ADVANCED C&I SYSTEMS
FOR SUPER CRITICAL
BOILERS
Presentation By
V.R.Krishnakumar
T.R.Rajagopalan
P.Rajesh
NTPC-SRHQ
Operation Services
Present Operational Scenario
Availability Based Tariff implemented
Continuous Availability and Operational
Reliability of Units are very crucial and
linked to Tariff
Fast response to load changes
demanded from Control centre is
assuming greater significance
AREAS IDENTIFIED FOR
IMPROVEMENT in SC BOILERS
Temperature measurement
MS Temperature Control using
advanced controllers
Intelligent Soot Blowing Systems
Fire side corrosion monitoring
COMMONLY USED TEMP.
TRANSDUCERS
Thermocouples
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Type J
Type K
Type R
RTDs
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PT 100
Cu 53
RTDs
A resistor that has a very large
temp.coefficient installed in a metal
protection sheath
The output of the resistor varies
proportionately with temp.
Pt is most commonly used as it has a
high melting point
It has a well defined resistance vs temp.
relationship
THE ISSUES IN STEAM TEMP.
MEASUREMENT
Drift in thermocouples over a period of time
RTDs have much better accuracy, but higher
cost and poor durability for rugged
applications
Performance acceptance test of power plants
warrants measurement within a certainty of
0.5ºC at 550 ºC
The temp.gradient over a T/C has a large
effect on its performance
3 ºC under estimation of MS temp.at 538 ºC
increases heat rate by about 0.1 per cent
THE ISSUES IN STEAM TEMP.
MEASUREMENT contd..
6 ºC over estimation of MS temp. can reduce
life expectancy of boiler tubes by up to 28%
Non-homogeneous T/Cs will give difft.
Outputs at difft. immersion depths
To achieve 0.5ºC accuracy it is necessary to
calibrate T/C at various insertion depths and
use only those which show same output at
difft. insertion lengths
PT100s are capable of meeting the accuracy
but their life expectancy in the industrial
environment is not known
STUDIES ON TEMP.SENSORS
(1) at PG & E and Duke Power
(2) at South African
energy company Eskom
RECOMMENDATIONS
Develop a database to keep history records of
all calibration data
For Performance tests T/Cs may not be used
at least for critical m/m like MS, RH, FW, Bled
steam etc.
A project should be initiated to scan
international trends in temp.m/m
T/Cs should be calibrated in situ to avoid
inhomogeneity effects
There are some internationally reputed
suppliers for PT100 RTDs suitable for steam
applications
STEAM TEMPERATURE
CONTROL - ISSUES
Use of conventional PID controllers
results in temperature excursions during
load fluctuations and takes around 30
minutes to stabilize the temp
This affects the life of critical
components in Boiler as well as Turbine
This also affects the heat rate/ economy
of generation
Main Steam Temp.Control
EPRI study in Mid
American’s George Neal
Station
RECOMMENDATIONS
Use of Advanced controllers such as Multi
Variable Controllers / Fuzzy Logic Controllers
in steam temp control applications
Optimization/Tuning of important boiler
controls may be taken up periodically
SH and RH steam sensors may be checked
for proper response on line. T/Cs exhibiting
time constants in excess of 30 seconds may
be removed and checked
PREDICTIVE ADAPTIVE
CONTROLLER
The dynamic modeling method is used to
represent the process and models the process
dead time also
Controller adapts to the changes in gain, time
constants and time delay(dead time)
happening in the process with MW changes
A custom gain scheduling that would
automatically load different sets of PID tuning
values depending on MW change
BENEFITS COMPARED TO
CONVENTIONAL PID
Performance improvement of around
50% i.e. deviations within 6 to 9 ºC
from set point compared to 12 to 19 ºC
during ramp load changes of 5-10
MW/min
SHIFT IN CONTROL
PHILOSOPHY FOR SC
BOILERS?
Firing control based on metal /steam
temperature measurement rather than
MW/Load set point
This may result in better match of load
and firing especially during ramp load
changes
FIRE SIDE CORROSION
MONITORING
Superheaters and Reheaters can fail due to
corrosion by molten alkali sulfates
Higher steam temperatures result in
increased susceptibility to this corrosion
On-line electrochemical corrosion monitors
developed for low temp.applications can be
adopted for monitoring of tube corrosions in
SC Boilers
Continuous monitoring is attractive as the
trend shows variations with load and coal
quality
FIRE SIDE CORROSION
MONITORING contd..
Local gas velocity and particulate
loading also influence corrosion rates
EPRI study results reveal that the
corrosion resistance of SS310 (25%Cr)
is superior to that of SS316 (18% Cr)
INTELLIGENT SOOT
BLOWING (ISB)
Conventional Soot Blowing based on a time
schedule rather than actual fouling conditions
in the boiler
ISB Uses a combination of modeling, neural
networks, expert systems and heat flux
sensors installed inside furnace
Heat flux sensors serve as a tool to deal with
spatial firing differences
ISB based on heat flux sensors has a positive
impact on tube life and also helps in better
control of Steam temp.
THANK YOU ALL