Modulating Control(System Description)

8/9/2019 Modulating Control(System Description)

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4 }

TOSHIB

3

PPLIC TION SOFTW RE

3 3 Contro l System


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CONTENTS

PART:

1

DISTRIBUTED

CONTROL

ND

INFORMATION SYSTEM

3 . APPLICATION SOFTWARE

(CONT D)

3 3 Cont ro l System

3 3 1 Genera l s o f

C60 Sof tware

3 3 2

Modula t ing Cont ro l

3 2 1

DCIS Modula t ing Cont ro l s System Desc r ip t ion

3 .

3 3

TOSMAP AT / D40

Opera t ion Manual

3 3 1

TOSMAP-AT/D40

O p era t in g I n s t r u c t i o n

(6F2B0022)


3/57

BANG PAKONG POWER STATION UNITS 3 4

DCIS MODULATING CONTROLS

SYSTEM DESCRIPTION

SYSDESCR DOC

Rev

0

2Sep91

Author: R McDermott


4/57

ONTENTS

INTRODUCTION .

1

1 TRACKING INITIALIZATION .

2

1.1 Type A Control Drives

1.2 Type B 1 Control Drives

1.3

Type B2 Control Drives

1.4

Type C Control Drives

1.5

Cascade Controls

2

TRANSMITTER DEVIATION SYSTEM .

4

2.1

Single

Measurement

2.2

Dual Measurement

2.3

Triple

Measurement

3

UNIT

MASTER

6

3.1

Coordinated Controls -

Introduction

3.2

Required Output Computation

3.3

Operating

Modes

3.4

Runback

System

3.5

Pressure

Set

Point

3.6

Governor Control

3.7

Firing Rate Demand

4 Affi FLOW CONTROL .

.

17

4.1 Process Measurements

4.2

Air

Demand

4.3

Excess

Air

Controls

4.4

Air

Flow Controller

4.5

Tracking

4.6

FD Fan Stall

4.7

Air

Heater

Cold End Temperature

4.8

Windbox Air Dampers

5

FUEL FLOW CONTROL . . . . . . . . . . . . . .

5.1

Fuel Measurements

5.2

Fuel Demand

5.3

Fuel Controllers

5.4

Fuel-Air Deviation Monitor


5/57

6

FURNACE PRESSURE .

. .

25

6.1

ID

Fan Speed

6.2

ID

Fan

Inlet

Dampers

6.3

Implosion Protect ion

7

STEAM TEMPERATURES

28

7.1 Main Steam Temperature

7.2

Reheat

Steam

Temperature

8

FUEL OIL PUMPS .

33

8.1

Fuel Oil Temperature

8.2

Fuel

Oil Header Pressure

8.3 Fuel Oil Heater Steam Pressure

8.4

Fuel

Oil

Transfer

Pump

Pressure

9

FEEDWATER . . . . . . .

34

9.1

Drum

Level

9.2 Feed Pump Minimum Flow

1 CONDENSATE DEAERATOR . . 38

10.1

Deaerator Level

10.2 Deaerator High Level

10.3

Deaerator

Pressure

10.4

Deaerator Temperature

10.5 Condenser Level

1 .6 ondensate Recirculation

1 .7

ondensate Pumps Recirculation

11

FEEDWATER HEATERS . . .

41

11.1

Feedwater Heaters Level

11.2 LP Heaters Drains

Tank

Level

11.3 LP Heaters Drains

Pump

Recirculation

11.4

HP

Heaters Drains

Pump Recirculation

12 MITSCELLANEOUS .

. .

43

12.1

Seal

Steam

Pressure

12.2

Seal Steam Temperature

12 .3

Closed Cycle Cooling Water Temp

12.4

Auxiliary Steam Pressure

13

SIMPLE

1NDEPENDENT LOOPS . . . . . . . . . . . . . . . . . .


6/57

BANG PAKONG UNITS 3 4 DCIS MODULATING CONTROLS

SYSTEM DESCRIPTION

INTRODUCTION

The

purpose

of this

document

is to

assist

in the understanding

of

the

design

principles used for

the

modulating controls. The document should be

read in

conjunction

with

the

following:

(a) Toshiba Drawing 7M1Z0218 Modulating Control Block (Functional)

Diagrams .

The

sheet numbers referred to in the following

text relate

to

these

diagrams.

(b) Toshiba descriptive

literature

for C60 controllers

and

870 computing

and

display system.

c)

Black Veatch International Project 14383

ang

Pakong

Thermal Plant

Unit

3

Piping and Instrumentation

Diagrams.


7/57

1 TRACKING AND INITIALIZATION

Changes in operating

mode

are

bumpless .

This

is achieved

by

automatically

initializing

the selected signal to

be

equal to the downstream signal prior to

transfer.

This principle is applied to

automatic-manual

selections of control

drives, cascade controls and boiler master

operating

mode changes.

The simplest case

is a controller

with

a single control drive.

When

control is

manual

the controller

output

is made to

track

the manually set position demand.

When control is automatic,

the manual

setter tracks

the output

to

the

control

drive from the controller.

Where

multiple drives are

used with

a single

controller,

the

tracking

signal depends on the control drive configuration.

Sheets

17, 18

and

19

show

the standard

tracking

systems

used

to initialize

automatic-manual

transfers for various configurations

of

multiple control drives.

The control drive

tracking

systems which follow

these

standard

systems

are

not

shown

on

the functional block diagrams. Tracking

systems

which differ from the

standard

are shown

on

the

relevant

functional diagram.

1 1

TYPE A

Refer

to Sheet

17.

This system

applies to

dual

drives which control auxiliary

plant

with

less

than

100% capacity

where both

drives

are

normally

in

automatic

[e.g.FD fans]. A bias setter allows changes to

the

relative loading; these changes

are

introduced

gradually

by using

a delay function.

Each

drive

has separate

auto-manual sub-window. Loop

gain

is constant for one or two drives in

automatic. f one drive is auto and

the

other manual

the

auto drive compensates

for

manual operation of the other.

For

example,

increasing

the manual drive

output

will decrease the

auto

drive the

same amount without waiting

for a

change in

the controlled process.

The

average

control drive position

is

used

for controller

tracking

. Feedforward

signals, if used, are added to the controller

output. It

follows that in tracking

mode, the feedforward must be subtracted from the tracking input to the

controller.

1 2 TYPE 81

Refer

Sheet

18.

This system

applies to

dual

drives

operated

from a single auto

manual sub-window; it follows that both drives must be

in

automatic

or

manual

These

usually operate

in

the

split-control configuration [e.g.Auxiliary

St ea m

Pressure]. The controller

tracks

the common manual demand signal to

the

two

drives.


8/57

3

1 3 TYPE 8

Refer to

Sheet

18.

This

system applies to dual 100% capacity drives

which

have

individual auto-manual subwindows. Only one

is permitted to be in

automatic;

the

other is available as a standby. With both drives

in

manual

the

controller

tracks

drive A

unless

drive B is selected to auto. A

short time

delay

on

B Auto

ensures that

the

tracking signal from B is established before transfer to auto

takes

place.

1 4 TYPE C

Refer to

Sheet

19.

This system

is used for configurations ofmore

than

two drives

where

any number

may be in automatic

[e.g.

Condensate

Pumps]. Loop gain is

kept

constant by

modifying the controller

error

to

be

inversely proportional to the

number

of

drives in automatic.

The controller output tracks

the

first drive to be selected to automatic. [Default

is drive A.] A

short time

delay before

transferring

to

automatic

operation

ensures that

the

tracking

signal is established.

The track

signal to the

remaining drives

on manual

comprises the controller output

plus

the difference

between

the

controller output and the

actual

position. The difference signal is

transferred via

the

track

input

of

an

integrator.

After selection to automatic,

this

difference

signal

at

the

integrator output

is

connected

n

reverse

to

the

integrator

input and slowly decays to zero.

This

decay is slower than the

response

of the control loop so

disturbance

to the process is minimal.

1 5 CASCADE CONTROLS

For a simple cascade loop, the primary controller tracks the secondary controller

process

variable

when

not auto.

This

forces

the

secondary controller

error

to zero

for

bumpless transfer.

Tracking

signals for cascade controls must include

the reverse

of any calculations

applied to the forward

path. For

example, feedforward signals

added

to

the

primary controller output must be subtracted from the track signal. Similarly,

multipliers become divisors and

the

inverse of any function

generators

in the

primary controller output path

must be

applied to the track signal. Because of

these

complications,

the

tracking

system

for

each

cascade control is fully shown

on

the

appropriate

functional diagram.


9/57

TRANSMITTER DEVIATION SYSTEM

All process

transmitters

used for modulating control functions are checked by

the

Transmitter Deviation System. an

abnormal

measurement condition is

detected,

all

dependent control loops are tripped to manual control. There

are

three transmitter

configurations: single

measurement

dual measurement

and

2 )

:.>-' 'L'

triple measurement. The details are shown on Sheets _.21, ..2 3 and 24. These

details

apply

to all relevant applications. The functional diagrams for specific

applications show only a simplified version comprising

signal

comparison and

resulting

input

to

the

auto permit logic.

Transmitter

deviations which affect fuel,

air or

governor control

trip the

coordinated control system to Manual mode as well as tripping

the

directly

affected loop.

The

coordinated loops

are

also monitored

by

the

fuel-air deviation

system, refer to Section 5.3.

2 1 SINGLE MEASUREMENT

}./

Refer Sheet 21. In this case, the signal is checked to ensure that it is within

the

normal range

with a tolerance

of

5%.

f it

outside

this

range an

alarm is

initiated and

any

control loops significantly affected

by

this signal

are

transferred to NOT AUTO status.

~ DUALMEASUREMENT

_a

Refer

Sheet

23.

The

dual measurements

are

compared and

if

they disagree by

more

than

a

preset

amount [typically 3%],

an

alarm is

initiated and

affected

loops are transferred to Not Auto status. The individual measurements are also

checked for in range,

if

outside by more

than

5% an alarm

is

initiated.

A CRT subwindow is provided for each transmitter pair. This enables the

operator

to monitor

each

input

and

select one

of the pair

for control. Logic

prevents selection of

an

out-of-range transmitter. a deviation occurs, the

operator

selects

the

good transmitter

and

disables

the

logic signal which trips

the

relevant control loops. n alarm reminds the operator that the monitor is

disabled.

2.3 TRIPLE MEASUREMENT

Refer Sheet 25. The median value for the three signals is derived.

f

any of the

three

disagree with

the median

an

alarm is

initiated and

the

relevant controls

are tripped to Not Auto status. The individual inputs are also checked for in-

range

i f outside by more

than

5%

an

alarm is initiated.


10/57

A CRT sub-window is provided for each triple measurement

This

enables the

operator to monitor all inputs and select the median or any one of

the

three

inputs

Logic

prevents

the selection of

an

out-of-range

transmitter f

a

deviation occurs the operator selects a good transmitter and disables

the

control

trip

.

n

alarm

reminds

the

operator

that

the

trip

is

disabled.


11/57

3 UNIT MASTER

3.1 COORDINATED CONTROLS INTRODUCTION

The fundamental requirement for coordinated boiler-turbine controls is to

automatically balance

the boiler

energy

production

against

the prevailing

energy

demand of

the

turbo-generator. The energy transfer is effected by the flow of

superheated steam

from the boiler to the

turbine where

the

heat energy

of

the

steam

converted

into

mechanical work.

The rate

of

transfer of energy between the

boiler

and

the

turbine can be

expressed in

terms

of energy

rates as

follows:

(i)

Et =

(Ef

+ Ew +

Er)- Eb-

Es [MW]

Where:

Et

=

Main and reheat steam

to

turbine

Ef = Fuel to boiler

Ew =Feedwater

to boiler

Es =

Change

in boiler

stored

energy

Er

=

Cold

reheat

steam

to

boiler

Eb =Boiler losses

For small to

moderate

load changes it

can

be assumed

that

Ew, Er and Eb are

proportional to boiler output. Simplifying (i):

(ii) Et =K(Ef- Es )

When

boiler

and turbine

are

in

balance, the rate

of

stored

energy

change is zero;

i.e. Es = 0

[Pressure

steady].

Turbine

input Et is controlled

by

the turbine

throttle

valve

through

the governor

and Ef is controlled by

varying

the

firing rate. Es is a function of the prevailing

out of balance between

boiler

and turbine. At higher loads and pressures,

the

steady

state

stored energy increases and additional fuel is needed until the

required

level is reached.

The

converse is true for falling loads. Temporary over

firing or

under-firing

is required

to accommodate this.


12/57


13/57

8

The target is constrained by the

maximum

and minimum limit

settings

. These

are mainly

used

to keep ADS control within

current

plant capability. The target

load is also limited to the capacity of the auxiliary plant in service [Target

Maximum]. When

the unit

is tripped

the

minimum

limit

is

set

to zero.

The target load tracks a load index

when

the coordinated loops are not in one of

the automatic modes. If MAN or BI mode is pre-selected target load tracks fuel

flow. IfBF or CO mode is pre-selected

target

load

tracks

unit MW

output

. The

selected signal provides a reference for system balancing when changing to an

automatic mode.

Changes in the target load are subjected to a rate-of-change limiting as set by the

operator.

This

operator selected rate will

be

over-ridden

i i t

is

higher

than the

current

turbine

rate

limit

setting.

If

a load runback

is

required because

of

an

auxiliary

plant

trip a fast runback

rate

will

be

selected [See Section 3.4]. A fast

rate is also selected when RO tracking is required.

A

further

constraint on Required Output

[RO]

is imposed

by

the Unit Capability

Monitor. This checks the process deviation for the major flow loops [fuel air

governor feedwater

and

condensate]. Should the deviation exceed a certain

threshold value the RO is blocked from moving in a direction which would

increase the error. This feature prevents mismatch of flow loops caused by poor

transient response and limiting or failure of regulating devices.

If the process deviations persist for longer than a preset time the Required

Output

is adjusted up or down so as to eliminate the deviation. This is called

Runup-Rundown action.

Under steady state conditions

with

system frequency at 50Hz the Required

Output normally equals Target Load as set by the operator or ADS provided that

there

are

no plant limitations.

When

the system frequency deviates from 50Hz

the

turbine

governor

takes

corrective action

by

increasing or decreasing load to

contribute

to

the frequency regulation of the interconnected power system. The

required output must reflect this adjustment otherwise the controls would see a

generation

error

and remove the unit s contribution. The frequency

bias

component of RO models the governor action from frequency deviations. Tuning

setter AOl

is adjusted

as

a function of governor droop setting.

[A

nominal 4

droop would produce 30MW/0.1Hz at

rated pressure

of 170 Bar.]


14/57

Governor

Steam Flow

Feedwater

Pumps

if Feed Flow

Condensate

Pumps

Required

Output

Fuel

Valves

Figure

3 2

F Fans

lade Pitch

I

Fans

Propagation ofR

Signal

to

Flow Control

Loops


15/57

The Required

Output

forms

the

basic demand for fuel

air and

governor as well

as providing

the

load index for pressure set point computation. The

required

output is propagated

indirectly to provide a feedforward demand signal

to

furnace

pressure

feedwater and condensate controls. This is

shown

in block

diagram

form

on Figure

3.2

3 3 OPERATING MODES

The required boiler-turbine balance

can be

achieved in several ways. The control

system

provides a choice

of

operating strategies for

the

co-ordination

of the

turbine governor which

sets

the energy demand rate and

the

boiler fuel/air

inputs which

provide the

required

rate

of

energy production to

match

the

demand. These different methods

of

operation are called SYSTEM MODES.

Refer to

sheets

35

and

38.

3

3 1

Coordinated [CO]

In

this

mode

the

boiler

inputs and the turbine

governor respond to

the

Required

Output signal [RO]. This is either set by the operator

or

the

automatic despatch

system

[refer Section 3.2]. Steady state boiler/turbine co-ordination is achieved

by

the use of the

common RO signal to

set

boiler

inputs

and turbine demand.

Refer to Fig. 3.3.1.

The Required

Output to

fuel and

air is

modified by dynamic compensation

signals which provide for

the

ensuing changes in stored energy when load and/or

pressure

are changed.

This

is effected

by

overfiring

or underfiring as

appropriate. Any residual unbalance is reflected by pressure changes as stored

energy

accomodates the

unbalance

. The RO to fuel and air

is

modified by a

pressure controller to eliminate pressure deviations.

The Required Output also provides the basic demand to

the

governor controller

which

regulates

turbine

energy

input

. The RO signal

is

modified

by

the

MW

controller so

as

to achieve the

required steady state

MW output.

The Coordinated Mode

is the normal

method

of

operation.


16/57

Setters

Generation

Correction

..

.

..

.

Pressure

Deviation

Block

Governor Control

ADS

t

Target Load

Limits

..

Auxiliary Plant

Rate

Capabil i ty ... Flow

Deviations

Frequency Bias

Dynamic

Compensation

Pressure

Correction

Excess Air

Correction

Fuel

Control

Air

Control

Fieu re 3 3 1

Coordinated Mode


17/57

Setters

0

0

0

Target

Limits

Rate

Capability

Frequency Bias

Generation

Correction

Pressure

Deviation Block

Governor Control

Auto Optional)

Auxiliary

Plant

Flow

-Deviations

Turbine

Demand

~ x S

T

r

Dynamic

Compensation

Pressure

Correction

Excess ir

Compensation

Fuel Control ir

Control

Figure

3 3 2

Boiler Follow Mode


18/57

10

3.3.2 Boiler Follow [BF]

This

mode allows for

o ordination of

boiler-turbine control

with or without the

governor on automatic. The coordinating signal is provided

by throttle

valve

pressure ratio compensated

for

pressure

set

point

[Pl Pt*Ps]; this

forms

the

basic

demand

to fuel

and

air, replacing Required Output. Refer to Fig. 3.3.2. Dynamic

feedforward

and pressure

correction

are

provided as

in

Coordinated Mode.

The

governor, i f selected

to automatic,

controls

MW

from

the

RO

signal as

for CO

mode.

Capability limiting is

also effective

when the

governor

is

on auto.

The

boiler follow mode allows for responsive control

when

the governor

IS

unavailable for auto operation. [Manual control ofMW.]

3.3.3 Base Input Turbine Follow

[ I]

Boiler

energy

input fuel

and

air)

is determined by

Required Output [3.2] only.

Frequency bias compensation to RO is not applied in this mode. Dynamic

compensation

and pressure

correction

are not

applied

to

boiler

inputs.

Refer to

Fig. 3.3.3

The

turbine

governor, i f selected to

auto,

controls

pressure

before

the throttle

valve

by regulating

the throttle valve position.

The turbine thus

follows boiler

inp

ut

energy

and

maintains

the

set

pressure.

The

resulting

MW

w ll

be

approximately

equal

to RO, depending on fuel heating value calibration.

f he

governor

is not auto and

the

throttle

valve is fixed,

the steady state turbine

output w ll follow boiler input energy, the MW w ll be approximately equal to RO

and the

pressure w ll

be proportional to RO. Pressure

can

be modified by

changing the

throttle

valve position

manually. This w ll cause temporary

disturbance

to MW

and steam temperature.

The

Base Input-Turbine

Follow mode

is

used when stable

boiler operation

is

required. f a

runback

occurs in CO or BF mode, control mode

is

automatically

transferred to BI mode.

3.3.4 Manual [MAN]

Governor manual and fuel

manual;

air

auto

optionally). The target load

tracks

fuel flow to provide RO

initial

status proportional to boiler output. [The rate

setter

is by-passed.]

With air on

auto,

the

air

demand

thus follows fuel flow.

Manual mode

is

normally

used at

sta.

rt up and

synchronizing until

stable

firing

conditions

are

achieved.


19/57

>

Ope rat

or

Sett

n s

Pressure

Correction

a

-

Pressure

Deviation Block

Governor

Control

Auto Optional)

Target Load

•

Limits

•

Rate

Capability

1

-

Plant Max

-

Run back

'------Flow

Deviation

Excess

Air

Correction

Fuel Control

Air Control

Figure

3 3 3

Base

Input

Mode


20/57

Control system

faults such

as

transmitter

deviation will trip

the

selected mode to

manual with

air

also manual.

3.3.5 Mode Selection

Fuel and governor control can only

have

auto status i f one of the

three automatic

modes

is

operative. The

term

Auto Permit refers to pre-conditions which

must

be satisfied prior to automatic operation. The permissives which must be

satisfied for each mode before auto operation is

enabled

are:

a) Coordinated [CO]

Not Manual mode * FW on Auto *

Steam

Temperature on Auto * Air Auto

Permit Fuel Auto Permit Governor Auto Permit * CO selected.

b) oiler Follow [BF]

Not Manual mode Air Auto

Permit

Fuel Auto Permit

* BF selected. [Governor Auto optional.]

c) ase

Input

[ I]

Not Manual mode Air Auto

Permit

Fuel Auto Permit

*

BI

selected. [Governor Auto optional.]

d) Manual [MAN]

No permits. [Air Auto optional.]

Following

the

selection

of

a mode,

the

process deviations for fuel, air and

governor loops

are

forced to zero to

ensure

bumpless transfer. A back-calculation

produces a tracking signal which is used to initialize

the

appropriate upper level

controllers [Pressure, Oxygen, MW] at

values

which force

the

fuel, air and

governor demand signals to

be

equal and opposite to the prevailing process

variable. [Refer to Section 1 Tracking

and

Initialization. ]

The

system logic checks that permissives are met and that deviations for fuel,

air

and governor are approximately zero for 5 seconds before Mode Auto status is

implemented. This

is

to

ensure

tracking

is

complete and bumpless transfer

ensues. After balance check, Mode Auto status allows pre-selected coordinated

loops (fuel,

air,

governor) to go to auto status. f a mode permit is lost, auto

control is

suspended and

an alarm

initiated.


21/57

12

3 4 RUNBACK SYSTEM

Refer to Sheet 29. The

auxiliary

plant capacity is computed from in-service

status

and plant rating for each type

of

auxiliary. For example, one motor driven

feed

pump plus

one

turbine

driven

feed

pump

would provide a nominal capacity

of 450 MW. The system selects

the

lowest calculated value from feed pumps,

circulation pumps condensate pumps, FD fans

and

ID fans as

the

auxiliary

plant

capacity. The

required

output computation selects

the

lower

of

this value and the

target

load setting [Refer 3.3]. Tuning setters A02 to AlO allow

the

nominal

maximum output for

each

type

of

auxiliary to

be

set.

The appropriate fast Runback-Rate

is

selected

i f

a runback

is

required to match

Required

Output

to

plant

capacity following

an

auxiliary trip. The requirement

for

runback

action

is

determined by

Target

Maximum being

less

than

the

prevailing

Required

Output. [Sheet

26]. When this occurs, the controls are

transferred to

BI

mode prior to runback action being initiated.

Each

auxiliary

plant group has a preset runback rate. The selected rate is

determined by

the

group which limits

the

unit capacity to less

than the

prevailing required output.

For example, consider

the

case mentioned

in

the

previous

paragraph

at a

load of

430 MW

i f the

motor driven pump trips.

The

pump

capacity is now 360 MW and

the target

load will reduce to this value. The

runback

system

will select

the

pump runback rate which overrides

the

operator

rate

setting

until

the

required

output

decreases to 360

MW n the

case

of

a

multiple

trip the

system

will choose the lowest target and the highest

rate.

t

should be noted that

i f an

auxiliary trip results in a

maximum

target greater

than

the

prevailing

required

output

then

no action

results.

This would be

the

case

in

the

above example i f

he

load

was

300 MW before

the

pump

trip.

Tuning

setters All

to A14 provide

the

runback rates for each auxiliary type.

3.5 PRESSURE SET POINT

Refer to

Sheet

32. The required output [RO]

is

used as

the

load index for

development of the

set

point for sliding pressure operation as

determined

by

the

turbine

manufacturer.

A function

generator

[F x)-04] computes

the

pressure

set

point from

the

prevailing RO. Tuning setter Al9 allows for adjustment of the

maximum

pressure.


22/57

3

f sliding

pressure mode is not selected,

the

fixed pressure

set

point is

set

at

the

master display

.

Sliding pressure operation is

available

in

CO

and BF

modes

only. The fixed pressure

set point

tracks actual pressure when in sliding

pressure or i f an automatic

mode

not

selected.

On transfer to BI

mode

the

pressure

set

point

is

held

at

the

pressure existing

at

transfer

.

The

rate

of

change

of pressure

set

point is limited. The limit [

per

min] is

set

by

tuning

setter

A23.

3.6

GOVERNOR CONTROL

Refer

to Sheet 35.

The

RO forms

the basic

demand signal

to

the governor

system.

The

action

of the

modifying controllers for

MW and

pressure

as

well

as

the

process feedback

depend on

the

selected

operating

mode; [See below].

The

governor controller output

is

subject to

directional blocking

from

pressure

deviations

. f

he

pressure is

greater

than set point by a preset amount,

then

the

governor

is prevented from decreasing. Likewise,

increase

is blocked on low

pressure deviation.

The

governor

controller output is transmitted via the auto/manual subwindow to

a pulse converter. This compares the controller output with the

calculated

throttle

valve position

[Pl Pt]

and

generates raise or lower pulses

.

The

raise/lower pulses are

integrated by

the

turbine

governor

system to form the

load

reference.

The governor controls are

operated

differently

depending

on whether -

ordinated, boiler

follow

or

base

input-turbine

follow

mode is

selected. [Refer

Section

3.2 for discussion

on mode

selection.]

a) Co-ordinated Mode:

n this mode

the

RO is the common demand

signal to

both turbine

governor and boiler

inputs ;

this

common

signal

provides

the required

boiler-turbine co-ordination.

The

process feedback to the governor controller is

turbine

first stage

pressure; this is an index of

turbine

energy input and is

closely

proportional to

steady state

MW output.

The generation controller

corrects

for

any residual

difference

between

RO

and the actual MW

output

after steam

has passed through the reheater and downstream

turbine

stages.

The generation controller adds a trimming

signal

to the

RO

.


23/57


24/57

15

3 7 FIRING

R TE DEM ND

Refer

to

Sheet 38. The basic firing rate demand computation is dependent on

the

current operating mode. This demand is transmitted in parallel to the air and

fuel control sub-loops.

a) Co-ordinated

Mode

In this mode, the governor is required

to

be

on automatic controlling MW

to

equal the prevailing Required Output [RO] signal. The basic firing

rate demand is also equal

to

Required Output. To this is added the

following modifiers:

i)

Heat

Rate Correction

This function compensates for the increase in unit heat

rate as

load

decreases. At lower load, proportionately more fuel is required because of

the lower efficiency.

ii)

RO

ate

This

compensates for the change in boiler stored energy at different load

levels. The component of firing

rate

to accomodate stored

energy

change

is proportional to

both

the firing rate

demand

and the rate of change of

firing rate demand. The amount of RO

Rate

feedforward is

set by

tuning

setter

A25.

ill)

Pressure

ate

This compensates for the change in

stored

energy due to different boiler

pressures.

The

component of firing rate

to

accomodate pressure changes

is proportional to the

rate

of change of pressure set point. This

component is introduced

in

sliding pressure mode only.

The amount of

Pressure

Rate feedforward is set

by

tuning setter A26.

iv)

Pressure

Correction

The pressure correction controller

responds

to pressure error and its

output recalibrates the steady-state firing

rate

demand

to

achieve zero

pressure error; [Boiler-turbine balanced].


25/57

16

b) Boiler Follow Mode

The basic

firing

rate demand is turbine

energy demand

computed

by

Pl/Pt x

Ps

where l is turbine first stage pressure

Pt is

pressure before

throttle

valves and

Ps is pressure

set point.

This

signal replaces RO

in

boiler follow mode. The signal modifiers are the same as for co-ordinated

mode

as

described

in

3.7.1 b), c), d). Tuning setter A24

calibrates

the

turbine energy demand signal.

n this mode,

the

unit MW

output may be

automatically controlled

by the

governor to

equal

RO or be

set

manually.

c)

Base

Input· Turbine Follow Mode.

n this

mode the

basic

firing rate

demand is

simply Required Output

[RO].

The

modifiers for heat rate dynamic compensation and pressure

correction

are not

applied.

Pressure

control

is

executed

by

the governor

controller i he governor is selected to automatic.


26/57


27/57

18

a) Lead/Lag

The

purpose

of this

function

is

to

ensure that the air

flow

is

always

in

excess of requirements when

the firing rate is

being changed.

The

lead/lag function accomodates

the

different

transient

response

characteristics of the fuel

and air

systems.

For

firing

rate

demand

increases, a lead signal

is

applied when a positive rate

of

change is

detected.

his

forces a

higher rate of

change to

the air

demand.

Conversely, for firing

rate demand

decreases, the air

demand

is subject to

lag .

This

delays

the

reduction

of the air demand

relative

to the

fuel.

b) Excess Air Correction:

In

order

to

ensure

complete combustion,

the

amount

of

air

supplied needs

to

be in

excess

of that

theoretically required to

burn

all

the

fuel. This

additional component

is

called excess air .

The

required excess air for a

given fuel and load is calculated

by

the boiler

manufacturer

. [Refer 4.3.]

c) Minimum Air Flow:

The

air

demand

is subject to a minimum limit [normally 30%]

and

a fuel

cross limit.

The

cross

limit

prevents a serious deficiency of air for

the

current

fuel flow.

Setter

A27

is

adjusted to

ensure this limit

action does

not

affect normal operation.

The resulting

control signal

is the

Air Demand.

The

selected

air

flow signal

is

subtracted

from the

demand to

form

the air

error to the air flow controller. A

high

air error

blocks

further

increase

in

RO. The

error must

be initialized

to

within

+-2%

of

zero before

air

auto

is

permitted.

4.3 EXCESS AIR CONTROL

The

amount of excess

air

can

be

determined by measuring the oxygen

[ 2]

in flue

gas. A function

generator

[Fx-13] calculates

the

base

oxygen

set

point

as

a

function of firing demand;

this

function

is based on

boiler performance

data

The

carbon monoxide [CO] concentration is used

to

determine

the

optimum excess

air

for maximum boiler efficiency. The desired O level is

maintained

by the O

controller.

The computed

base

oxygen set point is corrected

by

the O controller output.

This

correction signal is limited

to

+-2%

02 The

base

set

point plus correction

is

the

oxygen

set

point.


28/57

19

The desired percentage of excess air is calculated from

the 02

set point by

function generator F(x)-15. The percent excess air multiplied

by

the firing

rate

demand

calculates the absolute

amount

of excess air.

This

is then

added to

the

basic air

demand as

a feedforward.

In

order

to obtain the exact oxygen content, the desired oxygen concentration is

compared

with the measured

value

and the

resulting error is applied to the

oxygen controller. The controller

output trims the

excess air

demand

feedforward to obtain the required value of oxygen. The

02

trim signal is limited

to

+-3 .

4 4 AIR FLOW CONTROLLER

The Air

Flow Controller positions

the

pitch angle control drives so

as

to reduce

the air flow

error

to zero. The control drives have

auto/manual

selection, position

bias and

equalizing control.

Tuning setter

A29 adjusts

the amount

of direct

demand feedforward to the air control drives. The operation of the dual drive

configuration [Type ] is described in Section 1.1.

Air flow control pre-selected to automatic is a required auto permit for CO,

BF

and BI control

system

modes [Section 3.3]. The air flow

may

e selected to

automatic

n Manual

mode;

the

basic demand is derived from

total

fuel flow.

[Refer

Sheet

26].

This method

is

normally only used

at

start up to

stabilize

air

flow

at 30 .

t is a prerequisite that furnace

pressure

is on automatic before

auto air flow control is permitted.

4 5 TRACKING

f he air controller is not auto, the oxygen controller tracks a back calculation

that forces

the

air

error

to zero. [Refer to Section 1.5.] The back calculation

includes the inverse of F(x)-15. This ensures bumpless transfer when air control

is transferred

to automatic.

To

facilitate

this

initialization,

the

oxygen controller

output must be available to the

air

demand computation when air is not auto.

The

manual

adjustment

of

the

oxygen

trim

signal is therefore only permitted

when

air control is on auto.


29/57

2

4 6 FD FAN STALL

Axial flow fans

can

stall under certain

operating

situations.

This

condition is a

function of the

fan

blade angle and ir velocity through the fan. It

can

occur if a

fan is operated at

high

head and

low flow.

This

situation can be caused by

restrictions

in the

flow

path or

by unbalanced

parallel

operation

of

two fans.

Stalling causes severe vibrations to the

fan

and

ducting

and a sharp drop

in

fan

output.

Sheet

46 shows

the system

provided to

warn the

operator that operation

is

close

to stall point. The volumetric flow is calculated for each fan and a function

generator

calculates the maximum safe pressure for the prevailing flow. This

calculated value

is

compared to

the

actual pressure

and an

alarm is

initiated

if it

is

higher

than

the

maximum

safe value.

4 7 AIR HEATERS COLD END TEMPERATURE

Flue gas

from the

furnace

is used to heat the incoming combustion

air in

two

rotary regenerative ir heaters. To avoid plugging

and

corrosion from

sulphur

products,

i t

is essential to

operate

the cold end of the heaters above the acid dew-

point temperature. The

heater

cold end temperature

is

defined as

the

average of

the

air

inlet

temperature

and the

flue gas outlet temperature.

The cold end temperature

is

controlled

by

pre-heating

the

air from

the

FD

fans

with

hot

water from the

deaerator.

Water to

the

two heat exchangers is supplied

by three

pumps.

Two valves associated with each

heater

control

the

relative

amounts

of

water

returning

to

the

deaerator

and

recirculating through

the

pumps. The water flow

is

relatively constant its temperature

is

determined by

the

proportion of hot water from

the

deaerator to recycled

water.

The two valves

work from a common signal

but in

opposite directions.

The

cold

end temperature

is

calculated from

the

average

of

three

thermocouples

for each measurement

as

shown on

Sheet

171. This

is

compared to

the

temperature set point and the resulting

error

is applied to the cold

end

temperature controller, [Sheet 175]. The controller

output

positions control

drives; for low cold

end

temperature the proportion

of

recycled

water

is

decreased,

the

water temperature increases which increases

the amount

of

combustion

ir

preheat. The opposite occurs for high cold end

temperature.

Auto/manual

selection and

set

point

adjustment is made at

the

appropriate CRT

subwindow.


30/57

21

4 8 WINDBOX AIR DAMPERS

After leaving the

air heaters

the combustion ir is

distributed

to the furnace

from

the

furnace windbox through windbox air dampers. These dampers are of

two types; Auxiliary Air Dampers

and

Fuel Air Dampers. Refer to

Sheets

121-

127

and

521-530.

4 8 1 Auxiliary Air Dampers

The Auxiliary Air Dampers

are

controlled to maintain the required differential

pressure

from

the

windbox to

the

furnace.

The

differential pressure set point is

computed from

steam

flow

by

function generator F x)-44.

The

auxiliary air is

admitted

above and below

the

active burners. The controlled pressure ensures

adequate

air

velocity.

The

selection of which elevations

are

active is executed by

the

burner

management system. A single controller and associated auto/manual station

operates all elevations of dampers.

4 8 2 Fuel Air Dampers

The

Fuel Air Dampers control

the

flow of ir around each

burner.

The opening is

calculated as a function of burner pressure; F x)-42 for fuel gas and F x)-43 for

fuel oil.

The

selection for

g s

or oil is made

by

the burner

management

system.

The dampers for idle elevations are closed by the BMS.

Elevation 1 is arranged

to

permit firing of single burners. all other elevations

require a

minimum

of two [opposite] burners. Warm up oil is fired on elevation

1 When

warm-up oil is used, the elevation 1

damper

opening is fixed

by

tuning

setter A70.


31/57

22

5 FUEL FLOW CONTROL

Refer to

Sheets

53 - 62 for analog signals

and

Sheets 440 - 444 for digital logic.

The boiler can produce

rated

output firing natural gas

or

fuel oil

or

combinations

of

both fuels.

Dual100

capacity control valves

are

provided for

both

fuels.

5.1 FUEL METERING

a)

Gas

Flow

Dual, 100 capacity meter ing systems

are

provided for

gas flow;

under

normal conditions only one system

is

in

service.

Each metering

system

comprises a flow orifice,

dual

differential pressure

transmitters,

a

pressure transmitter and dual temperature transmitters.

A

tuning setter

A52 allows site

adjustment

of

the

specific gravity. From these inputs

the

volumetric flow is calculated at

standard

conditions [273.18 deg Kelvin,

1.0133 Bar Abs]. Tuning

setter

A48 calibrates

the

gas flow to match

the

firing

rate demand in per unit

values.

b) OilFlow

The main

[heavy] fuel oil flow

is

calculated from fuel oil to burners (+),

return oil from

burners -) and warm

up oil (

. The

signals

are

modified

to a common scale before

the

c6mputation

and tuning setter

A49

calibrates

the

total to equivalent

per unit

mass flow.

The

ignitor [light]

oil flow

is

also metered

and

converted to mass flow

by

A47.

The three

fuel measurements; gas,

m in

oil

and

ignitor oil,

are

converted to

equivalent heat flow

by tuning setters

A38, A39

and

A37. These setters provide

the

facility

to adjust

for changes in fuel

heating

values. Ignitor oil is added to

fuel oil to give total oil

heat

flow. Gas heat flow

is

added to the oil to give total

fuel

heat

flow.

5 2 FUEL DEMAND

The

fuel

demand is

computed by

the

coordinated control system from required

output

[CO

or

BI modes]

or turbine

demand [BF mode]. Refer to Section 3.7,

Firing

Rate Demand. The fuel demand

is

cross limited with

the metered

air flow,

the lower being selected. This

is to prevent

significant mismatch between air

and

fuel. [Fuel

demand>> air

flow.] Tuning

setter

A30 adjusts

the air

flow signal

so

that t

is normally

not

selected.


32/57

3

The fuel demand is apportioned to gas and oil fuel according to the Oil Ratio

setting by the operator.

The

oil demand is calculated from fuel

demand

times

the

ratio

setting

. The gas demand is calculated from total fuel demand less

the

oil

demand.

5 2 1 ombined Firing

When combination firing is being used, both fuels may

be

on automatic control or

one fuel on

auto

and the other on manual.

The

control system accomodates all

configurations

of automatic

operation. Transfer from one configuration to

another

is bumpless.

(a) Both Fuels Auto

The

required

proportion

of

oil fuel

is set

by

the operator

by

adjustment of

the ratio setter. The oil demand is computed from

the

total demand times

the

oil fuel

ratio

. The

remaining

demand is assigned to

the gas

fuel. The

oil fuel ratio servo is only available to

the

operator when both fuels are on

auto

.

If,

when

firing both fuels, one fuel becomes limited and the flow error

exceeds a preset threshold deviation, [because of insufficient burners in

service or any other reason], this error is

added

to the other fuel control

error. Should both fuels become limited, the ensuing total fuel deviation

will block

further

changes

in total

demand in

the

direction which would

increase the deviation.

b) One Fuel Auto the Other Manual

In this case,

the

oil ratio setter

tracks the

proportion

of

oil flow

of

total

fuel.

The

flow error of the fuel that is not-auto is added to the

auto

fuel

error.

Manual

change

to the

not-auto fuel flow causes

the

error

signal to

the auto fuel to change an equal and opposite amount.

The

total fuel flow

thus

remains constant.

(c)

Both

Fuels Manual

The co-ordinated control

system

will be in

the

Mode Not-Auto

status.

The ratio

setter

tracks oil flow, thus balancing

the

oil fuel sub-loop. Prior

to Mode Auto

status the

total fuel demand equals

total

fuel flow and

the gas

fuel sub-loop will also

be

balanced,

as

follows:


33/57

24

Total Demand =Total Fuel

Oil Demand = il Flow

Gas

Demand

= Total

Demand

- Oil Demand] =Gas Flow

5 2 2 Single Fuel Firing

In

this case the non-fired fuel flow will

be

zero

and

the oil

ratio se

t ter tracking

will run to either zero (oil off

or

100% (oil on).

The

fired fuel receives

the

total

fuel demand.

~ FUELCONTROLLERS

There is a separate controller for gas and oil fuel. Dual 100% valves

are

provided

for each fuel; only one of

the

pair is permitted on automatic at the same time.

[Refer Section 1.3]. The controller output position the selected gas and oil control

valves.

The gas flow and oil flow errors

are

modified i f necessary to hold the pressures

between

required

high

and

low limits;

in

accordance

with

NFPA 85B

and

85D.

Tuning setters A34, A35, A36 and A41 set the minimum and maximum header

pressures

.

5 4 FUEL·AIR DEVIATION MONITOR

Firing conditions which lead to a situation where

there

is insufficient air to

bum

the

fuel are potentially hazardous. This condition can

be

caused by incorrect

manual operation of fuel and

air

or control system faults. n

independent

control system supervises the fuel-air ratio. Two levels of abnormal fuel-air ratio

are detected; Fuel High and Fuel Very High . The Fuel High condition, after a

short delay, trips

the

coordinated controls to

Manual

mode

and

independently

trips both

fuel

and

air to manual

f

urther deviation occurs,

the Fuel

Very High

condition

initiates

fuel firing

rate

cutback.

The

cutback action continues

until

the Fuel

High

condition resets . Refer to Sheet 586 for logic.


34/57

5

6 FURNACE PRESSURE

The supply

of

combustion air and the removal of the products

of

combustion is

carried out by

the

forced dr ft and induced

draft

fans working

in

balanced

draft

configuration.

The

work

is shared

between

the

two sets

of

fans.

The pressure

inside the furnace

is

controlled to be slightly negative at all loads;

this ensures

the designed balance between FD and ID fans

is

maintained. t also prevents the

leakage of extremely hot furnace gases to the boiler external area through casing

and duct leaks.

Changes in

both

FD

and

ID

fan output

affect

both

air flow

and

furnace pressure;

however

it is

now standard practice to control

air

flow primarily by the FD fans

and furnace pressure by

the

ID fans. This is

in

accordance

with

the NFPA code.

To minimize furnace

pressure

deviations on load changes,

the

ID fans

output

follows the FD fans and the furnace pressure control trims the residual

unbalance

by

further

adjustment

of

the ID fans output. In effect, the

ir

flow

is

controlled

by

both FD and ID fans in parallel.

This is

true also

when

the FD fans

are in manual provided that the ID fans are auto.

The ID fans at Bang Pakong 3 4 can be regulated by either changing

the

fan

speed through a variable speed coupling

or

by inlet

damper

control [refer 6.1,

6.2]. A triple

measurement

system as described

in

Section 2.3

is

used for the

measurement of

furnace pressure. The control logic for

the

furnace

pressure

control

is

shown

on

Sheets

109-

117 [analog] and 513- 517 [digital].

6 1 ID FAN SPEED

The variable speed

feature

of the ID fans enables

the

fan to operate nearer to

optimum conditions over a wide load range which reduces losses

and

consequently improves efficiency. The response of speed control

is

somewhat

slower

than

the FD fan blade pitch control because of the need to change the

rotational inertia. The

inlet

dampers are

used

as the controlling device for

furnace pressure because of the better transient response.

The ID fan speed

PID

controller follows a set point computed from the total FD

fan pitch position

demand

by

the

function

generator

F x)-40. The process

feedback for

the

speed control loop is the total ID fan speed regulator control

drive demand. The total ID fan speed

thus

tracks the total FD

fan

blade pitch

and maintains balanced operation.


35/57

26

The two ID

fan

control drives are arranged n as dual drives, type A as described

in Section 1.1. Demand feedforward direct to the control drives

is

adjusted

by

tuning setter

A51. This improves

transient

response. The ID

fan

variable speed

couplings are controlled by a local loop which adjusts ID

fan

speed to match the

speed

demand

from

the

DCIS.

6.2 ID

F N INLET D MPERS

The

final

furnace

pressure is controlled by the ID fan

inlet

dampers.

The

ID

fan

speed is controlled to a

value

which places

the

inlet dampers at ·a steady

state

opening

of

about 60 over

the

normal load range with two-fan operation. On a

load change,

the

ID fan speed tracks

the

FD fans

and the

ID damper controls

correct furnace pressure transients.

The median furnace pressure signal is compared to the operator set point and the

error is attenuated for small deviations by function generator F(x)-39. [Furnace

pressure signals

tend

to

be

very noisy.]

This

has same effect as reducing PID

controller gain and prevents unnecessary controller action and wear to

mechanical components. For

larger

deviations, the error is not attenuated

The

controller is reverse acting.

Feedforward from

the

difference between total FD pitch position

and

total ID fan

speed

demand

is

applied. Normally

this

calculation produces zero feedforward;

the exception being when the

speed

control

is

on manual

or

operating

in

the flat

part of

the

speed demand function generator. The amount of feedforward is

adjusted

by tuning

setter

A50. The

damper

control drives are in dual

configuration, type A as described

in

Section 1.1.

6.3

IMPLOSION PROTECTION

Two basic mechanisms can cause a negative pressure excursion

of

sufficient

magnitude

to cause

structural

damage to

the

furnace

and

ducting. Firstly,

control malfunction

or

operator

error can

cause

high

suction to be applied with

restricted

air path into

the

furnace, e.g. FD dampers closed. The second

mechanism

is

the

result of

rapid

temperature drop

in furnace

gas temperature

following a

termination

of fuel input The temperature drop causes a decrease in

furnace pressure which may be sufficient to implode the furnace.


36/57

7

The

control system includes the following protective features

against

implosions

in accordance

wit

NFPA Code 85G:

a) Triple furnace

pressure

measurement system.

b)

Feedforward from FD fan pitch position demand.

c) Directional blocking of both FD and ID fans if an abnormal furnace

pressure error occurs. For example, an abnormally low pressure causes

blocking of ID increase

and FD

decrease.

d) A

Master Fuel trip

[MFf] initiates an override which reduces the ID fan

inlet

vanes to

a preset proportion of its prevailing value. The override

then decays over a

number

of seconds

and

allows furnace

pressure

control

to

resume. Tuning setter A90 adjusts the proportional transient

reduction applied

to

the inlet dampers control drives when MFT occurs.

The override is effective in both manual and automatic control.

e) FD

Fan

Stall

alarm

is provided

to

warn against the possibility of

uncontrolled air flow changes caused

by

a

fan

stall condition.


37/57

8

STEAM TEMPERATURE

Control of main steam

and

reheat

steam temperatures

is effected

by

a

combination of furnace

gas

recirculation tilting

burners

and

spray

desuperheating. The

gas

recirculation

and tilting burners both

affect the

relative

distribution of heat

between

evaporative

and

superheating elements. The

primary

temperature

control is

by gas

recirculation; the tilting

burners

are

regulated to

a pre-programmed position

that is

a function of

load and

proportion

of

oil firing.

The gas

recirculation flow

and

tilt position affect

both main and

reheat steam

temperatures. The gas

recirculation flow is

regulated

to control

reheat temperature. The main steam superheater is designed to absorb

sufficient excess heat to

require desuperheating

over the

normal load range. This

enables

the

main steam temperature to be controlled by desuperheating.

For

the

main steam there are

two

stages of spraywater

desuperheating;

these

follow

the

primary and secondary superheaters. Spray desuperheating is also

fitted

at the reheater

inlet

for emergency use. The

steam temperature

controls

are

shown

on Sheets 77-

98 [Analog]

and 471-

499 [Digital].

7 1 MAIN STEAM TEMPERATURE

The main

steam

temperature

is controlled in two stages; secondary

superheater

outlet

and tertiary

[final]

stage

outlet.

The desuperheaters

are

located before

the

secondary

and tertiary

stages.

desuperheaters is shown

on Fig

7 1.

The

arrangement

of

superheaters and

The

time

constants associated

with

steam temperature controls are significant.

Both

control systems

are arranged in

a cascade configuration

where

the

inner

loop controls desuperheater

outlet

temperature. This assists

in

stabilizing the

outlet temperatures against disturbances such as spray pressure

fluctuations

and

burner changes. To

further

improve the dynamic response

and

stability a

number of

anticipatory

[feedforward] signals are applied; the objective

being

to

reduce

the

amount ofcorrection

required

by the feedback process.

All temperature

and pressure

sensors

used

for the steam

temperature

controls

are

duplicated with transmitter deviation monitoring logic as described

in

section

2.2.


38/57

lA

ToTIJRBINE

TERTI RY

(FINAL)

:

;

:

I

i

:

... : ..

f

;

:

:

••E .

~

SECONDARY

' ' ' •-

•

••n•

d:notutttt

'

..

i

T

lB lC

PRIMARY

DRUM

Figure 7·1 MainSteam

u p ~ r h e a t e r

:

:

i

l

;

E

:

;

:

:

1

.

.

lD

:

1

.:..

:


39/57

9

7 1 1 Feedforward Signals

The

basic feedforward to each system is a calculated set

point

for

desuperheater

outlet temperature [inner loop] for the prevail ing load.

This

set point represents

the expected steady state

value

for sliding

pressure

operation with gas firing and

includes

the

expected

amount

of

gas

recirculation and excess air.

The

feedforward is added to the

primary

controller output; the controller modifies the

feedforward

to

achieve the required

outlet

temperature. ther feedforwards are

added to compensate for dynamic conditions and different operating conditions,

such as fixed

pressure

.

a) Drum Pressure

Increase

in

drum pressure

reduces

the enthalpy

of

the

saturated

steam

and creates higher mass flow for the same firing rate. This results in a

drop in steam

temperature

. Function generator F x)-69 computes the

steady-state drum pressure from steam flow for sliding pressure

operation. The deviation of actual pressure from

the

computed value is

added

to

the

feedforward summer. High

pressure

causes increase in the

desuperheater outlet set point to compensate for steam temperature drop.

Tuning setter A65 calibrates the level of feedforward.

b) Air Flow

Overfiring

and

underfiring on load changes

alters the

relationship

between heat input and cooling steam through the superheaters and

causes

steam temperature

variations. The relationship between steam

flow and ir flow is subject to transient change; this is

used

to

generate

a

feedforward signal. Increases in ir flow [firing rate] relative to the

steam

flow increases

temperature

;

the

feedforward decreases the

desuperheater

outlet set point.

Tuning

setter A68 calibrates

the

feedforward. This input also compensates for changes in excess air.

c) Pressure Set Point

Changes in

pressure

set point require over/under firing which affects

steam temperature. The effect is proportional to the rate of change of

pressure. Increasing pressure set point causes a

transient

increase

in

steam

temperature. The feedforward reduces

the desuperheater

outlet

set point by an amount proportional to the pressure set point rate.

Tuning

setter

A65 calibrates

the

feedforward signal.


40/57

3

d) Gas Recirculation

The amount of gas recirculation varies from the predicted value because

of factors such

as furnace

fouling. The difference between actual

and

calculated

gas

recirculation

generates

a feedforward signal.

Increasing

gas recirculation increases

the

steam

temperature; the

feedforward

decreases the

desuperheater outlet set

point. Tuning setter A61

calibrates the feedforward signal.

7.1.2 Secondary Superheater Outlet Temperature

The steam temperature

in each of the two links from

the

secondary

superheater

outlet header

has

its own control system.

There

are

four

desuperheaters

between

the primary and

secondary

superheaters.

Each of

the

two secondary

superheater

outlet temperature

controllers

operate

in cascade configuration with the

associated pair of desuperheater spray controllers which regulate desuperheater

outlet

temperature.

The arrangement

is

shown on Fig. 7.1.2. The

outlet links

make a cross-over; hence

Link

A temperature

is

controlled by

desuperheaters

C and D

and

Link B is controlled by A and B, [Refer Fig. 7.1].

The secondary superheater

outlet

controllers set point is

calculated as

a function

of steam flow [F(x)-36]. The calculated

set

point

may

be replaced by

an

operator

setting.

The

common

set point is

compared

with

each

of the

two secondary

outlet

temperatures and the resulting error is applied to the appropriate controller.

Each

controller output modifies the feedforward signal [7 .1.1]

to

form

the

set

points to the spray controllers. The feedforward comprises the basic set point

computed by

F(x)-14

and the

dynamic components

set by tuning setter

A66.

The setpoint to

the spray

controllers is auctioneered against

the

calculated

saturation temperature [F(x)-33] plus margin the higher being selected.

Each desuperheater

is equipped

with

two 100 capacity

spray

valves. Only

one

valve is

permitted

on

automatic operation at the

same time. Refer Section 1.3,

Type B-2.

7 1.3 Final Superheater Outlet Temperature

The steam temperature from each of the two tertiary [final] superheater outlets

has its own control system. There are two

desuperheaters

between the secondary

and

final

superheaters

. Each of the two final superheater

outlet

temperature

controllers

operate

in

cascade configurat-ion

with

the

associated

desuperheater

spray

controllers which

regulate

desuperheater outlet

temperature.

The

arrangement is shown in simplified form on Fig. 7.1.3.


41/57

LINK

A

STMTEMP

CDSHOUT

,

,

LINK2A

CONTROLLER

>

SECONDARY

SUPERHEATER

OU LET

SET POINT

FEEDFORWARD

SATURATION TEMP

SET POINT

lDDSHOUT

IADSHOUT

TEMP if

LINK2B

CONTROLLER

>

LINKB

STMTEMP

lBDSHOUT

, TEMP

EMP

1 lCDSH

CONTROLLER

TEMP

lDDSH I ~

CONTROLLER iE J

I lADSH

CONTROLLER

IB DSH I

O N T R O L L E R ~

IC

lD

DESUPERHEATER DSH)

SPRAY VALVES

lA

Figure 7• •2 Secondary Superheater Outlet

Temperature Control

lB


42/57

WEST BRANCil

STM

TEMP

WEST (A)

CONI ROU...ER

>

2ADSHOUf

MP .....--2-A..ILD-SH-...

CONI ROU...ER

2A

FEEDFORWARD

SATURATION TEMP

SET POINT

DESUPERHEATER (DSH)

SPRAY VALYES

EAST

(B)

CONTROLLER

>

EAST BRANCH

STMTEMP

._

2B DSH OUT

2BDSH TEMP

O N T R O L L E R ~

2B

Figure 7• •3 Final Superheater Outlet

Temperature Control


43/57

3

The final

superheater outlet

temperature

is set by the

operator. The common

set

point is compared to each of the two final

outlet

temperatures and the resulting

error is applied to the

appropriate

controller.

The

controller

output

modifies

the

feedforward signal [7 .1.1] to form the set points to the spray controllers. The

feedforward comprises

the

basic

set

point

computed

by

F(x)-20

and the

dynamic

components set

by

tuning

setter A67.

The set point to the spray controllers is auctioneered against the calculated

saturation

temperature [F(x)-34]

plus

margin, the

higher

being selected.

Each

desuperheater

is

equipped

with two 100

capacity

spray

valves. Only one

valve

is

permitted

on automatic operation at the same time. Refer Section 1.3,

Type B-2.

7 2 REHEAT STEAM TEMPERATURE

Reheater

outlet

steam temperature can be controlled

by regulating

gas

recirculation flow, burner

tilt

angle

or spraywater attemporation

.

The primary

control method is gas recirculation; spray attemporation is

used

only i the gas

recirculation

system is

unable to

maintain

control for any reason. [Introducing

spraywater into the reheat stage lowers the

unit

efficiency.] Burner

tilts

are only

used to

compensate

for the different characteristics of oil firing versus gas. The

reheat

temperature

controls

are

shown on sheets

92 - 98 [analog]

and

490

- 499

[digital].

7 2 1 Gas Recirculation

The reheater

steam

outlet temperature is normally controlled

by

regulating the

gas

recirculation flow.

Increased mass

flow over the convective surfaces

increases the heat absorption. Changes in gas recirculation also affect the main

steam

temperature,

see Section 7 .1.1.

The

average reheater temperature is compared to

the

set

point

and

the resulting

error

is auctioneered

against

the

fan

motor current deviation before being applied

to the GR

Fan

controller. Ifeither motor current exceeds the

maximum

set point,

the controller acts to reduce

the current

rather than control

temperature.

The amount of gas recirculation flow

is

controlled by positioning the inlet vanes

on

the

two GR

Fans.

A feedforward signal from function

generator

F(x)-32

is

added

to the controller output. This computes

the

expected GR Fan

vane

position

as

a function

of

load

for

gas

firing.

Function generator

F x)-32 addB

a

modifying

signal

proportional

t the amount of oil firing. The control

drives

for

these

fans

are fans

conform to Type A as described

in

section 1.1.


44/57

3

2.2 Reheat Sprays

The reheat

spray

controller

set point

is

the

GR Fan

set

point with a bias added.

Under

normal operation,

the

spray controller sees a low temperature

nd

holds

the valves closed. f he

temperature increases

above the set

point

by

an

8.Iilount

greater

than

the

bias,

the

spray controller becomes active.

The

bias

is

removed

if

the GR Fans are not auto.

The spray valves

are in

two pairs; each pair

is

configured as Type B-2, Section

1.1.

7 2 3 Burner Tilts

The

burner

tilt position follows a load

program

developed by F x)-41. This

program

is

modified by F x)-45 when oil

is

fired, the modifier being proportional

to the

oil ratio.


45/57

33

8 FU L OIL

PUMPS

8 1 FUEL OIL TEMPERATURE

There

are

two oil

heating

systems each

with

its own control system. Refer to

sheets 133 135 533 535. The base set point for the fuel oil temperature

controller is

set manually

and is biased

by

the fuel oil viscosity controller. The

bias

range

is limited to 1-5 deg. The temperature

is

controlled by regulating the

flow of heating

steam

to the oil heater. A feedforward signal proportional to the

oil flow is

added

to the

temperature

controller output.

tuning

setter 71

calibrates the feedforward signal.

8 2 FUEL OIL HEADER PRESSURE

The fuel oil pressure is maintained by recirculating oil from the fuel oil

pumps

discharge

header

back to the storage

tanks

Refer to sheets 139

and

539. Two

parallel control valves are operated in split control configuration from a single

controller and subwindow Type B-1 Section 1.1. The control is reverse acting;

increasing

pressure

causes the valves

to

open.

8 3 FUEL OIL HEATER STEAM PRESSURE

Steam is available

to

the oil heaters from two sources; IP extraction to deaerator

and cold

reheat

The oil

heater

steam pressure controller regulates the

extraction steam and cold reheat pressure control valves in split control

configuration to maintain the set value. Refer

Sheets

143 543

and

Section 1.1

type B-1.

8 4 FUEL OIL TRANSFER PUMP PRESSURE

The

fuel oil

pressure

from

the

transfer pumps is

maintained by

recirculating oil

from

the transfer

pumps discharge

header

back to the storage

tanks

Refer to

sheets 147 and 547. Two paralle l control valves

are

operated in split control

configuration from a single controller

and

subwindow Type B-1 Section 1.1.

The

control is reverse acting; increasing pressure causes

the

valves to open.


46/57

34

9 FEEDWATER

Boiler feedwater is supplied from a pumping system comprising two 60 capacity

turbine driven pumps and two 15 motor driven pumps. Control of

pump

output

is

by

varying pump speed. The pump turbines are equipped

with

variable speed

governors

and the

constant speed motors

are

connected to

their

pumps

by

a

variable speed hydraulic coupling.

The motor driven

pumps

are normally used for

start

up and low load operation.

Because of the differing capacities and response characteristics, the controls only

permit one type of pump to be on automatic operation at the same time. Refer to

Sheets 65 - 75 [analog] and 448 - 468 [digital].

9 1 DRUM LEVEL

The

drum level control system comprises a three-element cascade configuration.

The

steam

flow from the boiler is

used

as the basic demand to the feedwater flow

controller,

thus

balancing the

water input

to the

steam

output. Imbalances

caused by transient conditions and metering

errors

result in changes

to

the boiler

drum

level. A separate drum level controller

adds

a correcting signal to the

feed

water demand

to

keep

the

level

at

the set

value.

Steam

flow increases cause a temporary drop in drum pressure. This lowers

the

boiling temperature and increases the volume of steam bubbles in the water

which,

in

tum causes a

transient drum

level increase. Introducing feedwater

immediately to match the steam flow aggravates the level deviation. To

overcome

this

effect, the

demand

from

steam

flow is modified to first decrease

and

then

increase as a time lag function to the final value. The reverse occurs on

a load decrease.

9 1 1

rocess

Measurements

The three measurements required for the system are steam flow, feedwater flow

and

drum level.


47/57

35

a) Steam Flow

The

steam

flow

is

measured inferentially from

turbine

first stage

pressure This

provides a linear signal proportional to steam flow over

the

normal

operating

range

.

The

tuning setter

BO

calibrates

the

·

first

stage pressure to represent percentage

steam

flow. The calibration

is

subjected to transient offsets from cycle changes such as feedwater

heaters out of

service.

b) Feedwater Flow

The feedwater flow is metered by differenti

Documents

Modulating Control(System Description)