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CONTROL AND
INSTRUMENTATION IN
SUPER CRITICAL BOILERS
Types of boilers
Drum type
Once-through type
THE CONCEPT
The mass flow rate thru’ all heat transfer circuits from Eco. inlet to SH outlet is kept same except at low loads wherein recirculation is resorted to protect the water wall system
Once-thru Boiler
Major differences from Drum type boiler :
• Evaporator system
• Low load circulation system
• Separator
Once -thru Boiler
Evaporator system :• Formed by a number of parallel tubes• Tubes spirally wound around the furnace to
reduce number of tubes and to increase the mass flow rate thru’ the tubes
• Small tube diameter• Arrangement ensures high mass velocity
thru the tubes
Once -thru Boiler
Low load circulation system :
At part loads once -thru flow not adequate to
cool the tubes
To maintain required mass velocities boiler
operates on circulating mode at low loads
Once -thru Boiler
Low load circulation system :
The excess flow over the once-thru flow separated in separator and– Returned to the condenser or de-aerator
thru’ a heat exchanger
or– Recirculated back to the boiler directly by
a dedicated circulating pump
Once -thru Boiler
Separator :
Separates steam and water during the
circulating mode operation
Runs dry during once-thru flow mode
Smaller in size compared to drum in a
drum type boiler
Once -thru Boiler
Advantages:
• Better suited for sliding pressure operation• Steam temperature can be maintained over
wider load range under sliding pressure• Quick response to load changes• Shorter start up time• Higher tolerance to varying coal quality• Suitable for sub critical & super critical
pressures
Once -thru Boiler
Requirements :
Stringent water quality
Sophisticated control system Low load circulation system
Special design to support the spiral furnace wall weight
High pressure drop in pressure parts
Higher design pressure for components from feed pump to
separator
Effect on Boiler Components
• Evaporator (Furnace) walls
• Super-heaters
• Thick walled boiler components
• Steam piping
Effect on Boiler Components
• Higher pressure & temperature lead to increased thickness of :– Shells of separator, start-up system
components, SHO header..– Main steam piping.
• Higher thickness results in larger temperature gradients across walls
Feedwater ControlNatural circulation boiler Once-through boiler
Feedwatercontrol
Level
Feedwater
Steam flowm•
EvaEco
T
T
Enthalpy
Feedwatercontrol
Feedwater
TP
Eco
Eva
THANK YOU
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