Download - Auto Control Loops Presentation
Auto Control Loops of Large-sized Thermal Plants
Janmejay MahapatraHead (Instrumentation)
Avantha Power Infrastructure LTD.6th Nov 2011 – PMI, Noida
Structure of the Controller
Is my plant performing as per Design?
An eye on the various measured values of different processes:
Temperature Pressure Flow Level Water & Steam Parameters (e.g.
conductivity, pH, silica etc.)
Is my plant performing as per Design?
And, hands on the various means to regulate the different processes:
Valves Dampers Speed Regulators Stroke Controllers
The Onerous task of the plant operator
The huge number of processes involved Electrical Load has to be matched with the
fluctuating demand Creeping of inefficiencies as the process
deviates from the design Safety of the equipment at risk as the
process drifts beyond allowable limits Start-up & Shut down activities Two shift operation
The Basic Control Schemes
On-off Control Proportional Control Proportional-Integral ( P-I ) Control Proportional – Derivative ( P-D ) Control Proportional-Integral-Derivative ( PID ) Control Ratio Control Spilt Range Control Cascade Control Feed-forward Control Adaptive Control
The Basic Control Schemes : Responses
Let’s recognise these:
Roughly 75% of process control loops cause more variability running in the automatic mode than they do in the manual mode.
A third of them oscillate as a result of nonlinearities such as valve dead band.
Another third oscillate because of poor controller tuning.
The remaining loops oscillate because of deficiencies in the control strategy.
Some simple rules of Tuning the Auto Control Loops
Increase the gain to achieve tight control and to
speed up the set point response.
Decrease the gain to provide a smoother, slower,
and more stable response.
Whereas insufficient gain slows down the initial
approach, inadequate reset action slows down the
final approach to set point.
If the reset action is too large (reset time too
small), there is excessive overshoot.
Some simple rules of Tuning the Auto Control Loops
If there is excessive overshoot and oscillation,
decrease the reset action (increase reset time).
For level loops with low controller gains (< 5.0),
reset action must be greatly restricted (reset
time increased to 50 or more minutes) to help
mitigate nearly sustained oscillations.
Rate is primarily used in temperature loops that
have narrow span transmitters and slow scan
times.
Some simple rules of Tuning the Auto Control Loops
There is noise in every process variable.
If the noise is greater than the measurement
resolution limit, the loop sees it.
If it causes an output change greater than the
final element or control valve resolution limit,
loop variability will be worse in auto than in
manual, particularly if there is any rate action.
Tuning the Auto Control Loops
Test new tuning settings by changing the set
point in both directions.
Don’t leave these new settings over night
until you are sure they are right.
Before entering new settings, ask the
operator for permission to tune the loop.
Ask the operator what the maximum
allowable size of variation.
Put the controller in manual before tuning.
Feed Forward Control
Useful for processes with significant dead times. Controlled condition is affecting processes
other than that is being controlled. Controller is required to respond promptly but
the response of the measured value is slow. Usually it is the derivative of the external
measurement. The “Anticipatory” action is added to the output
of the controller. Normal feedback control after the passing off of
the transient.
Cascade Control
Cascade control is a type of control in which a secondary (slave or inner) loop is added.
Secondary Loop gets a set point from a primary (master or outer) loop.
The secondary loop response is normally faster than (about five times) the primary loop response.
The secondary loop can correct for upsets it can measure before they affect the primary loop.
BHEL - The Base Documents
Recommended Scheme of Auto Controls –
Trichy Document
APRDS Control Scheme – Trichy Document
Analog Control Schemes – PEM Document
HPBP Control Scheme – PEM Document
CMC Scheme – PEM Document
The Control Loops
Heater Level Controls LPH - 2 / 3 level controlHPH - 5A / 6A / 5B / 6B level control
Hotwell Level Control GSC minimum flow control Gland seal Water Header Pressure control Controls of Deaerator
Pressure control Level control
Dosing ControlAmmonia, Hydrazine, Phosphate
The Control Loops
Furnace Press Control Secondary Air Flow Control Drum level control Fuel Flow Control PA Header press Control Mill Air Flow Control Mill Air Temperature Control SH Steam Temp Control RH Steam Temp Control Burner Tilt Control AH Cold End Temp Control
The Control Loops
CBD Tank Level Control Atomising Steam Press Control HFO Flow Control LDO Flow Control HFO Heater – A O/L Temp Control HFO Pump Discharge Press Control LDO Pump Discharge Press Control
The Control Loops
HP Bypass Pressure Control HP Bypass Temp. Control APRDS Pressure Control APRDS Temperature Control
Co-ordinated Master Control (CMC)
Importance of Heater Level Control
Feed Water / Condensate Temp. Rise
Terminal Temp. Difference (TTD)
Tsat(steam) – TFW Outlet
Increase in TTD implies worse heat transfer
Drain Cooler Approach (DCA)
TDrain Outlet – TFW Inlet
Increase in DCA implies decrease in water
level.
Heater Performance Monitoring
LPH –2 LEVEL CONTROL
LPH –3 LEVEL CONTROL
HPH -5A LEVEL CONTROL
HPH -6A LEVEL CONTROL
HPH -5B LEVEL CONTROL
HPH -6B LEVEL CONTROL
HOTWELL LEVEL CONTROL
HOTWELL LEVEL CONTROL
GSC MIN FLOW CONTROL
GLAND SEAL COND PR CONTROL
Why is Deaerator Pegging Required
Heat the incoming feed water to the saturation point to reduce the solubility of entrained gases.
The gases are basically O2, CO2 and NH3 - very corrosive at elevated temperatures.
Set of trays that separates the water into thin sheets from which the gases can easily escape.
The gases are vented out of the deaerator to the atmosphere.
To ensure mixing & elimination of corrosive gases, the FW level & steam pressure controls are important.
Why is Deaerator Pegging Required
DEAERATOR PRESS CONTROL
DEAERATOR LEVEL CONTROL
DEAERATOR LEVEL CONTROL
Hydrazine is used as oxygen scavenger. Hydrazine is one of the most costly chemical. Hydrazine dosing done at De-aerator outlet (BFP
suction)
Feed water has to be alkalized to a pH of 9 or higher, to reduce oxidation.
Also helps in forming of a stable layer of magnetite on the water-side surface of the boiler, protecting the material underneath from further corrosion.
Achieved by dosing alkaline agents into the feed water, like NaOH or volatile NH3.
Ammonia dosing done at CEP Discharge.
Dosing in Condensate & Feed Water
AMMONIA DOSING CONTROL
HYDRAZINE DOSING CONTROL
It is the Drum which speaks in a tube type boiler! Loose reddish powder – Boiler corrosion Loose Greyish powder – Economiser / pre-boiler
corrosion
Purpose is to reduce hardness and facilitate scale control.
Phosphate is a highly soluble, powder compound – significantly effective for hardness & scale control
Binds to calcium – prevents scaling but forms a sludge.
Phosphate dosing done at boiler drum.
Chemical Dosing in Drum
PHOSPHATE DOSING CONTROL
FURNACE PRESS CONTROL
FURNACE PRESS CONTROL
FURNACE PRESS CONTROL
FURNACE PRESS CONTROL
FURNACE PRESS CONTROL
AIR FLOW CONTROL
AIR FLOW CONTROL
AIR FLOW CONTROL
AIR FLOW CONTROL
DRUM LEVEL CONTROL
Typical Drum parameters of 500/600MW sub-critical unit – 197 Kg/cm2 & 363 Deg C!
Density is proportional to Pressure. Density is inv. proportional to Temperature.
DP is affected by changing density of water & steam inside boiler.
Drum Level Measurement - Facts
DRUM LEVEL CONTROL
DRUM LEVEL CONTROL
DRUM LEVEL CONTROL
DRUM LEVEL CONTROL
DRUM LEVEL CONTROL
DRUM LEVEL CONTROL
DRUM LEVEL CONTROL
AIR FLOW CONTROL
FUEL FLOW CONTROL
FUEL FLOW CONTROL
PA HDR PRESS CONTROL
PA HDR PRESS CONTROL
MILL-A AIR FLOW CONTROL
MILL –A AIR TEMP CONTROL
SH STM TEMP CONTROL
SH STM TEMP CONTROL
SH STM TEMP CONTROL
BURNER TILT CONTROL
BURNER TILT CONTROL
RH STM TEMP CONTROL
RH STM TEMP CONTROL
RH STM TEMP CONTROL
Boiler Flue gas contains water vapour, sulfur & other chemicals.
Low Flue gas temp. (below dew point) at AH cold end may result in condensation of water vapour.
Combination with sulfur forms sulfuric acid.
Results in corrosion of the cold end metal.
Why AH Cold end needs to be protected?
AH COLD END TEMP CONTROL
AH COLD END TEMP CONTROL
CBD TANK LEVEL CONTROL
ATOMISING STEAM PRESS CONTROL
HFO FLOW CONTROL
LDO FLOW CONTROL
HFO HEATER – A O/L TEMP CONTROL
HFO PUMP DISCH PRESS CONTROL
LDO PUMP DISCH PRESS CONTROL
HP BYPASS SYSTEM PRESS CONTROL
HP BYPASS SYSTEM PRESS CONTROL
SPRAY BLOCK VALVE LOGIC
HP BYPASS SYSTEM MISC ALARMS
HP BYPASS SYSTEM TEMP CONTROL
HP BYPASS SYSTEM TEMP CONTROL
AUX STEAM HDR PRESS CONTROL
AUX STEAM HDR PRESS CONTROL
HT AUX STEAM HDR TEMP CONTROL
LT AUX STEAM HDR TEMP CONTROL
COORDINATED MASTER CONTROL SCHEME
COORDINATED MASTER CONTROL SCHEME
COORDINATED MASTER CONTROL SCHEME
COORDINATED MASTER CONTROL SCHEME
COORDINATED MASTER CONTROL SCHEME
LOGO
Heater Performance Monitoring
Heater Performance Monitoring
Off design final FWH temperature at a 500MW Coal fired plant
Outlet Temperature Target 225◦ C
Actual 214◦ C
Difference -11 ◦ C
CHECKED PERFORMANCE PARAMETERS Temp Rise Target 27◦ C
Actual 18◦ C
DCA Target -12.22◦ C
Actual -16.1◦ C
TTD Target -12.22◦ C
Actual -6.94◦ C