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)
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BANG PAKONG POWER STATION UNITS 3 4
DCIS MODULATING CONTROLS
SYSTEM DESCRIPTION
SYSDESCR DOC
Rev
0
2Sep91
Author: R McDermott
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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
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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 . . . . . . . . . . . . . . . . . .
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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.
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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.
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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.
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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.
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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.
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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.
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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.]
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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
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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.
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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
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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
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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.
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>
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
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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.
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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.
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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
.
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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].
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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.
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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.
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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.
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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.
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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.
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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.
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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:
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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.
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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.
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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.
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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.
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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.
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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
.:..
:
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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.
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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.
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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
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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
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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.
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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.
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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.
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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.
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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
top related