What is Hydraulic Turbine? | Civil Engineering Optional Notes for UPSC PDF Download

What is the Hydraulic Turbine?

• Hydraulic Turbine: The hydraulic turbine serves as a mechanical device that transforms mechanical energy into electrical energy. It is part of hydraulic machines, which either convert hydraulic energy into mechanical energy or vice versa.
• Hydraulic Machines: Devices that convert hydraulic energy into mechanical energy are called turbines, whereas those that convert mechanical energy into hydraulic energy are known as pumps.

In a Hydroelectric plant, as shown above:

• A Dam is constructed across a river or a channel for storing water. The reservoir is also known as the Head race.
• Pipes of large diameter called Penstock carry water under pressure from the storage reservoir to the turbines. These pipes are generally made of steel or reinforced concrete.
• Turbines have different types of vanes, buckets, or blades mounted on a wheel called a runner.
• Tail-race is a channel carrying water away from the turbine after the water has worked on the turbines. The water surface in the tail race is also referred to as the tail race.

Important Terms of Hydraulic Turbine

• Gross Head (H_g ): Vertical difference between headrace and tailrace.
• Net Head (H_net): The actual head available at the inlet to work on the turbine is called the Net Head. It can be mathematically expressed as H_net = H_g – h_L.
• Total Head Loss (h_L): h_L is the total head loss during the transit of water from the headrace to the tail race, mainly the head loss due to friction, and is given by, h_f = fLV²/2gD = 4f’LV²/2gD. Where, ‘f’ is the coefficient of friction of penstock, which depends on the type of material of penstock, ‘L’ is the total length of the penstock, ‘V’ is the mean flow velocity of water through the penstock, ‘D’ the diameter of penstock and g is the acceleration due to gravity.

Types of Efficiencies

Hydraulic Efficiency(ηh)

• The ratio of the power generated by the runner of a turbine to the power supplied at the inlet of the turbine is known as Hydraulic efficiency.
• As the power supply is in the hydraulic form, and the possible losses occur between the striking jet and vane, it is rightly called hydraulic efficiency.
• If R.P. is the Runner Power and W.P. is the Water Power:
  η_h = Runner Power/Water Power = R.P./W.P.

Mechanical Efficiency(η_m)

• The ratio of the power available at the shaft to the power generated by the runner of a turbine is called Mechanical Efficiency.
• The runner’s energy loss in the annular area between the nozzle and spear is due to slips and other mechanical problems.
• If S.P. is the Shaft Power:
  η_m = S.P./R.P.

Overall Efficiency(η_o):

• The ratio of the power available at the shaft to the power supplied to the inlet of a turbine is termed overall efficiency.
• Because this covers the overall problems of energy losses, it is known as overall efficiency.
• It depends on both the hydraulic losses and the slips and other mechanical problems that will create a loss of energy between the jet power supplied and the power produced at the shaft available for coupling of the generator.
  η_o = (S.P./R.P.)×(R.P./W.P.)

Volumetric Efficiency:

Classification of Hydraulic Turbines

• The hydraulic turbines can be classified based on the type of energy at the inlet, the direction of flow through the vanes, the head available at the inlet, discharge through the vanes, and specific speed.
• They are classified as:
  

1. Pelton Turbine

• The Pelton wheel turbine is an impulse turbine. In this type of turbine, the nozzle is connected at the exit of the penstock, which converts the available energy head into a velocity head.
  

Where,

• u= wheel velocity
• V= Jet velocity
• V_r= relative velocity
• V_w=whirl velocity
• φ=Angle by the relative velocity at the outlet
• β= Guide blade angle at the outlet and u₁=u₂=u=2πN/60 where N= no. of rotation of the wheel

Tangential flow Impulse Turbine

Basic components of Impulse Turbine are:

• Nozzle
• Runner and buckets
• Casing
• Breaking Jet

The power is given to the Pelton turbine = ρaV1(V_w1 ± V_w2)u Nm/sec
Hydraulic efficiency is given by

η_h = [(V₁ – u)(1 + kcosΦ)u](V₁²/2)

For maximum efficiency dη_h/du = 0
(η_h)_max = (1 + kcosΦ)/2

Point to Remember

• The velocity of jet: V₁=C_V×√(2gH) where C_V=0.98 to 0.99 and called the coefficient of velocity.
• The velocity of wheel: u=φ √(2gH) where φ = Speed ratio (u/V₁) (0.43 to 0.48)
• The velocity of wheel u= πDN (rps)
• Jet ratio (m) =D/d (pitch diameter ratio to jet diameter; generally 12).
• Number of bucket Z= 15+(D/2d) = 15+0.5 m
• The number of jets is the ratio of the total flow rate to the Rate of flow by a single jet.

2. Radial Flow Reaction Turbine

• In this type of turbine, the water strikes the runner, and it has both a kinetic energy head and a pressure energy head. Due to the KE head, the impulse will generate, which is similar to the case of an impulse turbine.
• Impulse: kinetic head
  Reaction: Pressure head
  

Let B₁, B₂ = width of the runner at the inlet and outlet
Area of flow at inlet: A₁ = πD₁B₁
Area of flow at inlet: A₂ = πD₂B₂
If vane thickness is taken into consideration:
Net Area of flow = [πD₁ – nt] · B₁ = kπD₁B₁
K is a factor for the net area, and n is the number of vanes.

Q = A₁V_f1 = A₂V_f2

Q = πD₁B₁V_f1 = πD₂B₂V_f2

F_radial = m(V_f1 – V_f2)

Since V_f1 = V_f2

Thus F_radial should be zero

Hence, πD₁B₁ = πD₂B₂

D₁B₁ = D₂B₂

D₁>D₂

B₁2

Note: If it is given that B₁ =B_2, then V_f1 ≠ V_f2

3. Francis Turbine

• Francis turbine is an inward flow reaction turbine.
• To maximize runner power, V₂ should be minimum. It can be done by converting the absolute velocity direction into the radial direction of the runner at the exit.

It means that

β 90°

V₂ = V_f2

V_w2 = 0

Hence, R.P. = ρQV_w1u₁

Hydraulic efficiency (η_h) = RP/HP = ρQV_w1u₁/ρQgH = V_w1 u₁

η_h = V_w1u₁/gH

4. Axial Flow Reaction Turbine

• Kaplan turbine and propeller turbines are examples of axial flow reaction turbines.
  RP = m(V_w1u₁ + V_w2u₂)

What is Draft Tube?

• A draft tube is a pipe of a gradually increasing cross-sectional area, linking the runner outlet to the tailrace, and is used to discharge water from the turbine exit to the tail race.
• An end of the draft tube is linked to the runner outlet, and the other end is submerged under the water level in the tail race.

Functions

• It permits a negative head to be established at the outlet of the runner and thereby increasing the net head on the turbine.
• It converts a large proportion of kinetic energy being rejected at the outlet of the turbine into helpful pressure energy.

Specific Speed

• The value of a specific speed for a turbine is the speed of a geometrically similar turbine that would produce unit power (one kilowatt) under a unit head (one meter).
• This value is provided by the manufacturer (along with other ratings) and will always refer to the point of maximum efficiency.
  N_s = N(P)^0.5/H^(5/4)
• Specific Speed is not a dimensionless number. It dimension [M^1/2L^–1/4T^-5/2].

Model Laws

What are Centrifugal Pumps?

• A hydraulic machine that converts mechanical energy into hydraulic energy or pressure energy is known as a pump.
• A centrifugal pump (also called a Rotodynamic pump or dynamic pressure pump) works on the principle of centrifugal force.
• In Centrifugal pumps, the liquid is subjected to a whirling motion by the rotating impeller, which is made of several backward-curved vanes.

Classification of Centrifugal Pumps

centrifugal pumps can be categorized based on different parameters. These are explained as follows:

Heads on a Centrifugal Pump

• Suction head (h_s)
  • It is the vertical distance between the liquid level in the sump and the center line of the pump.
  • It is expressed as meters.
• Delivery head (h_d)
  • It is the vertical distance between the center line of the pump and the liquid level in the overhead tank or the supply point.
  • It is expressed in meters.
• Static head (Hs)
  • It is the vertical difference between the liquid levels In the overhead tank and the sump when the pump is not working.
  • It is expressed as meters.
    Therefore: H_s= (h_s+ h_d)
• Friction head (h_f)
  • It is the sum of the head loss due to the friction in the suction and delivery pipes.
  • The friction loss in both pipes is calculated using Darcy‟s equation.
    h_f = fLV²/2gD = 4f’LV²/2gD
• Total head (H)
  • It is the sum of the static head H_s, friction head (h_f), and the velocity head in the delivery pipe (V²_d /2g). Where, V_d=velocity in the delivery pipe.
    H_m = h_s +h_d +h_fs + h_fd + V_d²/2g
• Manometric head(H_m)
  • It is the total head developed by the pump.
  • This head is slightly less than the heat generated by the impeller due to some losses in the pump.
    
• Power of pump
  (i).Power requirement of pump = shaft power (SP)
  (ii). Impeller power (IP) = SP – (mechanical frictional losses)
  (iii). Manometric power or output power (MP)= ρQgH_m
  SP > IP > MP

Applications of Hydraulic Turbine

Here are some common applications of hydraulic turbines:

• Generating electricity in hydroelectric power plants
• Providing mechanical power for industrial processes, such as milling and pumping
• Driving water pumps for irrigation and water supply systems
• Assisting in flood control and water management by regulating the flow of water in rivers and reservoirs
• Providing propulsion for ships and submarines using water propulsion turbines.