where, =Vb/V1
- 36. The kinetic energy supplied to the fixed blade=V1 2/2g The kinetic energy supplied to the moving blade= Vr2 2-Vr1 2/2 Therefore, Total energy supplied to the stage, h=(V1 2/2)+(Vr2 2-Vr1 2/2) For symmetrical tri-angels Vr2=V1 h= (V1 2/2)+(V1 2- Vr1 2/2 )= V1 2- Vr1 2/2
- 37. Considering the ABD from velocity diagram : Vr1 2= V1 2+ Vb 2-2VbV1 cosα h= V1 2-(V1 2+ Vb 2-2VbV1 cosα)/2 =(V1 2 +2VbV1 cosα-Vb 2)/2 = V1 2 /2[1+2Vbcosα/V1-(Vb/V1) 2 ] h=V1 2 /2[1+2 cosα- 2 ]
- 38. The blade efficiency of the reaction turbine is given by : ƞ=Work done(W)/Total energy supplied( h) =V1 2(2 cosα- 2 )/V1 2/2(1+2 cosα- 2) =2(2 cosα- 2)/(1+2 cosα- 2) =2 (2cosα- )/(1+2 cosα- 2) =2(1+2 cosα- 2)-2/(1+2 cosα- 2) =2-2/ 1+2 cosα- 2
- 39. The ƞb becomes maximum when factor 1+2 cosα- 2 becomes maximum : Therefore ,the required condition is d/d (1+2 cosα- 2 )=0 therefore,2cosα-2 = 0 =cosα i.e. The condition for maximum efficiency, =Vb/V1 =cosα
- 40. Substituting the value of in the equation, we get (ƞb)max=2-2/1+2cosαcosα-cos2α therefore, Maximum efficiency= (ƞb)max=2cos2α/1+cos2α
- 41. Reheat factor : The Thermodynamic effect on the turbine efficiency can be best understood by considering a number of stages between two stages 1 and 2 as shown in Figure The total expansion is divided into four stages of the same efficiency and pressure ratio. 22 1 P P P P P P P P zy y x x
- 43. The overall efficiency ( )of expansion is . The actual work during the expansion from 1 to 2 is Reheat factor (R.F.)= ( R.F is 1.03 to 1.04 ) WWa o )'21( )21( dropheatisentropic dropheatactual W Wa o )( )( overalldropheatIsentropic isentropicdropheatCumulative 12 1 _ . h hhhh FR zDycxBA
- 44. If remains same for all the stages or is the mean stage efficiency. We can say; s s zD z yc yz xB xy A x s h h h h h h h h 2 1 1 zDycxBA zyzxyx s hhhh hhhh 1 21 )(isentropicdropheatCumulative dropheatactual FRs .0
- 45. This makes the overall efficiency of the turbine greater than the individual stage efficiency. The effect depicted by eq (B) is due to the thermodynamic effect called “reheat”. This does not imply any heat transfer to the stages from outside. It is merely the reappearance of stage losses an increased enthalpy during the constant pressure heating (or reheating) processes AX, BY, CZ and D2.
- 46. Thank you..