FIELD OF INVENTION
The present invention generally relates to supplementary fired heat recovery
steam generators (HRSGs). More particularly, the invention relates to a method
for protection of super heater tubes in supplementary fired heat recovery steam
generators (HRSG).
BACKGROUND OF THE INVENTION
Heat Recovery Steam Generator (HRSG) is one type of boiler where the heat
from the hot waste gas, mostly the Gas Turbine (GT) exhaust is recovered for
steam generation. In HRSGs, whenever the load requirement becomes more
than that of Gas turbine exhaust input, a supplementary firing is introduced. The
fuel may be fuel gas or oil which is fired in Duct Burners. In Supplementary Fired
Heat Recovery Steam Generators, the typical configuration locates the Duct
Burner in the Gas Turbine exhaust path, upstream of the Heat Recovery Steam
Generator. Downstream of the Duct burner, the heat transfer sections are
located, with the Super heater section immediately following the Duct burner
section. The Supplementary firing fuel flow (gas/oil) varies in accordance with
the load demand. This is achieved using a plurality of Fuel Flow control valves.
The Command to the control valves is derived from the Main Steam Outlet
Pressure value. This is called Steam pressure control system and provided in the
Distributed Control System (DCS).

During normal operation within the maximum continuous rating (MCR)
parameters, not much problem is faced due to over firing in the Duct Burners
leading to unsafe operation including tube failure. In contrast thereto, as a
general practice duct burner operations at partial GT load are not allowed since
the burner under partial GT load operates in the off - design regime. However,
requirements may arise when the duct burner needs to be operated at partial
GT-load. In this case, the burner is provided with a not-to-exceed heat output
value that would permit a safe flame length/temperature distribution for the
minimal GT load under consideration. However, the device features need be
configured to ensure HRSG safety and reliability based on the tube metal
temperature, attemperator capacity, burner turndown, steam flow distribution,
etc.
Though the Fuel flow is decided based on the Steam demand, a check has also
to be built in the control function so that the Fuel flow is not increased beyond a
particular level so as to protect the immediately following Super Heater tube
sections. This has to be carried out even if the boiler demand keeps increasing.
This is called protection of Super Heater tubes.
In a typical HRSG control function, the additional requirement of Super Heater
tube protection is implemented based on the Temperature measurements at
various points viz. the Flue Gas Temperature at the outlet of the duct Burner and
is based the Metal Temperature measurements at the Super Heater tube skin.
The exact location of the Metal Temperature measurement is identified using the
flow pattern, the Heat transfer capability and the rate of heat transfer.

Event though the metal temperature measurements give the exact protection
criteria for the Control function, the difficulty may arises if the metal temperature
measurement fails, and ultimately the Flue Gas temperature is measured leading
to restriction of Fuel flow in the Duct Burners affecting the overall performance
of the boiler.
Another difficulty is faced in mounting the Metal Temperature measurement
instruments in case of closer modules in bigger size HRSGs where the access is
limited, and where the possibility of the probe lead getting disturbed is high. This
will lead to the non-availability of the critical measurement which results in non
protection of the super heater tubes leading to tube failure, and ultimately a shut
down of the Burner system or the HRSG itself.
US 6453852 (Bl) discloses a heat recovery system which includes an upstream
superheater section and a duct burner arranged to provide improved
performance. The upstream superheater section and duct burner are arranged to
reduce the negative effects of heat extraction by the upstream superheater
section on the duct burner operation. The arrangement provides a downstream
flow from the upstream SH section that is temperature stratified and positions
the duct burner elements in the areas of maximum temperature. The tubes of
the upstream superheater section and the duct burner elements are arranged in
a variety of ways to provide temperature stratification, for example, in the
horizontal or vertical directions.

US 4578944 (A) describes a heat recovery steam generator outlet temperature
control system for a combined cycle power plant. In the plant, an inlet
temperature is adjusted or controlled while concurrently modulating the IGV's
and the load of the gas turbine and the attemperator of the boiler, and
concurrent modulation of the IGV's, the gas turbine load, the attemperator is
effected as upon three independent control variables to adjust and/or maintain
the boiler outlet temperature independently of steam turbine operation and to
maximize gas turbine load control range. Control of the output temperature of
one boiler is also managed in relation with the mixed temperature of steam from
other boilers fluidly connected to the steam turbines.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a method for
protection of super heater tubes in supplementary fired heat recovery steam
generator (HRSG), which eliminates the disadvantages of prior art.
Another object of the present invention is to propose a method for protection of
super heater tubes in supplementary fired heat recovery steam generator
(HRSG), in which the super heater tubes are protected against the over heating
of the tubes due to over firing of the burners.
A still another object of the invention is to propose a method for protection of
super heater tubes in supplementary fired heat recovery steam generator

(HRSG), which enables control of the fuel input to burner by modulating the fuel
flow control valve based on the steam demand and the process temperatures to
protect the super heater tubes.
Yet another object of the invention is to propose a method for protection of
super heater tubes in supplementary fired heat recovery steam generator
(HRSG), which enables protection of the super heater tubes during part load
operation of the GT.
SUMMARY OF INVENTION
According to the invention, the Duct Burner firing rate is decided based on the
steam demand. However, during part loads of Gas Turbines, an out of regime
firing is required. Under this condition, a possibility of over firing in the HRSG
arises which in turn produces flames of higher length, and sometimes the flames
touches the super heater (SH) tubes, and if SH modules are arranged
immediately following the Duct burner or the heat is more than permissible it
affects the metallurgy of the SH tubes or differential heating leading to tube
failure. Hence a system is incorporated to control the flow, based on the steam
temperatures at Desuperheater (DESH) outlet, Main steam temperature, and the
Flue Gas temperature at the Duct Burner outlet, which limit the firing in the Duct
Burner thus protecting the Super heater tube section immediately following the
Duct Burner section, ie., the flow increase is inhibitated in case of abnormal Flue

Gas / DESH inlet / Main steam temperatures to safeguard the Super heater
tubes.
According to the invention, an increase in steam temperature indicates more
heat pickup in the SH tubes, and spraying of water in the DESH reduces the
temperature to the safe limits.
During increased steam demand, the Fuel Flow control valve is commanded to
increase the fuel flow to cater to the new steam requirement. At this juncture a
control is implemented to determine if the HRSG flue temperature after the duct
burner, main steam temperature and DESH inlet temperature are within the
prescribed limits.
If any of the above temperatures increases than the set values, the firing rate
control shall be reduced by 10% and is stay put and the control is changed over
to manual.
Thus the Super Heater tubes are protected by limiting the Fuel Flow to the Duct
Burner whenever the flue gas temperature is more than the set value and also
whenever the main steam temperature and DESH inlet temperature are more
than the set value.
For this reason, the duct burner firing rate is limited to 5... 10% increase in heat
input per minute, which avoids the negative impact on the HP SHTR and the
Drum. This ramp rate minimizes any negative effects and allows a safe and

reliable operations of the HRSG. Also manual increase of Flow under the
conditions of abnormal increase of temperature at these points, is inhibited.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 is a schematic diagram of the Steam Pressure Control System.
DETAILED DESCRIPTION OF THE INVENTION
As shown in figure-1, main Steam temperature (1) is measured and compared
with a set point (7). The data respecting to the fuel air flow and fuel air gas flow
(3) are also measured and transmitted to the respective controllers (8,9).
Whenever one of the flue gas temperature at the duct burner outlet (6),
desuperheater inlet temperature (5), and main steam temperature exceeds their
respective set points, the fuel air gas flow control valves (10,11) are closed and
an implementation of the inverse ramp (12) is introduced and continued, till the
said temperature reach a safe limit. During this inverse ramp operation which is
adjustable, any demand for "Fuel flow increase" is inhibited (Set aside). During
this inverse ramp operation, manual intervention is inhibited so that the FCV
cannot be opened manually. However, the auto manual station is transferred to
manual mode.
A positive ramp limiter (Adjustable 0-5% per minute) provided at the final control
signal transmitted to the Flow Control Valve ensures a predefined rate of
increase of the firing.

Thus, where the load requirement becomes more than that generated from the
GT exhaust input, a supplementary firing is introduced. Supplementary firing is
started only when the diverter is fully (100%) opened towards HRSG.
The fuel demand signal is generated in the Master pressure controller based on
the deviation between the preset and the prevailing HRSG main steam pressure.
The fuel demand is given to a fuel flow controller.
The HRSG flue temperature after the duct burner, main steam temperature and
DESHi/l temperature are checked periodically, particularly when the load on
HRSG is raised. In case the temperatures exceed the limit value, the firing rate
control is reduced by 10% and a stay put command is initiated, and the control
is changed over to manual.
Reasons for increase in the temperature is checked before putting back the firing
system on auto. In any case, the firing rate is restricted such that the
temperatures are within acceptable limit.
GT. Load is reduced to the required level (as envisaged in the operating
envelope) after reaching the required steam generation at full load but not
exceeding MCR steam generation, with the GT exhaust temperature being
maintained above steam outlet temperature. During this period of GT load
reduction, the supplementary firing is kept constant and is not increased, even if
the steam output decreases. Firing rate may be increased gradually to meet the
steam demand (supplementary heat input restricted to the specified level

depending upon the GT load / steam output) ensuring that the temperatures do
not exceed the set values. The steam generation is not allowed to go below the
recommended value at any point during the Supplementary firing Mode
operation.
The duct burner firing rate is limited to 5... 10% increase in heat input per
minute, in order to avoid the negative impact on the high pressure Superheater,
and the Drum. This ramp rate minimizes any negative effects and allows a safe
and reliable operation of the HRSG.
The process as described hereinabove is implemented by adapting a Steam
Pressure Control logic, which operates the Fuel Flow Control valve, whenever the
flue gas temperature at Duct burner outlet or De-Super Heater (DESH) inlet
temperature or Main steam temperature exceeds their respective set points, and
the Fuel Flow CV (FCV) is closed continuously following an inverse ramp
(adjustable) till the above temperatures reach the safe limits. During this inverse
ramp operation, any demand for "Fuel flow increase" is inhibited (Set aside).
During this inverse ramp operation, any manual intervention is inhibited so that
the FCV cannot be opened manually. However, the auto manual station is
transferred to manual.
A positive ramp limiter (Adjustable 0-5% per minute) at the final control signal
transmitted to the Flow Control Valve ensures a predefined rate of increase of
the firing.
WE CLAIM
1. A method for protection of super heater tubes in supplementary fired heat
recovery steam generator (HRSG) in steam pressure control system), the
method comprising the steps of:
(i) measuring the on line main steam temperature (1) and comparing the
measured temperature with a pre-set value;
(ii) measuring the fuel air flow and fuel air gas flow (3) corresponding to
the steam temperature (1) and transmitting the measured values to
respective controllers (8,9);
(iii) determining the fuel gas temperature at the duct burner outlet, or, the
temperature at inlet of the desuperheater, or the main steam
temperature;
(iv) comparing the temperatures determined in step (iii) with the
respective temperatures preset in the system;
(v) closing the fuel air gas flow control values (10,11) in case any of the
temperature values determined at step (iv) exceeds the preset values;
and
(vi) implementing an inverse ramp (12) and continuing the inverse
ramping till the value of the exceeded temperature reduced to the safe
limit.
2. The method as claimed in claim 1, comprising the step of transmitting a
control signal to the flow control valves (10, 11) incorporating a positive
ramp limiter to allow a predefined rate of increase in the firing.
3. A method for protection of super heater tubes in supplementary fired heat
recovery steam generator (HRSG) in steam pressure control system) as
substantially described and illustrated herein with reference to the
accompanying drawings.

The invention relates to a method for protection of super heater tubes in
supplementary fired heat recovery steam generator (HRSG) in steam pressure
control system), the method comprising the steps of (i) measuring the on line
main steam temperature (1) and comparing the measured temperature with a
pre-set value; (ii) measuring the fuel air flow and fuel air gas flow (3)
corresponding to the steam temperature (1) and transmitting the measured
values to respective controllers (8,9); (iii) determining the fuel gas temperature
at the duct burner outlet, or, the temperature at inlet of the desuperheater, or
the main steam temperature; (iv) comparing the temperatures determined in
step (iii) with the respective temperatures preset in the system;(v) closing the
fuel air gas flow control values (10,11) in case any of the temperature values
determined at step (iv) exceeds the preset values; and (vi) implementing an
inverse ramp (12) and continuing the inverse ramping till the value of the
exceeded temperature reduced to the safe limit.