Steam power plants

UNIT 3
Steam Power Plants
CO - Identify various componentsof steam, Diesel and Gas turbine Power Plants.

• SYLLABUS
• 3.1 Steam power plant introduction, components, advantages and
limitations.
• 3.2 Fuel handling system in power plant types and component
• 3.3 Electro-static precipitators.
• 3.4 Control systems of power plant elements, types, desirable
characteristics.
• 3.5 Steam temperature control and feed water control
• 3.6 Maintenance procedure of major components of steam power plant.

• A Steam Power Plant converts the heat energy of coal into electrical energy. Coal
is burnt in a boiler which converts water into steam. The expansion of steam in
turbine produces mechanical power which drives the alternator coupled to the
turbine. Steam Power Plants contribute maximum to the generation of Power for
any country.
• Steam Power Plants constitute 75.43% of the total installed captive and non-
captive power generation in India.
• In Steam generating stations coal, oil, natural gas etc. are employed as primary
sources of energy.
• Firstly the water is taken into the boiler from a water source. The boiler is heated
with the help of coal.
• The increase in temperature helps in the transformation of water into steam. The
steam generated in the boiler is sent through a steam turbine.
• The turbine has blades that rotate when high velocity steam flows across them.
This rotation of turbine blades is used to generate electricity.
Introduction of Steam Power plant

Introduction of Steam Power plant
• A generator is connected to the steam turbine. When the turbine turns, electricity is
generated and given as output by the generator, which is then supplied to the
consumers through high-voltage power lines.

Basic Components of steam power plant
Principle of Working: • A steam power plant converts chemical energy of fossil fuels,
such as, coal, gas etc. into mechanical energy, which can be further converted into
electrical energy by using an electric generator.
Basic Components of Steam Power Plant are:

Basic Components of steam power plant
1. Boiler: It is used to produce high pressure high temperature steam from water after
absorption of heat. This heat required is, obtained by burning the fossil fuel such as coal in
the combustion chamber.
2. Steam Turbine: High pressure high temperature steam produced in the boiler is sent to the
steam turbine. At the inlet of turbine, a nozzle is provided. While passing through the nozzle,
pressure energy of steam is converted in to kinetic energy. Thus, velocity of steam is
increased. These high velocity steam jets issued from nozzle are allowed to strike over the
blades mounted on the rotor of steam turbine. Expansion of steam makes the rotor to rotate
and produces mechanical energy.
3. Electrical Generator: Output shaft of steam turbine is coupled to shaft of electriCal
generator. This electrical generator converts the mechanical energy into electrical energy, i.e.
electricity.
4. Condenser: The exhaust steam leaving the steam turbine is passed into the condenser,
where it is cooled and condensed to give water as condensate with the help of a cooling
medium, such as, cold water or air . or combination of both water and air.
5. Feed water pump: The condensate, i.e. water leaving the condenser is pumped back to the
boiler with the help of feed water pump.

Major Component of a Modern Steam Power Plant
• A modern steam power plant consists of following main components:
1. Boiler with accessories such as Superheater, Air-preheater, Economiser etc.
2. Steam turbine.
3. Electric generator.
4. Condenser with condensate extraction pump.
5. Cooling tower.
6. Feed water pump (also known as Circulating water pump)..
7. Coal handling system.
8. Ash handling system.
9. Ash precipitators.
10.Boiler chimney.
11.Forced draught fans.
12.Feed water treatment plant.
13.Storage yard.
14.Control room.

Layout of Steam Power plant
1. Coal and ash circuit, .
2. Air and gas circuit.
3. Feed water and steam circuit.
4. Cooling water circuit.

Advantages and Disadvantages of steam power plant
Advantages of Steam Power Plant
1. Steam power plant can respond to rapidly
changing loads without difficulty.
2. A portion of the steam generated can be used
as a process steam in different industries.
3. Steam power plant can be located very
conveniently near the load center.
4. As steam power plant can be set up near the
industrial zone, transmission costs are
reduced.
5. Steam engines and turbines can work
continuously under an overload of 25 %.
6. Cheaper fuel can be used.
7. Less space is required as compared to hydro-
electric plants.
8. Cheaper in production cost as compared to
diesel power plant.
Disadvantages of Steam Power
Plant:
1. Maintenance and operating costs are high.
2. The cost of plant increases with increase in
temperature and pressure.
3. Long time required for erection and putting
into action (i.e. actual use).
4. A large quantity of Water is required.
5. Great difficulty experienced in coal
handling.
6. The plant efficiency decreases rapidly, if
plant runs below 75% load.
7. Presence of troubles due to smoke and
heat in the plant.

Fuel (coal) Handling
• Three types of fuels can be burnt in any type of steam generating plant:
1. Solid fuel such as coal,
2. Liquid fuel such as oil,
3. Gaseous fuel such as gas.

Rise in capacity of the plant faces a problem in coal supply system from coal mines to
the power stations. The coal from coal mines may be transported by the following
means:
1. Transportation by sea or river.
2. Transportation by rail.
3. Transportation by ropeways.
4. Transportation by road.
5. Transportation of coal by pipeline.

Factors to be Considered While Selecting a Coal Handling System
Following factors should be considered in selecting a coal
handling system:
1. Handling method should be simple requiring less labor and
movement of coal.
2. Handing should be centralized to facilitate inspection and
maintenance.
3. Duplicate handling of coal should be avoided.
4. Electrical motors should be used as prime movers, as they are
reliable and flexible with high residual value.
5. Working parts must be enclosed to avoid abrasion and
corrosion.
6. Coal handling system should be able to deliver required
quantity of coal during peak hours..
7. Coal handling system should have surplus capacity to meet
additional demand .
8. Coal handling system should be flexible and adaptable to load
changes.

Fuel (coal) Handling
• Coal is transported to
power station by rail or
road and stored in coal
storage plant and then
pulverized.
• The function of coal
handling plant is automatic
feeding of coal to the
boiler furnace.
• A thermal power plant
bums enormous amounts
of coal.
• A 200MW plant may
require around 2000 tons
of coal daily.

Types of fuels (coal) handling Equipment or Devices
• Unloading equipment
• Coal preparation equipment
1. Crushers
2. Sizers
3. Dryers
4. Magnetic separator
• Equipment’s related to transport
of coal
1. Belt conveyors
2. Screw conveyors
3. Bucket elevators
4. Grab bucket elevators
5. Skip hoists.

1. Unloading equipment
• The type of coal unloading equipment used
in power plants depends on the type of
out-plant handling mode such as road, rail
or sea transport.
• Lift truck use hydraulic mechanisms to
dump coal in bins or on conveyors directly
by filling the contained at the back side.
• In case of railway cars, the gates at the
base can be opened to allow the coal to fall
in hoppers.
• Other devices used for unloading include
car-towers, rotary car dumpers and
unloading bridges.
• Cranes are used on unloading bridges for
unloading large ships, whereas, unloading
conveyers are used with sonic modern
ships.

2. Coal Preparation Equipment
• The coal preparation plant is usually located near the
power plant.
• This equipment prepares the coal by drying out the excess
moisture and crushing the coal to proper size
• The coal preparation plant contains the following
equipment's.
1. Crushers: The coal crushers are necessary to prepare
coal of required size by crushing it. The capacity of coal
crushers can be as high as 600 tonnes per hour.
2. Sizer: Sizer are used to separate the sized and un-sized
coal. The un-sized coal is returned to the crusher.
3. Dryers: The sized coal is passed to a coal dryer by a belt
conveyer. The sized coal is dried by passing hot flue gas
over it.
4. Magnetic separators: The impurities like iron fillings,
metal scrap is removed from coal by magnetic separator
and dumped by another conveyer into a reject chute.

3. Equipment's Related to Transport of Coal
• Transfer of coal from one place to
another i.e. from unloading point to
storage site involves different types of
equipments.
• Some of them are,
(a) Belt conveyers.
(b) Screw conveyers.
(c) Bucket elevators.
(d) Grab bucket elevators.
(e) Skip hoists.
• The combustion in conventional
system become unstable, when the ash
exceeds 48%, but even 70% ashy
containing coal can be efficiently
burned in FluidizedtBed Combustion
boiler.

Layout of coal handling or stages in coal handling

1. Coal Delivery
• From the supply points, the coal
may be delivered to power station
through rail, road, river or sea.
• Plants situated near the river or
sea may make use of navigation
facilities.
• Stations, which cannot make use of
navigation facilities, may be
supplied coal either by rail or
trucks.
• Transportation of coal by trucks is
usually used in the following cases.
(i) The coal mines are not far off.
(ii) When the necessary railway
facilities are not available.

2. Unloading:
• The type of coal unloading equipment
used in the plant depends upon the type
of out-plant handling mode as road, rail
or ship.
• If coal is delivered by trucks, there is no
need of unloading device as the trucks
may dump the coal to the outdoor
storage.
• Coal is easily handled, if the lift trucks
with scoop are used as shown in Fig.
• When the coal is transported by sea, the
unloading equipment's normally used are
given below: 1. Portable conveyors 2.
Coal accelerators 3. Coal towers 4.
Unloading bridges 5. Self unloading
boats.

3. Preparation: If the coal Is not of proper size, the coal preparation may be carried out
by: 1. Breakers 2 .Crushers 3. Sizer 4. Dryers 5. Magnetic separator.

4. Transfer : Transfer means the handling of coal between the
unloading point and the final storage point, from where, it is
discharged to the firing equipment.
1. Belt conveyors 2. Screw conveyors
3. V bucket elevator and conveyor 4. Pivoted bucket conveyor
5. Grab bucket conveyor 6. Flight conveyor
7. Skip hoists 8. Mass flow conveyor

1. Belt conveyors
• A belt conveyor is very suitable means of transporting large quantities of coal over large
distance.
• Belt conveyor consists of an endless belt made of rubber, canvas etc. running over a pair of
end drums of pulleys and supported by a series of rollers (known as idlers) provided at regular
intervals.
• The return idlers are plain rollers, which are spaced wide apart. They are used to support the
empty belt.
• The initial cost of the system is not high.

Advantages of Belt Conveyor
1. Most economical method of coal transport
in medium and large capacity plants.
2. Smooth and clean operation.
3. Minimum repair and maintenance costs.
4. Able to discharge large quantities of coal
quickly and continuously.
5. Minimum power consumption.
6. The rate of coal transfer can be easily varied
by just varying the belt speed.
7. Coal being transferred is protected.
Disadvantages of Belt Conveyor
1. Not suitable for greater heights and short
distances.
2. Limited inclination (about 20°) at which,
coal can be transferred.

2. Screw Conveyor
• Screw conveyor consists of an endless helicoids screw fitted to a shaft.
• The driving mechanism is connected to one end of the shaft and the other end of shaft is
supported in an enclosed ball bearing.
• While rotating in a trough/housing, the screw transfers coal from one end to the other end.
• Specification :- 1. Diameter - 15 to 60 cm.
2. Speed - 70 to 120 rpm
3. Max. capacity - 125 tonnes/hour

Advantages of Screw Conveyor
1. Screw conveyor is suitable for the coal transfer
between small distances.
2. Screw conveyor requires minimum space.
3. Initial cost of screw conveyor is low.
4. Screw conveyor is very simple and compact.
5. It can be made dust tight.
Disadvantages of Screw Conveyor
1. Power consumption is very high.
2. Wear and tear is very high.
3. Life of screw conveyor is considerably short
as compared to belt conveyor.

3. V bucket elevator and conveyor
• In this type of elevator, V-shaped steel
buckets are rigidly fastened to an endless
chain going around sprockets.
• The buckets are equally spaced on the
chain, and receive their load by dipping
into coal pocket at the lower end of the
system.
• The material elevated in V-buckets is
discharged either by centrifugal force at
the top of the elevator or by drawing
back the buckets on the discharged side.
• The material is continuously handled and
can be both hoisted and conveyed.

Advantages of Bucket Elevator
1. Less power is required (or operating the
equipment (as the coal is carried, not
dragged).
2. Coal can be discharged at elevated
places.
3. Less floor area is required.
Disadvantage of Bucket Elevator
1. Its capacity is limited and hence, it is
not suitable for large capacity stations.

4.Pivoted bucket conveyor
1. This conveyor consists of malleable iron buckets suspended by pivots midway between the
joints of two endless chains, which are driven by a motor located at some convenient point,
usually at the top of a vertical rile.
2. While travelling horizontally, buckets maintain their position due to gravity and support the
joints.
3. The conveyor is loaded by passing below a crusher.
4. The coal is charged into the bunker by a tripping device.
Advantages
1. Low operational cost.
2. High capacity.
3. less floor area requirement DIE
Disadvantage
1. High initial cost

5. Grab bucket conveyor
• Grab bucket conveyor is a form of hoist,
which lifts and transfers the load on a single
rail or track from one point to another.
• This is a costly machine and it is justified,
only when, other arrangements are not
possible.
• Capacity of a grab bucket may be about 50
tonnes per hour
Advantages of Grab Bucket Conveyor
1. Less running cost.
2. It can be used to the locations, where other
types of conveyors cannot be used:
Disadvantages of Grab Bucket Conveyor
1. High initial cost.
2. Use of grab bucket conveyor for transferring
the coal is justified, only when, other
arrangements cannot be used.

Advantages of Coal Handling Systems:
1. Higher reliability.
2. Less labour required.
3. Economical for medium and large capacity plants.
4. Easy and smooth operation.
5. Easy to start
6. With reduced labour, management and control of plant becomes easy.
7. Minimum labour is put to unhealthy condition.
8. Losses in transport are minimized.
9. Can be economically adjusted according to the need.
Disadvantages of Coal Handling Systems:
1. Needs continuous maintenance and repair.
2. Capital cost of the power plant is increased.
3. In mechanical handling, part of the power generated by steam power plant is usually
consumed. Therefore, net power available for supply to consumers is reduced.

Flight conveyor
Skip hoists
Mass flow conveyor

Electrostatic Precipitator (ESP)

Electrostatic Precipitator
• Function: An electrostatic precipitator (ESP) is an electro-mechanical device, which
filters out fine particles like fly ash, dirt, dust etc. from dirty flue gas before releasing
out of a chimney.
• Working Principle: "Using Electrostatic force of a very high voltage negative charge
(DC)”, The negative charge is achieved using a Transformer-Rectifier Set (TR set).

Corona Discharge (also known as the Corona Effect) is an electrical discharge caused by
the ionization of a fluid such as air surrounding a conductor that is electrically charged.

Construction:
• Electrostatic precipitator consists of following basic elements.
1. Source of high voltage (440 V, 50 Hz, 3 6 supply).
2. Ionizing and collecting electrodes.
3. A high voltage transformer.
4. A rectifier to convert AC to DC.
• Electrostatic precipitator (ESP) has two sets of electrodes insulated from each other.
• One set has the rows of electrically grounded vertical parallel plates and they are called as
collecting electrodes.
• There is no "positive° charge in collecting electrodes. As they are grounded, they have zero
charge. Since zero is higher than negative; collecting electrodes act as a positively charged
electrodes.
• The second set of electrodes consists of wires and they are called as discharge or emitting
electrodes.
• The discharge or emitting electrodes are centrally located between each pair of collecting
electrodes.
• The discharge or emitting electrodes carry the negatively charged high voltage (40 to 80 kV) from
an external DC source. The negative charge is achieved using a Transformer-Rectifier Set (TP. set).

Working:
• The dirty gas containing dust is passed between these two electrodes, which are oppositely
charged.
• Ionization of gas occurs due to supply of high voltage of 40 to 80 kV and both positive and
negative are formed.
• The positive ions travel to the negatively charged wire. electrodes, :i.e. discharge or emitting
electrodes.
• The negative ions i.e. electrons travel towards the grounded collecting electrodes, but their
velocity decreases as they move away.
• As these negative ions i.e., electrons move to the collecting electrodes, they collide with fine
like fly ash, diet, dust etc. suspended in the gas stream and give them negative charge.
• These negatively charged particles are driven to the positively charged collecting electrodes
(plates) by( the electrostatic force. Since fine particles of ash, soot, dust, dirt etc. are
separated from the dirty gas and get accumulated on the collecting electrodes, we obtain
clean gas at outlet of electrostatic/ precipitator.

• The collecting electrodes have large contact surface. Accumulated particles of ash, soot, dust,
dirt etc. are removed from the collecting electrodes, when collecting electrodes are rapped
mechanically.
• The use of electrostatic
precipitators is increasing due
to strict pollution norms.
• Sometimes electrostatic
precipitators are used, in
addition, with cyclone
collectors, if the size of dust
particles is high.
• Electrostatic precipitators
have high collection efficiency
of the order of 90% and
above.

Advantages of Electrostatic Precipitator
1. Electrostatic precipitator can remove fine dust particles efficiently.
2. Low maintenance charges.
3. Easy to operate.
4. It is most effective with high dust loaded gas.
5. Dust is collected in dry form.
Disadvantages of Electrostatic Precipitator
1. Capital cost is high.
2. Operational charges are high.
3. Space requirement is more compared to other dust collectors.
4. If gas velocity above the designed speed increases, its collection efficiency is reduced.

Control System
• For optimum operation of the power plants, it is essential to depend on automation and
control, which utilizes very sophisticated controlling and monitoring techniques
• Unlike the olden-day plants, where control and monitoring systems were done by-operators
using simple "circuits, the modern-day power plants are controlled almost entirely on
computerized equipment, which has considerably simplified the control of processes involving
multiple variables.
• In order to maximize
the efficiency and
availability, control
and instrumentation
or automation in
'power plants is
compulsory.

Type of Control System
1. Process Control System:
• This is a closed-loop control system, which automatically collects plant data by reading
instruments, and monitors the physical and electrical parameters associated with the
boiler, turbine and generator continuously.
• The system is used to control pumps, valves and switches for routine functions and
during start-up or shutdown periods.
• Alarms and events are logged for better monitoring of the system.

2. Operational Monitoring System:
• This system collects the
operational data, which could be
utilized by the plant operators for
record keeping, report writing and
analysis.
3. Automatic Generation Control System:
• This system adjusts the generation capacity against load, thereby maintaining the
quality.
• It also minimizes the cost of energy production and transmission

4. Load Frequency Control System:
• This system is used to monitor generation load by maintaining frequency at the set
value.
• It also monitors net power interchanges with neighboring control areas at the set
values and power allocation among generating units at economical rates.
5. Power Plant Maintenance:
• This system stores the information required for analysis of maintenance costs and
evaluation of equipment performance. The system being an interactive one could be used
by the plant personnel to enter problem data, planning data and work execution data,
apart from entry and maintenance of an equipment database and for easy access to
equipment history.
6. Plant Monitoring System:
• This system collects data for the purpose of fuel monitoring and performance
calculations without performing any control actions.
• Data stored in the system may be retrieved to prepare reports and performance
analysis.

Pneumatic and Electrical Control Systems
Pneumatic Control Systems
• This system is widely used in large
stations. The controlling medium of
pneumatic system is dry, clean
compressed air.
• The pneumatic system of control
has proved to be reliable in
operation. It possesses quick
responding characteristics.
• Control drives for fans, pulverizes
and feeders are all generally
pneumatically operated, designed
to operate smoothly dampers and
other type of controls.
• The pneumatic controls, when
maintained with proper care and
maintenance, are safe and reliable.

Electrical Control System
• In this system, controlled conditions such as
level, temperature, pressure flow or any
other derived quantity is sensed by an
appropriate detecting element and
transmitted electrically to measuring unit,
where any deviations from its
predetermined or set value are converted in
to proportional alternating voltage. This is
then amplified in preamplifier.
• The amplified control voltage is now
rectified in a phase sensitive rectifier system
and its output fed to a servo-drive. The
servo-drives, in turn, operate various
regulating units such as fan, dampers, flow
control valves, desuperheater valves, to
obtain desired results.

Control Room for a Thermal Power Station

• Modern thermal power stations are becoming of higher capacity and require co-ordinated
master control and manual facility, surveillance of the control system, reduced critical
areas, and more exact tuning of control loops. In fact, there is a trend towards greater
automation.
• Modern trend in central power stations, equipped with up-to-date control equipment, is
to locate all remote controls and instruments in a central control room.
• Control room is the nerve center of a power station.
• All the controls, protective devices and indicators are housed in it. Reliability of
instrumentation and control in thermal power plant, is very important.
• To ensure that, the sophisticated electronic equipment control and instrumentation
system work with maximum efficiency, it is essential to provide the specified environment
such as air conditioning and dust free atmosphere in control room.
• The positioning of the instruments depends on their shape, size and number.
• It is desirable to group together the various controls in order to have easier regulation and
adjustment either automatic or manual

Control Equipment's Fitted in control
room
1. Controls
2. Voltage regulators.
3. Frequency stabilizers.
4. Load distributors.
5. Synchronizing equipment's.
6. Integrating meters.
7. Protective devices like (a) circuit
breakers, (b) relays, and (c)
emergency trippers.
8. Indicating devices include
instruments for indicating the
load, voltage, frequency, power
factor, winding temperatures,
open and closed position of circuit
breakers etc.

Control and Supervisory Instruments for steam Power Plant
• Main controls are provided for important parameters like Fuel, Feed water, Air, Steam, Ash,
Furnace temperature, Flue gases, Condenser cooling water etc., so that, a steam power plant
can run the plant smoothly and efficiently.
• Pressure Gauge
• Thermometers
• Speed and cam shaft
position recorder
• Vibration amplitude recorder
• Noise meter
• Flow meters
• Watt meters, voltmeters and
ammeters
• Hand wheels
• Governor controls

Main Protective Equipment's (Electrical) for steam power plant
1. Circuit Breakers: The function of
circuit breakers is to break a circuit,
when various abnormal conditions
arise and create a danger for the
electrical equipment in an installation.
Thus, a circuit breaker prevents
dangerous currents in high voltage
circuits. The circuit breakers may be oil
circuit breakers or air circuit breakers.
When the current carrying contacts
are separated, an arc is produced
between them. This arc provides for
the gradual change over from current
carrying to voltage isolating states of
the contacts.

2. Relays: The function of the relay in the
power supply system is to recognize a short
circuit and to initiate the operation of the
devices to isolate the defective element with
minimum disturbance to the normal power
supply system.
3. Isolator: It is only operated under no-load
condition. Its main purpose is to isolate a
portion of the circuit from the other. These are
generally placed on both sides of the circuit
breaker in order to make repairs and
maintenance on circuit breakers without any
danger.

4. Switchgear:
• The switchgear constitutes all parts or
equipment's of the power plant, whose
function is to receive and distribute electric
power. It comprises assemblies of switching
apparatus, protective indicating and
metering devices, relevant accessories etc.
• The functions performed by the switchgear
are listed below. (i) The faults are located
and the faulty plant is automatically
disconnected from system by the operation
of protective equipment's. (ii) To break
short-circuit without giving rise to dangerous
conditions. (iii) To facilitate re-distribution of
loads, inspection and maintenance on the
system.

5. Switch: It makes and breaks the circuit under full load or no-load conditions. Generally,
it is operated manually.

Desirable Characteristics of control system
1. Co-ordinates master control and manual
facility.
2. Surveillance of the control system.
3. Diminishing critical areas.
4. More exact tuning of control loops.
5. Reliability of instrumentation and control.
6. Improved thermal efficiency.
7. Maximum overall efficiency, i.e. Efficient
operation of power plant.
8. Improved response time.
9. Improved operator interface.
10. Improved access of plant data to
engineering and management personnel.
11. Low operating and maintenance cost.
12. High reliability of power plant.
13. Economic operation of power plant
14. Lowest cost of power generation.
15. Extension of life of equipments of power
plant, so that, they can serve for longest
time.
16. Safe operation of power plant.
17. Minimum environmental effects and
pollution control.
18. Reduced water requirement.

Steam Pressure Control System
• The steam pressure control system (also know as boiler master).
• It maintains steam pressure by adjusting flows of fuel and combustion air to meet the desired
Pressure. When pressure drop, the flows are increased.
• A steam pressure sensor acts directly on fuel and air low control, such as, the pulverized-coal
power drives and the forced draught fan which affect the desired changes.
• A trimming signal from fuel and airflow
sensor maintains the proper air-fuel
ratio.
• A steam flow sensor is substituted for
the fuel-flow sensor, when it is difficult
to obtain accurate fuel flows.
• Usually about a 5 second delay is
allowed, when changing flows of coal
and combustion air to ensure the
prevention of a momentary rich mixture
and thereby obtaining smoke-free
combustion.

Automatic Combustion Control
• By means of automatic
combustion control, it
becomes easy to maintain
a constant steam pressure
and uniform furnace
draught and supply of air
or fuel can be regulated to
meet the changes in steam
demand. The boiler
operation becomes more
flexible and better
efficiency of combustion is
achieved.
• This saves manual labour
also.

Feedwater Control System
• Feedwater control system is used to control the flow of feedwater and thereby, flow of steam
to meet the load demand by the turbine.
• At the same
time, feedwater
control system
maintains the
level of water in
the steam drum
within relatively
narrow limits.
• Normally, the
water level in the
drum is
maintained half
full up to the
diametral plane.

• A high steam consumption by the turbine, combined with low feed water supply would lower
the water level in the drum.
• A three-element automatic control system is shown in which, the 3 elements are, 1. Drum
level, 2. Feed water flow and 3. Steam flow.
• In the case of high steam consumption and low feedwater supply, the drum level will fall
below the set point (i.e. desired level of water in drum).
• The drum level sensor responds to such error and acts on the controller to increase
feedwater valve opening to meet the steam flow demand.
• This action may be too slow Therefore, it is supplemented by sensors for feedwater and
steam flow.
• The difference between -the signals from these two sensors predicts the changes in drum
level and a signal is sent to the contrller to actuate the valve in the desired direction.

Steam Temperature Control System
• An accurate control of superheat temperature is important for efficient power plant operation.
• The main variables affecting superheat temperature are:
1. Furnace temperature.
2. Temperature of gases entering the convective superheater.
3. Cleanliness of superheaters.
4. Mass flow rate of gases through the convective superheater.
5. Feed water temperature. 6.
6. Variation of load on the unit.
7. Build-up of slag or ash on heat-transfer surface.
• Though there are several reasons for temperature fluctuations, but the main reason is
“Change in the load”. the load.
• Prediction in steam temperature decreases the plant efficiency. Generally, it has been
observed that, a drop of about 20 C temperature results in about 1% increase in heat rate.
• On the other hand, an increase in steam temperature above designed temperature may result
In overheating and failure of super heater tubes, reheater tubes and turbine blades.
• The temperature of the saturated steam leaving the drum corresponds to the boiler pressure
and remains constant, if the steam-pressure controls are in working order.

Maintenance of Steam Power Plant
• The Maintenance is Carried out daily, Weekly and yearly by maintenance Dept.

Purpose of Maintenance:
• To maintain the steam turbine in order to achieve as high plant availability as possible at the
minimum cost.
• The maintenance of large modem unit must be organised to minimise forced outages (i.e.
when the plant is not running due to any problem), which affects availability.
• If the outages are unavoidable, the maintenance work should be planned so that, plant can be
put again to service in the minimum time. This may require round-the-clock working on some
activities.
• Plant efficiency is also a major aspect of maintenance team. It is the duty of maintenance
department to keep a close communication and working relation with the planning and
operation departments in observing the plant performance.
• It is essential to keep a close watch for faults, which are' inherent in the design of any
equipment of steam turbine power plant. These are generally known as 'type faults'.
• After careful study, its design may be improved with the help of manufacturing method.

Categories of Inspection Activities
• Maintenance of steam turbines used in power station may be classified broadly in three
categories. Each category needs different planning approaches
1. Rectification of defects:
• The defects may or may not be urgent.
• If immediate repairs are needed, these may be carried out on running units or the units
should be shut down. If the repair is not urgent, it may be carried out in planed maintenance
programmed of future work.
2. Planned preventive maintenance on running units:
• A system of routine maintenance is adopted to prevent the defects and breakdowns.
• The planning of such a system needs careful consideration in order to arrive at the optimum
levels of maintenance.
• The actual work content of each maintenance routine can only be determined after
experience with the plant over a period of time.
• After carrying out the preventive maintenance, it is recorded on a record card with full
particulars of the items attended.

3. Planned preventive maintenance on shut-down units:
• During shutdown, statutory surveys and major overhauls are the main activities.
• In order to minimize the maintenance costs of shut-down units, it is necessary to plan the
work to be done carefully in advance, so that, the duration of plant shut-down can be
minimized. This planning process should start some weeks or months before the scheduled
date of shut-down.
• A work list is prepared, on the basis of which, it is possible to programmed the course of the
overhaul. The technique for critical path analysis is now generally used for this purpose.
• A record in maintained after doing the maintenance for future use.

Steam power plants

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