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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


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


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


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


GO TO CC

Corex Main Flow FT-1 FT-2

1oo2

Computation Block

1oo2

1oo2

PT-1

PT-2

TT-1 TT-2


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


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)

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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

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