STEAM NOZZLE AND STEAM TURBINE

MSBTE DIPLOMAIN MECHANICAL ENGINEERING
SECOND YEAR MECHANICAL ENGINEERING

1. Unit 4 steam nozzle and steam turbine Prof. Dhammadip A. Kamble (M.E. (Mech. Design Engg.))

2. Steam Nozzle A steam nozzle may be defined as a passage of varying cross-section, through which heat energy of steam is converted to kinetic energy. Steam Nozzle major function is to produce steam jet with high velocity to drive steam turbines. Steam Turbine A steam turbine converts the energy of high-pressure, high-temperature steam produced by a steam generator into shaft work.

3. Steam Nozzle Types of Nozzles 1. Convergent Type- T he cross-sectional area decreases continuously from its entrance to exit . 2. Divergent nozzle: The cross sectional area of divergent nozzle increases continuously from its entrance to exit.  

4. Steam Nozzle 3. Convergent – Divergent nozzle: In this condition, the cross sectional area first decreases from its entrance to the throat and then again increases from throat to the exit. I n present day application, it is widely used in many types of steam turbines.

5. Mach Number Mach number can be expressed as M = v / c                                  where M = Mach number v = fluid flow speed ( m/s) c =  speed of sound (m/s)

6. Significance of Mach Number If the Mach number is = 1 , the flow speed is equal to the speed of sound - and the speed is  sonic . ( At throat of nozzle) If the Mach number is  < 1 , the flow speed is lower than the speed of sound - and the speed is  subsonic . ( T hen the nozzle is Convergent) If the Mach number is  > 1 , the flow speed is higher than the speed of sound - and the speed is  supersonic . ( Then the nozzle is D ivergent) If the Mach number is  >> 1 , the flow speed is much higher than the speed of sound - and the speed is  hypersonic .

7. critical pressure The maximum gas flow through a nozzle is determined by critical pressure . The pressure at which maximum discharge through nozzle is occurs is called critical pressure. p c  / p 1  = ( 2 / (n + 1) ) n / (n - 1) where p c  = critical pressure ( Pa) p 1  = inlet pressure (Pa) n = index of isentropic expansion or compression - or polytropic constant

8. Application of steam nozzle 1. Steam and Gas turbine 2. Jet Engine  3. Rocket Motors- 4. It is used to measure the discharge of fluid.- e.g. Venturimeter  5. Injectors for pumping feed water to boilers. 6. The supersonic gas turbine engine: for the air intake when the air requirement of the engine is high. 7. Rockets : for providing sufficient thrust to move upwards. 8. For removing air from the condenser using the injector. 9. Spray painting 10. Steam jet refrigeration system

9. Classification of steam turbine 1. According to working principle Impulse Turbine Reaction Turbine Impulse-Reaction Turbine 2 . According to stages of expansion of steam Single stage Turbine Multi stage Turbine 3. According to the position of shaft Horizontal Turbine Vertical Turbine

10. Classification of steam turbine 4 . According to direction of steam flow Axial Flow Turbine Radial Flow Turbine Tangential Flow Turbine 5 . According to exhaust steam pressure Condensing Type Steam Turbine Non-Condensing Type Steam Turbine

11. Impulse turbine

12. Working of Impulse turbine There is only one set of nozzles . The complete expansion of steam from the boiler steam pressure to the exhaust or condenser pressure takes place in the one set of nozzles. Thus the pressure in the moving blades chamber is approxi­mately equal to condenser pressure . The steam enters the moving blades chamber with a high velocity. The pressure in the set of moving blades remains constant. The ve­locity of steam is reduced as it passes over moving blades as some of the kinetic energy of the steam is used in producing work on the tur­bine shaft . For simple, impulse turbine, rotational speeds of the magnitudes of 20000 rpm may be obtained. This high speed of rotation will pro­duce high centrifugal force which will restrict the size of the turbine. Thus the De-Laval type turbine is of relatively small size and hence has a small power-output.

13. Working of reaction turbine A reaction turbine is a type of steam turbine that works on the principle that the rotor spins, as the name suggests, from a reaction force rather than an impact or impulse force . In a reaction turbine there are no nozzles to direct the steam like in the impulse turbine. Instead, the blades that project radially from the outer edge of the rotor are shaped and mounted so that the shape between the blades, created by the cross-section, create the shape of a nozzle.  These blades are mounted on the revolving part of the rotor and are called the moving blades . The fixed blades, which are the same shape as the moving blades, are mounted to the outer casing where the rotor revolves and are set to guide the steam into the moving blades . (1)  The steam enters through a section of curved blades in a fixed position. (2)  The steam then enters the set of moving blades and creates enough reactive force to rotate them, (3)  The steam exits the section of rotating blades. (4)  The direction of rotation.

14. Working of reaction turbine

15. Difference between Impulse Turbine and Reaction Turbine

16. Difference between Impulse Turbine and Reaction Turbine

17. Degree of reaction Degree of reaction is defined as the ratio of static pressure drop in the rotor to the static pressure drop in the stage. OR It is the ratio of static enthalpy drop in the rotor to the static enthalpy drop in the stage. static enthalpy drop in the rotor static enthalpy drop in the stage

18. Losses in Steam Turbines Residual Velocity Loss Presence of Friction Steam Leakage Loss Due to Mechanical Friction in Bearings Pressure Losses in Regulating Valves and Steam Lines Losses Due to Low Quality of Steam Radiation Loss

19. Compounding Compounding of steam turbine is used to reduce the rotor speed . It is the process by which rotor speed come to its desired value. A multiple system of rotors are connected in series keyed to a common shaft and the steam pressure or velocity is absorbed in stages as it flows over the blades. The steam produced in the  boiler  has sufficiently high enthalpy when  superheated. In all turbines the blade velocity is directly proportional to the velocity of the steam passing over the blade. Now , if the entire energy of the steam is extracted in one stage, i.e. if the steam is expanded from the boiler pressure to the condenser pressure in a single stage, then its velocity will be very high. Hence the velocity of the rotor (to which the blades are keyed) can reach to about 30,000 rpm , which is too high for practical uses because of very high vibration. Moreover , at such high speeds the centrifugal forces are immense, which can damage the structure. Hence, compounding is needed. The high velocity steam just strikes on a single ring of rotor that causes wastage of steam ranging 10% to 12%. To overcome the wastage of steam compounding of steam turbine is used.

20. Velocity Compounding

21. Velocity Compounding Moving blades are fixed to the shaft and fixed blades are attached to the casing . T wo or three rows of moving blades which are separated by fixed blades and these moving blades are just in reverse position from the fixed blades . Now steam pass through the nozzle or a set of nozzles where it is expanded from boiler pressure to the condenser pressure . Due to decreasing the steam pressure, its velocity becomes very high. This high-velocity steam first enters the first ring of moving blades where some portion of velocity is absorbed. Then it passes through the next ring of fixed blades . The fixed blades changed steam direction and direct to the second ring of moving blades. There is no change in steam velocity when it passes over the fixed blades . Now steam passes through the second row of moving blades and its velocity is further reduced. Steam loose its velocity every time when it passes through the moving blades. So, steam leaves the turbine with a low velocity. So, we can see that the steam's pressure can only drop at nozzle and further pressure drop occurs either in the moving or fixed blades. Velocity and pressure curves on a base represent the axis of the turbine. 

22. Pressure Compounding

23. Pressure Compounding It consists of a number of fixed nozzles which are placed between the moving blades which means the ring of moving blades are separated by the ring of fixed nozzles. The moving blades are keyed on a same shaft in series . Now the steam of boiler pressure enters the first set of nozzles but does not drop the total pressure in one set of nozzles. The total steam pressure is dropped by number of stages and each stage consists of a set of nozzles and a ring of moving blades . The steam is coming from the boiler and enters the first set of nozzles. A small amount of pressure is drop here by which it increases its velocity. At first steam passes through the first set of moving blades where its pressure does not change but its velocity decrease . After that it passes to the second set of fixed nozzles, its pressure is further reduced and expanded again. Now steam is directed to the second set of moving blades where the velocity of steam is almost absorbed. This process is continuing running until it reaches condenser pressure. Since steam pressure is reduced by each set of nozzle, so velocity of steam entering the moving blades is almost reduced and it reduces the rotor speed. Velocity and pressure curves on a base represent the axis of the turbine.

24. Pressure-velocity Compounding

25. Pressure-velocity Compounding This method is the combination of previous two methods. This method is the combination of both pressure and velocity compounding. Here the set of nozzle rings is fixed at the beginning of each stage and pressure remaining constant of each stage. Diameter is comparatively large in stage for increasing the volume of steam at lower pressure . The total pressure drop of the steam is divided into stages and velocity obtained in each stage is also compounded. A pressure velocity compounded turbine allows a bigger pressure drop in each stage. That's why pressure velocity compounded method need less stage as compared to the other method.

26. Regenerative feed heating or bleeding of steam

27. Governing of steam turbine Steam turbine governing system  is a method, used to maintain a constant steady speed of turbine. The importance of this method is, the turbine can maintain a constant steady speed irrespective of variation of its load. A turbine governor is provided for this arrangement. The purpose of the governor is to supply steam into the turbine in such a way that the turbine gives a constant speed as far as possible under varying the load. So , basically Steam turbine governing system is a process where turbine maintains a steady output speed irrespective of variation of load.

28. Throttle Governing

29. Throttle Governing Throttle Governing of steam Turbine is most popular and easiest way to control the turbine speed. When steam turbine controls its output speed by varying the quantity of steam entering the turbine is called Throttle Governing. It is also known as Servomotor methods . Assume that the turbine's load increases. It will decrease its speed which will decrease the centrifugal force of the turbine. Now fly balls of the governor will come down thus decreasing their amplitude. These fly balls also bring down the sleeve. The sleeve is connected to a control valve rod through a lever pivoted on the fulcrum. This down word sleeve will raise the control valve rod. Now oil is coming from the from the oil sump, pumped by gear pump is just stay at the mounts of both pipes AA or BB which are closed by the two wings of control valves. So, raise of control valve rod will open the mouth of the pipe AA but BB is still closed. Now the oil pressure is coming from the pipe AA. This will rush from the control valve which will move the right side of the piston. As a result, the steams flow rate into the turbine increases which will bring the speed of the turbine to the normal range. When speed of the turbine will come to its normal range, fly balls will come into its normal position. Now, sleeve and control valve rod will back to its normal position.

30. Nozzle Governing

31. Nozzle Governing In nozzle governing the flow rate of steam is regulated by opening and shutting of sets of nozzles rather than regulating its pressure .  In this method groups of two, three or more nozzles form a set and each set is controlled by a separate valve. The actuation of individual valve closes the corresponding set of nozzle thereby controlling the flow rate. In actual turbine, nozzle governing is applied only to the first stage whereas the subsequent stages remain unaffected .   Since no regulation to the pressure is applied, the advantage of this method lies in the exploitation of full boiler pressure and temperature.  As shown in the figure the three sets of nozzles are controlled by means of three separate valves.