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