Basic Civil and Mechanical - Unit 3 Notes

BASIC CIVIL AND MECHANICAL ENGINEERING

Unit III

(Power Plant Engineering, Pumps and Turbines)

Classification of Power Plants

 

Steam Power Plant

Steam is used to drive steam engines and steam turbines due to the following reasons:

1. Steam can be raised quickly from water
2. It does not react much with materials.
3. It is stable at temperatures required in the plant

Layout of Steam Power Plant:

The layout of steam power plant has the following circuits:

1. Fuel (Coal) and ash circuit
2. Air and flue gas circuit
3. Feed water and steam flow circuit
4. Cooling water flow circuit.

Coal and Ash Circuit:

 

Layout of Steam Power Plant

Water and Steam Circuit:

• The water is preheated by the flue gases in the economiser.
• This preheated water is then supplied to the boiler drum.
• Heat is transferred to the water by the burning of the coal.
• Due to this, water is converted into the steam.
• The steam raised in boiler is passed through a super heater.
• It is superheated by the flue gases.
• The turbine drives generator to produce electric power.
• The expanded steam is then passed through the condenser.
• In the condenser, steam is condensed into water the re circulated.

Cooling Water Circuit:

• The exhaust steam from the turbine is condensed in the condenser.
• In the condenser, the cold water is circulated to condense the steam into water.
• The steam is condensed by loosing its latent heat to the circulating the cold water.
• Hence the cold water gets heated.
• This hot water is then taken to a cooling tower.
• In cooling tower the water is sprayed in the form of droplets through nozzles.
• The atmospheric air enters the cooling tower from the openings provided at the bottom of the tower.
• This cold water is again circulated through the pump, condenser and the cooling
• Some amount of water may be lost during circulation.
• Hence make up water is added to the pond by means of a pump

Layout of Steam ( Thermal) Power Plant

Energy Conversion Process: 

Chemical Energy (Fuel/Coal) -- > Heat Energy (Boiler) --> Mechanical Energy (Turbine) -->Electrical Energy ( Generator)

Advantages of Steam Power Plant (Thermal plant)

• Life of plant is more (25-30 years ) compared to Diesel plant (2-5 years)
• Repair and maintenance cost is low when compared to diesel plant.
• Initial cost is less compared to nuclear plant.
• Suitable for varying load conditions.
• No radio active harmful wastes are produced
• Unskilled operators can operate the plant.
• The power generation does not depend on the water storage.
• There are no transmission losses, as they are located near load centres.

Disadvantages of thermal power plant:

• Less efficient than diesel plants.
• Starting up and bringing into service takes more time.
• Cooling water required is more.
• Space required is more.
• Storage required for the fuel is more.
• Ash handling is a big problem
• Not economical in areas which are remote from coal fields.
• Manpower required is more.
• For large units, the capital cost is more.

List down the factors to be considered for selection of site for thermal power plant:

Availability of coal:

• A thermal plant of 400M, capacity requires nearly 6000 tons of coal every day.
• Power plant should be located near coal mines.

Ash Disposal Facilities:

• Ash comes out in hot condition and handling is difficult.

The ash can be disposed into sea or river.

Water Availability :

• Water consumption is more as feed water into boiler, condenser and for ash disposal.
• Water is required for drinking purpose.
• Hence plant should be located near water source.

Transport Facility :

• Railway lines or other mode of transport for bringing heavy machineries for installation also for bringing coal.

Public Problems:

• The plant should be far away from residential area to avoid nuisance from smoke, fly ash and noise.

Nature of Land :

• Many power plants have failed due to weak foundations.
• Land (soil) should have good bearing capacity to withstand dead load of plant.

Thermal power plants in Tamil Nadu:

Neyveli

Tuticorin

Ennore

Mettur

Explain about the pollution caused by Thermal Power Plant (Steam Power Plant):

• Main pollutants from thermal plants are SO₂, CO₂, CO as minute particles such as fly ash.
• SO₂ causes suffocation, irritation to throat and eyes and respiratory for people. It destroys crop.
• CO is a poisonous gas.
• Dust particles cause respiratory troubles like cough, cold, sneezing etc.,

Thermal Pollution:

• Thermal plants produce 40 millions kJ of heat to the environment through condenser water and exhaust gases.
• Thermal pollution of atmosphere can be reduced using the low grade energy exhausted steam.

Noise Pollution:

• The sources of noise in a power plant are turbo alternators, fans and power transformers.
• Sound proofing can be done to reduce the noise.

Gas Power Plant

• A gas power plant uses gas turbine as the prime mover for generating electricity.
• It uses natural gas or kerosene or benzene as fuel.
• Gas plant can produce only limited amount of the electricity.
• Efficiency of the plant is only 35%
• Generally a gas plant is expensive to operate.
• Hence it is usually installed with steam power plant in closed combined cycle.
• It is generally used in combination with steam/thermal power plant during peak load
• When the gas power plant is combined with thermal/steam power plant efficiency of the plant is up to 60% - 70%
  
  
  Layout of the Gas turbine Power plant:
  
   Gas Power Plant – Working Principle:
  
  
  Combustion and generation of electricity:
• Gas turbine draws clean air into through air filter from atmosphere, with the help of a compressor.
• During the compression pressure of the air is increased.
• Compressed air is passed through to a combustion chamber along with fuel (Natural gas).
• The air fuel mixture is ignited at high pressure in the combustion chamber.
• Combustion takes place.
• The generated hot gas of compression is passed through the gas turbine.
• Hot gases expand, and the turbine blades are connected to the turbine shaft are rotated.
• The turbine shaft which is coupled to the shaft of the electrical generator at the other end also rotates and drives the electrical generator.
• A portion of the energy developed by the hot gases through the gas turbine is used to run the compressor.
• The residual hot gases from gas turbine are passed through a heat exchanger (heat recovery steam generator)
• The heat exchanger produces steam with high pressure with the help of a steam boiler.
• The steam is allowed to expand in the steam turbine.
• when it passes through the turbine blades, the turbine shaft is rotated. The shaft is coupled to the generator, which generates electricity.
• Gas turbine and steam turbine combination enables increased power generation.

Transmission and distribution :

• The generated electricity from both gas and steam turbines is fed to the step up transformer where its voltage is increased.
• Then the electricity is conveyed through transmission lines for distribution.

MERITS:

Natural gas is readily available.

• Setting up cost can be reduced if the plant is installed near the source of natural gas.
• Less gas storage cost
• Less space occupation.
• Compared to steam power plant, smaller in size.
• Low operating cost.
• Low maintenance cost.
• No standby losses.
• Cheaper fuels like natural gas.

Demerits:

• 2/3 rd of generated power is used for driving the compressor.
• Gas turbine has low thermal efficiency.
• Has starting problem.
• Efficient only in combined cycle configuration.
• Temperature of combustion chamber is too high, which results in shorter life time.
  
  
  Diesel Power Plant
  
  Working of Diesel Power plant:
  
  
• Air from atmosphere is drawn into the compressor and is compressed.
• The compressed air is sent to diesel engine through filter.
• In the filter, dust, dirt from air are filtered and only clean air is sent to diesel engine.
• Fuel oil from tank is passed through filter where it gets filtered and clean oil is injected into the diesel engine through fuel pump and fuel injector
• Mixture of compressed air and spray of fuel oil are ignited into the engine and combustion takes place.
• The heat energy is utilized for driving the generator, which produces power.

Main components of a Diesel power plant:

1. Fuel Supply system

It consists of fuel tank, fuel filter and fuel pump and injector.

2. Air Intake and Exhaust system

It consists of compressor, filter and pipes for the supply of air and pipes for exhaust gases. In the exhaust system silencer is provided to reduce the noise.

3. Cooling system

Circulates water around the Diesel engines to keep the temp at reasonably low level.

4. Lubricating system

It includes lubricating oil tank, pump, filters and lubricating oil.

5. Starting system

For initial starting the devices used are compressed air, battery, electric motor or self-starter.

Hydro Electric Power Plant:

Components of Hydro Electric Power Plant:

Reservoir :

• Water is collected during rainy season
• It is stored in the reservoir.
• A dam is built across the river adequate water head.

Penstock :

• It is a passage through which water flows from reservoir to turbine.

Surge Tank :

• It is installed along the penstock (between turbine and reservoir)
• To control or regulate the sudden water over flow and to protect the penstock from bursting.
• It reduces the pressure and avoids damage to the penstock due to the water hammer effect.
• When the load on the turbine is decreased there will be a back flow, which causes increase or decrease in pressure. It is known as water hammer.
• Power House :
• It is building that houses that water turbine, generator, transformer and control room.
• Water Turbine:
• Water turbines such as Pelton, Kaplan and Francis are used to convert pressure and kinetic energy of flowing water into mechanical energy.
• Draft Tube:
• It is connected to the outlet of the turbine.
• Tailrace:
• It refers to the downstream level of water discharged from turbine.
• Generator :
• It is a machine used to convert mechanical energy into electrical energy.
• Step up transformer:
• It converts the Alternating Current (AC) into high voltage current suitable for transmission.

Working Principle of Hydro Electric Power Plant:

• It uses the potential energy of water of water stored in a reservoir.
• The water from the reservoir through a penstock and then forced through nozzle or nozzles before reaching the turbine.
• The hydraulic turbine converts the kinetic energy of water under pressure into mechanical energy.
• The shaft of the turbine is coupled to a generator that generates electricity
• The electricity generated is fed to the step-up transformer to increase its voltage.
• Power is fed to the transmission lines for distribution.
• The output power of Hydel power plant depends on the head of water stored in the reservoir and the quantity of water discharged.
  
  
  Classification of Hydro Electric Power Plant:
  
  Factors to be considered for the location of hydro electric Power Plant:
  Availability of Water:
  Adequate water must be available with good head.
  Cost and type of Land:
  Bearing capacity of the land should be good to withstand huge structures and equipments.
  Storage of Water :
  A dam must be constructed to store the large quantity of water in order to cope with variations of water availability through out the year.
• Transportation Facilities :

The site should be accessible by rail and road for easy transportation of equipments and machinery.

• Pumped storage facilities :

The pumping facilities to reuse the water should be possible.

Merits of Hydro Electric Power Plant:

• Requires no fuels and hence pollution free.
• Low operating cost.
• Simple in construction and requires less maintenance.
• Very robust and durable.
• The reservoir and dam can also be used for irrigation.

Demerits of Hydro Electric Power Plant:

• Very high capital cost
• Skilled personnel is required for construction.
• High cost of transmission as plant is normally required far off from hilly areas.
• Period of delay causes the delay in the commissioning of the plant.
• Construction of new hydel plant may need rehabilitation of people and payment compensation for land acquisition.
  
  
  NUCLEAR POWER PLANT – LAYOUT
  
  
  
  • Nuclear power plant uses nuclear energy from radio active element for generating electrical energy.
  • More than 15% of the world’s electricity is generated from Nuclear power plants.
  • It is generally located far away from populated areas.
  • In future generation of electricity will be depending on Nuclear Power Plant, as it is economical.
  • 1 kg of uranium U -235 can produce electrical power electrical that can be produced by using 3000 -4500 tonnes of high grade coal or 2000 tonnes of oil.
  
  
  Components of Nuclear Power Plant:
  Nuclear Fuel :
  Normally used nuclear fuel is uranium (U²³⁵)
  Fuel Rods: 
  
  
  The fuel rods hold nuclear fuel in a nuclear power plant.
  Neutron Source: A source of neutron is required to initiate the fission for the first time. A mixture of beryllium with plutonium is commonly used as a source of neutron.
  
  
  Reactor:
• Nuclear fission takes place in the reactor only.
• Nuclear fission produces large quantity of heat.
• The heat generated in the reactor is carried by coolant circulated through the reactor.

Control Rods:

• They are used to control the chain reaction.
• They are absorbers of neutrons.
• The commonly used control rods are made up of cadmium or boron.

Moderator:

• Moderators are used to slow down the fast neutrons.
• It reduces 2 MeV to an average velocity of 0.025 eV.
• Ordinary or heavy water are used as moderators.
• Fuel Rods:
• The fuel rods hold nuclear fuel in a nuclear power plant.
• Neutron Reflectors:
• To prevent the leakage of neutrons to large extent.
• In PHWR, the moderator itself acts as reflectors.
• Shielding:
• To protect from harmful radiations the reactor is surrounded b a concrete wall of thickness about 2 to 2.5 m.
                                                              Nuclear Fission
   
  
  
  • It is a process of splitting up of nucleus of fissionable material like uranium into two or more fragments with release of enormous amount of energy.
  • The nucleus of U²³⁵ is bombarded with high energy neutrons
  U²³⁵+₀n¹ Ba ¹⁴¹+Kr⁹²+2.5₀n¹+200 MeV energy.
• The neutrons produced are very fast and can be made to fission other nuclei of U²³⁵, thus setting up a chain reaction.
• Out of 2.5 neutrons released one neutron is used to sustain the chain reaction.

1 eV = 1.6X10^-19 joule.

1 MeV = 10⁶ eV

Working Principle of Nuclear Power Plant:

• The heat generated in the reactor due to the fission of the fuel is taken up by the coolant.
• The hot coolant then leaves the reactor and flows through the steam generator.
• In the steam generator the hot coolant transfers its heat to the feed water which gets converted into steam.
• The steam produced is passed through the turbine, which is coupled with generator.
• Hence the power is produced during the running of turbine.
• The exhaust steam from the turbine is condensed in the condenser.
• The condensate then flows to the steam generator through the feed pump.
• The cycle is thus repeated.

Advantages of Nuclear Power Plant:

• Requires less space compared to steam power plant.
• Fuel required is negligible compared to coal requirement.
• Fuel transport cost is less.
• Reliable in operation.
• Cost of erection is less.
• Water required is very less.

Disadvantages of Nuclear Power Plant:

• Initial Cost is higher.
• Not suitable for varying load condition.
• Radioactive wastes are hazardous. Hence these are to be handled with much care.
• Maintenance cost is higher.
• Trained workers are required to operate the plant.

Nuclear Power Plants in India:

• IGCAR, Kalpakkam in Chennai.
• Rana Pratap Sagar in Rajasthan
• Narora in Uttar Pradesh
• Kakarpur near Surat at Gujarat

Kaiga Power Plant at Karnataka

Pumps

• A pump is a machine which is used to raise or transfer the fluids.
• It is also used to maintain the constant flow rate or constant pressure.
• It is normally driven by a engine or a motor.
• Pumps are rated by the horse power.
• Important specifications for pump maximum discharge flow, maximum discharge pressure, inlet size and discharges size.

Classification of pumps:

It is classified into positive displacement pumps and roto dynamic pumps.

• In positive displacement pumps, fluid is drawn or forced into a finite space and it is sealed.
• It is then forced out and the cycle is repeated.

In roto dynamic pumps, centrifugal force is used to move the fluid into a pipe.

Reciprocating Pumps:

• It is a positive displacement pump
• It uses a piston and cylinder arrangement with suction and delivery valves integrated with the pump.
• It can be single acting and double acting
• There may be single or multi cylinders also.
• It is a positive displacement pump
• It sucks and raises the liquid by actually displacing it with a piston/plunger that executes a reciprocating motion in a closely fitting cylinder.

Working of single acting Reciprocating Pump:

• During suction stroke the piston moves to the left, causing the inlet valve to open.
• Water is admitted into the cylinder through the inlet valve.
• During the discharge stroke the piston moves to the right closes the suction valve and opens the out let valve.
• Through the outlet valve the volume of liquid moved out of the cylinder.

Double Acting Reciprocating Pump – Working:

• Each cycle consists of two strokes.
• Both the strokes are effective, hence it is known as double acting pump
• Liquid is filled at one end and discharged at other end during forward stroke.
• During the return stroke, end of cylinder just emptied is filled and the end just filled is emptied.

Air Vessels

Air vessel is a closed chamber containing compressed air in the upper part and liquid being pumped in the lower part.

 

Purpose of using an Air Vessel:

1. To get continuous supply of liquid at a uniform rate.
2. To save the power required to drive the pump (By using an air vessel the acceleration and friction heads are considerably reduced)
3. To run the pump at much higher speed without any danger of separation. 

Advantages of reciprocating pump:

• Relatively compact design
• High viscosity performance
• Ability to handle high differential pressure.

Centrifugal Pumps

Components of Centrifugal pump:

• A rotating component comprising of an impeller and a shaft.
• A stationery component comprising a volute (casing), suction and delivery pipe.

Working Principle of Centrifugal pump:

Principle: When a certain mass of fluid is rotated by an external source, it is thrown away from the central axis of rotation and a centrifugal head is impressed which enables it to rise to a higher level.

Working:

• The delivery valve is closed and the pump is primed, so that no air pocket is left.
• Keeping the delivery valve still closed the electric motor is started to rotate the impeller.
• The rotation of the impeller is gradually increased till the impeller rotates at its normal speed.
• After the impeller attains the normal speed the delivery valve is opened when the liquid is sucked continuously upto the suction pipe.
• It passes through the eye of the casing and enters the impeller at its centre.
• The liquid is impelled out by the rotating vanes and it comes out at the outlet tips of the vanes into the casing.
• Due to the impeller action the pressure head as well as the velocity heads are increased.
• From the casing the liquid passes into the pipe and lifted to the required height.
• When pump is to be stopped the delivery valve is to be first closed, other wise there may be some backflow of water into the reservoir.

Types of casing

Volute and Vortex Casing

Volute and Diffuser casing

Volute Casing: In this type of casing the area of flow gradually increases from the impeller outlet to the delivery pipe.

Vortex Casing: If a circular chamber is provided between the impeller and volute chamber the casing is known as Vortex Chamber.

Diffuser C :

• The impeller is surrounded by a diffuser.
• The guide vanes are designed in such a way that the water from the impeller enters the guide vanes without shock.
• It reduces the vibration of the pump.
• Diffuser casing, the diffuser and the outer casing are stationery parts.

Priming of a centrifugal Pump:

• The operation of filling the suction pipe, casing and a portion of delivery pipe with the liquid to be raised, before starting the pump is known as Priming
• It is done to remove any air, gas or vapour from these parts of pump.
• If a Centrifugal pump is not primed before starting air pockets inside impeller may give rise to vortices and causes discontinuity of flow

Losses in Centrifugal pump:

Hydraulic Losses:

• Shock or eddy losses at the entrance to and exit from the impeller
• Losses due to friction in the impeller
• Friction and eddy losses in the guide vanes/diffuser and casing

Mechanical Losses:

• Losses due to disc friction between the impeller and the liquid which fills the clearance spaces between the impeller and casing
• Losses pertaining to friction of the main bearing and glands.

Specific speed of Centrifugal Pump:

• It is the speed in revolutions per minute at which a geometrically similar impeller would deliver one cubic meter of liquid per second against a delivery head of one meter.
   
  IMPULSE TURBINE:
• The steam coming out at a very high velocity through the nozzle impinges on the blades fixed on the periphery of rotor.
• The blades change the direction of steam flow without change in pressure.
• The resulting force causes the rotation of the turbine.

E.g Pelton wheel.

REACTION TURBINE:

• The high pressure steam from the boiler is passed through the nozzles.
• When the steam comes out through these nozzles, the velocity of steam increases relative to the rotating disc.
• The resulting force of steam on nozzle gives the rotating motion to the disc and the shaft.
• The shaft rotates in opposite direction of the steam.

E.g Francis Turbine, Kaplan Turbine.

Comparision between Impulse and Reaction turbine:

S.No.	Impulse Turbine	Reaction Turbine
1	It consists of nozzles and moving blades	It consists of fixed blades which act as nozzles and moving blades
2	Steam is expanded completely in the nozzle. All the pressure energy is converted into kinetic energy	Steam is partially expanded in the fixed blades. Some amount of pressure energy is converted into kinetic energy
3	Pressure of steam is constant over the moving blades.	Pressure drop takes place in the moving blades.
4.	Because of high pressure drop in the nozzles, blade speed and steam speed are high.	Because of small pressure drop, blade speed and steam speed are less.
5.	Low Efficiency	High Efficiency
6.	Occupies less space per unit power	Occupies more space per unit power.
7.	Suitable for small power requirements	Suitable for medium and high power requirements.

2 Mark Questions

1. List out the conventional power plants.
2. What are the main purposes of a condenser in a steam power plant?
3. Write three factors to be considered for the selection of a steam power plant site.
4. State two advantages and disadvantages of a nuclear power plant.
5. Briefly write the working principle of a gas turbine.
6. Give two advantages and disadvantages of a gas turbine power plant.
7. Give two examples for reaction hydraulic turbine.
8. What is the use of a surge tank in a hydraulic power plant?
9. What are the alternate sources of energy to fossil fuels?
10. What is the function of moderator in nuclear power plant?
11. Write short notes on nuclear waste disposal.
12. What is the use of turbocharger in diesel power plant?
13. What is priming?

Self study topics – Assignment -1

1.Cogeneration - Explain

2. What are topping and bottoming cycle in steam power plant?

3. State the methods of improving the thermal efficiency of a gas turbine cycle.

4. Explain greenhouse effect.

5. What are the causes and effects of global warming?

           6. Compare thermal power and hydro electric power. 

Prepared by

A.R. PRADEEP KUMAR,

Assistant Professor/Mech- S&H,

DHANALAKSHMI COLLEGE OF ENGINEERING