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 THERMAL STRESS

 THERMAL STRESS

HOW IT IS GENERATED IN TURBINE?HOW IT IS GENERATED IN TURBINE?

During operational changes of the Turbine say Start-up, Loading &

During operational changes of the Turbine say Start-up, Loading &

unloading the surface of the Turbine components gets heated or 

unloading the surface of the Turbine components gets heated or 

cooled immediately as it comes in contact with the steam.

cooled immediately as it comes in contact with the steam.

Whereas internals of the turbine components are not able to

Whereas internals of the turbine components are not able to

response that fast.

response that fast.

The result is a differential temperature between Surface (T

The result is a differential temperature between Surface (Tss) ) & & MidMid

metal (T

metal (Tmm)which generates Thermal stress.)which generates Thermal stress.

Because

Because Thermal StressThermal Stress

Ts – Tm (

Ts – Tm (

T)

T)

The more the value of 

The more the value of 

T the more will be the

T the more will be the

thermal stress

(4)

 TSE

 TSE

WHAT IS THE NEED ?WHAT IS THE NEED ?

• The The Turbine Turbine is is equipped equipped with with TSE TSE to to assist assist in in optimized optimized Start-

Start-up,operation & Shutdown without impairing the expected

up,operation & Shutdown without impairing the expected

operating life.

operating life.

Time is a prime importance while start-up,loading operation of Time is a prime importance while start-up,loading operation of 

Turbine.

Turbine.

•  At t At the he samsame tie time me it ait also lso very very necnecessessary ary to kto keeeep thp the te thermhermalal

stress

stress in in turbine turbine components components under under control.control.

TSE

TSE

is specifically designed for achieving both

is specifically designed for achieving both

the above mentioned objectives at the same

the above mentioned objectives at the same

time.

(5)

 The components of TSE

 The components of TSE

TSE

TSE

basically consists of three sections.

basically consists of three sections.

1. INPUT SECTION

1. INPUT SECTION

2. COMPUTING DEVICES

2. COMPUTING DEVICES

3.

(6)

INPUT SECTION

INPUT SECTION

Input section needs

Input section needs

Temperature inputs from Turbine components.

Temperature inputs from Turbine components.

ACTUAL LOAD

ACTUAL LOAD

(7)

 Temperature Inputs

 Temperature Inputs

•• TSE takes temperature inputs from five Turbine

TSE takes temperature inputs from five Turbine

components.

components.

They are

They are

1. Emergency Stop Valve.

1. Emergency Stop Valve.

2. HP Control Valve.

2. HP Control Valve.

3. HP Turbine casing.

3. HP Turbine casing.

4. HP Turbine Shaft.

4. HP Turbine Shaft.

5.

(8)

WALL TEMPERATURE SENSORS

WALL TEMPERATURE SENSORS

•• The The Temperature Temperature inputs are inputs are supplied supplied by by Ni Ni Cr-Ni Cr-Ni thermocouples thermocouples known asknown as WT SENSORS

WT SENSORS •

• Temperature inputs for stationery parts are obtained from WT Temperature inputs for stationery parts are obtained from WT SensorsSensors having Two legs.

having Two legs. •

• One leg is inserted at 95% One leg is inserted at 95% of the metal depth nearing surface measureof the metal depth nearing surface measure

surface temperature (T

surface temperature (TSS) & another leg is i) & another leg is inserted at 55% of materialnserted at 55% of material

thickness (T

thickness (Tmm).).

• For TFor TS &S & TTmm for rotating parts Tfor rotating parts TSS is taken from a plis taken from a place where the Radialace where the Radial

clearance between Casing & Rotor is minimum.

clearance between Casing & Rotor is minimum.

The TThe Tmm is calculated with fair degree of accuracy by means of followingis calculated with fair degree of accuracy by means of following

equation. equation. • •  Tm  Tm = Ts = Ts [ 1- [ 1- (0.6(0.692 e 92 e -t/T1 -t/T1 + 0.+ 0.131 131 e -t/e -t/T2 T2 + 0.+ 0.177 177 ee -t/Tk ) ] -t/Tk ) ] Where,

Where, Ts Ts : : Surface Surface Temperature Temperature T1 T1 : : 2408.312408.31 Tm

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• The millivolt (D.C.) output from thermocouple is fed to AnalogThe millivolt (D.C.) output from thermocouple is fed to Analog

Signal Conditioning Cabinet (CJJ05) where it is converted into

Signal Conditioning Cabinet (CJJ05) where it is converted into

4-20

4-20 mA mA signals signals are are fed fed into into TSE TSE CABINET CABINET (CJJ01).(CJJ01).

•  Act Actual sual speepeed md measeasured ured from from Halls Halls ProbProbe pe providrovided ed in tin turbinurbinee

front pedestal as 4-20 mA signal fed into TSE CABINET.

front pedestal as 4-20 mA signal fed into TSE CABINET.

•  Act Actual Lual Load oad of Tof Turbourbo-Ge-Generanerator itor is ms measeasured ured & a & a currecurrentnt

signal of 4-20 mA signal fed into TSE CABINET for actual

signal of 4-20 mA signal fed into TSE CABINET for actual

Load indication & computation of Load Margins.

(12)

Computation

Computation

• The five turbine components has got five computing channels inThe five turbine components has got five computing channels in

computing devices.

computing devices.

• Each Computing channel calculates the differenceEach Computing channel calculates the difference TaTa from Tsfrom Ts

& Tm.

& Tm.

Ta = Ts-Tm

Ta = Ts-Tm

• The calculated temperature differenceThe calculated temperature difference TaTa is compared with theis compared with the

permissible temperature difference

permissible temperature difference Tp.Tp.

TpTp is derived from limit curve of that particular componentis derived from limit curve of that particular component

already fed into TSE hardware.

already fed into TSE hardware.

• These Limit Curves are nothing but maximum permissibleThese Limit Curves are nothing but maximum permissible

temperature difference allowed w.r.t. Tm while heating & cooling.

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The difference between

The difference between

Tp &

Tp &

Ta is

Ta is

called margin.

called margin.

Comparing

Comparing

Ta

Ta

against

against

Tp

Tp

on the +ve side,

on the +ve side,

we get UPPER

we get UPPER

MARGIN & the same on the

MARGIN & the same on the

 –ve side we get LOWER M

(19)

Suppose at any particular condition

Suppose at any particular condition

Ts of

Ts of HP Casing HP Casing = = 300 300 deg cdeg c

Tm of HP Casing = 240 deg c

Tm of HP Casing = 240 deg c

∆ Ta= Ts-Tm= 300-240=60 deg c

∆ Ta= Ts-Tm= 300-240=60 deg c

From upper limit curve when Tm = 240 deg c

From upper limit curve when Tm = 240 deg c

Then Max upper permissible temp diff (∆ Tpu) = 100 deg

Then Max upper permissible temp diff (∆ Tpu) = 100 deg

c

c

Max lower permissible temp diff (∆ Tpl) = -60 deg c

Max lower permissible temp diff (∆ Tpl) = -60 deg c

So the Upper margin =

(20)

•• The inference of the above computation of upper &

The inference of the above computation of upper &

lower temperature is that Surface temp

lower temperature is that Surface temp T

T

ss

can

can be

be

increased by 40 deg c (to the level of 340 deg c) is

increased by 40 deg c (to the level of 340 deg c) is

known as

known as

Upper margin

Upper margin

.

.

•• Similarly

Similarly T

T

ss

can be decreased by 120 deg c is known

can be decreased by 120 deg c is known

as

as

Lower margin.

Lower margin.

Thus the upper & lower margin for all the five turbine

Thus the upper & lower margin for all the five turbine

components calculated in similar fashion.

components calculated in similar fashion.

The minimum upper margin & minimum lower margin

The minimum upper margin & minimum lower margin

among them is selected separately for display purpose

among them is selected separately for display purpose

& as well fed to EHC for controlling speed rate & load

& as well fed to EHC for controlling speed rate & load

rate.

(21)
(22)

•• TSE OUTPUT SIGNAL GOES TO

TSE OUTPUT SIGNAL GOES TO

TSE DISPLAY

TSE DISPLAY

TSE MARGIN RECORDER

TSE MARGIN RECORDER

 ATRS

 ATRS

CMC

CMC

•• EHC

EHC

SPEED CONTROLLER

SPEED CONTROLLER

LOAD CONTROLLER

LOAD CONTROLLER

(23)

TSE DISPLAY

TSE DISPLAY

TSE DISPLY

TSE DISPLY

TSE DISPLAY has two

TSE DISPLAY has two separate sections

separate sections

1. One is up

1. One is up

to synchronisation stage.

to synchronisation stage.

2. Another for

2. Another for

Load condition.

Load condition.

The sections are illuminated according to

The sections are illuminated according to

operating mode

operating mode

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ADMISSION OR TURBINE MODE HAS THE

ADMISSION OR TURBINE MODE HAS THE

MARKING ON WHITE SCALE INDICATES

MARKING ON WHITE SCALE INDICATES

THE ACTUAL SPPED OF THE TG

THE ACTUAL SPPED OF THE TG

SET.

SET.

THE UPPER BOUNDARY OF

THE UPPER BOUNDARY OF

TRANSPARENT SECTOR INDICATES THE

TRANSPARENT SECTOR INDICATES THE

UPPER MARGIN FOR SPEEDING UP.

UPPER MARGIN FOR SPEEDING UP.

•• THE TOP RECTANGLE(ADM. MODE) & LED

THE TOP RECTANGLE(ADM. MODE) & LED

(TURBINE MODE) GETS ILLUMINATED AND

(TURBINE MODE) GETS ILLUMINATED AND

INDICATES THE COMPONENT WHICH IS

INDICATES THE COMPONENT WHICH IS

CAUSE FOR IMPOSING MARGIN.

(25)

ADMISSION MODE

ADMISSION MODE

 ADMISSION MODE IS

 ADMISSION MODE IS

SELECTED BEFORE OPENING

SELECTED BEFORE OPENING

STOP VALVES.

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TURBINE MODE

TURBINE MODE

THIS MODE IS SELECTED BEFORE

THIS MODE IS SELECTED BEFORE

OPENING THE CONTROL VALVES FOR

OPENING THE CONTROL VALVES FOR

SOAKING OR SPEEDING UP.

SOAKING OR SPEEDING UP.

PRE-SELECTION SWITCH FOR SELECTION

PRE-SELECTION SWITCH FOR SELECTION

OF ADMISSION OR TURBINE MODE ARE

OF ADMISSION OR TURBINE MODE ARE

PROVIDED ON THE CONSOLE.

PROVIDED ON THE CONSOLE.

•• THE ABOVE TWO MODES ARE DISPLAYED

THE ABOVE TWO MODES ARE DISPLAYED

ON THE L.H.S. OF TSE DISPLAY.

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(31)

LOAD MODE

LOAD MODE

THE TSE INDICATOR SWITCHES OVER TO RIGHT HANDTHE TSE INDICATOR SWITCHES OVER TO RIGHT HAND

SECTION ONCE THE LOAD >2% MCR.

SECTION ONCE THE LOAD >2% MCR.

• DURINNG LOAD OPERATION THE DISPLAY INDICATESDURINNG LOAD OPERATION THE DISPLAY INDICATES

• ACTUAL LOAD ( ACTUAL LOAD ( MARKING)MARKING)

•• UPPER & LOWER LOAD MARGINS WHICH SIGNIFIESUPPER & LOWER LOAD MARGINS WHICH SIGNIFIES MAXIMUM LOADING & UNLOADING LIMIT AT

MAXIMUM LOADING & UNLOADING LIMIT AT THATTHAT MOMENT.

(32)
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(34)

•• TSE OUTPUT SIGNAL GOES TO

TSE OUTPUT SIGNAL GOES TO

TSE DISPLAY

TSE DISPLAY

TSE MARGIN RECORDER

TSE MARGIN RECORDER

 ATRS

 ATRS

CMC

CMC

•• EHC

EHC

SPEED CONTROLLER

SPEED CONTROLLER

LOAD CONTROLLER

LOAD CONTROLLER

(35)
(36)

Speed controller output (

Speed controller output (

EHC OUTPUT)

EHC OUTPUT)

gets blocked if 

gets blocked if 

Turbine speed >2850 r.p.m.

Turbine speed >2850 r.p.m.

and

and

TSE

TSE

GETS

GETS

FAULTED

FAULTED

LOWER MARGIN IS NOT USED IN SPEED

LOWER MARGIN IS NOT USED IN SPEED

CONTROLLER AS TURBINE COASTING

CONTROLLER AS TURBINE COASTING

DOWN IS NATURAL.

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(38)

NEGATIVE LOAD MARGIN CAN

NEGATIVE LOAD MARGIN CAN

UNLOAD THE MACHINE WHEREAS

UNLOAD THE MACHINE WHEREAS

REDUCRD LOWER MARGIN CAN

REDUCRD LOWER MARGIN CAN

PREVENT TURBINE FROM

PREVENT TURBINE FROM

UNLOADING.

(39)

TSE

TSE

INFLUENCE

INFLUENCE

TO

TO

ATRS

ATRS

SGC Turbine can not be made ON if TSE is N/A.SGC Turbine can not be made ON if TSE is N/A.

TSE upTSE upper mper margin is argin is one oone of criteria f criteria needed needed for thefor the

next step (No. 15) and subsequently speed raise

next step (No. 15) and subsequently speed raise

to 3000 r.p.m.

to 3000 r.p.m.

Speed raise is held up till upper margin is not

Speed raise is held up till upper margin is not

more than 30 deg c.

more than 30 deg c.

SGC Turbine start up programme gets switched off SGC Turbine start up programme gets switched off 

while Rolling (600-2850 r.p.m.)if TSE

while Rolling (600-2850 r.p.m.)if TSE Upper MarginUpper Margin

<0 deg c.

<0 deg c.

(40)
(41)

USE OF MARGIN IN CMC

USE OF MARGIN IN CMC

•• Minimum of 

Minimum of TSE lower margin

TSE lower margin

&

& Unit Load Rate

Unit Load Rate

(in CMC Console) is

(in CMC Console) is considered as allowable load

considered as allowable load

rate

rate at

at which

which the

the unit

unit would

would be

be unloaded.

unloaded.

•• Minimum of 

Minimum of TSE upper margin

TSE upper margin

&

& Unit Load Rate

Unit Load Rate

is considered as allowable load rate at which

is considered as allowable load rate at which the

the

unit would be loaded.

(42)
(43)

TSE TEST

TSE TEST

• For checking the proper functioning of the five computingFor checking the proper functioning of the five computing

channels from INPUT SECTION up to DISPLAY, known INPUT

channels from INPUT SECTION up to DISPLAY, known INPUT

signals can be applied (by pressing test buttons) to get a

signals can be applied (by pressing test buttons) to get a

predetermined results.

predetermined results.

•• Testing of five computing channels are possible only if Testing of five computing channels are possible only if 

1. NO EHC FAULT

1. NO EHC FAULT

2. NO TEST PROGRAMMING BLOCK FROM ATRS

2. NO TEST PROGRAMMING BLOCK FROM ATRS

If there is any deviation TSE should not be used till fault is cleared

(44)
(45)

Procedure adopted in case of lost upper Margin

Procedure adopted in case of lost upper Margin

The turbine should not be loaded further.

The turbine should not be loaded further.

Reduction of steam temperature by cutting

Reduction of steam temperature by cutting

down firing.

down firing.

(46)

Procedure adopted in case of lost lower Margin

Procedure adopted in case of lost lower Margin

Avoidance of further unloading.

Avoidance of further unloading.

Increase firing to increase steam temperature.

Increase firing to increase steam temperature.

(47)
(48)

References

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