U.S. patent application number 13/021457 was filed with the patent office on 2012-08-09 for air cooling system and method for a heat recovery steam generator inlet.
Invention is credited to Bryan F. Craig, AMY L. SIEBEN.
Application Number | 20120198846 13/021457 |
Document ID | / |
Family ID | 46599725 |
Filed Date | 2012-08-09 |
United States Patent
Application |
20120198846 |
Kind Code |
A1 |
SIEBEN; AMY L. ; et
al. |
August 9, 2012 |
AIR COOLING SYSTEM AND METHOD FOR A HEAT RECOVERY STEAM GENERATOR
INLET
Abstract
The air cooling system and method for a heat recovery steam
generator (HRSG) inlet provides a combined cycle power plant
utilizing a powerful fan coupled to ductwork connected to pipes
that enter the HRSG inlet duct coupled to an exhaust duct of a
Combustion Turbine (CT) for lowering the temperature of the CT
exhaust gas provided to the heat recovery steam generator by the
CT. The cool air injection system is utilized during low load
operation or startup of the CT to ensure that spray water from an
inter-stage desuperheater in an HRSG is fully evaporated prior to
entering the downstream superheater or reheater. A feedback system
includes temperature elements measuring the mix temperature that
regulates the cooling air injection rate into the HRSG inlet.
Inventors: |
SIEBEN; AMY L.; (Eden
Prairie, MN) ; Craig; Bryan F.; (Tampa, FL) |
Family ID: |
46599725 |
Appl. No.: |
13/021457 |
Filed: |
February 4, 2011 |
Current U.S.
Class: |
60/653 ;
60/666 |
Current CPC
Class: |
Y02E 20/16 20130101;
F22B 35/007 20130101; F01K 23/101 20130101; F22B 1/1815 20130101;
F01K 23/10 20130101; F22G 5/12 20130101 |
Class at
Publication: |
60/653 ;
60/666 |
International
Class: |
F01K 7/34 20060101
F01K007/34; F01K 7/42 20060101 F01K007/42 |
Claims
1. An air cooling method for a heat recovery steam generator (HRSG)
having a chamber containing inter-stage attemperators in a
desuperheater and a turbine engine exhaust HRSG inlet duct
accepting flow of exhaust gas from a heat source, the method
comprising the steps of: beginning a first cooling air flow
sequence when output from the heat source is greater than 20% load
in MW and the heat source has an increasing rate of change, the
first air cooling sequence injecting the cooling air flow into the
heat source exhaust or the HRSG inlet duct; and terminating the
first cooling air flow sequence when the heat source output is
greater than 80% load in MW or the heat source exhaust inlet duct
temperature is less than the steam set point plus 50.degree. F.
2. The air cooling method according to claim 1, further comprising
the step of beginning a second cooling air flow sequence when the
heat source output in megawatts is less than 80% and the inlet duct
temperature is greater than the steam set point plus 50.degree. F.,
the second air cooling sequence injecting the cooling air flow into
the heat source exhaust or the HRSG inlet duct.
3. The air cooling method according to claim 2, further comprising
the step of initiating a changeover from the first cooling sequence
to the second cooling sequence when the heat source output remains
below 80% and the HRSG inlet duct temperature goes above the steam
set point plus 50.degree. F. at any time during the first
sequence.
4. The air cooling method according to claim 3, further comprising
the steps of: transferring a lead temperature control to cooling
air after a predetermined time delay; limiting the minimum speed of
a source of the cooling air flow; and controlling a high-pressure
superheater temperature of the HRSG responsive to temperature
readings inside the HRSG inlet duct.
5. The air cooling method according to claim 4, further comprising
the step of automatic changeover from the second sequence to
desuperheater cooling water when the heat source output goes above
80% or the HRSG duct temperature goes below the steam set point
plus 50.degree. F. during the second sequence.
6. The air cooling method according to claim 5, further comprising
the steps of: isolating a transit path of the cooling air flow from
entering the HRSG inlet duct; preventing backflow of exhaust gases
from the heat source; and turning off the cooling air flow source
after a predetermined time delay.
7. The air cooling method according to claim 6, further comprising
the step of varying a rate of the cooling air flow based on
feedback parameters from the HRSG.
8. An air cooling system for a heat recovery steam generator
(HRSG), the HRSG having a chamber containing inter-stage
attemperators in a desuperheater and a heat source exhaust to HRSG
inlet duct accepting flow of exhaust gas from a heat source, the
system comprising: means for beginning a first cooling air flow
sequence when the heat source output in megawatts is greater than
20% and the heat source has an increasing rate of change, the first
air cooling sequence injecting the cooling air flow into the heat
source exhaust to HRSG inlet duct; and means for terminating the
first cooling air flow sequence when the heat source output is
greater than 80% or the turbine engine exhaust inlet duct
temperature is less than the steam set point plus 50.degree. F.
9. The air cooling system according to claim 8, further comprising
means for beginning a second cooling air flow sequence when the
heat source output is less than 80% MW and the inlet duct
temperature is greater than the steam set point plus 50.degree. F.,
the second air cooling sequence injecting the cooling air flow into
the heat source exhaust or the HRSG inlet duct.
10. The air cooling system for a heat recovery steam generator
inlet according to claim 9, further comprising means for initiating
a changeover from the first cooling sequence to the second cooling
sequence when the heat source output remains below 80% and the heat
source exhaust to HRSG inlet duct temperature goes above the steam
set point plus 50.degree. F. at any time during the first
sequence.
11. The air cooling system according to claim 10, further
comprising: means for transferring a lead temperature control to
cooling air after a predetermined time delay; means for limiting
the minimum speed of a source of the cooling air flow; and means
for controlling a high-pressure superheater temperature of the HRSG
responsive to temperature readings inside the heat source exhaust
to HRSG inlet duct.
12. The air cooling system according to claim 11, further
comprising means for automatic changeover from the second sequence
to desuperheater cooling water when the heat source output goes
above 80% or said HRSG duct temperature going below the steam set
point plus 50.degree. F. during the second sequence.
13. The air cooling system according to claim 12, further
comprising: means for isolating a transit path of the cooling air
flow from entering the HRSG inlet duct; means for preventing
backflow of exhaust gases from the heat source; and means for
turning off the cooling air flow source after a predetermined time
delay.
14. The air cooling system according to claim 13, further
comprising means for varying a rate of the cooling air flow based
on feedback parameters from the HRSG.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to cooling systems
for steam generators used in power plants and the like, and
particularly to an air cooling system and method for a heat
recovery steam generator inlet that provides temperature reduction
control for a heat recovery steam generator (HRSG) air inlet to
mitigate overspray during an inter-stage cooling procedure
performed in a desuperheater.
[0003] 2. Description of the Related Art
[0004] Combined cycle and cogeneration are processes that use the
exhaust heat discharged from a Combustion Turbine (CT) to provide
heat to a boiler, which, in turn, makes steam to power a Steam
Turbine (ST). A heat recovery steam generator (HRSG) is a boiler
that has an inlet coupled to an exhaust of a CT engine or other
heat source and typically includes an inter-stage
desuperheater.
[0005] Desuperheater overspray is a term used to describe a
condition where spray water from an inter-stage desuperheater in an
HRSG is not fully evaporated prior to entering the downstream
superheater or reheater and associated piping.
[0006] Water quenching from desuperheater overspray has caused a
number of superheater and reheater tube failures in HRSGs. Piping
damage has also occurred at some plants, due to un-evaporated spray
water quenching the pipe wall.
[0007] This problem is particularly prevalent in HRSGs behind large
CTs where exhaust gas temperature reaches a peak of 1200.degree. F.
when operating at low load or during startup. This operating
condition is challenging from a superheater or desuperheater design
perspective, as it results in a combination of low steam flow and
high spray water flow, which yields a high likelihood of
desuperheater overspray. While many systems use water injection to
cool the turbine exhaust, such cooling results in thermal fatigue
damage (quench) to non-pressure parts in the inlet duct, wastes
expensive treated boiler water that cannot be recovered, and is
insufficient to accomplish cooling of the turbine exhaust gas
without additional overspray in the desuperheaters.
[0008] Thus, an air cooling system and method for a heat recovery
steam generator (HRSG) inlet solving the aforementioned problems is
desired.
SUMMARY OF THE INVENTION
[0009] The air cooling system and method for a heat recovery steam
generator inlet includes a powerful fan coupled to ductwork
connected to pipes or ducts that enter the exhaust duct of a
Combustion Turbine (CT) for lowering the temperature of the CT
exhaust gas provided to the heat recovery steam generator by the
CT. The cool air injection system is utilized during low load
operation of the CT to ensure that spray water from an inter-stage
desuperheater in the HRSG is minimized so that it is not calling
for spraying to within 50.degree. F. of saturation prior to
entering the downstream superheater or reheater. This eliminates
the damaging thermal fatigue cycle in the downstream pressure
retaining components of the HRSG.
[0010] A plant's existing control system includes a desuperheater
cooling water flow control valve, and a HP SH (high-pressure
superheater), as well as a RH (reheat) steam outlet temperature
element that regulates each via a feedback loop. The cooling air
injection rate into the HRSG inlet is controlled by a variable
frequency drive taking its set point from the mixed gas temperature
measurement near the tube face. The existing control logic is not
altered, and the novel air cooling system is added as a completely
independent `plug and play` system. Minor additions to the plant's
control system are implemented for control of this system. Many
plants spend significant time operating at minimum CT load. This
minimum load operation results in very high exhaust temperatures,
which may also cause overspray in the HRSG's desuperheater. Plants
that suffer desuperheater overspray problems or thermal fatigue
damage of non-pressure parts (such as gas baffles, liner systems,
and tube ties) in the inlet duct from the CT to the HRSG benefit
from the system and method disclosed herein.
[0011] The system and method disclosed herein reduce the peak gas
temperature, thereby protecting the plant equipment. The system and
method has successfully mitigated overspray in a GE 7FA turbine
combined cycle system, where unchecked exhaust temperature reaches
a peak of 1200.degree. F. at low flow. The system and method are
theorized to work with any constant temperature CT or other direct
fired heat source.
[0012] These and other features of the present invention will
become readily apparent upon further review of the following
specification and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a perspective view showing a general arrangement
of an air cooling system for an HRSG inlet according to the present
invention.
[0014] FIGS. 2A, 2B, and 2C show a schematic diagram of piping and
instruments for a component control circuit of the air cooling
system for an HRSG inlet according to the present invention.
[0015] FIGS. 3A and 3B show a block diagram of the control circuit
of the air cooling system for an HRSG inlet according to the
present invention.
[0016] FIGS. 4A, 4B and 4C show a schematic diagram showing logic
flow of a digital electronic timing and control circuit of the air
cooling system for an HRSG inlet according to the present
invention.
[0017] FIGS. 5A, 5B and 5C show a schematic diagram showing logic
flow for seal air control electronic circuitry of the air cooling
system for an HRSG inlet according to the present invention.
[0018] FIG. 6 is a block diagram showing configuration of a
simplified HRSG and the inter-stage desuperheater.
[0019] Similar reference characters denote corresponding features
consistently throughout the attached drawings.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] Desuperheater overspray is a term used to describe a
condition where spray water from an inter-stage desuperheater in a
heat recovery steam generator (HRSG) is not fully evaporated prior
to entering the downstream superheater or reheater.
[0021] Water quenching from desuperheater overspray has caused a
number of superheater and reheater tube failures in HRSGs. Piping
damage has also occurred at some plants, due to unevaporated spray
water quenching the pipe wall.
[0022] The problem is most likely to occur when turbine exhaust gas
temperature reaches a peak of 1200.degree. F. when operating at low
load or start up, a frequent and unavoidably encountered operating
condition. This operating point yields low steam flow combined with
high spray water flow, resulting in the desuperheater spraying to
saturation in order to maintain the outlet steam temperature.
[0023] Plants suffering desuperheater overspray problems have had
limited options with minimal success, such as making control
changes or equipment modifications to prevent tube and/or piping
failures from occurring. This is due to a fundamental mismatch in
the heat source and the heat recovery surface area during startup
or low load conditions.
[0024] Based on analysis performed by the present inventors,
desuperheater overspray can be prevented by reducing the turbine
exhaust gas temperature between 50 and 100.degree. F., depending on
the severity of the overspray. As shown in FIG. 1, the air cooling
system 100 includes a variable frequency drive (VFD) motor 127
driving a powerful fan 122 coupled to ductwork 119 extending into
an insulated duct 113 connected to pipes (or alternatively duct
work) 107 that extend to and couple (with the aid of mechanical
penetration seals 109) to an HRSG air inlet 105, the air inlet 105
being connectable to an exhaust duct of a CT. This arrangement
forces ambient air into inlet 105 for lowering the temperature of
the CT exhaust gas provided to the heat recovery steam generator by
the CT. The intake of the powerful blower 122 may also be connected
to a silencer 121 for quieter operations.
[0025] Due to expected cooling and heating cycles within the
ductwork, a plurality of expansion joints 115 may be provided to
interconnect sections of ductwork. To prevent backflow of CT
exhaust gases when the powerful blower 122 is off, a seal damper
117 is disposed between the airflow duct 119 and the insulated duct
113. The seal damper 117 raises and lowers on command from the
existing plant distributed control system. The seal air fans 111,
being connected to the damper 117 via a seal air pipe 113, provide
the necessary pressure to aid in prevention positive seal at the
damper. The air cooling system 100 is utilized during low load
operation, including startup of the CT to ensure that spray water
from an inter-stage desuperheater in the HRSG is fully evaporated
prior to entering the downstream superheater or reheater. An
existing control system includes a desuperheater cooling water flow
control valve, and an HP SH (high-pressure superheater), as well as
an RH (reheat) steam outlet temperature element that regulates each
through a feedback loop. The cooling air injection rate into the
HRSG inlet is controlled by a variable frequency drive taking its
set point from the mixed gas temperature measurement near the tube
face. The existing control logic is not altered, and this system is
added as a `plug and play` system completely independent. Minor
additions to the plant's distributed control system are implemented
for control of this system. The system 100 may be designed into a
new plant or incorporated into an existing plant as a `plug-n-play`
option.
[0026] As shown in FIGS. 4A-4C, the air cooling system involves
electronic control and timing circuit 400a, which initiates a first
start sequence when the CT output in megawatts (MW) is greater than
20% and the set point is increasing.
[0027] During the start sequence the manual or auto selectable
cooling air blower 122 starts, based on a signal sent by the
electronic control circuit 400a. Additionally, the electronic
control circuit 400a limits blower speed to 100%. Moreover, the
circuit 400a commands the isolation damper 117 to open after a
delay for the fan to spin up to minimum speed. As shown in FIGS.
5A-5C, a seal air pressure switch, timing and control circuit 400b
stops the seal air blower when the isolation damper 117 is full
open.
[0028] The control circuit 400a issues logic signals, which control
cooling air blower 122. A seal air timing, control and alarm
circuit 400b controls the isolation damper 117 and the damper seal
air fans 111. Moreover, the control circuit 400a includes input
circuitry that allows operation to be enabled by a user (operator)
in conjunction with an indicated start-run signal issued by the CT
and the manual or auto selectable cooling air blower 122 set to
auto. Additionally, the electronic control circuit 400a allows for
the start sequence to be initiated when the cooling air blower is
started, e.g., manually, irrespective of CT MW setpoint.
[0029] In the stop sequence, the control circuit 400a delays stop
initiation in order to take the time to close the damper. Then the
control circuit 400b commands the seal air blower 111 to start.
Subsequently, the control circuit 400a disables 100% blower speed
and sends a signal that enables a blower speed ramp down (ramp rate
control is shown in function diagram 300 of FIG. 3B).
[0030] The electronic control and timing circuit 400a and the seal
air alarm, timing and control circuit 400b initiate a second
sequence when the CT MW output is less than 80% and the HRSG inlet
duct temperature is greater than the steam temperature plus
50.degree. F. It should be understood that the electronic control
circuits 400a and 400b are exemplary, and one having ordinary skill
in the art could utilize a variety of different options, including,
e.g., programmable logic arrays, field programmable gate arrays,
microprocessor controllers, and the like.
[0031] As shown in the control block and schematic diagrams 200 of
FIGS. 2A-2C, 4A-4C, and 5A-5C, the second start sequence, which is
controlled by the electronic control and timing circuit 400a and
the seal air alarm, timing and control circuit 400b, comprises the
steps of starting the cooling air blower 122; commanding the
isolation damper 117 to open after a one-minute delay; commanding
the seal air blower 111 to stop when the isolation damper 117 is
full open thereby controlling the HP SH temperature with the
cooling air blower. The water desuperheater's existing logic
remains intact, but does not call for excessive spray to saturation
as there is very little water needed to maintain steam
temperature.
[0032] Via command signals from control circuits 400a and 400b, the
system 100 provides a corresponding stop sequence that begins
responsive to the CT MW being greater than 80% or the HRSG inlet
duct temperature being less than the steam set point plus
50.degree. F., the stop sequence comprising the steps of:
commanding the isolation damper 117 to initiate a closing sequence,
commanding the seal air blower to start, and commanding the cooling
air blower 122 to stop after a delay.
[0033] When the isolation damper 117 is fully closed, the system
100 causes changeover sequence number one to transition to
changeover sequence number two.
[0034] The system 100 commands a changeover from sequence one to
two responsive to CT MW output remaining below 80% and the
temperature inside HRSG duct 105 going above the steam set point
plus 50.degree. F. anytime during sequence one. For example, when
the aforementioned power and temperature readings have been sensed
by the system 100, the lead temperature control transfers to
cooling air mode after a delay, the blower speed minimum is
enabled, and the system 100 resumes control of the HP SH
temperature.
[0035] The system 100, and more specifically, the seal air alarm,
timing and control circuit 400b, has alarms that are actuated if
the isolation damper fails to open in a predetermined allotted
time, if the isolation damper fails to close in a predetermined
allotted time, if the high cooling air duct temperature indicates
turbine exhaust gas (TEG) flow back toward cooling air blower, or
if seal air blower pressure is low.
[0036] The system utilizes a pre-existing high-pressure superheater
(HP SH) temperature controller which controls the desuperheater
cooling water flow, and, as shown in block function diagram 300 of
FIG. 3B, a gas temperature controller (the TIC-005), which controls
the cooling air blower. The desuperheater and its cooling water
flow temperature control point are diagrammatically shown in FIG.
6.
[0037] With reference to point descriptions of circuit 400a in FIG.
4C, if the input at H-9 goes high, the lead controller will change
from desuperheater cooling water to blower cooling air. If the
input at H-10 goes high, the cooling air blower will go to 100%. If
the input at H-12 goes high, the cooling air blower speed will ramp
down.
[0038] The system 100 may be used in conjunction with commercially
available turbine control modifications to further enhance
overspray mitigation at the facility. Moreover, the system performs
best in a combined cycle application having waste boilers, where
the surface area layout causes the attemperators to spray to
saturation. For example, the system is particularly useful in the
GE `F` Class turbines, where there is a temperature spike to
1200.degree. F. from 30-70% loads with varying degrees of reduced
full load mass flow. Additionally, many newly designed HRSGs
include features where the split of the surface arranged for the
duct burner emphasizes the overspray issue and causes significant
problems, which the system and method are designed to address.
[0039] The CT can be operated at low load with no modification to
the turbine controls or equipment. This system can also be used
with other heat sources, such as a fired kiln. When the heat source
is incongruous with the designed surface area of the HRSG it causes
attemperator overspray. The additional ambient air added by the air
cooling system 100 prevents overspray in the attemperators. This
prevents cracking of the main steam piping downstream and tube
failures that previously were a result of spraying to saturation
(no residual superheat in the steam) and the subsequent thermal
fatigue. The system 100 adds ambient air to the HRSG to keep the
non-pressure components in the HRSG inlet duct, such as tube ties,
liner plate, and flow distribution devices, from creep and
oxidation damage during extended duration at low load and high
temperature. The system, thus, affords protection of the HRSG from
thermal fatigue and oxidation damage.
[0040] Since the fan is preferably operated with a variable
frequency drive (VFD) motor 127, the exact output is between two
flows, depending on backpressure from the main heat source (CT
exhaust in the exemplary case). The VFD 127 has been integrated
into the system 100 to optimize air addition. However, the system
may also be effectively used without a VFD due to small variation
in fan flow in comparison to heat source flow from the CT. Thus,
the system focuses on reducing the overall bulk temperature to the
HRSG. Computational Fluid Dynamics (CFD) analysis has also been
done to prove that the resultant temperature distribution is
satisfactory. The system 100 focuses on conventional type HRSGs
with vertical tubes and separate superheater, evaporator, and
economizer tube bundles, but is applicable to other boilers.
[0041] The system 100 does not have a negative impact on the
emissions from the HRSG exhaust stack on a pound-per-hour basis,
since the addition of ambient air is the only change to the gas
stream. There are no changes necessary to CT or steam turbine
controls or components of the combined cycle system during startup
or low load operation.
[0042] The system 100 combines controlling desuperheater water
sprays and the percent of dilution air to the exhaust gas stream to
maintain a constant temperature to the HRSG so that the inter-stage
sections of the superheater and reheater of the HRSG downstream of
the spray water nozzle are not being moisture laden with overspray
from desuperheater cooling steam or water.
[0043] The system 100 includes integrated control of the fan, which
is optimized through the use of the variable frequency drive fan,
which is appropriately sized in order to provide the cooling
necessary to eliminate spraying to saturation while still allowing
control of the outlet steam temperature by the existing
attemperator. The system 100 utilizes in depth knowledge and
modeling of existing designed boiler surface areas as a basis for
fan size design and control parameter selection.
[0044] The emissions of the upstream CT are unaffected by the
system 100 on a pound per hour basis, other than allowing operation
to occur at low loads without damage to the HRSG components. The
benefit of residual superheat in the HRSG are as follows: (1)
minimize water being entrained with the steam as it enters the
downstream superheater and reheater sections of the HRSG causing
thermal fatigue failures, (2) minimize solids in the steam leaving
the superheater and reheater sections for increased purity of steam
to steam turbine or process, (3) minimize creep and oxidation
damage on non-pressure components in the gas stream inlet duet of
the HRSG.
[0045] The air cooling system and method for a heat recovery steam
generator inlet can be operated with the CT power level unaffected
(e.g., 30 to 80% load, as estimated by the load curves for a GE
Frame 7FA), but with a diluted exhaust temperature to the HRSG
inlet. It is particularly useful with the constant temperature CTs,
which maximize the temperature and control flow by manipulation of
the inlet guide vanes (such as the GE `F` class turbines).
[0046] The system 100 manipulates the inlet air dilution rate based
on thermocouples added to the inlet duct of the HRSG downstream of
the mixing.
[0047] The positive-sealing damper is disposed between the HRSG and
the ambient air fan in order to protect equipment and personnel
from backflow through the fan while the heat source is in normal
operation. A pressure switch in the ambient air duct is connected
to the digital electronic circuit, which sends control signals to
provide positive flow or flow direction.
[0048] It is to be understood that the present invention is not
limited to the embodiments described above, but encompasses any and
all embodiments within the scope of the following claims.
* * * * *