FIELD OF THE INVENTION
The present invention relates to a process of integrated operation of a Heat
Recovery Steam Generator (HRSG) with smooth change over from gas turbine
(GT) mode to Fresh air firing (FD) mode in combined cycle and Co-generation
systems.
BACKGROUND OF THE INVENTION
It is known that HRSGs comprise a fuel burning equipment which interalia
convert cold water to hot water including steam to meet the process requirement
HRSG system utilizes solely the GT exhaust gas as the medium to supply oxygen
to the burner system. The Burner is located in an internally insulated duct
enclosure.
In HRSG with Forced DRAFT FAN (F D FAN) operation, there are two modes of
operations applicable, namely

1. GT with supplementary firing mode in Figure 1
In this mode of operation, the HRSG system utilizes solely the GT exhaust
gas as the medium to supply oxygen to the burner system.
Thus, the GT is running (8), the Divertor damper is opened towards the
HRSG and FD Fan, and the other accessories remain switched off as per
Figure 1.
2. FD FAN mode in Figure 2
Fresh air is supplied to the forced draft fan when the gas turbine is not
in service. The fresh air supply is stopped once the GT becomes
operational.
US Patent no. 4151712 May 1, 1979, entitled Protective shutdown system for
combined cycle plant having a duel liquid fuel system, teaches that a gas turbine
and an after burner are operated with a dual liquid fuel system. During a
protective stop, logic controls are provided for initiating a transfer from operating
fuel to a flusing fuel after predetermined conditions are satisfied.

US Patent No.6957540 B1 OCT 25 2005, entitled Multimode complex power
plant, describes a power plant including a boiler unit portion and a gas turbine
unit portion operable in multiple modes, operation of the boiler unit portion being
independent of operation of the gas turbine unit portion. Exhaust gas from the
gas turbine is cooled in a heat recovery steam generator, where steam is
produced. The exhaust gas my also be provided to support combustion in a
separately fueled boiler.
Thus, the prior art fail to teach any method or apparatus for usage of fresh air at
ambient temp supplied in the FD fan mode along with GT exhaust gas in GT
mode.
French Patents FR84/14484 FR97/02586 deal with fuel burners used in HRSGs
and do not describe the auto changeover operational requirements from GT
mode to fresh air mode.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a process of integrated
operation of a Heat recovery steam generator (HRSG) with smooth change over
from gas turbine (GT) mode to Fresh air firing (FD) mode in combined cycle and
Co-generation systems.

Another object of the invention is to propose a process for integrated operation
of Heat recovery steam generator (HRSG) with smooth change over from gas
turbine (GT) mode of Fresh air firing (FD) mode in combined cycle and Co-
generation systems, which allows automatic and fast changeover of the
operational mode of the combined cycle and co-generation system from GT
mode upon tripping of the GT.
A further object of the invention is to propose a process of integrated operation
of a Heat recovery steam generator (HRSG) with smooth change over from gas
turbine (GT) mode to Fresh air firing (FD) mode in combined cycle and Co-
generation systems, without suspending the supplementary firing by the FD
fans.
SUMMARY OF THE INVENTION
Accordingly, there is provided a process of integrated operation of a Heat
recovery steam generator (HRSG) with smooth change over from gas turbine
(GT) mode to Fresh air firing (FD) mode in combined cycle and Co-generation
systems.
During automatic trip transfer, the FD Fan commences its inlet and outlet
dampers operations. GT Diverter damper close operations, and fuel flow control
for burners, are to be done in a pre defined sequential and timing logic. The
whole activities are to be completed within a short time (<40 sec), so that the
burners continue in service and sustain the steam generation.

Thus, an automatic transfer of GT mode to FD mode implementation for
selecting an Auto Trip Transfer logic in the DCS (Distributed control system), a
selector Switch being hard-wired in the DCS.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows a prior art Heat Recovery generator (HRSG).
Figure 2 - shows a heat recovery steam generator in operation according to the
process of the invention, that is the HRSG continuing steam generation even
after the gas turbine (GT) tripped.
Figures 3-4 - show process flow charts depicting the operation of automatic
transfer of operational mode to fresh air mode (FD - Fan mode) from GT - mode
upon tripping of the GP.
DETAIL DESCRIPTION OF THE INVENTION
As shown in figure 2, the HRSG operates according to the inventive process
shown in figures 3 and 4 as under:

Whenever GT trips, while HRSG in "GT with supplementary firing" mode of
operation, the "Auto Trip Transfer" control circuit of the invention automatically
changes the HRSG operation with FD FAN, and without putting off the burners.
Thus the steam generation continues and the load demand is met quickly.
During Auto Trip Transfer, the FD Fan (1) starting its inlet (4) and outlet (3)
dampers operations and GT Diverter damper (5) close its operations, and fuel
flow control for burners, are done in a pre defined sequential and timing logic.
The whole activities are completed within short time (<40 sec), so that the
burners continue to provide service and sustain operation.
When the GT trips while the HRSG is in service with Supplementary Firing in
operation, FD mode of Operation of the HRSG takes over automatically without
putting off the Supplementary firing and thereby maintain the outer steam
parameters quickly.
To achieve smooth and successful transfer to FD mode, following process steps
are implemented based on the auto-trip transfer control circuit logic built in the
distributed control system ( DCS ) of the HRSG:-

- The GT trip transfer action is switched on when the following conditions
are met.
> Auto trip transfer soft switch in DCS is selected on ON, position.
> FD fan (1) Auto/Manual switch is in " AUTO", mode,
> FD fan outlet gate (2) is open,
> FD Fan outlet damper (3) is closed, and it's A/M switch in "AUTO",
> FD F inlet damper (4) is closed.
> Diverter damper (5) is full open towards HRSG
> Left and Right burners (6) are in service; and
> GT (8) tripped
- On GT tripped condition, the Trip transfer action performs the following
> Gives " GT trip transfer exists signal"
> Initiate and monitor the total transfer time of less than 40 sec.
> Start command to FD fan
> Command to force auto the steam pressure control.
> Hold FD inlet damper at close for less than 20 sec .
> Commands to BMS (burner management system) not to trip burners on
Gas Turbine trip.

- Check whether inlet damper closed (4), outlet damper closed (3) in
the DCS/PLC as Part of boiler interlocks before starting the FD fan
(1)
- Once the FD fan (1) reaches 80% of its full speed the outlet
damper (3) is
- opened fast to avoid stopping of the fan, for example, after 13
secs. From Fan ON signal (from MCC (motor center)), the outlet
damper (3) is Commanded to open.
- Simultaneously command HRSG to fast close the diverter (5), to
achieve the air flow towards HRSG.
- Immediately after transmitting the "FD outlet damper open
command: (to ensure fan reaches 100% speed and Ready to take
loading), command to open FD fan inlet damper (4) to 25%
(opening limit to suit motor Full load current) is given.
- Immediately after transmitting the FD fan inlet damper (4) open
command, to ensure loading of FD fan, a command is given to
open and hold, full load control valve to 40% within 5 to 6 sec to
avoid heavy drops in Outlet steam pressure followed by 100%
close command to low load control Valve for quick transfer of F.L.
FCV in FD mode of operation.

- The trip transfer is completed once the following conditions are
achieved
> FD fan running (1);
> Diverter damper closed towards HRSG (5)
> FD outlet damper full open (3)
> FD inlet damper opened ( 25%) (4)
> Left and Right burners ON (6)
> (FD air flow >xx t/hr)
- After the trip transfer completion, the "Top transfer Exists" signal is
reset. Also the "hold" to Full load control valve position, the 'force auto'
in steam pressure control, and diverter damper Close command latches
are released. Thus further opening of full load control valve, can be done
Thro' auto/manual control to suit the load demand
- If the "Trip Transfer completed" is not achieved within after the pre-
determined secs. (Maximum allowable transfer time). From GT tripped,
then the "trip transfer" is treated as unsuccessful/failed. Alarm and
indication are given in DCS as Trip transfer failed.
- Once the "Trip transfer failed, command is given to Trip the HRSG and
also to stop the FD fan. By Switched OFF the Auto trip transfer select-
ion, Fresh FD mode of operation can be started Necessary.

- The FD fan outlet gate is kept all the time open. It will be kept
closed while doing any maintenance. On its upstream equipments
like FD fan, it's l/L and 0/L dampers etc when the GT is running.



Successful completion of GT trip transfer from GT mode to FD mode is achieved
after 50th second at best.

Successful completion of GT trip transfer from GT mode to FD is achieved after
30TH second.

WE CLAIM:
1. A process of integrated operation of a Heat recovery steam generator
(HRSG) with smooth change over from gas turbine (GT) mode to fresh
air firing (FD) mode in combined cycle and Co-generation systems
comprising the steps of :
switching on the select for switch connected to the DCS when the gas
turbine trip transfer action is to be initiated, the accessories of the FD
fan and those of the HRSG being restored in desired positions wherein
the GT is tripped;
performing the trip transfer activities;
checking the boiler (HRSG) interlocking before starting the FD-fan, the
boiler interlocking including closing the inlet and outlet dampers of the
(HRSG);

transmitting command to open the outlet damper upon the FD
fan attains at least 80% of its full capacity;
- simultaneously transmitting signal to the HRSG to close the diverter to
allow airflow towards the HRSG;
- commanding opening of the FD Fan inlet damper upon attaining the FD
Fan outlet damper 100% of its full speed; and
- sending an "open and hold" signal to the full load control valve of the
HRSG upto at least 40% to avoid high drop in outlet steam pressure
followed by a 100% close command to the low load control valve for
transfer of the full load control valve to FD mode of operation.
2. The process as claimed in claim 1, wherein the accessories for the FD
fan and the HRSG being FD fan inlet damper, FD fan outlet damper
(HRSG) diverter damper, FD fan outlet gate, left and right burners.

3. The process as claimed in claim 1 wherein the trip transfer step is
completed when FD fan running, diverter damper closed towards
HRSG, FD outlet damper full open, FD inlet damper opened, left and
right burners in ON state.

ABSTRACT

The invention relates to a process of integrated operation of a Heat recovery
steam generator (HRSG) with smooth change over from gas turbine (GT) mode
to fresh air firing (FD) mode in combined cycle and Co-generation systems
comprising the steps of switching on the select for switch connected to the DCS
when the gas turbine trip transfer action is to be initiated, the accessories of the
FD fan and those of the HRSG being restored in desired positions wherein the GT
is tripped; performing the trip transfer activities; checking the boiler (HRSG)
interlocking before starting the FD-fan, the boiler interlocking including closing
the inlet and outlet dampers of the ( HRSG); transmitting command to open the
outlet damper upon the FD fan attains at least 80% of its full capacity;
simultaneously transmitting signal to the HRSG to close the diverter to allow
airflow towards the HRSG; commanding opening of the FD Fan inlet damper
upon attaining the FD Fan outlet damper 100% of its full speed; and sending an
"open and hold" signal to the full load control valve of the HRSG upto at least
40% to avoid high drop in outlet steam pressure followed by a 100% close
command to the low load control valve for transfer of the full load control valve
to FD mode of operation.