boiler auto controls, mr. es salaikuberan

7/26/2019 Boiler Auto Controls, Mr. ES Salaikuberan

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CONTROLS & INSTRUMENTATION / FB 1

Bharat Heavy Electricals Limited, Tiruchirappalli-14

Presentation on

Boiler Auto Controls

By

E. S.Salaikuberan, DGM C&I


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Automatic Control

Automatic control is the maintenance of a desiredvalue of a quantity or condition by measuring the

existing value, comparing it to the desired value(set

point) and employing the difference to initiate actionfor reducing the difference.


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Basic Diagram of a Boiler

WATER

FUEL

AIR

MIXING OFFUEL & AIR

HEATTRANSFERSURFACE

FURNACE

STEAM/WATER SYSTEM

STEAM

BLOWDOWN

FLUE GAS

ASH


4/64CONTROLS & INSTRUMENTATION / FB 4


Boiler Auto Controls

Main control loops are:

- Unit coordinated control

- Combustion control : Fuel oil control, Coal flow control, Air flow

control- Feed water control

- Superheated & Reheated steam temperature control

- Furnace draught control

-Primary air pressure control

- Mill controls




SAMA Symbols

Scientific Apparatus Makers Association

ENCLOSURE SYMBOLSTable 1

Function Symbol

Measuring orReadout

Manual SignalProcessing

Automatic Signal

Processing

Final Controlling

Within a circle use a letter symbol from Table IIWithin other enclosures us a symbol from Table III






SAMA Symbols (contd)

MEASURING/READOUT LETTERSTable II

Process Variable FunctionA = Analysis**C = ConductivityD = Density

F = FlowL = LevelM = MoistureP = PressureS = Speed

T = TemperatureV = ViscosityW = WeightZ = Position

R = RecordingI = IndicatingT = Transmitter

RT = RecordingTransmitter

IT = IndicatingTransmitter

FRT

FR

**Self-defining symbols such as O 2, pH, etc., can be used in place of A.






SAMA Symbols (Table III)SIGNAL SIGNAL

FUNCTION PROCESSINGSYMBOL

FUNCTIONPROCESSING

SYMBOL

SUMMING or + INTEGRATE OR TOTALIZE QAVERAGING

/n

HIGH SELECTING

DIFFERENCE or - LOW SELECTING

PROPORTIONAL K or P HIGH LIMITING

INTEGRAL or I LOW LIMITING

DERIVATIVE d/dt or D REVERSE PROPORTIONAL -K or -PMULTIPLYING X VELOCITY LIMITING V

DIVIDING BIAS ROOT EXTRACTION

TIME FUNCTION

f(t)EXPONENTIAL X

n VARIABLE SIGNALGENERATION

A

NON-LINEAR FUNCTION f(x) TRANSFER T

TRI-STATE SIGNAL(RAISE, HOLD, LOWER) b SIGNAL MONITOR H/, H/L, /L



Bh t H El t i l Li it d Ti hi lli 14





SAMA Legend

T A

A

K

1. CONTROLLER

SET POINTGENERATOR

PROPORTIONAL

RESET

MANUAL SIGNAL

GENERATOR

AUTO./MAN.TRANSFER

SWITCH

T A

A

2. AUTO MANUAL + BIAS STATION

BIAS

ADJUSTMENT

MANUAL

SIGNAL

GENERATOR

AUTO./MAN.

TRANSFER SWITCH

Measuring or Readout

Automatic Signal Processing

Manual Signal Processing

Final Controlling

Signal Repeater

Summing /h Summing

d/d1

Derivative Difference Integral

K, -K Proportional, Reverse Proportional

X Multiplying Dividing Root Extracting

f(x) Non Linear or Unspecified Function

f(t) Time Function> High Selecting < Low Selecting

High Limiting Low Limiting

Velocity or Rate Limiter

+, -, BiasTransfer Analog Signal Generator

>| >

|

|

CONTROLS & INSTRUMENTATION / FB


8






Simple Feedback Control

PRIMARY VARIABLE

XT

K

A T A

f(x)

SET POINTPROCESS

MANIPULATED VARIABLE



9






Feedforward Plus Feedback Control

PRIMARY VARIABLE

XT

YT

SECONDARY

VARIABLE

A T A

f(x)


PROCESS

SET POIN T

K



10

Bharat Heavy Electricals Limited Tiruchirappalli 14





Cascade Control

PRIMARY VARIABLE

XT

ZT

K

K

SET POINT

A AT

PROCESS

f(x)


SECONDARYVARIABLE



11






Block Diagram of Boiler Control

+INPUT OUTPUT

FIRING

RATEDEMAND

FUEL DEMAND

AIR DEMAND

FEEDWATER CONTROL

STEAM TEMPERATURE

CONTROL

BOILER



12





Unit Coordinated Control

In unit Coordinated Control, target load dictated either by

AUTOMATIC LOAD DISPASTCH SYSTEM (ADS) or set manually

through UNIT MASTER.

In Auto position, the UNIT MASTER, accepts ADS signal while in

Manual position, target load set manually.

In case of Unit disturbance, Run down/Run back condition are

generated and unit load demand automatically reduced to low limit.

Unit load demand - fed to Boiler Master to generate Boilerdemand i.e. Air & Fuel requirement and Turbine Control to set

Turbine demand i.e. MW.

Bharat Heavy Electricals Limited Tiruchirappalli-14


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Envisages following modes of operation:

Coordinated mode:In this mode both Boiler Master and Turbine Control are put in

Auto and unit load demand fed to both Boiler and Turbine to

compute their individual load demands. Boiler is given ThrottlePressure error for adjustment of the demand.

Boiler Follow mode:

In this mode turbine takes load and boiler follows. Adopted when

turbine control is in Manual. Turbine sets MW load and boiler

controls throttle pressure as a follower.



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Turbine follow mode:

Boiler takes lead & turbine follows. Adopted during Run down/Run

back situation. MW controlled by boiler & throttle pressure

maintained by turbine.

Manual mode:

Both boiler master & turbine controller switched to manual & unit

controlled manually.

Run down/Run back:

Run down initiated automatically when Process error i.e. Feed

water, Air flow etc. becomes excessive & control element goes tomax. position making process uncontrollable. Unit load demand

reduced to a permitted value. Similarly, Run back initiated

automatically under Tripping of unit auxiliaries i.e. BFP, IDF etc. unit

demand reduced to match unit capabil ity.



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Bharat Heavy Electricals Limited, Tiruchirappalli 14


Turbine follow mode:

Boiler takes lead & turbine follows. Adopted during Run down/Run

back situation. MW controlled by boiler & throttle pressure

maintained by turbine.

Manual mode:

Both boiler master & turbine controller switched to manual & unit

controlled manually.

Run down/Run back:

Run down initiated automatically when Process error i.e. Feed

water, Air flow etc. becomes excessive & control element goes tomax. position making process uncontrollable. Unit load demand

reduced to a permitted value. Tripping of unit auxiliaries i.e. BFP,

IDF etc. unit demand reduced to match unit capabili ty.



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Unit Coordinated control

Up/Down directional blockings introduced when process error i.e.

Feed water, Air flow etc. becomes high or associated controlelement goes to max.position, to hold unit load demand till such

time the process recovers.

Under generator CB OPEN & turbine trip unit run under house load

operation.

Throttle pressure control:

Fixed or Variable Pressure mode of operation can be selected. With

FPO, set point be fixed manually while in VPO, steam flow signalrepresenting load index programmed to generate VP set point.



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COMBUSTION CONTROL



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y , pp

Combustion Control

Objective:

1) Regulate fuel input to boiler to maintain a continuous supplyof steam at a constant pressure.

2) Regulate air input to boiler in correct proportion to the fuel

input.

Combustion control system results in

* A more constant steam pressure and temperature, result ing inhigh efficiency.

* Fuel saving.

* Increase in life of equipments & pressure parts.




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y , pp

Block Diagram of Boiler Control

+INPUTBOILER

OUTPUT

FEEDWATER CONTROL

STEAM TEMPERATURE

CONTROL

FUEL DEMAND

AIR DEMAND

FIRING

RATE

DEMAND


y , pp

20



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y pp

Combustion Control

Divided into three sub loops:

Master demand.

Fuel flow control.

Air flow control.



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Combustion Control

MASTER DEMAND

Main steam pressure measured by a pressure transmitter.

Output of transmitter compared with set point. The error applied

to a PI controller.

Main steam flow used as feed forward feature & added to the

output from the controller.

Output of A/M station establishes air flow demand and fuel flowdemand signal.



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Master demand



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Combustion Control

AIR FLOW CONTROL

Secondary air flow to the boiler measured by aerofoils(left side &

right side of boiler). Compensated for pressure and temperature.

Total primary air flow, obtained by summing the air flow through

each mill in service.

Both added to obtain total air flow to boiler.

Air flow demand signal from master demand and actual fuel flow

signal compared and the higher of the two (lead-lag system)selected. This ensures an air-rich furnace.

This signal refined for maintaining correct amount of excess air.

Amount of excess air identified by the oxygen level in flue gas.



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Combustion Control

AIR FLOW CONTROL

High/Low limiters - used to limit the value in case oxygen

analyser out of service.

Under any circumstance the air f low should not be less than 30%

MCR airflow. Hence the signal from high/low limiter checked for

min.30% MCR airflow.

Resultant signal - developed SP for air flow control.

Difference between SP and actual total airf low signal - applied toa PI controller.

Controller output signal - connected to regulating device of the

corresponding FD fans in service through A/M station.



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Combustion Control

FUEL FLOW CONTROL

Fuel flow demand signal from Master Demand and actual total

airflow signal corrected for air-fuel ratio compared. Lower of twoselected as set point of fuel flow control ler(Lead-lag system).

This ensures that fuel flow is lower than airflow under all

circumstance.

Coal flow measured by adding speed of coal feeders in service.

Heavy oil flow in supply and return line measured and

difference between two corrected for calorific value.

Sum of coal flow and heavy oil flow gives actual total fuel flow.



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Cross-limiting combustion control strategy - ensures that there can neverbe a dangerous ratio of air and fuel within a combustion process.

Implemented by always raising air flow before allowing fuel flow to

increase or by lowering fuel flow before allowing the air flow to drop, asshown.

Air/fuel Cross-limiting

Cross-limiting combustion mechanism





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Firing Single Fuel/Cross Limiting

AIR CONTROL

FT PT FT

A

A T

P I

A T

P I

A T

P I

f(x)

f(x)

Fuel FLOW

STEAM

HEADER PRESSURE AIR FLOW

SP SP

LO SELECT HI SELECT

Fuel CONTROL

Fuel FeedAIR DAMPERCONTROLS & INSTRUMENTATION / FB

Air Feed

30




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Cross Limit Control With O2

Trim

GAS FLOW

STEAMFLOW

AIR CONTROLGAS CONTROL

HI SELECTLO SELECT

SP SP

STEAMHEADER PRESSURE AIR FLOW O2

AIR DAMPERGAS VALVE

FT PT FT AT

P

P P

T

T

A

A

A A

I

I I

P

TA

I

f(x)

f(x)





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Boiler Efficiency

To ensure complete combustion and

reduce heat loss, excess air to be

kept within suitable range.

Regulation of excess air provides:

Better boiler heat transfer rate.

An advance warning of flue gas

problems (excess air coming out of

the zone of maximum efficiency).

Substantial savings on fuel.

Excess Air Regulation




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FEED WATER CONTROL

(DRUM WATER LEVEL CONTROL)




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Block Diagram of BoilerControl

+INPUT OUTPUT

FIRINGRATE

DEMAND

FUEL DEMAND

AIR DEMAND

FEEDWATER CONTROL

STEAM TEMPERATURE

CONTROL

BOILER




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Feed Water Control

Feedwater - considered as life blood of boiler.

Erratic flow of feedwater - speaks on overall plant efficiency.

Many boilers today operate at steam temperature near the

maximum permissible metal temperature and temperature

fluctuations as a result of poor feedwater control will be

disastrous.



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Feed Water Control

FEEDWATER CONTROL TYPES:

Single Element Control:

Control system where one process variable - drum level -

used as the input.

Three Element Control:

Control system where three variables (Steam flow,

feedwater flow and drum level) used as inputs.



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Single Element Control

Should be used for start up control before steam flow is

delivered to the process.

The minimum feed water control system and applied

where steam flow is constant or at low loads when steam

flow measurement not available.

When single-element control is combined with three

element control - the mode selection may be automatic oroperator selected.




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Single Element Control

SET POINT K

T

f(x)

A

A

X

LT PT

f(x)

FINAL CONTROL DEVICE

M/A CONTROL STATION

PRESSURECOMPENSATION

DRUM PRESSUREDRUM LEVEL

Typical single-drive control system. For simplicity, redun-

dant transmitters have not been shown on this typicalcontrol drawing. See Figure 2A for ANSI/ISA-S5.1-1984format.





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Single Element Control Action

* INTERACTION WITH FIRING RATE CONTROL DUE TO IMBALANCEBETWEEN STEAM FLOW AND FEEDWATER FLOW.

% STEAM FLOW

FEEDWATER FLOWSTEAMFLOW

*

*

TIME00

25

50

75

100

NWL

SHRINKDRUM LEVEL

SWELL




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Three Element Control

Requires a pressure compensated drum level signal, a

desired set point signal for level, a feed forward signal from

a temperature compensated steam flow transmitter and a

signal from feedwater flow transmitter.

The feedwater flow transmitter should be temperature

compensated if the measurement is affected by feed water

temperature.

The control makes feedwater flow follow steam flow and use

the deviation in level as a resetting action to bring the

required water inventory back to balance.

Feedwater demand shall be derived from the error between

the feedwater / steam flow error and drum level.




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Three Element Control

PRESSURECOMPENSATION

f(x)

A

X

PT FT TE

f(x) f(x)

X

LT PT

f(x)

X

FT TE

f(x)

K

K

T A

FINAL

CONTROLDEVICE

M/A CONTROLSTATION

SET POINT

PRESSURE AND

TEMPERATURECOMPENSATION

STEAM

TEMPERATUREFLOWPRESSURE

Typical single-drive control system. For simplicity, redundanttransmitters have not been shown on this typical control drawing.See Figure 4A for ANSI/ISA-S5.1-1984 format.

LEVEL PRESSURE

DRUM

TEMPERATURECONDENSATION

FLOW TEMPERATURE

FEEDWATER

Typical single-drive control system. For simplicity, redundant transmitters

have not been shown on this typical control drawing.





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Performance of Three Element control




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Feed Water Control

PHILOSOPHY OF REVERSAL OF DRUM LEVEL CHANGE

Increase in boiler load (steam flow) results in SWELLING.

Swelling - an increase in drum water level due to an increase

in steam bubble volume.

Condition due to an increase in load, with a resulting

decrease in drum pressure and an increase in heat input.

Swelling also occurs during a cold start up as the specificvolume of water increases.



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Feed Water Control

PHILOSOPHY OF REVERSAL OF DRUM LEVEL CHANGE

A reverse phenomenon ie., SHRINKAGE takes place due to

boiler load reduction.

Shrinkage - decrease in drum level due to decrease insteam bubble volume.

Condition due to a decrease in load (steam flow), with a

resulting increase in drum pressure and a decrease in heatinput.



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STEAM TEMPERATURE CONTROL



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Block Diagram of BoilerControl

+INPUT OUTPUT

FIRING

RATEDEMAND

FUEL DEMAND

AIR DEMAND

FEEDWATER CONTROL

STEAM TEMPERATURE

CONTROL

BOILER


St T t C t l


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Steam Temperature Control

Need:

Important and critical factors that affect the operating efficiency of

a modern turbine is steam temperature.

Higher the temp. the better the efficiency.

However strength of steel decreases very rapidly at hightemperature hence maximum operating temperature limited.

Since temp. changes affect rate of thermal expansion of the close

tolerance turbine equipment, it is required to maintain a nearlyconstant steam temperature despite large load swings



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Basic Methods of Control

DESUPERHEATING (ATTEMPERATION)

BURNER TILT POSITIONING



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Control with Desuperheater

Desuperheating - a method of reducing temp. by

introducing hot water into the steam flow just prior to the

steam entering the secondary superheater or reheater.

Addition of water to steam - the steam temp. gets reduced

and the water is turned into steam by absorbing heat from

the steam.

A two element system (cascade control) is more suitable

for high capacity boilers where the design involves heavymass of superheaters with associated high thermal inertia

time.



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Control with Desuperheater

In two element control, a cascade control used to position the

spray CV.

The outer loop PI controller receives an error signal equal to thedifference between steam temp. SP.(e.g. 540C) and the main steam

temp.

The output of the controller establishes the set point for the SHDESH outlet temp.

This SP compared to measured SH DESH outlet temp. and the

resulting error applied to inner loop controller.

The outer loop controller regulates the SP of the inner loop to

control the main steam temp.


Block diagram and control loop of two element


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Block diagram and control loop of two element

control


Two element control


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Burner tilt Positioning

The second method of controlling the final steam temp. is with

burner tilts.

Steam temp. maintained by allowing nozzle tilt to respond to the

lower of either SH or RH outlet temp. with spray responding to the

higher.

If the burners tilted up - fire ball is raised upward within the furnace.

The result - less heat absorbed by water wall tubes and more heat

absorbed by super heaters.

This results in increasing the final steam temp.

Pointing the burner downward has an opposite effect.


Tilting in a Tangential Corner Firing


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Tilting in a Tangential Corner Firing

Furnace



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Other Interlocks


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Other Interlocks

Spray water CVs - to be interlocked to remain closed

at loads less than 20% MCR.

Nozzle tilts - to be interlocked to remain in Horizontalposition at loads less than 25% MCR.

Spray water block valves - to be interlocked to open

only when either associated spray water CV demand isgreater than 0%.

Increase in nozzle tilt demand - to be blocked if any

spray water CV is fully open.



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OTHER CONTROLS


Furnace Draft Control


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Objective:

To maintain furnace pressure constant at set value.

Achieved by changing flue gas flow by modulating guide vane and speed of ID fan.

Excessive furnace pressure - monitored for directional block on ID and FD fans.

Fuel demand signal used as feed forward feature.

For equal loading of ID fans, each ID fan motor current measured, averaged &

compared. Difference used for taking corrective action. The corrected signal used to

posit ion fan regulating units.

Guide vane position of each ID fan monitored & compared separately with

maximum and minimum position limit. Maximum position limit enables guide vane to

be kept with in its control range.

Furnace Draft Control



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PA Header Pressure Control (Cold PA System)


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( y )

Objective:

To maintain PA header pressure according to the feeder speed(the

feeder speed which is higher than others considered as set value).

Primary air header pressure measured and compared with the

selected feeder speed signal through a high signal selector to

maintain minimum header pressure.

Error applied to a PI controller and regulating device of each PA fan

modulated accordingly.

To have equal loading of two running PA fans, the PA fans motor

current measured,averaged & compared. Difference used to position

the PA fan regulating unit.



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Mill Outlet Temp. and Air Flow Control


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p

(Cold PA System)Objective:

To adjust mill air flow according to feeder speed & to maintain mill

outlet temp.at set value.

Mill air flow maintained by adjusting hot air reg. damper while mill

outlet temp. maintained constant by adjusting cold air reg.damper.

Temp.compensated mill air flow signal compared with variable air

flow set point as a function of feeder speed.

Error applied to a PI controller.Rate of change of fuel demand signal used as feed forward feature.

Mill outlet temp.measured & compared with set point. Error applied to

a PID controller.



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boiler auto controls, mr. es salaikuberan

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