turbo supervisory system

8/12/2019 Turbo Supervisory System

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S.No Parameter

measured

Type of Pick-up Location of the

Pick-up

I CASING

EXPANSION

1 HP casing

expansion

Inductance (Rope

and

Cam

arrangement)

HPT front bearing

Pedestal

2 IP casing

expansion

Inductance (Rope

and Cam

arrangement)

HPT rear bearing

Pedestal

II DIFFERENTIAL

EXPANSION

1 HP differential

expansion

Inductance

(non contact type)

Adjacent to HPT

Front bearing

2 IP differential

expansion

Inductance

(non-contact

type)

On coupling between

IP and LP rotor

3 LP differential

expansion

Inductance

(non-contact

type)

On coupling between

LP & Gen rotor

III AXIAL SHIFT Inductance

(non-contact

type)

On coupling between

HP & IP rotor

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2 IP control Valves

3 LP bypass control

valves

VI RELATIVE

VIBRATION OF TG

SHAFT

1 HP turbine front Eddy current

(non-contact

type)

Adjacent to HPT front

bearing

2 HPT rear Adjacent to Thrust

bearing

3 IPT rear Adjacent to IPT rear

bearing

4 LPT rear Adjacent to LPT rear

bearing

VII ABSOLUTE

VIBRATION OF

BEARING PEDESTAL

1 HPT front Seismic Mass

(Contac t type)

Pickups are provided

just above the

respective shaft

vibration pick-ups

2 HPT rear

3 IPTrear 8


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1 HP front Top L, Top-R Bot-L,

Bot-R (4 nos)

2 HPT rear Top L, Top-R Bot-L,

Bot-R (8 nos) Front &

Rear

3 IPT rear Bot-L, Bot-R (2 nos)

4 LPT rear Front-Bot Right,

Rear Bot Right (2nos)

5 Generator Front Front-Bot Right,

Rear Bot Right (2nos)

6 Generator Rear Bearing Liner (2nos)

3.0. Inductive Transducers:Inductive transducers are those in which the self-inductance of a coil or the

mutual inductance of a pair of coils is altered in value due to variations in the

value of the quantity under measurement.

The measuring principle is based on the fact that the impedance of a coil

with iron core depends on the size of air gaps in the magnetic circuit. The

curve of the characteristic ie., Impedance as a function of the air gap is

shown in fig: 2(a). The characteristic almost has the shape of hyperbola. It has

its maximum value when the air gap is zero and approaches a value called

Basic Impedance Zero as the air gap becomes larger. This strongly bent

characteristic is very steep when the air gaps are small. When the air gap

becomes larger no measuring effect can be observed 9


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These 2 coils L1and L2 are connected to form a measuring bridge as shown

in fig: (2c). A fixed a.c voltage is fed to the bridge across the terminals (2) and

(4).

For a particular position of the collar of the turbine shaft, the air gaps d 1and

d2are identical. Hence the bridge is in balanced/equilibrium. When the bridge

is in balanced state, there is no potential difference across the terminals (1)

and (3).

When the turbine shaft changes its position, the air gaps between the coils

L1and L2and the collar will vary. This varies the inductance of the coils L1and

L2resulting in unbalancing of the bridge. Hence a potential difference exists

between the terminals (1) and (3) and this output voltage is processed as

measured quantity. The characteristic of this measuring set up i.e., the output

voltage as a function of air gap is shown in fig: 2(d). The characteristic is

almost linear in the area of passage through zero and this area is used for

measuring the quantity.

This type of inductance transducers i.e., non-contact type is used for HP, IP,

LP differential expansions and axial shift of Turbine.

Inductance type transducers (contact type) are used for measuring HP, IP

casing expansions, HP, IP control valve positions and LP bypass control valve

positions.

4.0. Casing Expansion:

Wh t l i h t d it t d t d i ll di ti I th


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The fixed points of 210 MW turbine casing are as shown in the fig: 3. Supports

for the turbine casing are as detailed below.

IP rear pedestal and LP rear pedestal are anchored rigidly over the

concrete foundation. Since IP casing and LP casing are supported on these

pedestals, they act as absolute fixed points for the respective casings.

Moreover the middle of the LP turbine outer casing is supported on the

concrete foundation through key and key-ways (Left & Right). Thus the middle

point of LP turbine outer casing acts as absolute fixed point for thermal

expansion. This facilitates thermal expansion of one half of LP outer casing

towards IP turbine end and the other half towards the generator end. Since IP

rear pedestal and LP rear pedestal are fixed points the thermal expansion of

both halves of LP turbine outer casing (with reference to middle fixed point)

towards both the ends is accommodated by stainless steel bellows provided

for this purposes.

HP front and rear pedestals are located on the concrete foundation with a

sliding support. HP outer casing is seated over these two pedestals.

IP outer casing is seated over HP rear pedestal (sliding support) and IP rear

pedestal (fixed support).

4.1 IP Turbine Casing Expansion:

IP turbine outer casing, on heating, will expand towards HP turbine end

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4.2 HP Turbine Casing Expansion:

HP turbine outer casing, on heating, will expand towards HP front

pedestal with reference to IP rear pedestal as fixed point. But the effect of IP

turbine casing expansion will be super imposed on HP turbine casing

expansion. The pick up of HP turbine casing expansion is located on the

bedplate of HP front pedestal. Hence the value measured by this pick up will

be a total of HP turbine and IP turbine casing expansions and it is not an

absolute thermal expansion of HP casing alone.

The transducer used for measuring HP/IP turbine casing expansion is of

inductance type (contact type). The part which is subject to a linearmovement and whose movement is to be measured ie: HP front pedestal for

HP casing expansion / HP rear pedestal for IP casing expansion is connected

by a rope to a rotatable measuring device called as CAM as shown in figure:

(4). This measuring device (cam) forms the magnetic return path for the two

sensor coils. The linear movement of the HP front/rear pedestal will be

transmitted as a rotary movement of the cam through the rope. The air gaps

between the cam and the coils depend on the angle of rotation. As the air

gaps vary due to the rotation of the cam, the output voltage of the bridge will

also vary.

5.0. Differential Expansion:

Differential expansion is the difference in the thermal expansions of the


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The fixed points for the casing and the rotor shown in figure (3). It may be

observed that the inner casings of HP, IP and LP turbines are supported in their

respective outer casings in such a way that both the rotor and the casing

expand in the same direc tion in all three cylinders.

5.1 HP Turbine Differential Expansion:

Transducer of the HP differential expansion is located in the HP front

pedestal adjacent to the journal bearing in such a way that it measures the

linear movement of a measuring disc / collar which is provided in the HP rotor.

The linear movement of the measuring disc is due to the following thermal

expansions.

(i) Since the IP casing expansion leads to physical movement of the HPrear pedestal, the thrust bearing itself is subjected to a movement

towards the HP front pedestal end and hence the entire rotor itself.

Hence the measuring disc is also subjected to an equivalent physical

movement towards the HP front pedestal end.

(ii) Direc tion of HP rotor expansion with reference to the thrust bearing(fixed point) of the rotor is towards the HP front pedestal end and the

measuring disc hence moves towards HP front pedestal end due to

HP rotor expansion

(iii) Since the transducer is mounted on HP front pedestal, the effect ofHP and IP casing expansions together will result the movement of the

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expansion, the air gaps d1and d2between the measuring disc (in the rotor)

and the two coils will change. Thus the inductances L1and L2change as well

but inversely.

The coils are connected to form a measuring bridge as shown in fig: 5.

Alternating current is fed to this bridge, which is in equilibrium when the air

gaps are the same. The output voltage of the measuring set-up depends on

the changes of the air gaps and this will be measured as HP differential

expansion.

5.2 IP Turbine Differential Expansion:

Transducer for the IP Turbine differential expansion is located in the IP rear

pedestal adjacent to Hydraulic turning gear in such a way that it measures the

linear movement of a measuring disc/collar which is provided in the IP/LP

rotors coupling itself. The linear movement of the measuring disc is due to the

following thermal expansions:

1. Since the IP casing expansion leads to physical movement of the HP rearpedestal, the thrust bearing itself is subjected to a movement towards

the HP front pedestal end and hence the entire rotor itself. Hence the

measuring disc is also subjected to an equivalent physical movement

towards the HP front pedestal end.

2. Direc tion of IP rotor expansion with reference to the thrust bearing (fixedpoint) of the TG rotor is towards the Generator end and the measuring

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Inductance type (non-contact type), which is explained in sec: 5.1 for HP

Differential Expansion.

5.3 LP Turbine Differential Expansion:

Transducer for measuring the LP turbine differential expansion is located in

the LP rear pedestal at the coupling between the Generator rotor and the LP

rotor. The face of the coupling itself is of a double cone shape and the

transducer measures the linear movement of the double cone. The linear

movement of the double cone is due to the following thermal expansions:

1. Since the IP casing expansion leads to physical movement of the HP rearpedestal, the thrust bearing itself is subjected to a movement towards

the HP front pedestal and hence the entire rotor itself. So the measuring

double cone is also subjected to equivalent physical movement towards

the HP front pedestal end.

2. Direc tion of LP rotor expansion with reference to the thrust bearing (fixedpoint) of the rotor is towards the Generator end. The effect of IP rotor

expansion will be superimposed on LP rotor expansion since both the

rotors are coupled together. So the measuring cone moves towards the

Generator end due to LP rotor expansion as well as IP rotor expansion.

3.The transducer is mounted on LP rear pedestal and it is not subjected toany physical movement, as the LP rear pedestal is the absolute fixed

point for the LP turbine casing.

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parallel to the rotor collar, but in the double cone arrangement adopted here,

the measuring cone makes an angle of 15 deg with the horizontal line.

The transducer is positioned parallel to this angled face at a distance a.

Now let b be the actual displacement to be measured.

Then a =b sin =b sin 15

= 0.25 b

For a cone angle of = 15 , an axial displacement of results in an air

gap change of a, which is equal to 0.25 b. Thus the large changes in

displacement can thus be reduced to relatively small changes in the air gaps.

6.0 Axial Shift of Turbine:

The axial shift of turbine rotor is the physical shift of rotor due to the

action of any unbalanced axial thrust on it.

The axial thrust experienced by the multistage rotor in the direction of

steam flow depends on the rate of steam flow and pressure drop in the moving

blades in various stages of the Turbine. When the steam passes through the

moving blades of a reaction stage, the rotor experiences an axial thrust due to

pressure drop across the moving blades.

The cumulative axial thrust on the rotor needs to be balance exactly so

that the rotor acquires a correct and fixed position in its dynamic condition.

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The IP and LP turbine stages are of double flow type. Hence the axial

thrust gets balanced automatically since the rate of steam flow and theamount of pressure drop across the stages are opposite and equal. In HP

cylinder, which is of single flow type, the rotor would experience cumulative

axial thrust due to the pressure drop in all the stages of the moving blades. This

cumulative axial thrust is balanced by the provision of a Balance Piston.

To maintain proper axial position and to take up any unbalanced axial

thrust, the turbo-generator rotor is positioned and supported by a double collar

thrust bearing located in the HP rear pedestal. In the thrust bearing, 2 collars

which are integral with the rotor, rotate between the thrust shoes made up of

segments and thus they are supported. The clearance between the collar and

the shoes is small and the bearing is lubricated by forced circulation of lub oil. If

the rotor starts to move in either direction (due to unbalanced condition), the

axial thrust is transmitted to the shoes through the lub oil film.

Transducer for measuring the axial shift of the turbine rotor is located inthe HPT rear pedestal adjacent to the thrust bearing in such a way that it

measures the linear movement of a measuring disc provided in the coupling

outer surface between HP and IP rotors. Since the thrust bearing acts as a fixed

point for the rotor, the thermal expansion of the rotor here is zero. Hence the

effect of thermal expansion of the rotor will not affect the position of the

measuring disc. Movement of this measuring disc is only due to physical shift of

the rotor (due to unbalanced axial thrust). The transducer thus measures true

physical axial shift of the rotor 22


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7.0 Turbine Speed:

The speed of a turbine is a measure of revolutions in unit time and it is

measured as revolutions per minute. The operating speed of turbine is 3000

rpm.

Measurement and monitoring of the speed of turbine will be necessary

for the following:

i. For automatic control of the turbine control valves for speed raisingduring start-up

ii.

During normal operation, the optimum speed of the turbine is to bemaintained.

iii. To help the operator to run the turbine at an optimum speed duringemergencies such as load shedding, grid disturbances etc.,

iv. To achieve accurate speed control when the generator feeding onlyto its unit auxiliaries (house load condition).

v. For conducting the over-speed test.Transducer for the measurement of turbine speed works on the principle

of Hall Effect and it is called Hall probe. This Hall probe is located in the HPT

front pedestal at the coupling end of main oil pump adjacent to Hydraulic

speed transducer (Governor Impeller)

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changed with the changes in the distance between the object and the probe.

This leads to changes in the flux density. As this probe converts the magneticflux density into a proportional voltage, all the characteristic parameters of flux

densities changing in time can also be measured.

7.2 Measurement of Speed:

In this arrangement, a non-magnetic disc is fitted with 120 small magnets

of alternating polarity around its circumference. The disc is mounted on the

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shaft by means of the hub ring at the coupling end side of main oil pump. Four

Hall Effect generators (Hall probes) are placed opposite at a small distance

from the pole surface of the disc as shown in Fig: 8. These probes are of

contact less type and three out of four probes are used here. When the turbine

rotor is in rotation, the reluctance of the magnetic circuit gets changed

alternately and hence the flux density. This leads to the generation of an

alternating voltage whose frequency is proportional to the speed of the

turbine rotor.

7.3 Generation Of Actual Speed Signal:

The output signal of each Hall-effect generator is approximately

sinusoidal and its frequency is proportional to the speed of the turbine. The

output signal of each Hall probe is passed to a separate pulse converter. Each

of these pulse converters amplifies the Hall effect voltage and multiplies to

form 3 square wave signals. One of the 3 signals of each pulse converter is

routed to pulse amplifiers.

A pulse monitor continuously monitors the 3 output signals of the pulse

amplifiers for failures. This is implemented by cyclical scanning of the input

pulses. The second signal in the chronological order is selected in the channel

selection circuit as the actual speed signal and supplied to a

frequency/voltage converter. In this frequency/voltage converter, the square

wave signals are converted into analog signals.

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(HPSV) and HP Control valve (HPCV) just before HP turbine. The HP turbine

exhaust steam (Cold Reheat Steam) is taken to the reheater through two C oldReheat Lines. The reheated steam is sent to the IP turbine through two hot

reheat lines. A set of IP stop valve (IPSV) and IP Control Valve (IPCV) is

available in each hot reheat line. The exhaust steam of IP turbine is flowing to

the LP turbine through two cross around pipes. Finally the exhaust steam of the

LP turbine is sent to the condenser.

HPSVs and IPSVs are meant for instantaneous isolation of the turbine from

the boiler in case of turbine tripping. HPCVs and IPCVs are used to regulate the

steam flow into the turbine.

Indications about the position of the above mentioned turbine valves are

helpful for the turbine operation Engineer to understand the existing condition

of the turbine. Valve positions need to be monitored due to the following

reasons:

a.To verify whether the turbine generators execute their controllingfunctions effectively.

b.To supervise the healthiness of each valve.c.To observe and record the positions thereby knowing the operating

status of turbine.

d.To raise the speed of the turbine and also to control it while starting up

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movement is to be measured, ie, valve stem is connected by a rope to a

rotatable measuring disc called as C AM as shown in Fig: 4. This measuringdevice is similar to that explained in sec : 4.2 for the measurement of HP/IP

casing expansions.

ith the above type of transducer, the positions of the following valves are

measured:

i. HP control valve (Left)ii. HP control valve (Right)iii. IP control valve (Left)iv. IP control valve (Right)v. LP bypass control valve (Left)vi. LP bypass control valve (Right)

9.0 Vibrations:

All bodies possessing mass and elasticity are subjected to vibration. The

rotating machines are to be designed in such a way that the vibration levels

are acceptable. Reliability of Turbo-Generators performance depends greatly

upon the level of vibration. Higher level of vibration results in fast wear out of

components such as rotor, bearings, couplings etc. Since the vibration is an

application of alternating forces it will result in catastrophic failure of the 28


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an unbalance creates a centrifugal force that deflects the rotor. It results in

radial displacement of the rotor. A rotor is said to be perfectly balanced whenits mass distribution is such that the summation of all the centrifugal forces is

zero and the summation of the moments of these forces about the centre of

gravity is also zero.

Turbine rotors are manufactured with a high degree of prec ision. Still

some residual unbalance, it unavoidable. Main causes of Unbalance in a

new rotor are usually due to the factors such as machining tolerances,

assembly procedures etc. During operation, this unbalance results in vibrations,

which are transmitted through the bearing and the bearing pedestal on to the

foundation.

For the rotors, which are in service, Bow or Deflection is the cause of

unbalance. Some of the causes of shaft Bow are hereunder:

1. Lengthy horizontal turbine rotors, which are supported in oneposition, for a long period after shut down, may develop bow. This

is due to non-uniform distribution of its weight along the rotor.

2. During shut down, a bow may develop in the rotor because ofnon-uniform distribution of heat.

3. Rubbing in glands and seals will cause hot spots, which can bowthe rotor.

4 The weight shiftson rotor 29


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1. Misalignment of rotors due to improperly seated bearings.2. Mechanical looseness at the supports.3. Oil whirl in the fluid film bearings (oil whirl is caused by the instability of

the rotor supported in the fluid film)

9.2 Critical Speed:

All rotating machine components have more than one natural

frequencies of oscillation; the frequencies at which they will vibrate

enormously. When the speed of rotation is nearing its natural frequency, even

a small amount of unbalance in rotor causes unacceptable vibration leading

to resonance. The speed of the rotor is known as C RITICAL SPEED. At these

critical speeds only, all the unbalanced forces in the rotor resonate and c reate

maximum amplitude of vibration. Above or below each critical speed,

resonant effects diminish. Hence the turbo machines are designed such that

the natural frequencies do not coincide with the operating speed.

9.3 Need Of Measurement:

Vibration measurement and monitoring are warranted due to the following

reasons:

1. It is useful to diagnose the healthiness of the turbine to avoid failure. Anychange in quiet running means a deterioration of the balanced state of

the rotor 30


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4. It is useful to correlate with probable failures by having history of vibrationdata as a function of changes in the process parameters such as load,temperature etc.

5. It is useful to estimate how much longer a machine can be safely run byobserving the trend in vibration level

When the normal operating speed of the turbine rotor is less than its first

critical speed, the rotor is said to be a rigid rotor. When its normal operating

speed lies above its first critical speed, the turbine rotor is said to be a flexible

rotor. The critical speed values of 210 MW turbine rotors (Units 4 to 7 / TPS-II) are

indicated below:

CRITICAL SPEED (in rpm):

HP rotor IP rotor LP rotor Gen. Rotor Combined

1 First critical speed 4230 3858 1818 1370 1544

2 Second critical speed >3600 >3600 >3600 3400 2126

9.4 Absolute Vibration of Bearing Pedestal:

Pick-up used for measuring absolute vibration of bearing pedestal is of

SEISMIC MASS TYPE (contact type). This type is a velocity transducer. In this, the

mechanical vibration is converted to an electric signal which is proportional to

the velocity of the vibration.

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The plunger coil is suspended from the housing by a spring. The inherent

frequency of the spring/mass is

= 2f = (c/m)1/2

where m is mass of the plunger coil with spring and C is the spring constant.

When the vibrating frequency is above the inherent frequency, the

plunger coil is steady in space due to its mass inertia. Thus a fixed point in

space is created and the vibrations can be measured with reference to this

point.

Whenever this transducer (seismic device) is firmly connected to the

surface of the vibrating item to be measured, a relative motion is generated

between the permanent magnet and the plunger coil. When the permanent

magnet vibrates and the spring-suspended coil is stationary in space, the coil

cuts magnetic lines of force resulting in generation of voltage.

The voltage is proportional to the vibration veloc ity (the rate of

vibration); the strength of the magnetic field, and the number of turns of wire in

the coil.

e = B.l.v

Where e is Induced voltage,

B is magnetic induction of the permanent magnet,

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specification for the pick up. The sensor, provided here, generates 20mV for a

vibration velocity of 1 mm/sec.

The amplitude of vibration is usually indicated rather than the rate of

vibrations. This is achieved by integrating the sensor output voltage, which is

proportional to the rate of vibrations.

9.5 Relative Vibration of Shaft:

Relative shaft vibration is the radial periodic motion of the shaft with

respec t to the bearing or casing. Radial shaft position is the position of the shaft

with respect to the bearing or casing and depends on the operating state of

the turbine and on the condition of the bearings. The range within which the

radial shaft position may change is very much greater than the vibration travel

amplitude of the shaft. The relative vibration is thus superimposed over the

change of the radial shaft position.

The measurement is carried out in a contact less manner according tothe eddy current method. A non-contact sensor requires external electronic

circuitry to generate a very high frequency a.c signal and detect vibrations in

the a.c signal caused by the vibration of the shaft.

The electronic circuit, called a signal sensor, generates a very high

frequency electrical signal. This signal is applied to a small coil of wire in the tip

of the non-contact pick-up. The high frequency electric signal applied to the

coil generates a magnetic field. This electro magnetic alternating field

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closer the coil to the shaft, the greater the loading effect and the smaller the

amount of elec trica l signal.

As the shaft moves relative to the tip of the pick up, the strength of the

electrical signal changes proportional to the movement. The variation in the

strength of the electrical signal is thus proportional to the amount of vibration

and is further processed.

Schematic diagram of non-contact pick-ups is shown in the fig: 10. The

signal sensor consists of a high frequency oscillator, an amplitude detector

(Demodulator) and an amplifier.

When the incoming supply is ON, the transducer starts working. Theoscillator generates a very high frequency electrical signal, which is applied

through a cable to a small coil of wire in tip of the pick up. The variation in the

strength of this elec tric signal occurs as explained earlier and it is proportional

to the amount of vibration.

The amplitude modulated high frequency electric signal of the

transducer is demodulated by the amplitude detector to give a d.c voltage

signal proportional to the gap (the distance between the tip of the transducer

and the shaft is called Gap) and an a.c. signal voltage proportional to

vibration. These signals are then amplified to a high level to give usable output

voltages.

10. Measurement Of Temperature - Thermocouple (Ref Fig: 11)

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junctions of the pair of dissimilar metal wires. One end is fused together to for a

measuring junction called as the hot junction and the other end, the cold

junction or the reference junction and are connected to a measuring device.

The temperature at the hot junction is determined by measuring the voltage

appearing at the cold junction. The voltage developed is a function of the

difference in temperature between the hot and the cold junctions. Hence the

temperature of the cold end junction must be accurately known.

The desirable properties of thermo couples for industrial use are

i Relatively large thermal e m fii Prec ision of calibrationiii Resistance to corrosion and oxidation.iv Linear relation of emf to temperature.

In order to prevent the forming of a second hot junction, the wires of

thermo couples are insulated from each other.

Ni-Cr-Ni Thermo couples are provided for measuring turbine bearing

metal temperature for the following (Ref. Fig. (12), (13) & (14)

1. HP front journal bearing - Top/Left, Top/Right, Bottom/ left &Bottom/Right (4 Points)

2. HP rear thrust bearing - Top/Left, Top/Right, Bottom/ left &Bottom/Right, Front Side & Rear Side

(8 Points)3. IP rear journal bearing - Front & Rear (2 Points)4. LP rear journal bearing - Front & Rear (2 Points)5. Generator front journal - Front & Rear (2 Points)

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S.No. Conditions Values

1. Speed:Maximum speed for continuous operation. 51.5Hz

Minimum speed for continuous operation 47.5 Hz

2. Turbo Generator Bearing Metal Temperature:

Alarm Value Trip Value

a) Bearing Number 1 HPT Front 90 C 100 C

b) Bearing Number 2 HPT Rear 90 C 100 C

c) Thrust Bearings 90 C 100 Cd) Bearing Number 3 IPT Rear 100 C 110 C

e) Bearing Number 4 LPT Rear 100 C 110 C

f) Bearing Number 5 Generator Front 100 C 110 C

g) Bearing Number 6 Generator Rear 80 C 90 C

3. Turbine Bearing Vibrations: Alarm Value "Trip Value"

a) Absolute Brg Vibration in (0 Peak) 35 45

b) Absolute Shaft Vibration in (0 Peak) 120 200

4. Differential Expansion: Upper Limit Lower Limit

a) HP Differential Expansion. + 5 - 3.5

b) IP Differential Expansion. + 6.5 - 2.0


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turbo supervisory system

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