Automatic Boiler Controls

INTRODUCTION AUTOMATIC BOILER CONTROLS

62B208

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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 MEASURES STEAM PRESSURE

ACC GENERATES A MASTER DEMAND FOR FUEL AND AIR

STEAM DRUM

MANIPULATES THE FORCED DRAFT BLOWERS & FUEL OIL CONTROL VALVE 62B-20811 AUG 00

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 62B-20811 AUG 00

Combustion Control MEASURES STEAM PRESSURE

ACC MANIPULATES FUEL FLOW TO THE BOILER BY INCREASING/DECREASING FUEL FLOW TO THE BURNERS

FUEL OIL CONTROL VALVE 62B-20811 AUG 00

Combustion Control MEASURES STEAM PRESSURE

ACC FORCED DRAFT BLOWER

FUEL OIL CONTROL VALVE 62B-20811 AUG 00

Automatic/manual Control Stations PRIOR TO THE FINAL CONTROL ELEMENT TO ALLOW FOR HUMAN INTERVENTION

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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THREE ELEMENTS TO CONTROL FEEDWATER FLOW

SF T DLT

FWCV

DLT

FF T

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SF T DLT

FWCV

SF/FF RELAY

FF T FF C FWCV POS

FWC A/M

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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

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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 62B-20811 AUG 00

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 FD PUMP CONT.

MEASURES FEED PRESSURE FROM A COMMON HEADER TO ONE OR MORE BOILERS

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Feed Pump Controls MAIN FD PUMP CONT.

GENERATES A DEMAND SIGNAL TO THE FEED PUMP STEAM ADMISSION VALVE

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Feed Pump Controls MAIN FD PUMP CONT.

THROUGH THE A/M STATIONS ON THE CONSOLE 62B-20811 AUG 00

Feed Pump Controls MAIN FD PUMP CONT.

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Constant Pressure Regulator • Feed pressure directly opposes spring compression

From common Header 62B-20811 AUG 00

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 62B-20811 AUG 00

BOILER FLEX COMPONENTS F/ F

FW C

S/ F

SP T

FINAL CONTROL ELEMENTS D/ L FD B

F OCV

MF P

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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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NEEDLE VALVE

RESE T

INDIVIDUA L TRIP 40 VALVE PSI

MASTE R TRIP 65 PSI FROM A/M STATION

REDUCE R ALARM PRESSUR E SWITCH

125 PSI

75 PSI

80 PSI

THREE WAY VENT

TO FINAL CONTRO L 62B-20811 AUG 00 ELEMNT

Modes Of Operation • Automatic • Remote MAUNUAL • Local manual

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NOW WE KNOW OUR ABC’S

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Introduction To ABC System

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