Mukharji TSE

 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 (T s) & Mid metal (Tm)which generates Thermal stress. Because Thermal Stress

The more the value of  thermal stress.

Ts – Tm ( T) T the more will be the

 TSE

WHAT IS THE NEED ?

• The Turbine is equipped with TSE to assist in optimized up,operation & Shutdown without impairing the expected operating life.

Start-

• Time is a prime importance while start-up,loading operation of  Turbine. •  At the the sam same e time time it also also very nec necess essary ary to kee keep p the the therm thermal al stress in turbine components under control.

TSE is specifically designed for achieving both the above mentioned objectives at the same time.

 The components of TSE TSE basically consists of three sections.

1. INPUT SECTION 2. COMPUTING DEVICES 3. OUTOUT DEVICES

INPUT SECTION Input section needs • Temperature inputs from Turbine components. • ACTUAL LOAD • ACTUAL SPEED

 Temperature Inputs • TSE takes temperature inputs from five Turbine components. They are 1. Emergency Stop Valve. 2. HP Control Valve. 3. HP Turbine casing. 4. HP Turbine Shaft. 5. IP Turbine Shaft.

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 Sensors having Two legs. One leg is inserted at 95% of the metal depth nearing surface measure surface temperature (T S) & another leg is inserted i nserted at 55% of material thickness (Tm).

•

•

For TS & Tm for rotating parts T S is taken from a place pl ace where the Radial clearance between Casing & Rotor is minimum.

•

The Tm is calculated with fair degree of accuracy by means of following equation.

•  Tm = Ts [ 1- (0.6 (0.692 92 e -t/T1 + 0.131 0.131 e -t/T2 -t/T2 + 0.177 0.177 e -t/Tk ) ] Where, Ts : Surface Temperature Tm : Mid metal Temperature

T1 : 2408.31 T2 : 457.08

• 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  Actual spee speed d meas measured ured from Halls Prob Probe e provid provided ed in turbin turbine e front pedestal as 4-20 mA signal fed into TSE CABINET.

•  Actual  Actual Load Load of Turbo Turbo-Ge -Genera nerator tor is is meas measured ured & a curre current nt signal of 4-20 mA signal fed into TSE CABINET for actual Load indication & computation of Load Margins.

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 component already fed into TSE hardware.

• These Limit Curves are nothing but maximum permissible temperature difference allowed w.r.t. Tm while heating & cooling.

• The difference between called margin.

Tp &

Ta is

• Comparing Ta against Tp on the +ve side, we get UPPER MARGIN & the same on the  –ve side we get LOWER MARGIN MARGIN..

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 =100-60=+40 deg c

• The inference of the above computation of upper & lower temperature is that Surface temp Ts can be 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 margin among them is selected separately for display purpose & as well fed to EHC for controlling speed rate & load rate.

• TSE OUTPUT SIGNAL GOES TO • TSE DISPLAY • TSE MARGIN RECORDER •  ATRS • CMC • EHC SPEED CONTROLLER LOAD CONTROLLER

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

• ADMISSION OR TURBINE MODE HAS THE MARKING ON WHITE SCALE INDICATES THE ACTUAL SPPED OF THE TG SET. • THE UPPER BOUNDARY OF TRANSPARENT SECTOR INDICATES THE UPPER MARGIN FOR SPEEDING UP. • THE TOP RECTANGLE(ADM. MODE) & LED (TURBINE MODE) GETS ILLUMINATED AND INDICATES THE COMPONENT WHICH IS CAUSE FOR IMPOSING MARGIN.

ADMISSION MODE

• ADMISSION MODE IS SELECTED BEFORE OPENING STOP VALVES.

TURBINE MODE • THIS MODE IS SELECTED BEFORE OPENING 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.

LOAD MODE • THE TSE INDICATOR SWITCHES OVER TO RIGHT HAND SECTION 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 THAT MOMENT.

• TSE OUTPUT SIGNAL GOES TO • TSE DISPLAY • TSE MARGIN RECORDER •  ATRS • CMC • EHC SPEED CONTROLLER LOAD CONTROLLER

• 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 COASTING DOWN IS NATURAL.

• NEGATIVE LOAD MARGIN CAN UNLOAD THE MACHINE WHEREAS REDUCRD LOWER MARGIN CAN PREVENT TURBINE FROM UNLOADING.

TSE INFLUENCE TO ATRS • SGC Turbine can not be made ON if TSE is N/A. • TSE upper upper margin margin is one of of criteria needed for the next step (No. 15) and subsequently speed raise to 3000 r.p.m.

• Speed raise is held up till upper margin is not more than 30 deg c. • SGC Turbine start up programme gets switched off  while Rolling (600-2850 r.p.m.)if TSE Upper Margin