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AUTOMATIC BOILER CONTROLSINTRODUCTION

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Functions

MEASURE

COMPARE

COMPUTE

CORRECT

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What Makes It Work?

• Low pressure air system

• Continuous supply

• High quality

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Automatic Boiler Controls

• Consist of three sub-systems

• Automatic combustion control• Automatic FEEDWATER control• Main feed pump constant pressure

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Combustion Control

• Maintains steam drum or SUPERHEATER outlet pressure

• Within 5 PSI of SETPOINT during steady state conditions

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Combustion Control

ACC MEASURES

STEAM PRESSURE

GENERATES A MASTER DEMAND FOR FUEL AND AIR

MANIPULATES THEFORCED DRAFT BLOWERS &FUEL OIL CONTROL VALVE

STEAMDRUM

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Combustion Control

MEASURES STEAM

PRESSURE

ACC

FORCED DRAFT BLOWER

MANIPULATES AIR FLOW TO THE BOILER BY INCREASING/DECREASING THE SPEED

OF THE FORCED DRAFT BLOWER

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Combustion Control

ACC

FUEL OIL CONTROL VALVE

MEASURES STEAM

PRESSURE

MANIPULATES FUEL FLOWTO THE BOILER BY

INCREASING/DECREASINGFUEL FLOW TO THE BURNERS

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Combustion Control

MEASURES STEAM

PRESSURE

FORCED DRAFT BLOWER

ACC

FUEL OIL CONTROL VALVE

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Automatic/manual Control Stations

PRIOR TO THE FINAL CONTROL

ELEMENT TO ALLOW

FOR HUMAN INTERVENTION

FINAL CONTROLELEMENT

AUTOMATIC SYSTEM

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Automatic FEEDWATER Control

• Maintains water level in boiler steam drum

• Plus or minus one inch from normal

• All steady state load conditions

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Automatic FEEDWATER Control

3 Element Feed

Water Level

+/-1 inch of Normal

MANIPULATES THE FEEDWATER

CONTROL VALVE TO REGULATE FEED FLOW

TO THE BOILER

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SFT

DLT

FFTFWCV

DLT

THREE ELEMENTS TO CONTROL

FEEDWATER FLOW

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SFT

SF/FFRELAY

DLT

FFT

FFC

FWCA/M

FWCVPOS

FWCV

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Compensates For Shrink And Swell

• Steam flow and drum level will respond in the same direction upon a boiler load change

• Increase to increase

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Shrink & Swell

• During up PWR MANUEVERS, a temporary rise in BLR WTR LVL occurs -- called swell

• During DWN PWR MANUEVERS, a temporary drop in BLR WTR LVL occurs -- called shrink

Swell

• Increase in STM demand:– More STM leaves BLR.– Drum press starts decreasing.– STM bubbles in GEN tubes expand.– WTR level appears to “swell”.

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Swell• Automatic combustion CONTRL system (acc

sys) senses decrease in drum press and increases firing rate

• Drum press increases and regains SETPOINT• WTR LVL decreases as bubble size decreases

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Swell

• WTR reaches normal and get slightly below normal

• Automatic boiler control system (ABC sys) will increase feed flow to match STM flow

• WTR LVL achieves normal

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Shrink

• During a DWN PWR MANUEVER shrink has the opposite effect :– Co wants all stop -- man overboard!– Throttle closes– STM drum press rises– WTR level lowers or “shrinks”

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Shrink

• Acc sys decreases firing rate • Drum press regains SETPOINT• WTR LVL increases• WTR LVL reaches normal, and slightly

above normal• ABC sys decreases feed flow

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Main Feed Pump Controls

• Maintain a constant pressure FEEDWATER supply to the boiler while on line

• Designed to supplement the FEEDWATER control valve

• 150 -200 PSI above steam drum pressure

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Main Feed Pump Controls

• The output of the header pressure controller will be directly proportional to the actual header pressure

• Increase - increase

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Feed Pump Controls

MAIN FDPUMP CONT.

MEASURES FEEDPRESSURE FROM A COMMON HEADERTO ONE OR MORE

BOILERS

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Feed Pump Controls

MAIN FDPUMP CONT.

GENERATESA DEMAND

SIGNAL TO THEFEED PUMP STEAMADMISSION VALVE

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Feed Pump Controls

MAIN FDPUMP CONT.

THROUGH THE A/MSTATIONS ON THE

CONSOLE

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Feed Pump Controls

MAIN FDPUMP CONT.

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Constant Pressure Regulator

• Feed pressure directly opposes spring compression

From common

Header

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Boiler Control Test

• On-line alignment and verification• Boiler flexibility test

• Overall system reliability

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Final Control Elements

• Fail safe positions

• Combustion to minimum• Water level to maximum• FEEDWATER control valve open• Feed pump steam admission valve fully

open

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FWCF/F

S/F SPT

D/L

FOCV

FDB

MFP

BOILER FLEX COMPONENTS

FINAL CONTROL

ELEMENTS

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Air Pressure Failure

• Air lock system

• Holds final control elements

• Allows WATCHSTANDERS to take control of the plant

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ALARM PRESSURE SWITCH

RESET

MASTERTRIP

NEEDLE VALVE

REDUCER THREE WAYVENT

INDIVIDUALTRIP VALVE

FROMA/M STATION

TOFINAL CONTROLELEMNT

40 PSI

65 PSI

80 PSI

125 PSI75 PSI

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Modes Of Operation

• Automatic

• Remote MAUNUAL

• Local manual

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NOW WE KNOW OUR ABC’S

THE END

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Introduction To ABC System

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