FIELD OF THE INVENTION
The present invention generally relates to a steam generator meant for
generating steam at a temperature suitable to the downstream equipment or the
process requirement. This invention particularly relates to a steam generating
system utilizing water in a desuperheater as the means for steam temperature
maintenance. More particularly, the present invention relates to a steam
temperature maintenance apparatus in steam generators.
BACKGROUND OF THE INVENTION
Steam generators are configured with a plurality of heating tubes for absorbing
the heat from combustion of variety of fuels for converting cold water into hot
water including water vapour corresponding to some defined input and output
conditions. Whenever there are deviations from these input conditions and
output requirements, the parameters of hot water and hot vapour also deviate
from the designed values.
To maintain the properties of the output medium within an usable range, a
mechanical apparatus interfaced to a computerized operating system is provided.
One such apparatus is used to maintain the vapour temperature within the
usable range by sprinkling and mixing low temperature water from upstream of
the steam generator with the steam.

As shown in figure-1, the prior art method of steam temperature maintenance
comprises sprinkling of water at a constant and lower temperature (higher
degree of subcooling) extracted from a feed water manifold (2) which is placed
at the exit of a water pressurizing device (1), but at the upstream of a feed
water pressure balancing device (3). The feed water manifold (2) suitably
distributes the water flow between a feed water pipe (4) supplying water to an
economiser (5) and a sprinkling water line (16). The water from an outlet pipe
(6) of the economiser in totality enters a steam drum (8) and the wet steam
from an (5) evaporator system (9) is separated in the steam drum (8). The dry
steam flows through a saturated steam pipes (10) and enters a primary
superheating circuit (11) and the low temperature water is sprinkled and mixed
with the steam from the primary superheater (11) before entering a secondary
superheater (13). A sprinkling water pressure balancing device (17) regulates the
water flow rate through the sprinkling water line (16) to a steam temperature
maintenance apparatus (12) so as to maintain the steam temperature at a
process manifold (15).
Thus, the prior art steam temperature control systems utilizes the feed water
immediately downstream of the feed pump for mixing with the steam to
reduce/maintain the steam temperature within the desired range.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a steam temperature
maintenance apparatus in a heat recovery steam generator (HRSG), which
eliminates the disadvantage of prior art.

Another object of the invention is to propose a steam temperature maintenance
apparatus in a heat recovery steam generator (HRSG), which is enabled to utilize
water of variable degree of subcooling.
A still another object of the invention is to propose a steam temperature
maintenance apparatus in a heat recovery steam generator (HRSG), which can
be adapted in steam generators other than HRSG.
Yet another object of the invention is to propose a steam temperature
maintenance apparatus in a heat recovery steam generator (HRSG),which
employs a feed water pressure balancing device to make selection between low
or high or intermediate degree of subcooling based on the system requirement.
A further object of the invention is to propose a steam temperature maintenance
apparatus in a heat recovery steam generator (HRSG), which can be deployed at
the outlet of the superheater or at one or more locations between the portions of
the superheater.
A still further object of the invention is to propose a steam temperature
maintenance apparatus in a heat recovery steam generator (HRSG), which is
enabled to tap and utilize water of low and variable degree of subcooling from
several locations of the economizer.
SUMMARY OF THE INVENTION
The hot exhaust gas line from the gas turbine is connected to the Heat Recovery
Steam Generator (HRSG) system for absorbing the heat available in the flue gas.

Generally the first section to receive the hot gas in the HRSG is the super heater
system. The super heater system may comprise one or more superheaters
formed of a plurality of tubes; a de super heater system consisting of a single or
plurality de super heaters; one or more evaporators; and one or more water
heating sections. Further, one or more stages of fuel burning equipment(s) in the
gas path may be provided to raise the output from the HRSG.
The gas turbine exhaust parameters are subjected to several variations due to
ambient and operating conditions and hence the absorption in individual sections
also vary at different operating conditions. Particularly in the superheater, a
higher heat absorption leads to higher than required steam temperatures and is
not desirable. Hence a method of maintaining the steam temperature within an
acceptable range is necessary.
In accordance with the present invention, a temperature maintenance apparatus
enabled to utilize water at varying degree of subcooling from outlet or
intermediate locations in the economiser in the Heat recovery steam generator
(HRSG) is provided. The apparatus is adaptable for operation of the super heater
system for regulating the outlet steam temperature in an acceptable range
resulting in operational improvements for example, a lower flue gas temperature
in an acceptable range resulting in operational improvements for example, a
lower flue gas temperature at the HRSG outlet a lower fuel consumption in the
HRSG and an enhanced controllability of the Steam temperature.
The apparatus employs super heaters for steam heating and allows the
temperature maintenance apparatus to always consume significant quantity of
spray water in the normal operating condition.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure-1 is a schematic view of a steam temperature maintenance apparatus as
per prior art.
Figure-2 is a schematic view of a steam temperature maintenance apparatus
enabled to utilize water of lowest degree of subcooling configured in accordance
with a first embodiment of the present invention.
Figure-3 is a schematic view of a steam temperature maintenance apparatus
enabled to utilize water of lowest degree of subcooling configured in accordance
with a second embodiment of the present invention.
Figure-4 is a schematic view of a steam temperature maintenance apparatus
enabled to utilize water of variable degree of subcooling configured in
accordance with a third embodiment of the present invention.
Figure-5 is a schematic view of a steam temperature maintenance apparatus
enabled to utilize water of variable degree of subcooling configured in
accordance with a fourth embodiment of the present invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION.
Figure 2 shows a first embodiment of the inventive apparatus. The apparatus
comprises a feed water system having a feed water pressurizing device (1)
including a feed water pressure balancing device (3) which regulates the flow
through a feed water pipe (4) and the economiser (5). The heated water from

the economizer (5) flows to a hot water manifold (7) from where a sprinkling
water line (16) is tapped off, and is utilized in a temperature maintenance
apparatus (12) after flowing through a sprinkling water pressure balancing
device (17) located downstream in a sprinkling water line (16). The remaining
water from the hot water manifold (7) flows into a steam drum (8) which
supplies water to and receives wet steam from an evaporator (9). The dry
vapour from the drum (8) passes through a primary superheater (11) and then
through a secondary superheater (13). The heated vapour is delivered to a
process manifold (15).
what is required, the sprinkling water pressure balancing device (17) is further
opened up till the temperature at the process manifold (15) is brought down to
the acceptable level. On the contrary, if the temperature of the vapour delivered
at the process manifold (15) is less than what is required, the sprinkling water
pressure balancing device (17) is gradually closed down till the temperature at
the process manifold (15) is reduced to the acceptable level.
Figure 3 shows the second embodiment of the inventive apparatus, where the
feed water pressure balancing device (3) is located between the hot water
manifold (7) and the drum (8). In this arrangement, since the water pressure
available is more compared to the arrangement shown in Figure 2, the size of
the sprinkling water pressure balancing device (17) can be reduced.
In both the above embodiments illustrated in Figure 2 & Figure 3, the degree of
subcooling (degrees below saturation temperature) is minimum as the water

from the economiser outlet is used. Due to water extraction from the economiser
outlet, irrespective of the extent of sprinkling water flow rate the water flow
through the economiser (5) remains unchanged. This results in lower fuel firing
rate for the same amount of steam output as compared to the arrangement as
per prior art illustrated in Figure-1.
Whenever, water of low degree of subcooling is used as the sprinkling water, the
water flow rate required is more and vice versa. Hence, in different operating
environments, the source of water for sprinkling can be selected from various
locations of the economizer for example, outlet, inlet or intermediate locations as
shown in Figure-4 & Figure-5.
The feed water pressure balancing device (3) can be placed upstream of the
economiser (5) and at a location between the outlet of the economiser (5) and
the drum (8) as shown in Figure-3 & Figure-5.
According to the invention, the apparatus uses water of lowest degree of sub
cooling which magnifies the sprinkling water flow requirement including a small
reduction in vapour temperature. Further, the innovative arrangement results in
lower steam pressure drop in the superheater, a reduction in the outlet
temperature of water at economiser outlet, and lessening the possibility of
steaming in the economizer (5).

By varying the degree of sub cooling of water from lowest level to the highest
level a single sprinkling water pressure balancing device (17) can be used for
extended range of vapour temperature maintenance.
The superheaters (11,13) may comprise one or more rows of tubular coils of
bare or finned or combination of bare and finned tubes. At least one temperature
maintenance apparatus is provided within a flow circuit based on functional
requirement, and may be used in a single pressure, dual pressure or triple
pressure steam generator and HRSG system. The temperature maintenance
apparatus (12) may be used in the reheater of a HRSG and in a steam generator
system including one or more interstages and/or the outlet of superheater
and/or reheater.

WE CLAIM :
1. A steam temperature maintenance apparatus (12) enabled to utilize
water with a low degree of sub cooling from the outlet of an
economiser (5) in a heat recovery steam generating (HRSG) system,
regulating temperature of the vapour medium at the outlet of a
process manifold (15), the apparatus comprising :
a sprinkling water manifold (7) receiving water at low degree of sub
cooling from outlet of the economiser (5) and distributing to a steam
(8) drum and to the apparatus, the manifold (7) comprising one or
more incoming links originating from one of the outlet, inlet and
intermediate location from the economiser (5); and
a sprinkling water pressure balancing device (17) capable to handle
water at low degree of sub cooling or water at saturated state or wet
steam with a fraction of vapour content, the device (17) being
disposed upstream of the economiser (5).
2. The apparatus as claimed in claim 1, comprising a feed water pressure
balancing device (3) disposed upstream of the economizer (5) at a
location between the outlet of the economizer (5) and the steam drum
(8).

3. A steam temperature maintenance apparatus (12) enabled to utilize
water with a low degree of sub cooling from the outlet of an
economiser (5) in a heat recovery steam generating (HRSG) system,
as substantially described and illustrated herein with reference to the
accompanying drawings.

The hot exhaust gas line from the gas turbine is connected to the Heat Recovery
Steam Generator (HRSG) system for absorbing the heat available in the flue gas.
Generally the first section to receive the hot gas in the HRSG is the super heater
system. The super heater system may comprise one or more superheaters
formed of a plurality of tubes; a de super heater system consisting of a single or
plurality de super heaters; one or more evaporators; and one or more water
heating sections. Further, one or more stages of fuel burning equipment(s) in the
gas path may be provided to raise the output from the HRSG.
The gas turbine exhaust parameters are subjected to several variations due to
ambient and operating conditions and hence the absorption in individual sections
also vary at different operating conditions. Particularly in the superheater, a
higher heat absorption leads to higher than required steam temperatures and is
not desirable. Hence a method of maintaining the steam temperature within an
acceptable range is necessary.
In accordance with the present invention, a temperature maintenance apparatus
enabled to utilize water at varying degree of subcooling from outlet or
intermediate locations in the economiser in the Heat recovery steam generator
(HRSG) is provided. The apparatus is adaptable for operation of the super heater
system for regulating the outlet steam temperature in an acceptable range
resulting in operational improvements for example, a lower flue gas temperature
in an acceptable range resulting in operational improvements for example, a
lower flue gas temperature at the HRSG outlet a lower fuel consumption in the
HRSG and an enhanced controllability of the Steam temperature.
The apparatus employs super heaters for steam heating and allows the
temperature maintenance apparatus to always consume significant quantity of
spray water in the normal operating condition.