FIELD OF THE INVENTION
The present invention relates to a method of operating a coal / oil / gas fired
steam generator. More particularly, the invention relates to a method for change-
over of the operation of forced draft fan from electrical steam mode in a steam
generator during the plant black-out condition.
BACKGROUND OF THE INVENTION & PRIOR ART
Boilers are employed worldwide by thermal power plants and many other
industries which require steam and / or power for its operation. The boilers are
designed to burn different fuels like coal / oil / gas either separately or in
combination. The fuels are admitted to the boiler furnace through the burners
disposed in different locations in the furnace walls. In some known types of
boilers, the burners are located in the four corners of the furnace at different
levels (known as corner fired boiler furnaces). In electric power plants operated
on the balanced draft principle, a forced draft fan (FD fan) is used to push air
into the boiler for the boiler combustion process. An induced draft fan draws the
outlet air or gas flow from the boiler into the chimney for discharge. In the
pressurized furnace type units Induced draft fan is not provided.
In a pressurized furnace type boiler, the inlet and outlet air/gas flows have to be
controlled both to support combustion at the operating load level while avoiding
unsafe operating conditions. Too much air flow results in energy wastage while
too little air flow results in a fuel rich condition which causes pollution and

possibly a boiler explosion. In addition, if too much gas is drawn from the boiler,
a boiler trip will occur to avoid an implosion. If too little gas is drawn from the
boiler, the boiler becomes over pressurized and dirt is blown out through small
openings in the boiler structure polluting the atmosphere. To avoid these
problems, the air flow is typically controlled to hold the boiler pressure in the
furnace and a trip occurs if the boiler pressure swings either side above the safe
thresholds. In certain types of steam generator, one of the two forced draft fans
is driven by an electrical speed control drive and the other forced draft fan is
driven by a steam turbine drive. In case of plant blackout conditions there exists
a need to switch over from electrical speed control driven forced draft fan to
steam turbine driven forced draft fan.
Patent GB1238921, describes an incinerator plant installation comprising a
furnace chamber having evaporatively cooled walls which form part of a steam
generating circuit, power-operated means for circulating evaporant through the
cooling walls of the furnace chamber, forced draught means for supporting
combustion within the furnace chamber, induced draught means for maintaining
the required furnace pressure conditions for the removal of the products of
combustion, and condenser means operatively associated with the steam
generating circuit wherein, in use, said power operated circulating means, said
forced draught means and said induced draught means are driven by steam from
the steam generating circuit, and any steam in excess of requirements is

condensed by said condenser means. The forced draught means and the induced
draught means are driven selectively, by steam turbines or electric motors.
Patent GB190215431, describes electric generators driven by steam motors, and
the method of operating the electric generators. The method comprises the step
of varying the steam pressure depending on load on the steam motor to regulate
the output of the electric generator.
The aforesaid prior arts does mention the steam turbine driven forced draught
means but does not discuss about the method of change over from electrical to
steam or vice versa during different modes of operation of the draft control
system.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method for change-over of
the operation of forced draft fan from electrical steam mode in a steam
generator during the plant black-out condition.
Another object of the invention is to propose a method for change-over of the
operation of forced draft fan from electrical steam mode in a steam generator
during the plant black-out condition, when both forced draft fans are operating
with one fan in manual control mode and the other fan in electrical speed control
in active mode.
A still another object of the invention is to propose a method for change-over of

the operation of forced draft fan from electrical steam mode in a steam
generator during the plant black-out condition, when both fans are operating
with one fan in steam turbine driven speed control active mode and the other
fan in electrical speed control in active mode.
Yet another object of the invention is to propose a method for change-over of
the operation of forced draft fan from electrical steam mode in a steam
generator during the plant black-out condition, when the turbine-driven fan
operating with speed control in active mode and the electrical speed controlled
fan is tripped.
A further object of the invention is to propose a method for change-over of the
operation of forced draft fan from electrical steam mode in a steam generator
during the plant black-out condition, when the turbine-driven fan operating with
de-activated speed control mode and electrical speed controlled fan operating
with de-activated electrical speed control mode.
A still further object of the invention is to propose a method for change-over of
the operation of forced draft fan from electrical steam mode in a steam
generator during the plant black-out condition, in which the electrical speed
control of the fan switching over from inactive to active mode while the turbine-
driven fan operating with inactive speed control mode is also included.
SUMMARY OF THE INVENTION
Accordingly, there is provided a method for changeover to operating at least one

steam turbine-driven forced draft fan in a power plant boiler during plant
blackout conditions wherein another forced draft fan is provided with electric
speed controlled drive, said method encompassing different operating conditions
including but not limited to a) both fans operating with one fan in manual control
mode and the other fan in electrical speed control in active mode b) both fans
operating - one fan in steam turbine driven speed control active mode and the
other fan in electrical speed control in active mode c) Turbine-driven fan
operating with speed control in active mode and electrical speed controlled fan is
tripped, d) Turbine-driven fan operating with speed control not in active mode
and electrical speed controlled fan operating with electrical speed control not in
active mode. The method for change-over of the electrical speed control of the
fan operating from inactive to active mode while the turbine-driven fan operating
with speed control not in active mode is also included.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 - Shows a schematic block diagram of the draft system in a steam
generator according to the present invention.
DETAILED DESCRIPTION OF A PREFFERED EMBODIMENT OF THE
INVENTION
Forced draft Fan Change-over method:
a) Condition- 1 When both Forced draft fans (1,2) are in operation
with the turbine driven forced draft fan in manual speed control

mode and the electrical speed controlled forced draft fan in
electrical speed control mode:
i. Turbine driven forced draft fan & Electrical speed controlled forced draft
fan regulating inlet damper will be maintained in maximum position and
disconnected from Automatic control.
ii. Turbine driven forced draft fan & Electrical speed controlled forced draft
fan speed control will be connected to auto-controls.
b) Condition- 2 - When both Forced draft fans are in operation with
the turbine driven forced draft fan in speed control mode & the
electrical speed controlled forced draft fan in electrical speed
control drive mode:
The electrical speed control drive system trips:
i. Electrical speed controlled forced draft fan shall trip.
ii. Electrical speed controlled fans' outlet damper shall close & regulating
inlet damper shall brought to minimum position.
iii. Boiler load shall be reduced to 60% MCR, if boiler load is greater than
60%MCR through auto control system.
iv. Turbine driven forced draft fanto continue in speed control mode.
c) Condition- 3 - When turbine driven forced draft fan is operating

in speed control mode & the electrical speed controlled forced
draft fan is OFF due to electrical speed control drive trip and if
Boiler load requirement is > maximum value that can be catered
to by one FD fan operation :
In this case the following sequence shall be followed for taking Electrical speed
controlled forced draft fan in operation in electrical speed control drive bypass
mode:
i. Boiler master control shall be taken to manual mode.
ii. Turbine driven forced draft fan speed shall be adjusted by operator
through A/M station taking turbine to manual.
Turbine driven forced draft fan regulating damper shall be set at position based
on load requirement.
(Note: speed change to rated rpm & damper position change should be gradual
and this is to be taken care remote manually by operator)
iii. Turbine driven forced draft fan regulating damper shall be connected
to auto.
iv. Boiler Master control shall be connected to automatic control mode.
v. Electrical speed controlled forced draft fan shall be started in electrical
speed control drive bypass mode (Motor shall be started in DOL) &
motor shall run at rated rpm.

vi. Electrical speed controlled forced draft fan regulating damper shall be
connected to automatic control.
d) Condition- 4 - When Turbine driven forced draft fan is ON
(Constant speed mode) & Fan -B is ON in VFD bypass mode & if
boiler load