boiler logics
DESCRIPTION
It shows basic controls of boilers and how the same is achieved.TRANSCRIPT
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PROCESS CONTROL
The task of planning and regulating a process, with the objective of performing it in an efficient, effective and consistent manner.
General feedback control loop
Controller
FCE
Process
Transducer
ED
SP
PV
Controlled Output
The following control loops are used in Boiler#9
Drum level control Furnace draft control Steam temperature control- attemperator-1 Steam temperature control- attemperator-2 Combustion air pressure control BFG header pressure control Corex header pressure control Combustion control
Drum level control Drum level is measured by 3 DP transmitters. Output of each transmitter is given to compensation block for calculating compensated drum level. Out of 3 compensated level signals 2 signals are taken for level control.
Drum level compensation The compensated drum level is calculated by the following formula
Compensated drum level (h) =
Where, P = Differential pressure measured by transmitter. h = Compensated drum level signal. Dw = Density of water. Ds = Density of steam. H = Water head on LP side wet head leg, which is to be feed
as constant=800mmWC. Da = Wet leg density; water density at 30 C
(Constant=0.996 g/cm^3)
P+H(Da-Ds)
(Dw-Ds)
The density of water and steam depends on the pressure.
Water and steam densities corresponding to pressure are given below.
Drum level is controlled by 2 modes:
Single element control mode
(Drum level)
Three element control mode
(Drum level, Steam flow and Feed water flow)
Pressure
(Kg/cm^2)
Water density
(g/cm^3)
Steam density
(g/cm^3)
40 0.798 0.02010
68 0.7436 0.03537
90 0.7051 0.04880
100 0.6884 0.05540
Single Element Control Mode
LT = Level Transmitter
PT = Pressure Transmitter
LCOM = Level compensation
LC = Level Controller
LT-1 LCOM
LT-2
LCOM
LCOM
LT-3
LC 2oo3 PV
LSP
30% CONTROL VALVE
Reverse
PT-1
PT-2
1oo2
Three element control mode
Compensated Drum level
Compensated Main Steam
Flow
Compensated Feed water
flow
Feed forward
summation block
Level controller
LSP
PV
Flow controller
PV
RSP
100% control valve
Reverse Reverse
30% CONTROL
VALVE
Compensated Main Steam flow
FT-1
FT-3
FT-2 2oo3 Computation
Block Compensated
main steam flow
1oo2
1oo2
PT-1 PT-2
TT-1 TT-2
FT = Flow Transmitter PT = Pressure Transmitter TT = Temperature Transmitter
Compensated Feed water flow
FT-1
FT-3
FT-2 2oo3 Computation
Block Compensated feed water flow
1oo2
TT-1 TT-2
FT = Flow Transmitter PT = Pressure Transmitter TT = Temperature Transmitter
Compensated steam Flow Compensated Steam Flow =
Actual steam flow x
Where,
P1 = Measured pressure signal.
P2 = Design Pressure. ( P2=95 Kg/cm^2)
T1 = Measured temperature signal.
T2 = Design Temperature. (T2=540 C)
Auto Change over
It is done by soft switch in DCS.
The switch has two modes
1) 1E to 3E
2) 3E to 1E
P1+1.029 T2+273
P2+1.029 T1+273
Before choosing mode-1 Ensure the following:
(30% A/M) in auto mode.
1E controller o/p should go to (30% A/M).
3E controller o/p should track 1E controller o/p.
3E controller o/p should not reach (100% A/M) and input to (100% A/M) should be zero.
(100% A/M) should be in auto.
After choosing mode-1 following actions occur automatically:
(30% A/M) o/p should ramp down to 0% in 15 Secs.
1E controller o/p should go to 3E Controller through feed forward summation block.
(100% A/M) o/p should ramp up to 3E Controller o/p in 15 secs.
1E controller o/p should not reach (30% A/M).
Once 15 secs ramp up time finished 100% control valve will be in action and 30% control valve will be a stand by.
Choosing the Mode-2 will be totally an inverse action of choosing Mode-1
Controller:
Action : Reverse Type : PID LSP : From manual (50%) 0 mmWC RSP : From Feed forward summation block LAL : -150 mmWC HAL : +150 mmWC
Control valve:
Action : Air to close Fail Action : Air fails to open.
Furnace draft control Furnace draft control is performed by SPLIT RANGE CONTROL.
The Hydro coupling and the damper actuator are in split range operation to cater the min and max ID fan air flow requirement.
Controller Action : Direct
Type : PID
Set point : -5mmWC
Low Alarm : -100mmWC
High Alarm : +100mmWC
Damper Actuator Action : Double acting
Fail Action : Air fails to lock and tend to stay at last position
Furnace draft control
PT-3
PV
50% to 100%
ID fan A Fluid oil Coupling
system
2oo3
PC
Function block
ID fan A damper Actuator
ID fan B damper actuator
Function block
PT-2
ID fan B Fluid oil Coupling
system
PT-1
LSP
-5mmWC
PT= Pressure Transmitter
PC= Pressure Controller
0% to 50%
0% to 100% 0% to 100% 0% to 100% 0% to 100%
Steam Temperature control Attemperator-1
The temperature between PSH-1 and PSH-2 is controlled by spraying feed
water into the steam after the PSH-1.
The set point to this temperature controller is a RSP.
The RSP is derived from the functional block, where it is calculated based on
the load(%), steam flow and fuel.
Load(%)
Steam Flow
(TPH)
BFG Alone
( Deg C )
Corex Alone
( Deg C )
BFG + Corex
Alone ( Deg C )
20 40 395 375 385
50 100 389 371 375
100 200 371 - -
WATER SPRAY
A/M A/M
TC
1oo2
TT-2
T/C-2
TT-1
T/C-1
PSH-2 PSH-1
CV-1
CV-2
FEED WATER
FUNCTION BLOCK
RSP
mV mV
4-20mA 4-20mA
Compensated Steam Flow
T/C = Thermocouple
TT = Temperature Transmitter
TC = Temperature Controller
PSH = Primary Super Heater
Controller
Action : Direct
Type : PID
Set point : From Function block
Low Alarm : 380 Deg C
High Alarm : 400 Deg C
Control valve
Action : Air to close
Fail Action : Air fails to lock and then tend to open
Steam Temperature control Attemperator-2
Steam temperature after PSH-2 is controlled by spraying water into the steam after the PSH-2. The desired main steam temperature (i.e.) SSH outlet temperature is achieved by controlling the temperature of PSH-2 outlet. The set point is local set point.
Controller
Action : Direct Type : PID Set point : 540 Deg C Low Alarm : 530 Deg C High Alarm : 550 Deg C
Control valve
Action : Air to close Fail Action : Air fails to lock and then
tend to open
WATER SPRAY
A/M A/M
TC
1oo2
TT-2
T/C-2
TT-1
T/C-1
PSH-2
CV-1
CV-2
FEED WATER
LSP
540 Deg C
mV mV
4-20mA 4-20mA
T/C = Thermocouple
TT = Temperature Transmitter
TC = Temperature Controller
PSH = Primary Super Heater
SSH
Combustion air pressure control
Combustion air pressure control is performed by two modes 1) VFD mode 2) Damper mode VFD mode:
Damper actuator A/M station is in manual mode and the damper is in 100% open condition.
The air pressure is controlled by VFD. Damper mode:
The fan is started by bypass starter and it is to be run at full speed.
The damper actuator is in auto mode and it controls the air pressure.
Change over:
The change over from VFD to Damper mode and Damper mode to VFD mode is to be carried out manually by the operator.
1oo2
VFD
A/M DAMPER
A/M
FD FAN
MOTOR
DAMPER ACTUATOR
VFD
PC
PT-2 PT-1
FD FAN DISCHARGE
LSP
FD FAN A FD FAN B
VFD
A/M
FD FAN
MOTOR
VFD
DAMPER
A/M
DAMPER ACTUATOR
Controller
Action : Reverse Type : PI Set point : 335 mmWC Low Alarm : 315 mmWC High Alarm : 355 mmWC
Control valve
Action : Double acting Fail Action : Air fails to lock and tend to stay at last position.
BFG Header pressure control
1oo2
PC
PT-2 PT-1
BFG LINE
LSP 1oo2
TT-2
TC-2
TT-1
TC-1
mV mV
4-20mA 4-20mA
1oo2
PI
PT-2 PT-1
TI
BFG Flow Compensation
PCV
Controller
Action : Reverse Type : PI Set point : 250 mmWC Low Alarm : 230 mmWC High Alarm : 270 mmWC
Control valve
Action : Air to open Fail Action : Air fail to close
Corex header pressure control
1oo2
PC
PT-2 PT-1
COREX LINE
LSP 1oo2
TT-2
TC-2
TT-1
TC-1
mV mV
4-20mA
1oo2
PI
PT-2 PT-1
TI
Corex Flow Compensation
PCV
4-20mA
Controller
Action : Reverse Type : PI Set point : 3000 mmWC Low Alarm : 2800 mmWC High Alarm : 3200 mmWC
Control valve
Action : Air to open Fail Action : Air fail to close
Combustion control
It is a lead lag combustion control
This control always maintain the air flow more than the fuel flow for proper combustion of fuel.
The combination firing can be done strictly adhering to following
operational procedures:
All burners are loaded equally in normal running condition
Same fuel is fired in all the running burners.
The block diagram for the combustion control is given below.
BFG Flow
Corex support Flow
Corex main Flow
Total heat value computation
Stoich Air fuel ratio computation
Combustion air flow Control valve
BFG Flow
Control valve
Flow controller
Main steam Pressure
Corex support Flow Control valve
Corex Main Flow Control
valve
Excess Air Ratio
> <
Flow controller
Pressure controller
Nullification block
Total Air Demand
Air Fuel Ratio low
Alarm Block
Combustion air flow
From Curve
c
a
O2 Analyzer
d
b O2 Controller
From Curve
f
LSP
Fuel firing limit block
a
RSP
PV
(c x d x f)
BFG Flow
FT-1
FT-2
1oo2 Computation
Block BFG flow
1oo2
1oo2
PT-1 PT-2
TT-1 TT-2
FT = Flow Transmitter PT = Pressure Transmitter TT = Temperature Transmitter
GO TO CC
Corex Main Flow FT-1 FT-2
1oo2
Computation Block
1oo2
1oo2
PT-1
PT-2
TT-1 TT-2
FT = Flow Transmitter PT = Pressure Transmitter TT = Temperature Transmitter
Corex common header flow
+ Corex Main flow
Corex Support
flow
GO TO CC
Corex support Flow
FT-1
FT-2
1oo2 Computation
Block
Corex support
flow
1oo2
1oo2
PT-1 PT-2
TT-1 TT-2
FT = Flow Transmitter PT = Pressure Transmitter TT = Temperature Transmitter
GO TO CC
Compensated Flow Calculation
Compensated BFG Flow = Actual BFG flow x
Compensated Corex Main Flow
Actual Corex
Main flow = x
Compensated Corex Support Flow
Actual Corex
Support flow
P1+1.029 T2+273
P2+1.029 T1+273
P1+1.029 T2+273
P2+1.029 T1+273
P1+1.029 T2+273
P2+1.029 T1+273 = x
Where, P1 = Measured pressure signal. T1 = Measured temperature signal. P2 = Design Pressure. (For BFG P2 = 800 mmWC) (For Corex P2 = 6000 mmWC) T2 = Design Temperature. (For BFG T2 = 40 Deg C)
(For Corex T2 = 40 Deg C)
GO TO CC
Total heat value computation
Actual heat value is generated in Boiler (Kcal/hr)= { ( BFG Flow (Nm^3/hr) x Gross Calorific value of BFG (Kcal/Kg) x Density value of BFG (Kg/Nm^3) ) + (Corex main Flow (Nm^3/hr) x Gross Calorific value of Main Corex gas (Kcal/Kg) x Density value of Main Corex gas (Kg/Nm^3) ) + (Corex Support Flow (Nm^3/hr) x Gross Calorific value of Support Corex gas (Kcal/Kg) x Density value of Support Corex gas (Kg/Nm^3)
) }
GO TO CC
Gross Calorific value of BFG = 608.9 Kcal/Kg Gross Calorific value of Main Corex gas = 1830 Kcal/Kg
Gross Calorific value of Support Corex gas = 1830 Kcal/Kg
Density value of BFG = 1.340 Kg/Nm^3 Density value of Main Corex gas = 1.207 Kg/Nm^3 Density value of Support Corex gas = 1.207 Kg/Nm^3
The expected heat value at 100% MCR = 156.6 MKcal/hr
GO TO CC
Fuel firing limit block The expected heat value per burner at 100% MCR =
18.9MKcal/hr The actual heat value generated per burner in Mcal/hr = Total heat value in MKcal/hr No of burners in operation In case of excess loading the output of the above equation will exceed 18.9Mcal/hr and an alarm will be generated in DCS and latch the last highest output of controller. Alarm should be generated in DCS that controller is in latched mode. During this time the operator should start the next burner and maintain the pressure. Once next burner started, the latched mode is released and again the combustion controller will control the load.
GO TO CC
Air flow control
FT = Flow Transmitter
FT-1
FT-3
2oo3 Air flow
Controller FT-2 PV
The following Digital signals are generated from air flow indication block:
Air flow very low trip signal to BMS logic.
Air flow >50% for furnace purge signal to BMS logic.
Air flow >25%<30% Signal to BMS logic.
GO TO CC
Air flow remote set point calculation
The output of high selector block is passed to the controller through the air fuel ratio multiplier as remote set point.
The purpose of air fuel ratio multiplier is as follows:
To adjust stoichiometric air ( theoretical air ) depending on fuel being fired. The stoichiometric air fuel ratio is different for different fuel. (a)
To adjust excess air requirement, this is function of burner load. (b)
Trimming of air flow set point based on oxygen in the flue gas. (d)
GO TO CC
Stoichiometric air fuel ratio computation
Stoich air fuel ratio for BFG = 1.11 Stoich air fuel ratio for Corex gas = 1.0 In dual firing mode, the Stoich air fuel ratio for BFG and corex = BFG load % * 1.110 Corex load % * 1.00 100 100
BFG load % and Corex load % can be calculated by the following
equation. Input X1= Fuel heat rate of BFG being fired. X2= Fuel heat rate of corex (main + support) being fired. Output Y1= BFG load in % =
Stoichiometric air fuel ratio (a) =
+
X1+X2
X1*100
+ Y1*1.11 (100-Y1)*1.0
100 100
GO TO CC
Excess air adjustment:
In any burner system it is necessary to have air flow in addition to theoretical air flow to ensure proper combustion of the fuel being fired.
The requirement of excess air during low load is more than the requirement of excess air in high load.
The excess air requirement is the function of oxygen content at flue gas outlet.
The total air requirement is calculated by the following formula
Total air requirement (c) = Stoich. air (a) * excess air multiplying factor
GO TO CC
The excess air multiplying factor (EAMF) is taken from the table given below corresponding to the load.
BFG excess air curve
Load(%) Steam flow (TPH)
Fuel flow (Nm^3/hr)
Excess air (%)
EAMF (b)
100 200 184405 25 1.25
80 160 147607 30 1.3
50 100 91377 40 1.4
10 20 21200 50 1.5
Corex excess air curve
Load(%) Steam flow (TPH)
Fuel flow (Nm^3/hr)
Excess air (%)
EAMF (b)
100 200 NA NA NA
80 160 NA NA NA
50 100 33141 40 1.40
10 20 6750 52 1.52
GO TO CC
Oxygen Trimming The oxygen content at the flue gas outlet is measured by the
analyzer and it is given to the controller as process variable.
The set point for this controller is remote set point which is derived from the table given below corresponding to the load.
Based on the remote set point the controller output varies from 0.8 to 1.2
Load
(%)
Steam flow (TPH)
BFG Corex
O2 % vol (WET)
O2 % vol (WET)
100 200 2.28 NA
80 160 2.61 NA
50 100 3.23 5.00
10 20 NA 5.90
GO TO CC
Total air demand The total air demand is calculated by the following formula
Total air demand = { c * d * f }
Where,
c = Excess air ratio.
d = Oxygen controller output.
f = Air demand from high selector.
Nullification block
To nullify the multiplication effect while comparing the air and master demand in low selector block, the air flow going to low selector block is divided by the value of ‘c’ and ‘d’.
The output of nullification block = { Air flow / ( c * d ) }
GO TO CC