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THERMAL STRESS HOW IT IS GENERATED IN TURBINE?
During operational changes of the Turbine say Start-up, Loading &
unloading the surface of the Turbine components gets heated or
cooled immediately as it comes in contact with the steam.
Whereas internals of the turbine components are not able to
response that fast.
The result is a differential temperature between Surface (Ts) & Mid
metal (Tm)which generates Thermal stress.
Because Thermal Stress Ts Tm ( T)
The more the value of T the more will be the
thermal stress.
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TSE WHAT IS THE NEED ?
The Turbine is equipped with TSE to assist in optimized Start-
up,operation & Shutdown without impairing the expected
operating life.
Time is a prime importance while start-up,loading operation of
Turbine.
At the same time it also very necessary to keep the thermal
stress in turbine components under control.
TSE is specifically designed for achieving both
the above mentioned objectives at the same
time.
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The components of TSE
TSE basically consists of three sections.
1. INPUT SECTION
2. COMPUTING DEVICES
3. OUTOUT DEVICES
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INPUT SECTION
Input section needs
Temperature inputs from Turbine components.
ACTUAL LOAD
ACTUAL SPEED
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Temperature Inputs
TSE takes temperature inputs from five Turbinecomponents.
They are
1. Emergency Stop Valve. 2. HP Control Valve.
3. HP Turbine casing.
4. HP Turbine Shaft.
5. IP Turbine Shaft.
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WALL TEMPERATURE SENSORS
The Temperature inputs are supplied by Ni Cr-Ni thermocouples known as
WT SENSORS
Temperature inputs for stationery parts are obtained from WT Sensorshaving Two legs.
One leg is inserted at 95% of the metal depth nearing surface measure
surface temperature (TS) & another leg is inserted at 55% of materialthickness (Tm).
For TS & Tmfor rotating parts TS is taken from a place where the Radialclearance between Casing & Rotor is minimum.
The Tmis calculated with fair degree of accuracy by means of followingequation.
Tm = Ts [ 1- (0.692 e -t/T1 + 0.131 e -t/T2 + 0.177 e-t/Tk ) ]Where, Ts : Surface Temperature T1 : 2408.31
Tm : Mid metal Temperature T2 : 457.08t : Time in minutes Tk : 56.62
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The millivolt (D.C.) output from thermocouple is fed to Analog
Signal Conditioning Cabinet (CJJ05) where it is converted into
4-20 mA signals are fed into TSE CABINET (CJJ01).
Actual speed measured from Halls Probe provided in turbine
front pedestal as 4-20 mA signal fed into TSE CABINET.
Actual Load of Turbo-Generator is measured & a currentsignal of 4-20 mA signal fed into TSE CABINET for actual
Load indication & computation of Load Margins.
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Computation
The five turbine components has got five computing channels in
computing devices. Each Computing channel calculates the difference Ta from Ts
& Tm.
Ta = Ts-Tm The calculated temperature difference Ta is compared with the
permissible temperature difference Tp.
Tp is derived from limit curve of that particular componentalready fed into TSE hardware.
These Limit Curves are nothing but maximum permissible
temperature difference allowed w.r.t. Tm while heating & cooling.
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The difference between Tp & Ta iscalled margin.
Comparing Ta against Tp on the +ve side,
we get UPPER MARGIN & the same on the
ve side we get LOWER MARGIN.
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Suppose at any particular condition
Ts of HP Casing = 300 deg c
Tm of HP Casing = 240 deg c
Ta= Ts-Tm= 300-240=60 deg c
From upper limit curve when Tm = 240 deg c
Then Max upper permissible temp diff ( Tpu) = 100 deg
c
Max lower permissible temp diff ( Tpl) = -60 deg c
So the Upper margin = ( Tpu- Ta) =100-60=+40 deg c& the Lower margin = ( Tpl- Ta) =-60-60 =-120 deg c
Th i f f h b i f &
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The inference of the above computation of upper &
lower temperature is that Surface temp Tscanbe
increased by 40 deg c (to the level of 340 deg c) is
known as Upper margin. Similarly Ts can be decreased by 120 deg c is known
as Lower margin.
Thus the upper & lower margin for all the five turbine
components calculated in similar fashion.
The minimum upper margin & minimum lower marginamong them is selected separately for display purpose
& as well fed to EHC for controlling speed rate & load
rate.
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TSE OUTPUT SIGNAL GOES TO
TSE DISPLAY
TSE MARGIN RECORDER
ATRS
CMC
EHC
SPEED CONTROLLER
LOAD CONTROLLER
TSE DISPLAY
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TSE DISPLAY
TSE DISPLY
TSE DISPLAY has two separate sections
1. One is up to synchronisation stage.
2. Another for Load condition.
The sections are illuminated according to
operating mode
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ADMISSION OR TURBINE MODE HAS THEMARKING ON WHITE SCALE INDICATES
THE ACTUAL SPPED OF THE TG SET.
THE UPPER BOUNDARY OF
TRANSPARENT SECTOR INDICATES THEUPPER MARGIN FOR SPEEDING UP.
THE TOP RECTANGLE(ADM. MODE) & LED(TURBINE MODE) GETS ILLUMINATED ANDINDICATES THE COMPONENT WHICH ISCAUSE FOR IMPOSING MARGIN.
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ADMISSION MODE
ADMISSION MODE IS
SELECTED BEFORE OPENING
STOP VALVES.
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TURBINE MODE
THIS MODE IS SELECTED BEFOREOPENING THE CONTROL VALVES FOR
SOAKING OR SPEEDING UP.
PRE-SELECTION SWITCH FOR SELECTION
OF ADMISSION OR TURBINE MODE ARE
PROVIDED ON THE CONSOLE.
THE ABOVE TWO MODES ARE DISPLAYED
ON THE L.H.S. OF TSE DISPLAY.
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LOAD MODE
THE TSE INDICATOR SWITCHES OVER TO RIGHT HANDSECTION ONCE THE LOAD >2% MCR.
DURINNG LOAD OPERATION THE DISPLAY INDICATES
ACTUAL LOAD ( MARKING)
UPPER & LOWER LOAD MARGINS WHICH SIGNIFIES
MAXIMUM LOADING & UNLOADING LIMIT AT THATMOMENT.
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TSE OUTPUT SIGNAL GOES TO
TSE DISPLAY
TSE MARGIN RECORDER
ATRS
CMC
EHC
SPEED CONTROLLER
LOAD CONTROLLER
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Speed controller output ( EHC OUTPUT)gets blocked if
Turbine speed >2850 r.p.m.
and
TSE GETS FAULTED
LOWER MARGIN IS NOT USED IN SPEED
CONTROLLER AS TURBINE COASTINGDOWN IS NATURAL.
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NEGATIVE LOAD MARGIN CANUNLOAD THE MACHINE WHEREASREDUCRD LOWER MARGIN CAN
PREVENT TURBINE FROMUNLOADING.
TSE INFLUENCE TO ATRS
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TSE INFLUENCE TO ATRS
SGC Turbine can not be made ON if TSE is N/A.
TSE upper margin is one of criteria needed for thenext step (No. 15) and subsequently speed raiseto 3000 r.p.m.
Speed raise is held up till upper margin is notmore than 30 deg c.
SGC Turbine start up programme gets switched offwhile Rolling (600-2850 r.p.m.)if TSE Upper Margin
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USE OF MARGIN IN CMC
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USE OF MARGIN IN CMC
Minimum ofTSE lower margin & Unit Load Rate(in CMC Console) is considered as allowable load
rate at which the unit would be unloaded.
Minimum ofTSE upper margin & Unit Load Rate
is considered as allowable load rate at which the
unit would be loaded.
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TSE TEST For checking the proper functioning of the five computing
channels from INPUT SECTION up to DISPLAY, known INPUTsignals can be applied (by pressing test buttons) to get apredetermined results.
Testing of five computing channels are possible only if1. NO EHC FAULT
2. NO TEST PROGRAMMING BLOCK FROM ATRS
If there is any deviation TSE should not be used till fault is cleared
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Procedure adopted in case of lost upper Margin
The turbine should not be loaded further.
Reduction of steam temperature by cuttingdown firing.
Soaking the turbine for sufficient time period.
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Procedure adopted in case of lost lower Margin
Avoidance of further unloading.
Increase firing to increase steam temperature.
Soaking the machine.
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