5. francis turbine

francis turbine

1. FLUID MACHINES Chapter 3: Water Turbines Francis Turbine Presented by Keshav Kumar Acharya Teaching Assistant TU, IOE Purwanchal Campus

2. Introduction Modern Francis turbine is an inward mixed flow reaction turbine i.e. the water, under pressure, enters the runner from the guide vanes towards the center in radial direction and discharges out of the runner axially Operates under medium head and requires medium quantity of water A part of the head acting on the turbine is transformed into kinetic energy and the rest remains as pressure head The pressure at the inlet is more than that at the outlet

3. Main Components of Modern Francis Turbine Penstock It is a water way to carry water from the reservoir to the turbine casing

4. Main Components of Modern Francis Turbine Spiral Casing or Scroll Casing It is designed with a cross – sectional area reducing uniformly around the circumference; being maximum at the entrance and nearly zero at the tip This is to distribute the water around the guide ring evenly In case of big units, the inner circumference of spiral casing has stay vanes To avoid the loss of efficiency, the flow of water from the penstock to the runner should be such that it will not form eddies

5. Main Components of Modern Francis Turbine Spiral Casing or Scroll Casing

6. Main Components of Modern Francis Turbine Spiral Casing or Scroll Casing

7. Main Components of Modern Francis Turbine Guide Vanes or Wicket Gates A series of airfoil shaped vanes, arranged inside the casing to form a number of flow passages between the casing and the runner blades Arranged in such a way that the energy of water is not consumed by eddies and other undesirable flow phenomenon Direct the water onto the runner at an appropriate angle They can swing about their axis, varying the flow passage and thus the discharge between two consecutive runner blades

8. Main Components of Modern Francis Turbine Guide Vanes or Wicket Gates

9. Main Components of Modern Francis Turbine Guide Wheel and Governing Mechanism Governing mechanism changes the position of guide vanes to change the water flow rate The guide vanes are connected to the regulating ring by means of a link and a lever The ring is connected to the regulating shaft by means of regulating rod, generally two in number By rotating the regulating shaft, the guide vanes can closed or opened thus allowing variable amount of water according to need The penstock pipe feeding the turbines is often fitted with a relief valve, also known as pressure regulator; it opens when the vanes are suddenly closed, diverting the water to the tail race

10. Main Components of Modern Francis Turbine

11. Main Components of Modern Francis Turbine Runner and Runner Blades Mixed flow type of turbine i.e. the inlet is radial and the exit is axial The runner consists of crown, shroud and blades Water flow passage is formed between crown and shroud in one direction and between consecutive blades on the other The higher specific speed runner is wider than the one which has a low specific speed because the former has to work with a large amount of water The runners are made of cast iron for small output, cast steel/ stainless steel for large output The disposition of shaft is horizontal for smaller units and vertical for larger units

12. Main Components of Modern Francis Turbine Runner and Runner Blades

13. Main Components of Modern Francis Turbine Runner and Runner Blades

14. Main Components of Modern Francis Turbine

15. Main Components of Modern Francis Turbine Draft Tube Integral part reaction turbines i.e. Francis turbine, Kaplan turbine, etc. Conduit with gradually increasing cross sectional area connecting the runner exit to the tail race Serves two functions: decreases the pressure at the runner exit to a value less than atmospheric pressure and thereby increasing the working head recovers a portion of the exit kinetic energy which otherwise goes waste to the tail race

16. Main Components of Modern Francis Turbine Draft Tube

17. Draft Tube Theory Consider the turbine that is not fitted with draft tube: Water from the runner exit then discharge freely into the tail race Evidently pressure P 2 at the runner exit equals the atmospheric pressure P a Again Kinetic Energy (V 2 /2g) goes waste to the tail race. In low head turbines with large through flow, this losses may be half or more of the energy entering the turbine 4 to 25% for mixed flow turbines & 20 to 50% of the total head for axial flow turbines

18. Draft Tube Theory Contd.. Consider the turbine fitted with draft tube: Turbine runner is sealed from atmospheric pressure and the flow occurs from section 2-2 to section 3-3 Section 2-2 corresponds to the runner exit (or inlet section of the draft tube) and the section 3-3 represents the draft tube exit which is located at depth y below the level of tail race

19. Draft Tube Theory Contd.. Pressure at section 3-3 will be Apply Bernoulli’s equation between section 2-2 and 3-3 Rewriting the above expression for p 2 /w And further substituting the value of

20. Draft Tube Theory Contd.. The term (y 2 - y) which represents the vertical distance between the runner exit and the tail water level is called the suction head of draft tube and is denoted by h s The above expression indicates that the pressure at the runner exit drops below atmospheric pressure (p 2 /w works out to be negative) if draft tube is of diverging section (V 3 < V 2 ) and the suction head h s is positive. Consequently the available head on the turbine increases and the turbine gives a higher output and efficiency

21. Draft Tube Efficiency Defined as ratio of actual conversion of kinetic head into pressure head in the draft tube to the kinetic head at the inlet of the draft tube By suitably shaping the draft tube, kinetic energy of out flowing water may be rendered as small as desired

22. Different Types of Draft Tubes Straight divergent tube The shape of the tube is that of the frustum of a cone It is employed for low specific speed vertical shaft Francis turbine The maximum cone angle is 8 ( or α = 4 ) If the angle is more than 8 , the water detaches away from the inner wall of the tube while flowing downwards forming vertices and causing loss of head The tube must discharge sufficiently low under tail water level Maximum efficiency is about 85%, because the kinetic head to be recovered is less

23. Different Types of Draft Tubes Moody’s Bell mounted Draft tube Moody’s spreading tube or Hydracone : Moody suggested a bell mouthed draft tube having solid conical core in the entire central portion of the tube , thus allowing a large exit area without excessive length. When turbine works at part load or due to high velocity at runner exit, the discharging water velocity has a whirl components, it is likely to cause eddy losses. The central cone arrangement is made to reduce the whirl action of discharging water.

24. Different Types of Draft Tubes Simple Elbow Tube In order to down the cost of excavation, particularly in rock, the vertical length of the draft tube should be minimum Since the draft tube exit diameter should be as large as possible to recover the kinetic head and at the same time the maximum value of cone angle is fixed, the draft tube must be bent to keep its definite length

25. Different Types of Draft Tubes Elbow tube with circular inlet and rectangular outlet This type of draft tube has been designed to turn the water from the vertical to the horizontal direction with minimum depth of excavation and at the same time having a high efficiency The transition from the circular section in vertical leg to rectangular section in the horizontal leg takes place in the bend The horizontal portion of the draft tube is generally inclined upwards to lead the water gradually to the level of the tail race and to prevent entry of air form the ext end. The exit end of the tube must be totally immerged in water

26. Energy conversion in a Francis turbine Out let draft tube Outlet runner Inlet runner Outlet guide vane Inlet guide vane

27. Energy conversion in a Francis turbine The total absolute sp. energy is the difference from the pr. side to the suction side of the turbine = E = g H The lower part of the diagram shows pressure energy and upper part shows the kinetic energy On RHS of the diagram before reach the inlet of the GV ~ 95 % energy is pr. with increasing of KE towards the GV inlet where swirl flow is dominating Through the GV cascade the rotational energy increases to match the circumferential speed of the runner blades Also meridional velocity increase because the water is flowing towards the decreasing diameter

28. Energy conversion in a Francis turbine As a result the pr. energy decrease to ~ 50 % of the total energy at the runner inlet From the runner inlet the energy is gradually converted to mechanical energy by the runner Towards the outlet of the runner the absolute pressure decreases bellow the pr. outside the draft tube and the rotational KE decreases to zero at BEP theoretically At the runner outlet the pressure normally decrease to a value below atm. pr., however, the KE decreases from the runner outlet to the draft tube outlet, where pr. increase to atm. pr. plus the pr. of the water level above the ref. level

29. Cavitation The phenomenon of formation, growth, travel and sudden collapsing of the vapor bubbles When the vapor bubbles collapse, very high pr. is created The metallic surfaces, above which these vapor bubbles collapse, is subjected to these high pr., which cause pitting action on the surface Cavities are formed on the metallic surface and also considerable noise and vibration are produced

30. Precaution Against Cavitation The pr. of the flowing liquid in any part of the hydraulic system should not be allowed to fall bellow its vapor pr. The special materials or coatings such as aluminium -bronze and stainless steel, which are cavitation resistant materials, should be used Runner may be kept under water. But it is not advisable as the inspection and repair of the turbine is difficult The other method to avoid cavitation zone without keeping the runner under water is to use the runner of low specific speed The cavitation free runner may be designed to fulfill the given conditions with extensive research The cavitation may be avoided by selecting a runner of proper Sp. Speed for given head

31. Effects of Cavitation The metallic surface are damaged and cavities are formed on the surface Due to sudden collapse of vapor bubble, considerable noise and vibration are produced The efficiency of turbine is decreases

32. Hydraulic machine subjected to Cavitation Reaction turbines and centrifugal pumps Affected by the cavitation in three ways Roughening of the surface takes place due to loss of material caused by pitting Vibration of parts is caused due to irregular collapse of cavities The actual volume of liquid flowing through the machine is reduced (since the volume of cavities is many times more than the volume of water which they are formed) causing sudden drop in output and efficiency

33. Appearance of Cavitaion and Sand Erosion damage The appearance of cavitation damage has pit holes with sharp edges which bleeds finger Sand erosion surface are with shinning luster and wavy scales or ripples But the appearance in combined effect of these two is not clear

34. NPSH Net Positive Suction Head [m] h v vapor pressure head [m] H A atmospheric pressure head [m] z 2 Height above ref. line at location 2 [m] z 4 Height above ref. line at location 4 [m] c 2 mean velocity at location 2 [m/s]  s loss coefficient [ - ] NPSH Net Positive Suction Head

35. z 4 4 Losses

36. z 4 4 Let us introduce the vapor pressure, h v :

37. NPSH Net Pressure Suction Head z 4 4 Atmospheric pressure: H A = h 4

38. Suction Head h s z 4 4

39. Submergence of a turbine NPSH Net Positive Suction Head [m] h v vapor pressure head [m] H A atmospheric pressure head [m] H S Submergence [m] c 2 mean velocity at location 2 [m/s]  s loss coefficient [ - ]

40. NPSH NPSH NB: H S has a negative value in this figure.

41. Relative path Absolute path with the runner installed Absolute path without the runner installed

42. c u1 u 1 v 1 c 1 b 1 c m1 u 2 v 2 c 2 b 2 D 2 D 1 Francis turbine