mahalakshmimahalakshmiengineeringcollege.com/pdf/eee/ivsem/ee2252/unit 2.pdf ·...

EEE DEPT/MAHALAKSHMIENGINEERING COLLEGE,TRICHY-621213

MAHALAKSHMI

ENGINEERING COLLEGE

TIRUCHIRAPALLI – 621213

QUESTION BANK

--------------------------------------------------------------------------------------------------------------

Sub. Code : EE2252 Semester : IV

Subject : POWER PLANT ENGINEERING Unit : II

--------------------------------------------------------------------------------------------------------------

PART-A

1.Enumerate advantages and disadvantages of hydropower plants?(AU NOV 12)

Advantages:

No fuel charges

It is highly reliable

Maintenance and operation charges are very low

Running cost of plant is low

It has no stand by losses

Disadvantages:

The initial cost of plant is very high

It takes the considerable long time for the erection of such plants.

They are located far away from the load center and as such they require long

transmission lines to deliver power ,subsequently the cost of transmission line

and losses in them will be more.

2.What is the surge tank? (AU NOV 11)

surge tank is an open tank which is often used with the pressure conduit of

considerable length.


The main purpose of providing surge tank is to reduce the distance between

the free water surface and turbine, there by reducing the water hammer effect on

penstock protect upstream tunnel from high pressure rises.

It also serves as serves a supply tank to the turbine when the water to the pipe

is accelerating during increased load conditions and as storage tank when the water is

decelerating during reduced load conditions.

3.What is the function of surge tank in hydroelectric power plant? (AU APR 12)

The surge tank is introduced between the dam and power house to keep in

reducing the sudden rise in the penstock.

4.What is the purpose of trash rack? (AU APR 12)

The water is taken from the dam or from the canal is provided with trash rack to

prevent the entry of derbis which damage the wicket gates and turbine runners or choke

up the nozzles of the impulse turbine.

5.Mention the merits of hydro electric power plants? (AU APR 11)

Water is renewable source of energy and available at free of cost.

Fuel and ash handling problems are about and hence no problem.

Plant can be started and stopped quickly

Operating cost and running is considerably lower than that of thermal plants.

6.Classify the hydro electric turbines with respect to high medium and low head?

(AUAPR 13)

High head-Impulse turbine.Example:Pelton wheel

Medium head-Reaction turbine.Example:Francis turbine

Low head-Reaction turbine.Example:Kaplan turbine

7.What is the function of draft tube in hydroelectric power plant? (AU NOV 11)

(i)It is the diverging passage connects the trail with turbine exit.

(ii)The water flowing the tube is decelerated. It comes out with minimum kinetic energy

,thus it utilizes most of kinetic energy by increasing work output.

8.What are the different types of spill way ? (AU NOV 10)

solid gravity or outflow spillway


chute or traugh spillway

Side channel spillway

shaft spillway

siphon spillway

9.List any four pumped storage hydro electric plants in India?

Bhira,Maharashtra,150 MW

Kadamparai,coimbatore,Tamilnadu,400 MW(4×100 MW)

Nagarjuna sagar PH,Andra pradesh,810 MW

Tehri dam,uttranchal,1000 MW

10.List the advantages of impulse turbine ?

Greater tolerance of sand and other particles in the water

Better access to working parts

No pressure seals around the shaft

Easier to fabricate and maintain

Better part -flow efficiency

PART-B

1.Explain with a neat sketch a pumped storage plant? (AU NOV,APR 10,11,12)

Pumped storage plants are employed at the places where the quantity of water

available for power generation is inadequate. Here the water passing through the

turbines in 'tail trace pond' .

During low load periods this water is pumped back to the head reservoir using the

extra energy available. This water can be again used for generating power during

peak loads.

Pumping of water may be done seasonally or daily depending upon the

conditions of the site and the nature of load on the plant.

Such plants are usually interconnected with steam or diesel engine plants so that

off peak capacity of interconnecting stations is used in pumping water and the

same is used during peak load periods.


Of course, the energy available from the quantity of water pumped by the plant is

less than the energy input during the pumped operation.

Again while using pumped water the power available is reduced on account of

losses occurring in prime movers.

Advantages :

There is substantial increase in peak load capacity of the plant at comparatively

low capital cost.

Due to load comparable to rated load on the plant ,the operating efficiency of the

plant is high.

There is an improvement in the load factor of the plant.

The energy available in the peak loads is higher than that of during off peak

periods so that the in spite of losses incurred in pumping there is over all gain.

Load on the hydro-electric plant remains uniform.


The hydro-electric plant becomes partly independent of the steam flow

conditions.

Under the pump storage projects almost 70 percent power used in

pumping the water can be recovered. In this field the use of "Reversible Turbine Pump"

units is also worth noting.

These units can be used as turbine while generating power and as pump

while pumping water to storage. The generator in this case works as motor during

reverse operation.

The efficiency in such case is high and almost the same in both the

operations. With the use of reversible turbine pump sets, additional capital investment

on pump and its motor can be saved and the scheme can be worked more

economically.

2.What is the function of hydraulic turbine? How are the turbines classified

?Explain any one of them with suitable sketch. (AU NOV 10)

Hydraulic turbine:

A hydraulic turbine converts the potential energy of water into mechanical io

Classification of Hydraulic turbines:

(i)According to the head and quantity of water available.

(ii)According to the name of the originator

(iii) According to the action of the water on the moving blades

(iv) According to the direction of flow of the water in the runner

(v) According to the disposition of turbine shaft.

(vi) According to the specific speed Ns.

1.According to the Head and Quantity of water available:

(i)Impulse turbine-requires high head and small quantity of flow.

(ii)Reaction turbine-requires low head and high rate of flow.

Actually there are two types of reaction turbines ,one for medium head and

medium flow and the other for low head and large flow.

2.According to the name of the originator:


(i)Pelton turbine-named after the tester Allen pelton of California(USA).It is an impulse

type of turbine and is used for high head and low discharge.

(ii)Francis turbine-name after James bichens Francis .It is a reaction type of turbine for

medium high to medium low heads and medium small to medium large quantities of

water.

(iii)Kaplan turbine-named after Dr.Victor Kaplan .It is a reaction type for low heads and

large quantities of flow.

3.According to the action of water on the moving blades:

4.According to the direction of flow of water in the runner:

(i)Tangential flow turbine.(pelton wheel turbine)

(ii)Radial flow turbine(no more used)

(iii)Axial flow turbine(Kaplan turbine)

(iv)Mixed(radial and axial)flow turbine(Francis turbine)

In tangential flow turbine of pelton wheel type the water strikes the runner tangential

to the path of rotation.

In axial flow turbine water flows parallel to the axis of the turbine shaft.kaplan

turbine is an axial flow turbine.

If the runner blades of the axial flow turbines are fixed ,these are called "propeller

turbines".

In mixed flow turbines the water enters the blades radially and comes out axially

,parallel to the turbine shaft.

According to the disposition of the turbine shaft:

Turbine shaft may be either vertical or horizontal. In modern turbine

practice ,pelton turbines usually have horizontal shaft whereas the rest especially the

large units ,have vertical units.

According to specific speed:

The specific speed of turbine is defined as the speed of a geometrically

similar turbines that would develop one brake horse power under the head of one


metre.All geometrically similar turbines will have the same specific speed when

operating under the same conditions of head and flow.

Specific speed=Ns=N tP /H(5/4)

Where N-the normal working speed in rpm

Pt-power output of the turbine and

H-the net or effective head in meters

Turbines with low specific speeds work under a high head and low discharge

condition ,while high specific speed turbines work under low head and high discharge

conditions.

Description of impulse turbine:

Pelton wheel ,among the various impulse turbines that have designed

and utilized ,is far the imporant.The pelton wheel or pelton turbine is a tangential flow

impulse turbine. It consists of a rotor ,at the periphery of which are mounted equally

spaced double -hemispherical or double -ellipsoidal buckets. Water is transfered from

the high head source through penstock pipes.

A branch pipe from each penstock pipe ends in a nozzle ,through which

the water flows through high speed jet. A needle or spear moving inside the nozzle

controls the water flow through the nozzle and at the same time, provides a smooth flow

with negligible energy loss.

All the available potential energy is converted into kinetic energy before

the jet strikes the buckets. The pressure all over the wheel is constant and equal to

atmosphere ,so that energy transfer occurs due to purely impulse action.

The jet energizing from the nozzle hits the splitter symmetrically and is equally

distributed into the two halves of hemispherical bucket as shown. The bucket centre

line cannot be made exactly like a mathematical cusp, partly because of manufacturing

difficulties and partly because of jet striking the cusp ,partly because of manufacturing

difficulties and partly because the jet striking the cusp invariably carries particles of sand

and other abrasive materials which tend to wear it down.

The inlet angle of the jet is therefore between 10 and 30 ,but it always assumed to

zero in all calculations.Theoritically,if the bucket were exactly hemispherical ,it would

deflect the jet through 180.Then ,the relative velocity of the jet leaving the bucket

Cr2,would be opposite in direction to the relative velocity of the entering jet Cr1.


This cannot be achieved in practice since the jet leaving the bucket would

then strike the back of succeeding bucket to cause splashing and interference so that

the overall performance of turbine efficiency would fall to low values .Hence in practice

,the angular deflection of the jet is limited to about 165 or 175 and the bucket is

therefore slightly smaller than a hemisphere in size.

3.What are the factors to be considered while selecting a site for hydroelectric

power plant? (AU APR 12)

(i)Water availability:

Availability of water and rainfall data are considered ,the rounded value of

these data should be available.

(ii)Water storage:

Storage capacity of the plant is always necessary for continuous generation

of power as there is wide variation in rainfall during the year.

(iii)Water head:

Topography of the area decides the availability of water head. An every

increase in head for the given output reduces the quantity of water to be stored and

therefore ,the capital cost.

(iv)Various geological investigations:

The tight basin of ample size.

A narrow outlet requiring dam of less volume.

A strong foundation able to support the dam structure.

Availability of materials for constructing dam.

(v)Environmental aspects:

(i)To ensure safe operation,helpful,productive and culturally pleasing surrounding.

(ii)To eliminate health hazards or other undesirable and unintended surrounding.

(iii)To preserve important historic ,cultural and natural aspect of site.


4.Draw the layout of hydroelectric power plant and explain its function of

components? (AU APR 11)

Water reservoir:

The quantity of water flow is extremely large during rainy season whereas

the quantity of water flow is very little during the summer season of the year.

But ,continuous availability of water is basic necessity for hydroelectric plants.

The main purpose of the reservoir is to store the water during rainy season and

supply the same to the turbine continuously throughout the year.

The capacitor of the reservoir of natural sites is determined with the help of

topographic surveys as its shape is always irregular.Water surface in the storage

reservoir is known as headrace.

Dam:

The dam is used in the hydro power plant to increase the height of water level

and thereby increasing the capacity of reservoir. The dam also helps to increase

the working head of the power plant .

The dam can be built is several different forms and each form uses the different

materials to build it .The form of the dam and its material depends upon the

various factors such as topographical ,geographical ,availability of materials -

economical factors and many others.


The dam must be stable ,water tight, low in maintenance cost. Simple in

construction and economic for its use. Modern dams like ballow dams are more

safe, aesthetic in design and also economical.

Spillway:

Water after certain level in the reservoir overflows through spillway without the

increase in water level in the reservoir during rainy conditions.

A spillway acts as a safety valve for the dam.It must have the capacity to

discharge major floods without damaging the dam.

Trash rack:

The water is taken from the dam or from the canal is provided with trash rack to

prevent the entry of debris which might damage the gates and turbine runners or

choice up the nozzles of the impulse turbine.

Fare bay:

It serves as temporary regulating reservoir which is used to store water when

the load on the plant is reduced and provide water for initial increment of an

increasing load while water in the canal is being accelerated .

In some cases ,the canal itself is large enough to absorb the flow variations.

This can be considered as naturally provided surge tank as it does the work of

surge

Water Tunnel:

Water tunnel carries water from the reservoir to surge tank. The tunnels may

have either of circular cross section or of horse shoe section.

A concrete or steel liner is necessary to prevent the collapse ,if the tunnel

material is weak. The tunnels carried through the rock are not lined by

smoothened to improve the hydraulic efficiency. The tunnels act as open

channels when flowing partially full and act as pressure conduits when flowing

full.

Canals:

A water channel is required to carry the water to the power house where

additional head is developed over and above that created by the dam. The water

channel may consist of canal penstock a combination of two.


canal and closed conduits are used as non pressure tunnels .Canals are

generally used where the quantity of water carried is too large because pipe

would be too large and too costly.

Penstock:

A penstock laid between the surge tank and prime mover is known as penstock.

It is commonly made of reinforced concrete or steel .The penstock is designed

for bearing very high pressure on inside surface during decreased demand on

generator and on outside surface during increased load conditions.

Penstock are usually -equipped with the head gates at the inlet which can be

closed during repair of the penstock.

The inlet valve downstream from the gate is provided to prevent collapse of pipe

immediately after the head gate is closed.

Surge Tank:

There is a sudden increase in pressure of the penstock due to sudden backflow

of water as load on the turbine is reduced when the load on the generator

increases .

This sudden rise of pressure in the penstock above the normal due to reduced

load on the turbine is known as water hammer.

When the load on the generator is increases due to high demand of power ,the

turbine gates are opened suddenly to produce more power.

At that time ,water has to rush through the penstock and there is tendency to

cause vacuum in the water system. The penstock must withstand the positive

hammer due to sudden closing of turbine gates and no vacuum should be

produced when the gates are opened suddenly due to increase in load condition.

Water turbine:

Water through the penstock enters into the turbine through inlet valve.water

turbine converts kinetic energy of water into mechanical energy to produce

electrical energy.

Prime movers which are commonly used are pelton turbine ,Francis turbine and

Kaplan turbine. The mechanical energy is available at the turbine shaft is used

to run the electric generator .

Draft tube:


The draft tube is connected at the outlet of water turbine. It allows the turbine to

be placed over the tailrace level. Therefore ,simply a draft tube is a pipe or passage of

gradually increasing cross-sectional area which connects the runner exit to the tail race.

The pressure at the exit of the runner of a reaction turbine is generally less than

the atmospheric pressure. By passing water through the draft tube ,the outlet velocity of

water is reduced considerably and gain in useful pressure head is achieved, Thus ,the

net working head on the turbine increases and therefore the output of the turbine also

increases. It is used to stop the remaining kinetic energy of water coming out of the

turbine.

Tailrace:

It is a waterway to lead the water discharged from the turbine to the river.

Transformer:

The transformer is to be raise the voltage generated at the generator terminal

before transmitting the power to consumers and workstations

5.Describe the selection factors for a hydraulic turbine? (AU APR 12)

The factors to be considered while selecting the hydraulic turbine.

(i)Rotational speed of the turbine

(ii)Specific speed

(iii)Maximum efficiency

(iv)part load efficiency

(v)head

(vi)type of water

(vii)runaway speed

(viii)cavitation

(ix)number of turbine units

(x)overall cost

(i)Rotational speed of the turbine:


The speed of the generator is assumed to be the same speed of turbine

speed by considering negligible transmission losses.

It is given by

Ns=60f/P

Ns-synchronous speed

f-frequency

P-number of poles

(ii)Specific speed:

The specific speed is defined by unit power is taken one KW and unit head is one

metre.

Ns= 5

4

N P

H

Specific speed Types of turbine

10-30 Pelton with single jets

17-50 Pelton with two jets

24-70 Pelton with four jets

70-257 Francis turbine

257-858 Kaplan turbine

(iii)Maximum efficiency:

the efficiency of any turbine is calculated based on the power developed. The

turbine which can give maximum power output developed. The turbine which an give

maximum power output for given input will give maximum efficiency.

(iv)part load efficiency:

Full load is defined as the load at which the turbine can produce maximum

efficiency. If the load increases above the full load ,it is known as overload, if it

decreases

below the full load ,it is known as under load or part load.

It is constant for all types of turbines.


(vii)runaway speed:

The speed of turbine increases with increases with increase in head.All generators

are designed for full run away speed.

(vi)type of water:

Reaction turbines are not suitable for dirt and sand water because it will cause

erosion of the runner blade. Because of erosion ,more friction will be produced.

(viii)cavitation:

Whenever the specific speed of the turbine increases beyond the recommended

values ,the cavitation might occur. To avoid this dense metal like cast steel and

stainless steel can be used rather than porous material like cast iron.

(x)overall cost:

The good design should be in such a way to give maximum efficiency with minimum

cost.

Total cost=capital +running cost

6.Neatly explain the hydro electric energy resources in India?

Hydro electric energy sources in India:

Hydropower is a commercial source of energy which supplies 22% of total electricity

of the world in 1980.It has been estimated that currently a merely 17% of world hydel

potential is tapped.

The Indian rivers can be divided into categories as rain fed and rain as well as

snow fed rivers.

The rain fed rivers are characterized by heavy discharged during the monsoon

month but flows dwindles to a mere trickle during the summer.

Rain and snow fed rivers originating in Himalayas ,the potential feed of snow

reduces the wide variation of inflow to some extent.

The installed hydroelectric capacity was 1300 MW before the

independence .Now has been raised to 15000 MW.Apart from the being perennial and


inexhaustible source of energy, it represents the cheapest source of energy in one

country.

The major difficulty in development of hydroelectric project is that it takes

relatively longer time for its hydrological ,topographical, geological and economical

investigation.

out of hydroelectric potential,33.4% is located in northern region,35.5%

in north-eastern region,7.8% in western region,14.4% southern region and 8.9% in

eastern region.

7.Explain the neat sketch the function of surge tank in hydroelectric power

plant? (AU APR12)

It reduces the distance between the free water level of the reservoir and turbine

and also reduces the intensity of water hammer that huge chamber to such an

extent above the water hammer effect can be neglected in the design of tunnel.

The surge tank acts as a relief value when load on the turbine is reduced and the

pressure in the pipe suddenly increases by diverting the main conduit flow partly

or wholly into this tank.

Te surge tank acts as a temporary reservoir during increased load on the

turbine.It provides sufficient water to enable the turbine to pick up the new load

quickly and safely and to keeping it running at the increased load until the water

level in the surge tank falls below its original level.

8.How are the dams classified? What are the factors to be considered in

selecting type of dams? (AU APR 10)

The dams are classified as follows

(i)Masonary dams

(ii)Gravity dam

(iii)butress dam

(iv)arch dam

Earth fill dams


Rock fill dams

Factors to be considered in selecting type of dam:

Topographical

Geological

availability of material

labour cost

transportation

9.Mention the advantages and disadvantages of hydropower ?

mahalakshmimahalakshmiengineeringcollege.com/pdf/eee/ivsem/ee2252/unit 2.pdf ·...

Documents