Impulse Turbine MCQ Quiz - Objective Question with Answer for Impulse Turbine - Download Free PDF

Explanation:

Energy Possessed by Steam Before Entering a Turbine

• Before steam enters a turbine, it primarily possesses thermal energy.
• Thermal energy is the internal energy of a system due to the kinetic energy of its molecules.
• In the context of steam, this energy is manifested as the high temperature and pressure of the steam generated in a boiler.
• In a typical power plant, water is heated in a boiler to produce steam.
• This steam is then directed to a turbine. The high-pressure, high-temperature steam contains significant thermal energy.
• As the steam expands through the turbine, this thermal energy is converted into mechanical energy, which in turn drives an electrical generator to produce electricity.

Explanation:

Hydroelectric Power Plant

• A hydroelectric power plant is a facility that generates electricity by harnessing the energy of flowing or falling water.
• It typically involves the conversion of kinetic energy from water into mechanical energy using turbines, which is then converted into electrical energy through generators.
• In a hydroelectric power plant, water is stored in a reservoir behind a dam.
• The water is released through a controlled outlet called a penstock.
• The kinetic energy of the flowing water drives turbines connected to generators, producing electricity.
• The water is then discharged back into the river or a tailrace downstream of the plant.

Components:

• Dam: Structures that store water in a reservoir, creating a height difference essential for generating energy.
• Forebay: A basin that acts as a buffer, regulating the flow of water to the turbines.
• Penstock: Large pipes or conduits that carry water from the reservoir or forebay to the turbines.
• Turbines: Mechanical devices that convert the kinetic energy of water into mechanical energy.
• Generators: Devices that convert mechanical energy from the turbines into electrical energy.
• Spillway: A structure used to provide controlled release of water from the dam to prevent overflow.
• Tailrace: A channel that carries water away from the turbines and back into the river.

Explanation:

Hydro Penstock Design

• A hydro penstock is a large pipe or conduit that directs the flow of water from a reservoir or dam to the turbines in a hydroelectric power plant.
• The design of the penstock is crucial in maximizing the efficiency of the hydroelectric power plant, as it directly affects the water flow rate and pressure, which in turn influences the power generation.
• The primary function of the penstock is to transport water from a higher elevation to the turbines at a lower elevation.
• The potential energy of the stored water is converted into kinetic energy as it flows through the penstock.
• This kinetic energy is then used to drive the turbines, which generate electricity.
• The efficiency of this process depends on several factors, including the design and construction of the penstock.

The length and diameter of the penstock directly influence the flow rate and pressure of the water reaching the turbines. Here’s a detailed explanation of why these parameters are so important:

• Length of the Penstock: The length of the penstock affects the head loss due to friction. A longer penstock will have more frictional losses, which reduces the water pressure and flow rate. Properly designing the length to minimize these losses is essential for maintaining high efficiency.
• Diameter of the Penstock: The diameter of the penstock determines the flow capacity. A larger diameter allows more water to flow through with less frictional resistance, leading to higher efficiency. However, increasing the diameter also increases the cost and structural requirements, so an optimal balance must be achieved.

Additional Factors: While the length and diameter are the most crucial aspects, other factors also play a role in the efficiency of the penstock design:

• Material: The material of the penstock affects its durability and resistance to corrosion and pressure. Common materials include steel and reinforced concrete.
• Shape and Alignment: The shape and alignment of the penstock can influence the flow dynamics. Smooth curves and proper alignment minimize turbulence and energy losses.
• Supports and Anchors: Proper support and anchoring of the penstock are necessary to withstand the water pressure and prevent structural failures.

Explanation:

Pelton wheel or turbine

It is a tangential flow impulse turbine. The water strikes the bucket along the tangent of the runner. The energy available at the inlet of the turbine is kinetic energy. The pressure at the inlet and outlet of the turbine is the atmosphere. This turbine is used for high head and low discharge.

The main parts of Pelton wheel are:

• Nozzle and flow regulating arrangement.
• Runners and buckets.
• Casing
• Breaking jet
  When the nozzle is completely closed by moving the spear in the forward direction, the amount of water striking the runner reduces to zero. But the runner due to inertia goes on revolving for a long time. To stop the runner in a short time, a small nozzle is provided which directs the jet of water on the back of the vanes. This jet of water is called a breaking jet.

Jet ratio:

It is defined as the ratio of the pitch diameter (D) of the Pelton wheel to the diameter of the jet diameter (d).

It is given as \(m = \frac{D}{d}\)

Additional Information

• The speed ratio of the Pelton wheel varies between 0.43 to 0.48.
• The value of the jet ratio is generally taken as 12.
• Number of buckets Z, can be found from simple relation, Z = 15 + 0.5m.

Explanation:

Forebay:

• A forebay is a basin area of a hydropower plant where water is temporarily stored before going into the intake chamber. The storage of water in the forebay is decided based on the required water demand in that area.

Intake Structure or Dam:

• An intake structure is a structure that collects the water from the forebay and directs it into the penstocks. There are different types of intake structures are available and the selection of the type of intake structure depends on various local conditions.

Penstock:

• Penstocks are like large pipes laid with some slope that carry water from the intake structure or reservoir to the turbines. They run with some pressure so, sudden closing or opening of penstock gates can cause a water hammer effect on the penstocks. So, these are designed to resist the water hammer effect apart from this penstock is similar to the normal pipe. To overcome this pressure, a heavy wall is provided for short-length penstocks, and a surge tank is provided in case of long-length penstocks.

Hydraulic Turbines:

• A hydraulic turbine is a device that can convert hydraulic energy into mechanical energy which is again converted into electrical energy by coupling the shaft of the turbine to the generator. The mechanism in this case is, whenever the water coming from the penstock strikes the circular blades or runner with high pressure it will rotate the shaft provided at the center and it causes the generator to produce electrical power.

Draft Tube:

• If reaction turbines are used, then a draft tube is a necessary component that connects the turbine outlet to the tailrace. The draft tube contains a gradually increasing diameter so that the water is discharged into the tailrace with safe velocity. At the end of the draft tube, outlet gates are provided which can be closed during repair works.

Tailrace:

• Tailrace is the flow of water from turbines to the stream. It is good if the powerhouse is located nearer to the stream. But, if it is located far away from the stream then it is necessary to build a channel for carrying water into the stream.

Spiral casing:

• The spiral casing around the runner of the turbine is known as the volute casing or scroll case. Throughout its length, it has numerous openings at regular intervals to allow the working fluid to impinge on the blades of the runner.

Thus the correct sequence of hydraulic power station items (given in question) in the direction of flow is

Penstock → Guide Wheel → Runner → Scroll Casing → Draft tube.

Concept:

Pelton wheel:

It is a tangential flow impulse turbine in which the pressure energy of water is converted into kinetic energy to form a high-speed water jet and this jet strikes the wheel tangentially to make it rotate.

Taygun's formula:

It is used to determine the number of buckets on the runner in the Pelton wheel turbine. It is given by the below formula:

\(Z = 15+{ D\over 2d}\)

Where D = Pitch or mean diameter, d = Nozzle or Jet  diameter

Calculation:

Given,

D = 1 m, d = 0.1 m

The number of buckets on the runner by Taygun's formula

\(Z = 15+{ D\over 2d} = 15+{ 1\over 2\times 0.1}\)= 15 + 5 = 20

Concept:

The overall efficiency η_o of turbine = volumetric efficiency (η_v)× hydraulic efficiency (η_h)× mechanical efficiency (η_m)

\({\eta _o} = {\eta _v} \times {\eta _h} \times {\eta _m}\)

\({{\rm{\eta }}_{\rm{v}}} = \frac{{{\rm{volume\;of\;water\;actually\;striking\;the\;runner}}}}{{{\rm{volume\;of\;water\;actually\;supplied\;to\;the\;turbine}}}}\)

\({{\rm{\eta }}_{\rm{h}}} = \frac{{{\rm{Power\;deliverd\;to\;runner}}}}{{{\rm{Power\;supplied\;at\;inlet\;}}}} = \frac{{{\rm{R}}.{\rm{P}}}}{{{\rm{W}}.{\rm{P}}}}\)

\({{\rm{\eta }}_{\rm{m}}} = \frac{{{\rm{Power\;at\;the\;shaft\;of\;the\;turbine}}}}{{{\rm{Power\;delivered\;by\;water\;to\;the\;runner}}}} = \frac{{{\rm{S}}.{\rm{P}}}}{{{\rm{R}}.{\rm{P}}}}\)

Overall efficiency: \({\eta _o} = \frac{{S.P}}{{W.P}}\)

Water Power = ρ × Q × g × h 

Calculation:

Given:

η_o = 0.8, Head h = 10 m, and Q = 1 m³/s.

\({\eta _o} = \frac{{S.P}}{{W.P}} = \frac{{S.P}}{{\rho \times Q \times g \times h}}\)

\(0.8 = \frac{{S.P}}{{1000 \times 1 \times 9.81 \times 10}} \Rightarrow S.P = 78480\;W \approx 78\;kW\)

Concept:

Specific speed (N_s):

It is defined as the speed of the turbine which is identical in shape, geometrical dimensions, blade angle, gate openings etc. with the actual turbine but of such size that it will develop unit power when working under the unit head.

\({{\rm{N}}_{\rm{s}}} = \frac{{{\rm{N}}\sqrt {\rm{P}} }}{{{{\rm{H}}^{5/4}}}}{\rm{\;\;\;}}\left( {{\rm{for\;single\;jet}}} \right)\)

\({\left( {{{\rm{N}}_{\rm{s}}}} \right)_{{\rm{multiple}}}} = \frac{{{\rm{N}}\sqrt {{\rm{nP}}} }}{{{{\rm{H}}^{5/4}}}}{\rm{\;\;\;\;}}\left( {{\rm{for\;multiple\;jet}}} \right)\)

where P = Power in kW, H = Head of water in metres.

Calculation:

Given:

(N_s)_single = 5, n = 2

\({{\rm{N}}_{\rm{s}}} = \frac{{{\rm{N}}\sqrt {\rm{P}} }}{{{{\rm{H}}^{5/4}}}}\)

∴ N_s directly proportional to the square root of Power.

\(\frac{{{{\left( {{{\rm{N}}_{\rm{s}}}} \right)}_{{\rm{single}}}}}}{{{{\left( {{{\rm{N}}_{\rm{s}}}} \right)}_{{\rm{multiple}}}}}} = \frac{{\sqrt {\rm{P}} }}{{\sqrt {{\rm{nP}}} }}\)

\(\therefore \frac{{{{\left( {{{\rm{N}}_{\rm{s}}}} \right)}_{{\rm{multiple}}}}}}{{{{\left( {{{\rm{N}}_{\rm{s}}}} \right)}_{{\rm{single}}}}}} = \sqrt 2 \)

\( \Rightarrow {\left( {{{\rm{N}}_{\rm{s}}}} \right)_{{\rm{multiple}}}} = 5\sqrt 2 = 7.071 \approx 7\)

Explanation:

Specific speed: 

• It is defined as the speed of a similar turbine working under a head of 1 m to produce a power output of 1 kW.
• The specific speed is useful to compare the performance of the various type of turbines.
• The specific speed differs for the different type of turbines and is the same for the model and actual turbine.

\({N_s} = \frac{{N\sqrt P }}{{{H^{\frac{5}{4}}}}}\)

Following are the range of specific speed of different turbines

• The specific speed of Pelton wheel turbine (single jet) is in the range of 10-35
• The specific speed of Pelton wheel turbine (multiple jets) is in the range of 35-60
• The specific speed of Francis turbine is in the range of 60-300.
• The specific speed of Kaplan/propeller turbine is greater than 300.

Important Points

Explanation:

Given;

Total discharge through pelton turbine = 3 m³/s

Area of jet = 0.02 m²

Velocity of jet = 75 m/s

Calculation:

Discharge through jet = 0.02 × 75 = 1.5 m³/s

\(Number\;of\;jet = \frac{{Total\;discharge}}{{Discharge\;through\;on\;jet}}\) = \(\frac{3}{{1.5}}\) = 2 

Concept:

The overall efficiency of the Pelton wheel is given by,

\({η _o} = \frac{{Shaft \;Power}}{{Water\; Power}}\)

Water Power = ρQgH 

Calculation:

Given:

Shaft Power = 800 kW, η_o = 0.8, H = 40 m

ρ = 1000 kg/m3, g =10 m/s2

\({η _o} = \frac{{Shaft \;Power}}{{Water\; Power}}\)

\({0.8} = \frac{{800}}{{Water\; Power}}\)

Water Power = 1000 kW

Water Power = ρQgH 

1000 × 10³ = 1000 × Q × 10 × 40

Q = 2.5 m3/s.

Explanation:

Impulse turbine: 

• If the energy available at the inlet of the turbine is only kinetic energy, the turbine so known as impulse turbine.
• The available energy at the inlet is only kinetic energy if the inlet pressure and outlet pressure is the same equal to atmospheric pressure.
• Example: Pelton Turbine

Reaction Turbine: 

• At the inlet of the turbine, the water possesses kinetic energy as well as pressure energy
• Example: Francis Turbine, Kaplan Turbine

Important Points

Impulse:

Impulse is the sudden change of momentum of an object when the object is acted upon by a force for an interval of time. 

Specific speed:

• It is defined as the speed of a similar turbine working under a head of 1 m to produce a power output of 1 kW.
• The specific speed is useful to compare the performance of the various types of turbines.
• The specific speed differs for a different type of turbines and is the same for the model and actual turbine.
• Specific Speed of Turbine, \({N_s} = \;\frac{{N \times \sqrt P }}{{{H^{\frac{5}{4}}}}}\)
• Where N = speed of turbine, P = power generated, and H = head generated

Calculation:

Given:

P =  kW, N = 300 rpm, H = 100 m

\({N_s} = \;\frac{{300 \times \sqrt {150000} }}{{{{100}^{\frac{5}{4}}}}} = 367.42 = 367\)

Explanation:

Impulse Turbine: 

If at the inlet of the turbine, the energy available is only kinetic energy, the turbine is known as impulse turbine.

Example: Pelton wheel turbine.

Pelton Wheel Turbine:

It is a tangential flow impulse turbine in which the pressure energy of water is converted into kinetic energy to form a high-speed water jet and this jet strikes the wheel tangentially to make it rotate.

The Main parts of the Pelton wheel turbine are:

1. Nozzle and Flow Regulating Arrangement
2. Runner and Buckets
3. Casing
4. Braking Jet
5. Penstock

Draft Tube is not a part of the Pelton wheel turbine, it is the main component of an axial flow reaction turbine.

Additional Information

Reaction Turbine: 

If at the inlet of the turbine, the water possesses kinetic energy as well as pressure energy, the turbine is known as a reaction turbine.

Example: Francis and Kaplan turbine.

Tangential flow turbines: 

Radial flow turbines: 

In this type of turbine, the water strikes in the radial direction. accordingly, it is further classified as

• Inward flow turbine: The flow is inward from the periphery to the centre (centripetal type). Example: Old Francis turbine
• Outward flow turbine: The flow is outward from the centre to the periphery (centrifugal type). Example: Fourneyron turbine

Axial flow turbine: 

The flow of water is in the direction parallel to the axis of the shaft.

Example: Kaplan turbine and propeller turbine.

Explanation:

Number of buckets on a runner: \(Z = 15 + \frac{D}{{2d}}\) 

Given: Jet ratio = 12 i.e D/d =12

Now,

Number of buckets on a runner = 15 + 6

∴ Number of buckets = 21

Important Points

Design parameters of Pelton wheel turbine:

1. Velocity of jet: at inlet \({V_1} = {C_V}\sqrt {2gH} \) where Cv = coefficient of velocity = 0.98-0.99

2. Velocity of wheel: \(u = \emptyset \sqrt {2gH} \) where φ is the speed ratio = 0.43-0.48

3. Angle of deflection: is 165° unless mentioned.

4. Pitch or mean diameter: D can be expressed by \(u = \frac{{\pi DN}}{{60}}\)

5. Jet ratio: \(m = \frac{D}{d}\) (12 in most cases/calculate), d = nozzle diameter or jet diameter

6. Number of buckets on a runner: \(Z = 15 + \frac{D}{{2d}}\) (Tygun formula) or, \(Z = 5.4\sqrt m \), m = 6 to 35

7. Number of Jets: obtained by dividing the total rate of flow through the turbine by the rate of flow through single jet. The number of jets is not more than two for horizontal shaft turbines and is limited to six for vertical shaft turbines.

8. Size of bucket: length of bucket L = 2.5d, width of bucket B = 5d, depth of bucket Db = 0.8d