Turbine

NEC, NARASARAOPETA TURBINE

TURBINE

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ABSTRACT:

A turbine is a type of engine that can extracts energy from a fluid, such as water,

steam, air, or combustion gases. It can be contrasted with a piston engine, which

uses a piston instead of a turbine to extract energy.

The steam extracted from the boiler is sent to the turbine. The steam energy is

converted to the mechanical energy in three steps. First send to the high pressure

turbine then to the intermediate pressure turbine and then to the low pressure

turbine. This is then converted to electrical energy. The turbine plays an important

role in the power generating station. The design of the blades are made in such a

way that the efficiency of the turbine increases.

In this report we are going to give the brief description on steam turbines and how

it is useful in the power generation. The classification of steam turbine gives that

on what factors does they function such as according to the direction of steam flow,

number of cylinder, method of governing, the principle of action of steam, etc.

The turbine is divided into three parts as HP turbine (high pressure turbine), IP

turbine (intermediate pressure turbine) and LP turbine (low pressure turbine). The

components of the steam turbine are pedestals, base plates and fixed points,

casings, bearings, rotors, glands, blades, coupling, turbine gear, emergency stop

valve and control valve.

Governing system controls the steam flow to the turbine in response to the controls

signals like speed error, power error. It can also be configured to respond to

pressure error.

Different governing oils and their purpose, components of the governing system

load controlling. Lubrication oil system of steam turbine.

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OVERVIEW OF THERMAL POWER PLANT:

Thermal power plants are the power generating stations. These plants situations are decided basing on the main concepts, they are

1) load sector and

2) Material required for the running of plant

Thermal stations are of two types:

Pit head station: These stations are which are near to the source of fuel.

Load head stations: These stations are which near to the load centers.

The main principle of the works of the thermal station is that the steam energy is converted to the electrical energy as the below process

At the initial stage the main fuel of this plant i.e., coal is supplied to the plant mainly trough trains. Coal from the wagons are unloaded to the coal handling plant in which the raw coal is sent to the mills through belt conveyers in which it is crushed to the pulverized form.

From here this pulverized coal is sent to the boiler with the help of air, in the boiler as the water flows through that, when the combustion takes place then the water converts to steam.

This steam is sent to the turbo generator, this turbo generator starts rotating due to the pressure of the steam. As the turbine and generator are inter connected due to the rotation of the turbine generator starts rotating i.e., the mechanical energy is converted to the electrical energy. The electricity produced is supplied to the switch yard, in which this power is supplied through grid, feeder, etc.The ash produced during combustion is either sent to the ash handling plant if it is wet one or used for preparation of bricks or for laying roads.

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INTRODUCTION TO TURBINE:

Steam turbines play key role in the thermal power stations. Turbines are rotating

machines which convert heat energy to mechanical energy. Generally, different

capacities of turbines are used in India, such as 15MW to 500MW. The turbine is a

reaction, condensing type tandem compound with throttle governing and

regenerative system of feed water heating. The turbine has one single flow HP, one

double flow IP and one double flow LP cylinders. The critical speeds of HP and IP

rotors are well above the operating speed while that of LP rotor is below operating

speed.

The steam from boiler is admitted to the HP turbine through two valves,

which are main steam stop and control valves. Check valves are provided at the HP

exhaust to the reheater because to avoid the flow of hot steam from reheater back to

the HP turbine. The steam from reheater goes to IP turbine through stop and control

valves and the exhausted from the IP turbine goes to the LP turbine through two

cross around pipes one either side of the turbine at the operating floor level.

Under changing load or grid or steam conditions also the synchronous speed

should be maintained, so the KWU turbine is equipped with hydraulic governor.

The speed of the turbine generator can be controlled either by hydraulic governing

or by electro-hydraulic governing system. When electro hydraulic governing is

controlling the speed the hydraulic-governing system acts as back up and comes

into operation automatically in case of failures. The changing of one governing

system to other is possible during operation. Governing is the one of the important

factor in the functioning of steam turbine. Lubrication is another important factor

effecting safety and availability of generating plant.

WORKING PRINCIPLE OF TURBINE:

When the steam expands in a narrow orifice, then it attains kinetic energy at the

expense of its enthalpy. This kinetic energy is changed to the mechanical energy

through the impact of the steam against the blades. The blades of the turbine are

designed in such a way that steam will glide on and off the blade without any

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tendency to strike it. From this we get the clear information that the motive force

will not attained due to the impact of the steam.

When the steam moves over the blades, its direction is continuously changing and

centrifugal pressure exerted as the result is normal to the blade surface at all points.

The motive force of the blade can be given asthe sum of the centrifugal forces and

the change of momentum and this cause the rotational motion of the blades.

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CLASSIFICATION OF TURBINES:

Turbines can be classified basing on different factors such as depending on

construction, the progress by which heat drop is achieved, the initial and final

conditions of steam used etc.

Let us discuss some of the factors in detail:

1) ACCORDING TO THE DIRECTION OF STEAM FLOW:

Taking the direction of into consideration, there are two types

One is the axial flow turbine which is that the steam flows in a direction parallel to

the axis of the turbine. Another is the radial flow turbine which is that the steam

flows in a direction perpendicular to the axial of the turbine.

2) ACCORDING TO THE HEAT BALANCE ARRANGEMENT:

a) Condensing turbines with regeneration:

The steam from the turbines is directed to the condenser at a pressure less

than atmospheric pressure. The pressure is also extracted from the intermediate

stages for the feed water heating.

b) Back pressure turbine:

The exhaust steam from these turbines is utilized for industrial or heating purpose.

c) Topping turbines:

These turbines are same as that of the back pressure turbine but the difference

is that the exhaust steam is further utilized in the small and low pressure

condensing turbines. Generally these turbines operate at the high initial conditions

of steam pressure, temperature and used during the extension of the plant capacity

in order to better efficiencies. Extending the plant capacity is that addition of the

high pressure boiler i.e., these turbines are additioanlunits.

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3) ACCORDING TO THE METHOD OF GOVERNING:

There are different types of governing systems

a) Turbines with throttle governing:

In this method the steam flow is controlled by opening and closing of control

valves simultaneously to the extent required by load.

b) Turbines with nozzle governing:

In this type the steam flow is controlled by sequential opening and closing of

the control valves allowing steam to flow to associated nozzle groups.

c) Turbine with by-pass governing:

In this the steam which is being fed to the first stage is also led to one, two or

even three intermediate stages of the turbine.

4) ACCORDING TO THE PRINCIPLE OF ACTION OF STEAM:

Basing on the principle of action of steam on turbine, these turbines are

mainly classified as impulse, reaction and also compounding, velocity

compounding, pressure compounding, pressure-velocity compounding and

multistage turbines.

a) Impulse:

In impulse turbine, the steam is expanded in fixed nozzles. The high velocity steam

issuing from the nozzles does work on the moving blades, which cause the shaft to

rotate.

b) Reaction:

In reaction turbine, the both fixed and moving blades acts as nozzles and

these are of same size. The pressure is reduced in both fixed and moving blades. In

this type of turbine the work is done by the impulse effect due to the reversal of

direction of the high velocity steam and a reaction effect due to the expansion of

steam through the moving blades.

c) Compounding:

Several problems crop up if the energy of the steam converted in one step, i.e. in a

single row of nozzle blade combination. With all heat drop taking place in one row

nozzles the steam velocity becomes very high and even supersonic. In order to

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convert the energy of steam with practical steam range it is necessary to convert it

in several steps and thus reducing the velocity of stem and rotor speed.

d) Velocity compounded impulse turbine:

This has only one set of nozzles and entire steam pressure drop takes place there.

The kinetic energy of high velocity steam issuing from nozzles is utilized in a

number of moving row of blades with fixed blades in between them .

e) Pressure compounded impulse turbine:

This is nothing but the series connection of impulse turbines on this same

shaft. The exhaust of one steam turbine entering the nozzle of the next turbine. The

total pressure drop is divided equally between all of them. Steam is first passed to

first nozzle ring in which it is partially expanded; it is then passed over the first

moving blades wheel. This process continues till the last blade until the whole of

the pressure has been absorbed.

f) Pressure velocity compounded impulse turbine:

It is the combination of both velocity compounded and pressure compounded

impulse turbine and has the advantages of allowing the more pressure drop in each

stage and less number of stages. The diameter of the turbine is increased at each

stage to allow for the increasing volume of steam. It is rarely used due to the low

efficiency.

g) Multistage reaction turbine:

It consists of number of rows of moving blades attached to the rotor and an

equal number of rows of fixed blades attached to the casing. Each stage utilizes a

portion of energy of steam.

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COMPONENTS OF STEAM TURBINE:

MAIN TURBINE

The main turbine is divided into three components

1) HP TURBINE:

High pressure turbine is the primary turbine through which the steam flows

first. The outer casing of the HP turbine is of barrel type construction. This

construction avoids mass accumulations due to absence of flanges. The inner

casing carries the guide blades and is axially split and kinematically supported. The

space between the inner and outer shells is sealed from the neighboring, spaces by

sealing rings. The inner casing is fixed in the horizontal and vertical planes in the

outer casing so that it can freely expand radially in all direction and axially from a

fixed point when heating up while maintaining eccentricity. The connections of the

main steam piping with the HP turbine are by means of sleeve joints having

buttress threads. These threads are located in the outer casing and connection with

the piping is made through breech nuts.

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2) IP TURBINE:

The intermediate pressure turbine is the next turbine after the HP turbine.

The casing of the IP turbine is split horizontally and is of double shell and double

flow construction. The inner casing carrying the guide blades and kinematically

supported within the outer casing. The reheated steam enters the inner casing

through the top and bottom. Although the casings are of split design they do not

restrict the start up timings and rapid load changes due to the provision of relieving

grooves built in the inner casing.

3) LP TURBINE:

The low pressure turbine casing is of triple shell fabricated construction. The outer

casing consists of the front and rear end walls; two side members called

longitudinal girders and top cover. The twin shell inner casing is supported

kinematically at each end by two support arms resting on the side members of the

outer casing. The inner shell of the inner casing carries the guide blade carrier of

the first stages of the turbine.

COMPONENTS OF STEAM TURBINES:

1) PEDESTALS, BASE PLATES AND FIXED POINTS:

The HP, IP and LP turbines rest on four pedestals. The four pedestal

houses the main oil pump and emergency governor. The central pedestal houses the

combined journal and trust bearing and the other pedestals containing one journal

bearing each. The front and the center pedestal slide over their respective base

plates, which are fixed, to the foundation. While the LP front and rear pedestals

themselves are fixed to the foundation.

2) CASING:

Turbine casing are essentially pressure vessels, their weight being supported at

each end. These are therefore, designed to with stand hoop stresses in transverse

plane and to be very stiff in longitudinal direction to maintain accurate clearance

between stationary and rotating components. Usually casings are of two types

1) Single shell casing 2) Multi shell casing

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Single shell casings take pressure drop from steam pressure to

atmospheric pressure in single shell and hence required thick wall and heavy

flanges at parting planes. This causes very large incremental thermal stresses

during transients resulting in shower start up and shut down.

In multi shell casings there is intermediate pressure between the shells

and hence pressure drop is shared by two shells resulting in thinner walls and

lighter flanges at parting planes. This type of casing has lower incremental thermal

stresses during transients resulting in quicker start ups and shut down. Multi shell

casings are now commonly used for HP and IP turbines.

3) BLADING:

The turbine blading are designed for maximum efficiency and reliability.

Blades are single most costly elements of turbine blades that are fitted to the

stationary part are called as the guide blades or nozzles and those are connected to

the rotor are called as moving or working blades. The three main types of blades

that are used are

1. Cylindrical blade

2. Tapered cylindrical blades

3.Twisted and varying profile.

The blades of HD and IP Turbines are designed for fifty percent reaction

and therefore, both fixed and moving blades have the same profile. They are

machined from rectangular bars stock with integral shroud and tee root. The root

and shroud have rhomboid shape in order to have an additional clamping force at

root and a moment at shroud for increased rigidity of blade assembly. The blades

are locked radially into grooves of casing/ rotor with the help of fitting pieces

called caulking brass. The guide blades in the medium and low temperature regions

are made from solid drawn material and have a hook type root. In this case

shrouding is separate and riveted in position. The root is brazed on to the blade.

These blades are locked axially in the casing groove, with the help of precision cast

clamping pieces to achieve tight fitting.

The constructional features of the blading of first 5 stages of LP Turbine are

similar to those of the drum stages of HP Turbine and IP Turbine. The moving

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blades of the last three stages are free standing and the root fastening is of firm tree

type. Which fits into milled axial groove in the LP rotor. These LP blades also

suitably twisted along their height to suit the different peripheral velocities from

the root to tip. The profiles are also made tapered to obtain most favourable stress

distribution in the profile. The trailing edge of these blades are very thin to avoid

formation of streams of water.

The axial distance between the last stages is kept large to facilitate breaking

of any water droplets, which may still remain. The leading edges of the last stages

are flame hardened to give protection against erosion. Suitably sized longitudinal

section slits are provided in the last stage guide blades to remove the water to the

condenser accumulated on the profile boundaries due to wetness of steam.

4) BEARINGS:

The bearings are made in two halves and are all elliptical type. The HP rotor

is supported by two bearings. The IP and LP rotors have a journal bearing each at

the end of their shafts. These are self- adjusting bearings. The bearings of the HP

rotor are with spherical supports with surface contacts. The IP and LP bearings

have line contact on the spherical supports obtained with the help of torus pieces.

Bearing babbit temperatures are measured by the thermocouples directly

under the white metal. The temperature of the thrust pads is measured by

thermocouples in two opposite pads on both turbine side and generator side. Lube

oil is admitted in the oil spaces that are milled into the bearing shells at the

horizontal joint and are open to the shaft journal.

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5) ROTORS:

If the turbine is impulse type the rotor is disc type i.e. blades are carried in the

discs, which may be integral forged with shaft or shrunk on the shaft. If the turbine

is reaction type then the rotor is drum type i.e. blades are directly carried on the

rotor. The HP, IP, LP rotors are of one piece single forging of suitable alloy steels

with integral coupling. The rotor carry the moving blades. The shaft seals are axial.

All the rotors are dynamically balanced to every fine degree of precision, this

ensures that there are minimum vibrations and dynamic loading of bearings.

Labyrinths with the sealing strips caulked into the shafts. Sealing in turbine casings

is provided to check steam leakage from HP and IP turbine and air leakages into LP

turbine.

6) GLANDS:

If the HP and IP cylinders the seal consists of a series of sealing strips

caulked alternatively in the shaft and into stationary rings. In the case of LP

cylinder glands sealing strips are fitted in the stationary rings only. Each sealing

ring consists of six or eight segments and is carried in grooves in the casing to

allow radial movement. Each segment is held in the position against a shoulder by

two coil springs. Both fixed and moving blades are fitted with a continuous shroud

in which steps have been machined to produce a labyrinth. The sealing strips are

caulked into the casing and shaft opposite the blade and are of stainless steel which

can be easily replaced.

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Turbine shaft glands are sealed with auxiliary steam supplied by an electro

hydraulically controlled seal steam pressure control valve. Above a load of 80MW

the turbine becomes self sealing. The leak off steam from HP or IP turbine glands

is used for sealing LP turbine glands

The steam pressure in the header is then maintained constant by means of a leak off

control valve which is also controlled by the same electro hydraulic controller,

controlling seal steam pressure control valve. The last stage leak off of all shaft

seals is sent to the gland steam cooler for regenerative feed heating.

7) COUPLING:

The shaft is made in small parts due to forging limitations and other

technological and economic reasons, the couplings are required between any two

rotors. The coupling permits angular misalignment, transmits axial thrust and

ensures axial location. the couplings are either rigid or semi flexible the former

neither permits angular nor lateral deflection while the later permits only angular

deflection. No of critical speeds depend upon the modes of vibration and hence the

type of coupling between rotors.

8) EMERGENCY STOP VALVE AND CONTROL VALVES:

Turbine is equipped with emergency stop valves to cut off steam supply and

with control valves regulate steam supply, emergency stop valves are provided in

the main steam line and interceptor valves are provided in the hot reheat line.

Emergency stop valves are actuated by servo motor controlled by the protection

system. Emergency stop valves remain either fully open or fully close. Control

valves are actuated by the governing system through servo motor to regulate steam

supply as required by the load. Valves are either single seat type or double seat

type. Singe seat type valves are preferred through those required higher force for

opening or closing

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9) TURNING GEAR:

Barring gear rotates the turbine at slow speed when turbine is being started

or shut down, thus allowing uniform heating or cooling of the rotors to avoid any

distortion of rotors. Usually barring gear consists of motor driven driving unit,

which consists of speed reduction assembly and automatic engaging and

disengaging arrangement. Sometimes the turbine is provided with a hydraulic

turning gear. This avoids mechanical linkage and more reliable. The shaft system is

rotated by a double row blade wheel, which is driven by high pressure oil provided

by the auxiliary oil pump. The oil passes via a check valve into thenozzle box and

then into the nozzles.

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SIEMENS TURBINE GOVERNING SYSTEM

Governing system controls the steam flow to the turbine in response to the controls

signals like speed error, power error. It can also be configured to respond to

pressure error. It is a closed loop control system in which control action goes on till

the power mismatch is reduced to zero. Hence the speed of the turbine generator set

can be controlled either by Hydraulic governing system or by Electro hydraulic

governing system.

COMPONENTS OF THE GOVERNING SYSTEM:-

The main components of the governing system and the brief description of their

functions are as follows:

REMOTE TRIP SOLENOIDS:-

It is a part of turbine protection unit. It has two operated valves. Under their normal

de-energized conditions, the control oil for governing is free to pass through them

to main trip valves. The solenoids gets energized whenever any electrical trip

command is initiated or turbine is tripped manually from local or UCB. Under

energized conditions the control oil supply gets connected to drain the solenoids

can be reset by resetting the UTRS (unit trip relay).

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MAIN TRIP VALVES:-

The main trip valves are the main trip gear of the turbine protective circuit. The

control oil from remote trip solenoids is supplied to them. Under normal conditions

this oil flows into two different circuits, the trip oil and aux.trip oil circuit. The trip

oil is supplied to the stop valve and aux.secondary oil circuit. The aux.trip oil

flows in a closed loop removed by main trip valves and turbine hydraulic

protection devices.

STARTING AND LOAD LIMIT DEVICES:-

The starting and load limit devices are used for resetting the turbine after tripping

for operating the stop valves and releasing the control valves for opening. The

starting device basically consists of a pilot valve that can be operated either

manually by means of a hand wheel or from remote by means of an electric motor.

Before start up the pilot valve is brought to its bottom limit position by reducing

the starting devices to 0% position. This causes the hydraulic governor bellows to

be compressed thus blocking the buildup of secondary oil pressure. This is known

as “control valve close” position. A buildup of oil pressure in these circuits can be

observed while bringing the starting device to zero position. This opens the start up

oil and auxiliary starts up oil circuits are drained. This opens the stop valves.

SPEEDER GEAR:-

The speeder gear is an assembly of a bellow and spring, the tension of which can

be adjusted manually from PCR by an electric motor locally by a hand wheel. The

bellow is also subjected to the primary oil pressure which is the feedback signal for

turbine speed. The speeder gear position determines the hydraulic governor set

point. The bellow and spring assembly is rigidly linked to the sleeves of the

auxiliary follow up piston valves. The sleeve position changes with the equilibrium

position of the bellow.

AUX.FOLLOW UP PISTON VALVES:-

There are two valves and are in parallel. The trip oil is supplied through orifice to

the aux.follow- up valves. The sleeve position determines the drain of trip oil

through the aux.follow-up valves.

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AUX.SECONDARY OIL CIRCUIT.

It consists of a pilot valve and an amplifier piston valve. The position of the pilot

valve spool depends upon the aux.secondary oil pressure. The other side of the

amplifier is connected to the drain. The movements of the amplifier piston are

transformed into rotation of camshaft through a piston rod and a lever assembly.

FOLLOW UP PISTION VALVES:-

The trip oil is supplied to the follow up piston valves through orifice and flows in

the secondary oil piping to control valves. The secondary oil pressure depends

upon position of sleeves of follow-up piston valves, which determines the amount

of drainage of trip oil.

There are in all twelve follow up piston valves. Six of them are associated with

hydraulic amplifier and six of them with Electro-hydraulic converter in the

governing system. The follow up piston, valves constitutes a minimum valve gate

for both the governor does. This means the governor with lower reference set point,

is effectively in control. This is also termed as hydraulic minimum selection of

governors.

SEQUENCE TRIMMING DEVICE:-

The function of the sequence trimming device or HP\IP trim device is to prevent

any excessive HP turbine exhaust temperature. It changes the time and setting

response of main and reheats control valves. When the reheat pressure is more than

32Kg/cm^2 and load less than 20% the IP turbine tends to get loaded more than HP

turbine. The trim device operates at this moment trimming the IP turbine control

valve. The control valve of HPT open more to maintain flow stem, reducing the

HPT exhaust temperature.

SOLENOIDS FOR LOAD SHEDDING RELAY:-

A pair of solenoid valves has been incorporated in the secondary oil lines on

control valves in order to prevent the turbine from reaching dangerous speed in the

event of sudden turbine load throw-off. The control valves are operated by the load

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shedding relay when the rate of load reduction exceeds a certain valve. The HP

control valves are closed due to draining of aux.secondary oil before the hydraulic

amplifier, by the second solenoid valve.

TEST VALVES:-

Each of the HP&IP stop valves servo motors receives trip oil through their

associated test valves. The test valves have got port opening for trip oil as well as

start up oil. The test valves facilitate supply of trip oil pressure beneath the servo

motor disc. When start up oil pressure is reduced the test valve moves up draining

trip oil above the servo-motor piston and building the trip oil pressure below the

disc, thus opening the stop valve. A hand wheel is also provided for manual

operation of the test valves.

COLD REHEAT SWING CHECK VALVE:-

One swing check valve is provided on the cold reheat lines from which the

extraction stem is drawn for HPH-6. These NRVs prevent charging steam in

turbine when tripped. The cold reheater BRVs are operated by their pilot valves via

their rotary servo motor in proportion to secondary oil pressure. They open out

fully when main control valves open up corresponding to 5-10% of maximum

turbine output.

Even when the pressure of secondary oil has not built up sufficiently, NRVs can be

opened up like safety valves when the upstream pressure rises the down stem side

pressure by one bar.

ELECTRO HYDRAULIC CONTROLLER:-

The parameters to be controlled i.e. speed, load and turbine throttle pressure

measured are converted into electrical signals by means of suitable transducers.

These signals are then fed to various control loops where they are processed as per

requirement of the operator and the turbine conditions. The control signal

generated is then fed to the Electro-hydraulic operated final control elements i.e.

the control valves. The electrical turbine controller basically consist of subsidiary

controllers for main steam control valve lift, speed, load and boiler pressure are

superimposed to main steam control valve position loop.

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SPEED CONTROLLER:-

The speed controller is used for starting the turbine upto synchronization and block

loading. It can also operate over the full load range, during emergency such as

generator tripping from full load to house load operation or during rapid load

throw-off and or severe frequency fluctuations. The speed controller is always kept

in readiness for operation even when it is not directly controlling the turbine by

tracking the signal generated by the other controller in service.

Speed set point changes at a gradient of 2160 rpm upto a setting of 2800rpm. After

2800rpm the set point changes at a gradient to facilitate exact adjustment of speed.

The speed reference value is indicated on two instruments, one with a range of 0-

3300rpm and other with range of 2800-3300rpm.

The control device for the speed reference value generates the time dependent

speed reference value NRTD/NRLIM which influence the speed controller. The

speed of the reference value is tracked depending on two operating conditions:

After turbine trips speed controller follows the actual speed with a difference of

120rpm. This follow mode ensures that the set point is below the actual speed, once

the turbine is tripped and rolling does not take place till raise command is given to

speed set point.

After synchronization, when load controller takes control, speed reference follows

the actual speed between 48.5 HZ to 51.5 HZ. This ensures that speed controller

does not interfere with load controller as output of speed controller will remain to

0%. The gradient of signal NRTD however is modified accordingly to available

TSE margins.

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NO LOAD SPEED CORRECTION:-

As a result of proportional control behavior of the speed controller, a control error

between the actual value and the reference value. A pre-feed which is function of

boiler pressure is provided to achieve identical speed at synchronizing point.

DROOP OF SPEED CONTROLLER:

5 % i.e.2.5Hz above and below 50 Hz. So for a changing of ±150 rpm of reference

speed, the speed controller output will vary from 0 to ±10volts.

STOPPING OF SPEED REFERENCE VALUE CONTROL:-

When the generator circuit breaker is “closed” stopping of the speed reference

value is inhibited. The speed reference value control is also stopped in an

analogous manner to that described above when the unit is operating in pressure

control mode.

SPEED MEASUREMENT AND PROCESSING:-

For speed measurement, four numbers of stationary hall probes are mounted

around a disc containing 120 magnets in front bearing pedestal. By means of these

probes the magnet pulses of the passing magnets are converted into an AC voltage

with a frequency proportional to the rotational speed of the TG. Advantages of this

system are that in one rotation of disc 60+ve pulses will be generated. So by

counting the pulses for one second we can directly the turbo set speed in rpm.

DN/DT MONITROING:-

During rolling of the turbine, if between the speeds 600rpm to 2850rpm the rate of

speed rise is very low i.e. less than 108rpm/min then DN/DT monitoring operates

giving appropriate alarms in the PMS/SOE. It blocks any further rise in speed and

brings back the speed reference to 600rpm. This is incorporated to avoid low

acceleration rates when the turbine speed lies in the critical speed range (600-

2850rpm).

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LOAD CONTROLLER:-

The load controller is used for controlling turbine during load operation. For

selecting the load controller a push button module LOAD CONTROLLER,

ON/OFF is provided. The load controller must be switched ON if it has to come

into action set point, TSM influence, frequency influence and pressure influence

are considered and a final reference is developed. An indication LOAD

CONTROLLER ACTIVE is provided on the turbine control panel when this lamp

glows, which shows that the load controller is in service.

LOAD CONTROL LOOP:-

Load reference value is from a reference setter module on the turbine panel, by

means of a motorized potentiometer and the output of reference setter pr is

displayed by the UCB desk indicator. The device for the load reference value

contains a proportional channel parallel to 1st channel. On the account of this

addition the response of the device is proportional to small changes of load

reference value and for large changes it is proportional integral. Control device for

the load reference value generates time dependent load reference value

PRTD/PRLIM which influences the load controller. This signal rises during start

up at a rate selected through load gradient setter until the final load reference value

PR has been reached.

LOAD GRADIENT AND TSC INFLUENCE:-

Load gradient is an additional feature available in load controller. The load gradient

acts in parallel with TSC margin via a minimum selector. Load gradient range is 0-

25MW/min. while TSC margin 0-30ºK corresponds to 0-25MW/min both load

gradient and TSC margin have got ON/OFF switch, so if anyone is desired to taken

out of service it can be switched off. If both are ‘OFF’ then PRTD changes at a

fixed rate of 25 MW/min.

LOAD REFERENCE VALUE CONTROL:-

By means of the reference value control, the maximum permissible gradient

for the turbo set is introduced. This gradient is limited by means of stress evaluator

or by load gradient limit. If the turbo-set is not synchronized with the power

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system, tracking to the value is effective in order that the controller be capable of

taking over control of the turbo-set independent of the actual reference value which

has been set.

If the load gradient limit is effective, the proportional action is switches off.

The output signal of the integrator is continuously with p-act by means of an

automatic circuit as long as the generator circuit breaker is not closed this provision

facilitate smooth transition from speed control mode to load control mode. The

load controller is equipped with ‘load limiter’ module. The output of load limiter

limits the sum of reference value transmitted to load controller to the pre-selected

value. Even the reduction in grid frequency cannot cause the turbo-set to exceed

the preset power level. The output of load limiter PR max is adjusted by means of a

setter on UCB and also indicated on the console.

The required dependence of load on frequency in accordance with the

frequency V/s load characteristics is achieved by means of additional load

reference value PRDF furnished unit. The unit is highly linear. The sensitivity of

the response with respect to the change in power grid frequency is less than 5Millie

HZ.

The frequency dependent load reference value can be connected and

disconnected by means of a push button switch inside controller cabinet. The status

of operation mode is displayed on the desk/PMS. An indication displays the PRDF

in MW. All reference values described so far totally in summing amplifier of the

load controller. The total load reference value as it is limited by the load limiter is

displayed on the control desk in megawatts as PR.

If the power plant together with a section of the power grid becomes isolated

during load control, the load controller automatically switches to a proportional

response with 5% droop. This arrangement permits safe operation of the turbo-set

under these conditions. The arrangement permits safe operation of the turbo-set

under these conditions. The status of operation of load under these conditions

described above is displayed on the control desk.

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PRESSURE CONTROLLER:-

The signal for the actual pressure and the reference pressure are furnished by boiler

load control system. Pressure influence signal is generated by a proportional

integral pressure controller and affects directly the lift of the main steam control

valves via a selection circuit. The pressure controller controls the turbine load with

respect to the main steam pressure deviation, and prevents, e.g. during a quick load

increases large pressure drop. These are two modes of operation on pressure

controller.

Initial pressure mode:-

It may be selected from control panel. In initial mode pressure controller

tries to maintain initial pressure turbine inlet i.e. reduce the difference between set

pressure and actual pressure to zero, by sacrificing load. The load delivered by the

turbo-set is determined by the boiler capability upto a maximum of power level set

by load controller. Load increases above this level are blocked.

Limit pressure mode:-

In the limit pressure mode the boiler storage capacity is utilized. The

pressure controller influences the MSCV to support boiler pressure control only if a

preset main steam pressure deviation is expected. This provision allows the load

controller to handle small quick load variation until pick up limit of the limit

pressure control is reached. This pick up limit is seldom reached in the usual

frequency supported load control mode since boiler pressure control holds pressure

deviation within narrow limits. The DP is displayed on the desk. If the alarm “

limit pressure reached” is present additional load increases is blocked by means of

the control unit for reference value load.

VALVE PRESSURE CONTROLLER:-

The outputs from the speed controller and the load controller are compared

in a MAX-MIN selector and the output from this is again compared with the

pressure controller output in a MINIMUM selector. The output from this is fed to

the valve position controllers. Therefore the signal from the speed controller, load

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controller and pressure controller are superimposed and selected to give an output

to the valve position controller. The feedback signal from the valve lift controller is

derived from the differential transformer which is housed in the Electro-hydraulic

converter and measure the position of the power piston in the amplifier.

ELECTRO HYDRAULIC CONVERTER:-

The electro-hydraulic converter or EHV is the connecting link between

electrical/electronic and hydraulic parts in Electro-hydraulic governing system. The

electrical signal from the valve position controller is taken to the electro-hydraulic

converter, which is processed and gives a command to the actuators. It consists

mainly of a moving coil system, sleeve and pilot valve assembly, Amplifier piston

and a differential transformer.

The electrical signal from governor control circuit operates the sleeve and pilot

valve spool. The sleeve slides up and down on spool, changing the relative

overlap between them, this varies the trip fluid drain. Under steady position the

pilot is in central position. When it is deflected from its position, the control oil is

admitted above or below the amplifier piston, as in the case of hydraulic amplifier.

The motion of amplifier piston is transmitted via a lever to a cam shaft which

actuates the sleeves of follow up piston valves, which regulate the secondary oil

pressure to the HP and IP turbine control valves separately.

HYDRAULIC GOVERNOR:-

On the shaft of main oil pump, a specially designed primary oil pump is

provided. The changes in primary oil pressure can be taken as proportional to small

changes in turbine speed. This primary oil pressure acts on diaphragm of hydraulic

speed governor against the set of speed setting spring which is tensioned by

speeder gear. Travel in diaphragm is limited by a start up and load limiting device.

The movement of diaphragm is transmitted by a link mechanism to the auxiliary

follow-up drain valves, the piston of auxiliary follow up drain valves is held oil is

essentially trip oil fed from trip circuit and drained through a port formed between

the piston and the sleeve.

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The piston of the hydraulic converter assumes a position corresponding to

aux.secondary oil pressure. A secondary oil pressure corresponding to the position

of the sleeve related spring tension is built in the ‘follow-up pistons of the

hydraulic converter’. Any change in the position of link results in the proportional

change of secondary oil pressure in the follow up piston of hydraulic converter.

The secondary oil circuit is also fed oil from trip oil circuits through reducing

valves/orifices. The varying secondary oil pressure in the ‘follow up pistons of

‘hydraulic converter’ operates the control valve.

CHANGEOVER FROM EHG TO HG CONTROL:-

If the turbine is to be rolled on hydraulic governor, the speeder gear is kept

minimum (0%) piston and the EHC output is adjusted to its 100% position. The

turbine is rolled upto 2800rpm by means of raised, isolating valves on the

secondary oil lines coming from EHG are to be closed, while operating on

hydraulic governor mode.

The changeover from Electro-hydraulic to hydraulic governor on load conditions is

done in the reserve sequence as mentioned in earlier section. First speeder gear

position is lowered until secondary has taken over the control. Then reference set

points of electric controller oil lines from EHG are closed. Now hydraulic governor

is fully effective and operate over the entire load range.

CHANGEOVER FROM HG TO EHG CONTROL:-

Changeover from one control system to the one is possible during normal

operation of turbine, since the two systems are brought in to parallel connection,

after associated follow up pistons which represent a minimum value selection open

sufficient to start draining secondary oil to control valves, meaning that the system

with the lower reference valve always becomes the controlling one.

When bringing in the electro-hydraulic control system, the reference set point of

electrical controller should be reduced slowly until secondary oil pressure drops

slightly. When this occurs, the electro-hydraulic converter has taken over. The

speeder gear of the hydraulic converter is now fully effective and can operate over

the entire output range. The hydraulic speed governor acts as a speed/load limiter

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in the event of electrical controller developing a fault. In this case, operation of the

turbine may be immediately changed over to hydraulic governor.

STARTING PROCEDURE:-

The turbine is started and brought upto synchronization speed by means of

main steam control valves and reheat steam control valves. Reference speed setter

is set to minimum speed at this point if the hydraulic speed governor is to be used.

In this case the speed

Reference of the electro-hydraulic controller is set to minimum. If on the

other hand, start up is to be effected with electronic controller, hydraulic reference

speed setter is set at maximum and the speed reference of the electro-hydraulic

controller is at minimum.

The stop and control valves are in the closed position as the oil circuit is not

yet pressurized. By turning hand wheel clock-wise or by operating motor of SLL

device in the close direction. Spring in auxiliary follow up piston is auxiliary oil

pressure. Thus there is no secondary oil pressure build up when the main trip

valves are latched in.

By further turning hand wheel of starting and load limiting device is moved

downwards whereby control oil is admitted first in to the start up oil circuit and

then in to the auxiliary start up oil circuit. The startup oil flows to the space above

the piston of test valves and forcing them down against the action of the valves,

thereby permitting trip oil to flow to that testy valves and to the main and reheat

stop valves.

The trip oil can now flow to space above the pistons of the steam stop valves

and reheat stop valves pressing them on the lower piston disc. Operation of the

startup load limiting is continued until it attains lowest position.

Subsequently by turning back, the hand wheel or by operating the motor of SLL

device in to open direction, the pressure oil is first discharged from auxiliary

startup oil circuit and then from the startup oil circuit. The piston of the test valves

move upwards by springs and as a result trip oil builds up in the space below piston

and is slowly discharged from the space above piston. The pressure difference thus

occurring lifts both pistons together in to their upper end positions thus causing

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main and reheat stop valves are now held their upper position by the oil pressure of

the trip oil under their stepped, differential pistons.

After the stop valves have opened, further turning of hand wheel in the open

direction after certain amount of dead range will enable the speed governor

diaphragm to move down words, causing – increase in the auxiliary secondary oil

pressure. This would cause the increase secondary oil pressure above hydraulic

converter with follow up to approximately 85 to 95% of rated speed. At this point

hydraulic speed governor will take over and maintain this speed. Starting and load

limiting device is then set at maximum position.

LUBRICATING OIL SYSTEM OF STEAM TURBINE

Good lubricating is one of the most important factors effecting the safety and

availability of generating plant and considering damage can occur if the correct

care and attention is not paid to the lubrication systems. Because of high capital

cost of plant, heavy losses are incurred if efficient plant is kept out of service for

lengthy repairs.

Friction:-

Whenever one body slides over another, friction occurs and a resistance to

motion is set up, at the contacting surfaces. This friction exerts a drag in the

opposite direction to the motion. If the friction between 2 bodies is sufficient to

prevent motion it is known as static friction. When motion exists, overcoming the

friction it is known as kinetic friction.

Effect of friction:-

The force necessary to overcome friction represents a loss of mechanical

energy, this loss to appearing as heat. In turbine bearing this heat must constantly

remove, otherwise overheating and eventually complete seizure will take place.

This heat can be removed by adopting lubrication systems. The best method is to

completely separate the bearing surfaces by a film of finite thickness. Once this

film has been established there is no metal to metal contact.

Film lubrication is one of the method used in turbine for both journal and thrust

bearings and also for the bearings in auxiliaries such as fans and feed pumps. The

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minimum thickness of oil film depends upon details of design but an average value

is approximately 0.002 of an inch per 1” radius of the shaft.

DESSIPATION OF HEAT FROM BEARING:-

Friction is responsible for the generation of heat with in the bearing and with

film lubrication, this is generated in the oil film. In the steam turbine, especially in

H.P. turbine bearings, heat is also conducted from the steam along the shaft to the

bearings, and radiated from the casing to the bearing. Therefore, the oil used as

lubricant must also as a coolant to extract this heat from the bearings and keep

them at a reasonable operating temperature. So, the oil flow through an

H.P.bearings is up to 10 times that necessary for lubrication, the extra oil flow

being necessary for cooling purpose. The actual flow rates for systems will vary

with different manufactures. About 1/3 of the total flow is used for lubrication and

cooling the bearings and the remaining 2/3 of the flow in the relay system

(governing system).

Requisite of Good oil:-

The oil in modern steam turbine is subjected to severe conditions and its break

down, could involve considered mechanical damage to the turbine. To meet this

section the oil used should posses some requirements. The main requirements of

good oil are

Oxidation stability: The oil should not form into oxides with heat bearing

surfaces.

Demusibility: Ability of oil to separate rapidly from water.

Prevention of Rust: Not from corrosion and there by rust.

Antifoaming characteristics: Due to crushing charging of oil crushing

continuously at high speeds foam is formed.

Correct viscosity: It is a property of a liquid. Suppose block ‘ A’ is sliding over

the surface ‘B’ which is entirely supported , on oil. The layer of oil adjacent to

block ‘A’ is moving with ‘A’ at almost same speed, where as the layer of oil

adjacent to ‘B’ is stationary between these 2 layers are an infinite number of layers

each sliding over the next. The resistance of oil to this sliding is called viscosity.

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The finished oil shall be clear and free from water, suspended matter, dirt,

sediments and other impurities. The same oil is used for actuating the governing

system as well as lubrication. The properties of the oil should not be affected by

centrifuging or by filtering which is essential to keep the oil in worm up condition.

For topping up of the oil system, whenever there is shortfall, it is preferable to use

the same brand and quality as that of already using in the system.

CENTRIFUGES:-

Centrifuges are the most useful method of cleaning oils and provided they

are operated and maintained correctly, they will keep oil in good condition for a

very long period. Centrifugal oil purifiers take advantage of the effect of

centrifugal force on materials having different specific gravities in different

weights for the same volume.

The oil is rotated at high speed in the centrifuge and solids having a higher specific

gravity than oil are thrown to the sides of the bowl. The solids thus throw out

collect on the sides of the bowl, and are removed by hand or in some cases

automatically. The water in the oil is also thrown out through the water discharges,

leaving the clean oil to return to the system through the oil discharges.

OIL PURUFICATION SYSTEM:-

Turbine oil should be maintained in good condition, otherwise its life will be

considerably shortened. In the lubrication system the oil is constantly exposed to

contaminating influences and these should be removed. The various methods of

purification used to ensure long life of the oil are:

Periodic sweetening: This method involves removing upto 10% of the oil in the

turbine oil system and replacing it with new oil. This system has got some merit in

small sets, but in large sets it is a costly process.

Filtration: The whole of the oil is withdrawn from the system and replaced with

fresh filtered oil.

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LUBRICATION SYSTEM IN STAGE 1 TURBINE:

Mineral oil is recommended by M/s BHEL for the lubrication system. The same oil

is used for the purpose of governing system. The various elements provided in the

system are:

M.O.P : It is mounted on the turbine shaft and directly driven by it. It comes in to

service fully at 2800rpm of the turbine shaft. This is the prime source of oil supply

to the system.

S.O.P : It is driven by the AC motor. It will serve the purpose of M.O.P during

startup and after trip out of the turbine.

AC LOP/ DC LOP : Exclusively for lubrication system 2 pumps are provided one

A.C driven and D.C driven as a back up to AC LOP. It will come into service at 0.6

kg/cm². of lube oil pressure and DC LOP will come in to service at 0.5 kg/cm² in

auto.

SELF-REGULATING DRAIN VALVE: The valve is mounted on the oil tank

and maintains at 1-kg pressure in the lubrication line at the bearings.

OIL GUAGE: The oil gauge is mounted in the oil tank and maintain at the turbine

floor.

M.O.T: Oil is stored in this tank with holding capacity of 28000 liters. The tank

consists of 2 Nos. injectors and 2 rows filters for cleaning the oil containing out

from bearings and relay systems. First injector provides suction pressure to M.O.P

at 1 kg/cm² and the second injector provides lubrication oil at 3kgs/cm².

LUBRICATION OIL SYSTEM IN STAGE-II TURBINE:-

The oil system fulfills the following functions.

Lubricating and cooling the bearings.

Driving the hydraulic turning gear during interruptions to operation on start-up and

shutdown.

Jacking up shaft system at low speeds.

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MOP:-

Main oil pump is mounted directly on the H.P. turbine shaft and comes into

service as the speed attains 2850rpm. All other pumps are installed on the top of

the oil tank and submersible type, with their prime mover on the cover. The main

oil tank has a direct reading fluid level indicator and fluid limit switch. A duplex

oil filter is provided additionally through which oil supply to thrust bearing is

arranged.

OIL COOLERS:-

The oil of the lubrication and the governing system is cooled in the oil

coolers. The cooling medium for these coolers is clarified water. The pressure of

the cooling water is kept lower than that of oil to avoid its mixing with oil in the

event of tube rupture.

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