MMHES - Impulse.pptx for Micro mini hydel sytems

impulse turbine introduction

1. IMPULSE TURBINE, SELECTION CRITERIA, EFFICIENCY AND COMPARISON Subject: Mini and Micro Hydal Systems 1

2. What is a TURBINE??? A turbine is a rotary mechanical device that extracts energy from a fast moving flow of water, steam, gas, air, or other fluid and converts it into useful work. A turbine is a turbo-machine with at least one moving part called a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades so that they move and impart rotational energy to the rotor. 2

3. WORKING PRINCIPLE The working principle is very much simple. • When the fluid strikes the blades of the turbine, the blades are displaced, which produces rotational energy. • When the turbine shaft is directly coupled to an electric generator mechanical energy is converted into electrical energy. • This electrical power is known as hydroelectric power 3

4. Basic types of turbines 4

5. IMPULSE TURBINE • In an impulse turbine , fast moving fluid is fired through a narrow nozzle at the turbine blades to make them spin around. • The blades of an impulse turbine are usually bucket-shaped so they catch the fluid and direct it off at an angle. • In an impulse turbine, the fluid is forced to hit the turbine at high speed. 5

6. TYPES OF IMPULSE TURBINES Pelton Impulse Turbine Cross-flow Impulse Turbine 6

7. PELTON IMPULSE TURBINE • These are usually used for high head, low flow power plants. • It was invented by Lester Ella Pelton in the 1870s. • Nozzles are direct forceful, high speed streams of water against a rotary series of spoon-shaped buckets, also known as impulse blades, which are mounted around the circumferential rim of a drive wheel also called a runner. 7

8. DESCRIPTION OF PELTON IMPULSE TURBINE • As the water jet hit the bucket-blades, the direction of water velocity is changed to follow the contours of the bucket. • Water impulse energy exerts torque on the bucket and wheel system, spinning the wheel; the water stream itself does a "uturn“ and exits at the outer sides of the bucket. • Pelton wheels operate best with Drop height: (50 - 2000 m) and Flow rate is (4 - 15 m3/s) 8

9. PELTON IMPULSE TURBINE COMPONENTS PENSTOCK- It is a large size conduit which conveys water from high level reservoir to the turbine . The penstock may be of wood ,concrete ,or steel. SPEAR AND NOZZLE-Nozzle is used to convert hydraulic energy into kinetic energy .Spear is so arranged that it can move forward or backward there by decreasing or increasing the annular area of nozzle passage. 9

10. PELTON IMPULSE TURBINE COMPONENTS CASING-Casing is provided to prevent strong splash of water ,which scatter in all directions and to guide the water to the tail race. This casing also acts as a safeguard against accidents. RUNNER WITH BUCKET- The turbine rotor, called the runner , is a circular disk carrying a number of cup shaped buckets which are arranged equidistantly around the periphery of the disk. For low heads the buckets are made of cast iron ,but for higher heads they are made of bronze ,cast steel, or stainless steel . 10

11. BREAKING JET When the nozzle is completely closed ,the amount of water striking the runner reduces to zero. But the runner due to inertia goes on revolving for long time . To stop the runner in short time , a small nozzle is provided which directs the jet of water on the back of vanes . This jet of water is called breaking jet. PELTON IMPULSE TURBINE COMPONENTS 11

12. GOVERNING MECHANISM Speed of the turbine runner is required to be maintained constant so that the electric generator coupled directly to the turbine shaft runs at constant speed under varying load conditions. 12

13. DESIGN OF PELTON TURBINES 13

14. DESIGN OF PELTON TURBINES NUMBER OF JETS: Generally a pelton wheel has one nozzle or one jet . However a number of nozzles may be employed when more power is to be produced with the same wheel .The nozzles are spaced evenly around the same runner. Theoretically 6 nozzles can be used with one pelton wheel. Practical considerations ,however ,limit the use of not more than 2 jets per runner for a vertical runner, and not more than 4 per runner when it is in horizontal position. 14

15. NUMBER OF BUCKETS: The number of buckets should be few as possible so that there is little loss due to friction. The jet of water must be fully utilized so that no water from the jet goes waste i.e no water escapes without striking the buckets . Z=15+D/2d where D= Pitch Diameter, d=Diameter of jet. DEPTH AND WIDTH OF BUCKETS: Width of bucket =5d Depth of bucket =1.2d DESIGN OF PELTON TURBINES 15

16. PELTON WHEEL 16

17. APPLICATIONS • Pelton wheels are the preferred turbine for hydro-power, when the available water source has relatively high hydraulic head at low flow rates. • Pelton wheels are made in all sizes. For maximum power and efficiency, the wheel and turbine system is designed such that the water jet velocity is twice the velocity of the rotating buckets. • There exist in multi ton Pelton wheels mounted on vertical oil pad bearing in hydroelectric power. 17

18. CROSS-FLOW IMPULSE TURBINE • It is developed by Anthony Michel, in 1903 and is used for low heads. (10–70 meters) • As with a water wheel, the water is admitted at the turbine's edge. After passing the runner, it leaves on the opposite side. • Going through the runner twice provides additional efficiency. • The cross-flow turbine is a low-speed machine that is well suited for locations with a low head but high flow. 18

19. APPLICATIONS • The peak efficiency of a cross-flow turbine is somewhat less than a kaplon, francis or pelton turbine. • It has a low price, and good regulation. • As water going through the runner twice, provides additional efficiency. • Cross-flow turbines are mostly used in mini and micro hydropower units. • Its good point as When the water leaves the runner, it also helps clean the runner of small debris and pollution. 19

20. IMPULSE TURBINE SELECTION CRITERIA 20

21. Water turbines use water pressure to rotate its blades and generate energy. Selecting the appropriate type of turbine depends primarily on available head and flow rate. Turbine is always selected to match the specific conditions under which it has to be operate and attain the maximum efficiency SELECTION CRITERIA 21

22. Following factors affect the selection of hydraulic turbine Head and Quantity of Water Specific Speed Rotational Speed Turbine Efficiency Cavitation Disposition of turbine shaft Part load operation SELECTION CRITERIA 22

23. Head and Quantity of Water Performance of the turbine is ideal at design head. Turbine efficiency falls at head higher and lower than the design head. According to head and quantity of water available, the turbines classified mainly into High Head Turbines Medium Head Turbines Low Head Turbines SELECTION CRITERIA 23

24. SELECTION CRITERIA Head Head Range Suitable turbine Explanation Very low head 3 – 10 m Bulb Turbine Kaplan turbines are also suitable but uneconomical for very low heads Low head 10 – 60 m Kaplan Turbine Propeller turbines are also suitable up to 15m head but there should not be load variations. Medium head 60 – 150 m Francis Turbine – High head 150 – 350 m Impulse or Francis Turbine One of them is decided based on the specific speed. Very high head >350 m Impulse Turbine – 24

25. Specific Speed Speed of a unit power when working under unit head . Symbolically, it is denoted by N s SELECTION CRITERIA N= Speed (RPM) P = Power developed H = head under which the turbine is working 25

26. Specific Speed The Specific speed is high for turbines which works under high heads and low flow rate. Similarly, specific speed is low for a turbine which works under the low head and high flow rate . SELECTION CRITERIA Type of Turbine Specific speed range Impulse Turbine 10 – 70 m/s Francis Turbine 80 – 400 m/s Kaplan Turbine 300 – 1000 m/s Bulb Turbine 1000 – 1200 m/s 26

27. Efficiency of Turbine Turbine efficiency varies with load. Necessity of operating turbine at part loads influences choice of turbines in the overlapping head ranges. The efficiency of the turbine selected should be as high as possible and it is considered for various working conditions of turbines. Different turbines in decreasing order of their overall efficiency are as follows: Efficiency of turbines = Francis Turbine > Kaplan Turbine > Impulse Turbine SELECTION CRITERIA 27

28. Rotational Speed The rotational speed of the turbine depends upon the specific speed of the turbine, frequency and number of pair of poles in the electric generator. So, the specific speed of the selected turbine should produce the same amount of rotation speed of the generator. SELECTION CRITERIA 28

29. Cavitation Cavitation mainly occurs in the case of reaction type turbines. So, while selecting the turbine, cavitation factor should be determined for that particular turbine to check whether it is in a safe zone or not. Cavitation factor depends upon the specific speed of the turbine. SELECTION CRITERIA 29

30. Disposition of Turbine Shaft It is recommended that the horizontal shaft arrangement is best suitable for large size impulse turbines such as Pelton turbine etc. Similarly in case of large size reaction turbines such as the Kaplan turbine, vertical shaft arrangement is recommended Part Load Operation In general, the efficiency of the turbine is maximum when it is running with a designed load condition. When the part-load or overload condition arises the efficiency reduces which is uneconomical. In that case, Kaplan turbine is recommended. SELECTION CRITERIA 30

31. Disposition of Turbine Shaft It is recommended that the horizontal shaft arrangement is best suitable for large size impulse turbines such as Pelton turbine etc. Similarly in case of large size reaction turbines such as the Kaplan turbine, vertical shaft arrangement is recommended Part Load Operation In general, the efficiency of the turbine is maximum when it is running with a designed load condition. When the part-load or overload condition arises the efficiency reduces which is uneconomical. In that case, Kaplan turbine is recommended. SELECTION CRITERIA 31

32. IMPULSE TURBINE EFFICIENCY AND COMPARISON 32

33. EFFICIENCY OF IMPULSE TURBINE DEFINITION Generally, in engineering, efficiency of turbo-machinery is defined as the ratio of work done by the turbine to the energy supplied. Efficiency of Turbomachinery= Work output/Energy Supplied (1) To measure the efficiency of Impulse Turbine, we can use following three types of efficiency. Hydraulic Efficiency Mechanical Efficiency  Overall Efficiency  33

34. EFFICIENCY OF IMPULSE TURBINE TYPES OF EFFICIENCY Hydraulic Efficiency It is the ratio of work done on the wheel to the energy of the jet. Mechanical Efficiency In practical, it is observed that all the energy supplied to the wheel does not come out as the useful work. Some energy is wasted to overcome the friction of bearing and other moving parts. Hence keeping this point in mind, the mechanical efficiency is the ratio of actual work available at the turbine to the energy imparted to the wheel. Overall Efficiency It is the ratio of actual power produced by the turbine to the energy actually supplied by the turbine. With this efficiency, you can measure the performance of the turbine. 34

35. EFFICIENCY OF IMPULSE TURBINE EULER TURBOMACHINE EQUATION The power output of a Pelton wheel turbine by using the Euler turbomachine equation is given as (2). [1] ω (2) = power available at shaft = Angular speed of rotor = Volume flow rate = Outlet radius , = Inlet radius = Outlet absolute tangential velocity = Inlet absolute tangential velocity   35

36. EFFICIENCY OF IMPULSE TURBINE TORQUE The Torque of shaft is given by equation (3): Sign convection Must be careful of negative signs since this is an energy- producing rather than an energy- absorbing device. For turbines, it is conventional to define point 2 as the inlet and point 1 as the outlet.   36

37. EFFICIENCY OF IMPULSE TURBINE ASSUMPTIONS FOR CALCULATIONS The center of the bucket moves at tangential velocity r ω (Figure 1a). There is an opening in the outermost part of each bucket, the entire jet strikes the bucket that happens to be at the direct bottom of the wheel at the instant of time under consideration. (Figure 1b) since both the size of the bucket and the diameter of the water jet are small compared to the wheel radius, we approximate and as equal to r . The water is turned through angle without losing any speed; in the relative frame of reference moving with the bucket, the relative exit speed is thus (the same as the relative inlet speed). (Figure 1b)   37

38. Figure 1: Schematic diagram of a Pelton-type impulse turbine ; the turbine shaft is turned when high-speed fluid from one or more jets impinges on buckets mounted to the turbine shaft. ( a ) Sideview, absolute reference frame, ( b ) bottom view of a cross section of bucket n , rotating reference frame. [2] 38

39. EFFICIENCY OF IMPULSE TURBINE VELOCITY TRIANGLE The tangential component of velocity at the inlet, is simply the jet speed itself, . According to velocity diagram as given in Fig. 2 we can calculate the tangential component of absolute velocity at the outlet, as given in equation (4) . After some trigonometry, sin = r ω + (4)   39

40. Figure 2: Velocity diagram of flow into and out of a Pelton wheel bucket. We translate outflow velocity from the moving reference frame to the absolute reference frame by adding the speed of the bucket ( r ω ) to the right. [3] 40

41. EFFICIENCY OF IMPULSE TURBINE OUTPUT POWER By substituting Eq.4 into Eq.2 , Eq.5 yields: = ω (5) By simplifying Eq.5, we get Eq.6 = ω (6)   41

42. EFFICIENCY OF IMPULSE TURBINE EFFICIENCY FACTOR The maximum power is achieved theoretically if 180°. However, if that were the case, the water exiting one bucket would strike the back side of its neighbor coming along behind it, reducing the generated torque and power. It turns out that in practice, the maximum power is achieved by reducing b to around 160° to 165°. The efficiency factor due to b being less than 180° is : Efficiency factor due to = (6) When 160°, for example, = 0.97—a loss of only about 3 percent.   42

43. EFFICIENCY OF IMPULSE TURBINE CLOSING REMARKS We see from Eq.5 that the shaft power output is zero if r 0 (wheel not turning at all). Shaft output power is also zero if r (bucket moving at the jet speed). Somewhere in between these two extremes lies the optimum wheel speed. By setting the derivative of Eq.5 with respect to r to zero, we find that this occurs when r /2 (bucket moving at half the jet speed). For an actual Pelton wheel turbine, there are other losses besides that of Eq.6, like: mechanical friction, aerodynamic drag on the buckets, friction along the inside walls of the buckets, nonalignment of the jet and bucket as the bucket turns, back splashing, and nozzle losses. The efficiency of a well-designed Pelton wheel turbine can approach 90 percent. In other words, up to 90 percent of the available mechanical energy of the water is converted to rotating shaft energy.   43

44. COMPARISON IMPUSLE TURBINE  In impulse turbine only kinetic energy is used to rotate the turbine. In this turbine water flow through the nozzle and strike the blades of turbine. Pressure energy of water converted into kinetic energy before striking the vanes.  REACTION TURBINE In reaction turbine both kinetic and pressure energy is used to rotate the turbine. In this turbine water is guided by the guide blades to flow over the turbine. In reaction turbine, there is no change in pressure energy of water before striking. 44

45. COMPARISON IMPULSE TURBINE  The pressure of the water remains unchanged and is equal to atmospheric pressure during process.  In impulse turbine casing has no hydraulic function to perform because the jet is at atmospheric pressure. This casing serves only to prevent splashing of water. REACTION TURBINE The pressure of water is reducing after passing through vanes.  Casing is absolutely necessary because the pressure at inlet of the turbine is much higher than the pressure at outlet. It is sealed from atmospheric pressure. 45

46. IMPUSLE TURBINE  Water may admitted over a part of circumference or over the whole circumference of the wheel of turbine. This turbine is most suitable for large head and lower flow rate. Pelton wheel is the example of this turbine. REACTION TURBINE  Water may admitted over a part of circumference or over the whole circumference of the wheel of turbine. This turbine is best suited for higher flow rate and lower head situation. COMPARISON 46

47. References R.S Khurmi Mechanical Engineering https://theconstructor.org/practical-guide/pelton-turbine-parts-working-design-aspects/2894/ [1][2][3] Çengel, Yunus A. Fluid mechanics : fundamentals and applications / Yunus A. Çengel, John M. Cimbala.—1st ed. p. cm.—(McGraw-Hill series in mechanical engineering) ISBN 0–07–247236–7 1. Fluid dynamics. I. Cimbala, John M. II. Title. III. Series

48. THANK YOU