Hydraulic Turbines | Mechanical Engineering SSC JE (Technical) PDF Download

Hydraulic Turbines

  • Turbines are hydraulic machines which convert hydraulic energy into mechanical energy. 
  • Penstock is a pipe of large diameter which carry water under pressure from storage reservoir to the turbine chamber. 
  • Surge Tank is an open operation tank provided in midway of Penstock to absorb water pressure hammer during sudden stoppage of flow in the turbiness. It should be located as near to turbines as possible.

Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)

                                                 Layout of a hydroelectric power plant

Efficiency to Turbines 

  • Hydraulic efficiency (hh)
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
  • Mechanical efficiency (hmech)
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
  • Volumetric efficiency (hvol)
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
  • Overall efficiency (ho)
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)

Classification of Turbines on the basis of energy at inlet 

  • Impulse Turbines: These have only kinetic energy at inlet.
    Example: Pelton Wheel, Gurard Turbine, Banki Turbine, Jonval Turbine, Turgo Impulse Wheel. 
  • Reaction Turbines: These have kinetic energy and pressure energy both at inlet.
    Example: Francis Turbine, Propeller Turbine, Kaplan Turbine, Thomson Turbine, Furneyron Turbine.

Classification of Turbines on the basis of type of flow within turbine 

  • Tangential Flow Turbine: Example: Pelton Wheel, Turgo Impulse Wheel. 
  • Radial Flow Turbines: Example: Inward-radial flow turbines are Francis turbine, Thomson Turbine, Guard Turbine. Outward-radial flow turbine is Fourneyron turbine 
  • Axial Flow Turbines: Example: Jonval Turbine, Propeller Turbine, Kaplan Turbine 
  • Mixed Flow TurbineExample: Modern Francis Turbine
  • Specific speed of turbines
Turbine                                                                        Specific Speed (MKS Unit)
Pelton Turbine                                                                        10 to 35                         Francis Turbine                                                                       60 to 300               Kaplan & Propeller Turbine                                                    300 to 1000
  • Head of turbines
Turbine                                                                                         Head
Pelton Turbine                                                                         above 250 m                   Francis Turbine                                                                       60 m to 250 m                 Kaplan & Propeller Turbine                                                      Below 60 m

Pelton Turbine

  • It is tangential flow impulse turbine.
  • It is high head low discharge turbine.
  • The bucket of a pelton wheel is double semi-ellipsoidal in shape. The jet of water impinges at the centre of the bucket & deflects through 160-170°, (The deflection angle is 160° to 170°).
  • The advantage of having double cup-shaped buckets is that the axial thrust neutralizes each other. A notch is cut at lower tip of bucket, which prevents the jet striking the preceding bucket and avoid the deflection of water towards the centre of bucket. 
  • Theoretically six jets can be used with one pelton wheel but practically not more than two jets for a vertical runner and not more than four jets for horizontal runner should be used.
  • Work done per second = Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
    V= Velocity of jet coming out of nozzle
    u1 = u2 = u = tangential velocity of vane
    k =  Bucket friction coefficient 

Efficiency of Pelton wheel
(i) Nozzle efficiency
Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
(ii) Hydraulic efficiency

Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
(iii) Mechanical efficiency

Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
(iv) Overall efficiency

ho = hN × hh × hmech 

  • Velocity of jet, V 1  Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
    Cv = coeff of velocity = 0.97 – 0.99
  • Speed Ratio
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
    K= 0.43 – 0.47 
  • Jet ratio (m) =
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
    A jet ratio of 12 is normally adopted 
  • No. of Buckets (Z) =
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
    This formula is called Tygun formula. 
    • If P is power developed by the pelton wheel when working under head (H) & having one jet only, the power developed by the same wheel will be (n. P) in n jets are used under the same head.

Francis Turbine

  • It is inward radial flow reaction turbine. 
  • Scroll casing is used to evenly distribute the water along the periphery & maintaining the constant velocity for the water 
  • Draft Tube - The water after passing through the runner flows to the tail race through a draft tube which is a passage of gradually increasing cross-section area which connects the runner exit to tail race. The basic function of draft tube is to convert kinetic head into the pressure head.

Purpose of draft tube 
(i) Permits the negative suction head to establish at the runner exit
(ii) It acts as a recuperator of pressure energy.
The angle of straight divergent type draft tube should not be more than 8° otherwise eddies will be formed & efficiency will be reduced. 

  • Inward Flow reaction turbine
  • Work done per sec
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
  •  For best efficiency flow should be radial at outlet, Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
    i.e., work done per sec
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
  •  Degree of reaction
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)

V & V are whirl velocities at inlet and outlet u1 & u2 are peripheral velocities of blades at inlet and outlet.

  •  Flow ratio  =
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical) = 0.15 0.30
  •  Speed ratio = 
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical) = 0.60 0.90
  •  Discharge through the turbine
    Q = pD1 B1 Vf4
     = pD2 B2 Vf2
    B1, B2 are width of inlet & outlet
    D1, D2 are diameter at inlet & outlet
    Vf4 , Vf2are velocities of flow at inlet & outlet

Kaplan Turbine

  • It is axial flow reaction turbine. 
  • Runner of kaplan turbine has four to six blades. 
  • Runner blades of propeller turbines are fixed but of kaplan turbine can be turned about axis.
  • At part load, high efficiency can be obtained in kaplan Turbine. 
  • No. of blades are 3 to 6 
  • If frictional resistance is ignored, then Vr1 = Vr2 
  • Velocity of flow remains constant. 
  • Its velocity triangle & calculation of efficiency is similar to Francis turbine. 
  • Discharge through the turbine
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
  •  Flow ratio = y = 0.70 
  • Usually
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical) = 0.35 to 0.61
    u1 = u2 =  Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
    Do = outer diameter of runner
    Db = inner diameter or diameter of boss or hub

Diagonal Turbine

  • This is intermediate between the mixed flow and axial flow turbine because the flow of water as it passes through the runner is at an angle of 45° to the axis and hence is known as diagonal turbine. 
  • Head range is 30 m to 150 m 
  • Blades are adjustable. 
  • It can be used as turbine as well as pump.

Tubular Turbine

  • Adjustable or non-adjustable runner blades. 
  • Head range 3 m to 15 m. 
  • Scroll casing is not provided. 
  • Runaway speed For a turbine working under maximum head & full gate opening, if the external load suddenly drops to almost zero value and at the same time, governing mechanism fail, turbine runner will tend to race up and attains maximum possible speed. This limiting speed of turbine is known as runaway speed.
    For Pelton Turbine, Runaway speed is 1.8-1.9 times of normal speed.
    For Francis Turbine, Runaway speed is 2-2.2 times of normal speed
    For Kaplan Turbine, Runaway speed is 2.5-3 times of normal speed
  • Performance of Turbines/Unit Quantities The concept of unit quantities is required to (i) predict the behaviour of turbines working under different conditions. (ii) make comparison between the same type but different size of turbines.

(iii) co-relation and use of experimental data.

  • Unit speed, Nu = 
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
  • Unit discharge, Qu =
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
  • Unit power,
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
  • Specific speed (Ns) of any turbines is the speed in rpm of a turbine, geometrically similar to the actual turbine but of such a size that under corresponding conditions it will develop unit power when working under unit head.
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
    Note that specific speed is not a dimensions parameter, its unit is [M1/2L–1/4T–5/2] 
  • Dimensionless form of specific speed is called shape number (S).
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
  • Model Laws of Turbines
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
    The parameter
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
     is called flow coefficient. or capacites co-efficient.
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
    The parameter
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
    is called head coefficient.
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
    The parameter
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
    is called power coefficient.
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
    Note: Similarity of model and prototype turbines are based on the assumption that efficiency of model is equal to that of the prototype. But the efficiency of prototype is higher than the models efficiency. This is known as scale effect.
  • Thoma’s cavitation factor (s)
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
    Ha = Atmospheric head
    Hv = Vapour pressure head
    hs = Suction head (at the outlet of reaction turbine)
    H = Working head
    NPSH = Net Positive Suction Head 
  • Critical value of Thoma’s number (sc) depends upon Ns and is given by:
    (a) For Francis Turbine,
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
    (b) For Propeller Turbine,
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
    (c) For Kaplan Turbine, sc is 10% higher than sc for properller turbine. 
  • Condition for no cavitation
    Hydraulic Turbines | Mechanical Engineering SSC JE (Technical)
The document Hydraulic Turbines | Mechanical Engineering SSC JE (Technical) is a part of the Mechanical Engineering Course Mechanical Engineering SSC JE (Technical).
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FAQs on Hydraulic Turbines - Mechanical Engineering SSC JE (Technical)

1. What is a hydraulic turbine?
Ans. A hydraulic turbine is a mechanical device that converts the energy from flowing water into rotational mechanical energy. It is widely used in hydroelectric power plants to generate electricity.
2. How does a hydraulic turbine work?
Ans. A hydraulic turbine works by utilizing the kinetic energy of water flowing under pressure. The water strikes the blades of the turbine, causing them to rotate. The rotating motion is then transferred to a generator, which converts it into electrical energy.
3. What are the different types of hydraulic turbines?
Ans. There are several types of hydraulic turbines, including the Pelton turbine, Francis turbine, and Kaplan turbine. Each type is designed for specific water flow conditions and has unique characteristics that make it suitable for different applications.
4. What are the advantages of using hydraulic turbines for power generation?
Ans. Hydraulic turbines offer several advantages for power generation, including high efficiency, low maintenance requirements, and the ability to generate electricity from a renewable energy source. They are also flexible in terms of water flow rates and can operate in a wide range of hydraulic conditions.
5. Are there any environmental considerations associated with hydraulic turbines?
Ans. While hydraulic turbines produce clean energy and do not directly emit greenhouse gases, there are some environmental considerations. The construction of dams for hydroelectric power plants can lead to habitat disruption and affect aquatic ecosystems. Additionally, fish passage can be an issue, as turbines can pose a risk to fish during migration. Various measures, such as fish bypass systems, are implemented to mitigate these impacts.
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