U.S. patent application number 14/539902 was filed with the patent office on 2015-09-17 for furnace combustion cross limit control with real-time diagnostic features.
The applicant listed for this patent is SYNCRUDE CANADA LTD. in trust for the owners of the Syncrude Project, as such owners exist now and. Invention is credited to JOSEPH AMALRAJ, ASHISH SHAH, XI SUN.
Application Number | 20150260398 14/539902 |
Document ID | / |
Family ID | 53178080 |
Filed Date | 2015-09-17 |
United States Patent
Application |
20150260398 |
Kind Code |
A1 |
SUN; XI ; et al. |
September 17, 2015 |
FURNACE COMBUSTION CROSS LIMIT CONTROL WITH REAL-TIME DIAGNOSTIC
FEATURES
Abstract
The present invention monitors and controls various
operation/process variables of a furnace or boiler, which may be,
for example, fuel flow, fuel pressure, air flow, box pressure, feed
outlet temperature, etc., and reacts to certain operation issues. A
method and system is provided for monitoring and controlling the
operation of a furnace in real-time which may prevent unnecessary
tripping of the furnace.
Inventors: |
SUN; XI; (Edmonton, CA)
; SHAH; ASHISH; (Fort McMurray, CA) ; AMALRAJ;
JOSEPH; (Fort McMurray, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SYNCRUDE CANADA LTD. in trust for the owners of the Syncrude
Project, as such owners exist now and |
Calgary |
|
CA |
|
|
Family ID: |
53178080 |
Appl. No.: |
14/539902 |
Filed: |
November 12, 2014 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61903310 |
Nov 12, 2013 |
|
|
|
Current U.S.
Class: |
110/341 |
Current CPC
Class: |
F23N 5/006 20130101;
F23N 2231/10 20200101; F23N 2229/04 20200101; F23N 5/18 20130101;
F23N 5/242 20130101 |
International
Class: |
F23N 5/24 20060101
F23N005/24 |
Claims
1. A method for monitoring and controlling the operation of a
furnace in real-time, comprising: (a) obtaining a measurement of a
controlled process variable (PV) from a measuring device that
senses the PV in a controlled process; (b) obtaining an output
reading (OP) from a controller that is in an auto-mode and is
continuously controlling the controlled process by maintaining the
PV within a desired range; (c) determining whether there has been a
change (dPV) in the PV relative to a previously obtained controlled
process variable (PV.sub.p) obtained from the measuring device; and
(d) determining whether or not dPV was initiated by the
controller.
2. The method as claimed in claim 1, further comprising: (e)
switching the controller into a manual mode if it is determined
that dPV was not initiated by the controller
3. The method as claimed in claim 2, further comprising: (f)
sending a signal to an operator alerting the operator that the
controller has been switched to the manual mode.
4. The method as claimed in claim 1, wherein the controlled process
variable (PV) is selected from the group consisting of box
pressure, fuel gas pressure, O.sub.2 concentration, valve stiction,
air flow transmission, and fuel gas transmission.
5. A method for monitoring and controlling the operation of a
furnace in real-time, comprising: (a) obtaining a measurement of a
controlled process variable (PV) from a measuring device that
senses the PV in a controlled process; (b) obtaining an output
reading (OP) from a controller that is in an auto-mode and is
continuously controlling the controlled process by maintaining the
PV within a desired range; (c) determining whether there has been a
change (dPV) in the PV relative to a previously obtained controlled
process variable (PV.sub.p) obtained from the measuring device; (d)
determining whether there has been a change (dOP) in the OP
relative to a previously obtained output reading (OP.sub.p)
obtained from the controller; and (e) determining whether dPV is
less than or greater than a predefined maximum controlled process
variable (PV.sub.max) and whether the dOP is less than or greater
than a predetermined minimum output reading (OP.sub.min) in order
to determine whether a change in the process was initiated by the
controller.
6. The method as claimed in claim 5, further comprising: (f)
switching the controller from the auto-mode into a manual mode if
it is determined that the change in the process was not initiated
by the controller
7. The method as claimed in claim 6, further comprising: (g)
sending a signal to an operator alerting the operator that the
controller has been switched to the manual mode.
8. The method as claimed in claim 5, wherein the controlled process
variable (PV) is selected from the group consisting of box
pressure, fuel gas pressure, O.sub.2 concentration, valve stiction,
air flow transmission, and fuel gas transmission.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method and system for
monitoring and controlling the operation of a furnace. More
specifically, the invention relates to a method and system for
operating a furnace, which may be a component in a process for
refining hydrocarbons, to run substantially continuously and close
to maximum capacity safely, reliably and efficiently.
BACKGROUND OF THE INVENTION
[0002] Industrial furnaces, such as those used in hydrocarbon
refineries, use conventional cross limit controls that are intended
to run in automatic mode and be monitored by a human operator.
However due to limitation of equipment capacities, field instrument
issues, fluctuations in operation variables, including ambient
temperature, feed flow rate, fuel pressure, fuel flow rate, fuel
quality, air flow rate, etc., some of the controllers in the
control system are mostly operated in manual mode. The term
"furnace" used herein includes furnaces and fired heaters. When a
certain operation variable changes, the human operator is trained
to react by adjusting other operation variables to compensate for
the change, in order to help maintain the furnace or boiler running
within the range of safe operating conditions. However, when the
furnace is operated manually, the furnace is sometimes supplied
with either too much or too little air for O.sub.2 requirement.
When there is not enough air in the furnace, some fuel is left
without being fully combusted in the furnace. When there is too
much air in the furnace, the excess air cools down the furnace
which reduces the furnace's efficiency. Further, in manual mode,
the furnace box pressure may be allowed to increase beyond a
certain threshold, thereby tripping the furnace. The furnace can
also trip if the fuel gas pressure is too high or too low based on
the predetermined set values. Other issues that can cause the
furnace to trip or otherwise disrupt the operation of the furnace
include transmitter malfunction and valve stiction. Any one of the
above identified issues may prevent the furnace from running
continuously in auto mode and/or at close to its maximum capacity.
Every time a furnace trips, the furnace needs to be restarted and
the start-up time for an industrial furnace can be lengthy.
Therefore, the tripping of a furnace not only interrupts the
operation of the furnace itself, but can also cause significant
delays in a process wherein the furnace is a component thereof.
[0003] In the prior art, there are some cross limit control patents
relating to furnaces and boilers, which are designed to handle some
special operating condition. For example, U.S. Pat. No. 4,369,026
describes a cross limit control design for controlling the fuel
flow and an oxygen-containing fluid to a combustion process, in
which it employs a positive polarity limiter and a negative
polarity limiter to maintain the fuel/oxygen ratio for a combustion
process. U.S. Pat. No. 4,064,698 gives a boiler combustion control
method in power plants with fuels having variable heating values.
U.S. Pat. No. 6,984,122 shows a combustion control with temperature
compensation. U.S. Pat. No. 4,498,863 gives a design for the
combustion control with feed forward function. U.S. Pat. No.
4,473,490 demonstrates a control of a reforming furnace.
[0004] For the constraint methods in process control schemes, which
are used to ensure safety of process operations, the method of
setting fixed high/low limits on controller output (OP) is
generally used. Since there are variations in operation conditions,
normally, these kinds of limits are very conservative. In the
multivariable process control methods, including the multivariable
mode predictive controllers, most implement the constraint
functions by changing the related other controlled variable' inputs
based on the process model relationships. Since the process dynamic
responses are normally not quick enough, there are always required
more safety rooms for any constraints, which result in the
constraints being somewhat far away from the optimum constraint
limits to avoid the possible process trips. Therefore, the fixed
limit method and the method of getting the constraint by changing
other controller inputs are not the optimum constraint methods,
that is, the both methods cannot maximize the furnace capacity.
[0005] In the area of valve stiction detection, one U.S. Pat. No.
8,145,328 is found to detect the valve stiction using controlled
variable (PV) and controller output (OP), in which a filter and a
mathematic analysis technique are employed to determine whether
there is valve stiction or not. Comparative studies of the valve
stiction detection have been described by Garcia, et al (2013),
Choudhury, et al (2006), Nallasivam, et al. (2010), Singhal and
Salsbury (2005), Chitralekha, et al (2010) and Zakharov, et al
(2013). All those studies have required the several cycles of valve
PV and OP data to detect the possible valve stiction, that is,
before determining whether the valve is stiction or not, the
methods in those papers need the data that valve stiction at least
happens several times. In the furnace operation situation, it may
cause furnace trip or O.sub.2 deficiency (safety problem) when
there is a big stiction issue such as 10% valve stiction if the
valve stiction detection methods in those papers are used. Thus,
those methods presented in those papers cannot be used in the
real-time/on-line valve stiction detection to prevent the upset or
trip caused by the valve stiction.
[0006] A few valve compensation methods are given by Cuadros, et al
(2012) and Mohammad and Huang (2012) to reduce the effect of valve
stiction. However, those methods are effective only for this kind
of valve stiction caused by friction nonlinearity around 0% valve
opening. In most cases, the valve stiction can happen at any
percent of valve opening, then, those methods can only extend the
stiction happening period and cannot reduce/remove the effect of
the stiction. Therefore, those methods are difficult to use in the
furnace combustion control.
[0007] The literature on fault detection and diagnosis schemes is
very extensive. There are three main approaches to fault detection
and diagnosis, namely, the model-based method, the knowledge-based
method, and the statistical analysis method. The model-based method
is the conventional method of fault detection which uses static or
dynamic models of the process such as Samy et al (2011) and
Insermann (1984). When the process models are unknown, the
knowledge-based method can be used to detect faults (Frank, et al
(1990)). Statistical analysis method is simple and effective for
specific faults, including frequency analysis method, data
characteristic analysis method, partial least-squares (PLS)
analysis and principle-component analysis (PCA) analysis methods
(Gertler, et al (1999)). Those methods are very effective for the
faults happened comparative slowly. However, for the sudden big
changing fault, the methods may have some time delay to find
it.
[0008] In the conventional furnace cross limit control, O.sub.2
controller normally trims the air to fuel ratio from minimum to
maximum (10 to 20), which means that if the O.sub.2 analyzer fails,
it may cause a big upset or unit trip. For the furnaces used in the
oil sands industry, the O.sub.2 analyzer is not very reliable due
to the harsh operating conditions. Thus, the effect of O.sub.2
failure is one of the big concerns in furnace operation.
[0009] However, no literature was found to handle the furnace
operation with real time dynamic constraints to deal situations
such as high box pressure, high/low fuel gas pressure; with real
time valve stiction detection; with real time transmitter failures
detection and/or with real time dynamic trimming of air to fuel
ratio.
SUMMARY OF THE INVENTION
[0010] A novel cross limit control scheme is developed to handle
the process and/or mechanic issues existed in furnace combustion
such as the limitation of equipment capacity, partial plugging in
air inlet/outlet line, transmitter failure and valve stiction, and
to ensure that the furnaces can safely run at/close its maximum
capacity. The novel cross limit control scheme of the present
invention focuses on one or more of the following criteria:
[0011] (1) Real-time dynamic constraints for fuel gas high/low
pressure, box pressure high and low excess O.sub.2 protections.
[0012] Currently, there are several ways to run furnaces, for
example: [0013] (a) Air and fuel gas are both in manual control
mode. In this way, the furnace outlet temperature cannot maintain
its desired setpoint when feed flow changes; [0014] (b) Air is in
manual control but fuel gas is cascade to temperature control. This
method may cause that either O.sub.2 is too high or O.sub.2 goes to
deficiency when feed flow suddenly increases, thus, the furnaces
either lose their efficiency or go to unsafe operation condition;
[0015] (c) Furnace is controlled by the conventional cross limit
control scheme. In this case, the furnace may trip at high box
pressure or high fuel gas pressure when the box pressure or fuel
gas pressure is on the high limit if feed flow increases or some
other operational condition(s) changes; [0016] (d) Furnace is
maintained by using a multivariable model predictive control
scheme. As mentioned in the Background of the Invention, this kind
of control cannot maximize the furnace's capacity.
[0017] In the invention of the real-time dynamic constraints, the
fuel gas pressure high/low constraint values and the box pressure
high values can be changed in a computer system. For the case of
high box pressure protection, the maximum allowed box pressure
value is pre-defined by operation and can be changed from DOS
(Distributed Control System) system. When the box pressure reaches
the allowed maximum value, current controllers' outputs (OPs) can
be set as the controller's maximum OP limits for air flow
controller, fuel gas flow controller, O.sub.2 controller and
temperature controller. Thus, the air and fuel gas flows cannot
increase any more but can decrease. It prevents the possible
furnace trip on high box pressure.
[0018] For the case of high fuel gas pressure protection, the
maximum allowed fuel gas pressure value is pre-defined by operation
and can be changed from DCS (Distributed Control System) system.
When the fuel gas pressure reaches the allowed maximum value,
current controller's outputs (OPs) will be set as the controller's
maximum OP limits for fuel gas flow controller and temperature
controller. Thus, the fuel gas flow cannot increase any more but
can decrease. It prevents the possible furnace trip on high fuel
gas pressure.
[0019] For the case of low excess O.sub.2 protection, the minimum
excess O.sub.2 value is predefined by operation and can be changed
from DCS system. When the excess O.sub.2 reaches the minimum value,
current controller's outputs (OPs) will be set as the controller's
maximum OP limits for fuel gas flow controller and temperature
controller. Thus, the fuel gas flows cannot increase any more but
can decrease. It prevents the possible furnace flooding.
[0020] When the box pressure is back to normal (less than the
allowed maximum box pressure), the high limits on air and O.sub.2
controllers will be removed. When the box pressure, fuel gas
pressure and excess O.sub.2 are back to normal, the high limits on
fuel gas and temperature controllers will be released.
[0021] For the case of low fuel gas pressure protection, the
minimum allowed fuel gas pressure value is pre-defined by operation
and can be changed from DCS (Distributed Control System) system.
When the fuel gas pressure reaches the allowed minimum value,
current controller's outputs (OPs) will be set as the controller's
minimum OP limits for fuel gas flow controller and temperature
controller. Thus, fuel gas flow cannot decrease any more but can
increase. It prevents the possible furnace trip on low fuel gas
pressure.
[0022] When the fuel gas pressure is back to normal (greater than
the allowed minimum fuel gas pressure), the low limits on fuel gas
and temperature controllers will be removed.
[0023] Note that for the same pressure constraint, the OP
maximum/minimum limits will be different due to the variation of
environment temperature and operating conditions. The real-time
dynamic constraint can catch the limit changes since all the limits
are dynamically set using the current OPs whenever any constraint
conditions reach. Therefore, the invention ensures the furnace can
operates at its maximum capacity more safely, reliably, and
efficiently.
[0024] (2) Real-time valve stiction detection.
[0025] Valve stiction is a common issue in furnace operation. Valve
stiction may be defined as the prescience of non-linear behavior in
a valve. This is attributed to static friction in the valve, which
impedes the motion of the valve until a force sufficiently great
can overcome the static friction. If the valve stiction is great
enough, it may trip the furnace when the valve stiction
happens.
[0026] For the valve stiction detection method of the present
invention, OP.sub.max is defined as the maximum allowed controller
opening change during a n sampling time period. PV.sub.min is the
possible minimum flow change when the controller opening changes
the amount of OP.sub.max. OP.sub.max and PV.sub.min are chosen so
that, when the controller suddenly opens or closes the amount of
OP.sub.max, it will not cause operational problems, and the
controlled variable's (PV) change is at least larger than the
amount of PV.sub.min. When the change of a controller opening (OP)
is larger than OP.sub.max during the n sampling time period but the
change of the controlled variable (PV) is smaller than PV.sub.min,
then one can conclude that the valve stiction exists since the OP
change doesn't get the response of PV. This method doesn't need to
wait for actual valve stiction to occur and, thus, it prevents the
possible furnace trip.
[0027] (3) Real-time transmitter failure detection.
[0028] Transmitter failure is another common issue in furnace
operation. If a transmitter fails, it causes furnace upset or
trips.
[0029] In furnace operation, the slow fuel gas or air transmitter
failure generally will not cause a large disruption since those
kinds of failures can be compensated by temperature or O2
controller. But a sudden big change of transmitter reading may
cause the furnace trip.
[0030] The transmitter failure detection method of the present
invention is mainly used to find a sudden change of the transmitter
readings due to transmitter failure. The present transmitter
failure detection method defines that PV.sub.maxF is the maximum
controlled variable (PV) change during a certain time period.
OP.sub.minF is the required minimum controller opening change when
the PV changes the amount of PV.sub.maxF. PV.sub.maxF and
OP.sub.minF are chosen by that, when the PV suddenly increases or
decreases the amount of PV.sub.maxF, the OP change is at least
larger than the amount of OP.sub.minF. When the change of PV is
larger than PV.sub.maxF during a certain time period, but the
change of OP is smaller than OP.sub.minF, then it concludes that
the transmitter fails since the PV change is not caused by the OP
change. This method can quickly catch the sudden transmitter
failure, and thus reduces the effect of transmitter failure on the
furnace operation.
[0031] (4) Real time dynamic trimming of air to fuel ratio.
[0032] The conventional O.sub.2 control generally trims air to fuel
ratio from minimum air requirement to maximum air requirement (10
to 20). Since the reliability of O.sub.2 analyzer is one of the
problems in furnace control, the present design trims the air to
fuel ratio within predefined small range. Thus, if the O.sub.2
analyzer fails, the effect of the failure will be greatly reduced.
If the O.sub.2 analyzer is out of range and shows a poor PV, the
new control will set the air flow controller and fuel flow
controller in manual mode. If the air to fuel ratio is outside the
predefined range, the air and fuel gas controllers will also be set
to manual mode and operator notified. If the output (OP) of the
O.sub.2 controller is greater than OP.sub.maxop or less than
OP.sub.minop, where OP.sub.maxop and OP.sub.minop are the
predefined maximum OP and minimum OP, the new control method will
inform the operator and operator can decide whether the O.sub.2
controller is needed to reset its function or not. When O.sub.2
control is reset, its OP is set at 50%, and air to fuel ratio is
initialized and set equal to the actual filtered air to fuel value
at that time.
[0033] (5) Furnace box high trim control.
[0034] This control function provides a further over pressure
protection. When the furnace box pressure is larger than a
predefined value, the controller starts to reduce the fuel gas flow
which eventually results in reduction in air as well by O.sub.2
controller. Thus, the furnace box pressure will decrease and
controlled.
[0035] Thus, in one aspect, a method for monitoring and controlling
the operation of a furnace in real-time is provided, comprising:
[0036] obtaining a measurement of a controlled process variable
(PV) from a measuring device that senses the PV in a controlled
process; [0037] obtaining an output reading (OP) from a controller
that is in an auto-mode and is continuously controlling the
controlled process by maintaining the PV within a desired range;
[0038] determining whether there has been a change (dPV) in the PV
relative to a previously obtained controlled process variable
(PV.sub.p); and [0039] determining whether or not dPV was initiated
by the controller.
[0040] In one embodiment, the method further comprises switching
the controller from the auto-mode into a manual mode if it was
determined that dPV was not initiated by the controller. In another
embodiment, if dPV was not initiated by the controller, the method
further comprises sending a signal to an operator alerting the
operator that the controller has been switched to the manual
mode.
[0041] In one embodiment, the controlled process variable (PV) is
selected from the group consisting of box pressure, fuel gas
pressure, O.sub.2 concentration, valve stiction, air flow
transmission, and fuel gas transmission.
[0042] In another aspect, a novel cross limit control scheme is
provided, comprising one or more of the following: [0043] a real
time dynamic constraint method for fuel gas high/low pressure, box
pressure and low excess O.sub.2 protection; [0044] a real-time
valve stiction detection method; [0045] a real-time transmitter
failure detection method; [0046] a real-time dynamic trimming of
air to fuel ratio; and [0047] a furnace box high trim control.
BRIEF DESCRIPTION OF THE DRAWINGS
[0048] Referring to the drawings wherein like reference numerals
indicate similar parts throughout the several views, several
aspects of the present invention are illustrated by way of example,
and not by way of limitation, in detail in the figures,
wherein:
[0049] FIG. 1 is a typical schematic of a furnace in communication
with the control system of the present invention.
[0050] FIG. 2 shows a functional block diagram of the novel cross
limit control with the dynamic constraints and diagnosis
features.
[0051] FIGS. 3A and 3B illustrate a decision flow diagram of the
methodology for the setting the dynamic high/low constraints based
on the box pressure and fuel gas pressure.
[0052] FIG. 4 is a decision flow diagram of the methodology for the
detection of and response to air valve stiction.
[0053] FIG. 5 is a decision flow diagram of the methodology for the
detection of and response to air flow transmitter issues.
[0054] FIG. 6 is a decision flow diagram of the methodology for the
detection of and response to fuel gas transmitter issues.
[0055] FIG. 7 is a decision flow diagram of the methodology for the
detection of and response to oxygen analyzer issues.
[0056] FIGS. 8a to 8e are graphical representations of sample data
collected from the operation of a furnace used in a process of
refining hydrocarbons, without the system of the present
invention.
[0057] FIGS. 9a to 9d are graphical representations of sample data
collected from the operation of a furnace used in a process of
refining hydrocarbons, with the system of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0058] The detailed description set forth below in connection with
the appended drawings is intended as a description of various
embodiments of the present invention and is not intended to
represent the only embodiments contemplated by the inventor. The
detailed description includes specific details for the purpose of
providing a comprehensive understanding of the present invention.
However, it will be apparent to those skilled in the art that the
present invention may be practiced without these specific
details.
[0059] The present invention relates generally to a method and
system for monitoring and controlling the operation of a furnace or
boiler. More specifically, the present invention monitors and
controls various operation variables of the furnace or boiler,
which may be for example, fuel flow, fuel pressure, air flow, box
pressure, feed outlet temperature, etc., and reacts to certain
operation issues. The figures and description herein describe the
present invention in relation to the operation of a furnace.
However, the present invention may be configured for monitoring and
controlling the operation of a boiler.
[0060] As used herein, a "controlled process variable" or "PV"
refers to a process variable that is controlled by a controller
used to continuously or automatically maintain the controlled
process variable or PV within a desired range. Examples of
controlled process variables useful for monitoring and controlling
the operation of a furnace or boiler include, but are not limited
to, temperature, pressure, flow rate, oxygen (O.sub.2)
concentrations, and the like.
[0061] As used herein, a "measuring device" refers to a device that
senses a process variable through the medium of a sensor or other
measuring element. Examples of measuring devices include, but are
not limited to, flow meters, pressure sensors, O.sub.2 analysers,
stiction detectors, and the like.
[0062] As used herein, an "output reading" or "OP" refers to a
reading or output given by a controller that is controlling a
process, which reading or output is useful to troubleshoot and
diagnose issues.
[0063] Referring to FIG. 1, a system includes a furnace 10 having a
combustion chamber 12, a flue stack 14, and a pilot light 15. The
furnace is configured to receive fuel from a fuel source via a fuel
supply line 16. In one embodiment, the fuel is natural gas or a
natural gas mixture. However, other types of fuel may also be used.
The furnace can also receive air from an air source via an air
supply line 18. In the embodiment, the air provides O.sub.2, or is
a mixture of O.sub.2 and at least one other gas. An air valve 19
may be included to control the supply of air into the furnace. In
the illustrated embodiment, the furnace is used to heat a feed,
which may be a hydrocarbon mixture, to or close to a desired
temperature. The feed is fed into the furnace via a feed-in line 20
and heated feed exits the furnace via a feed-out line 22. Although
the embodiments described herein relate to a furnace for heating
feed, it can be appreciated that the system of the present
invention may be used for other furnaces.
[0064] Flue 14 allows any exhaust from the combustion chamber to
exit directly into the surroundings, but flue 14 may include a
valve 24 for selectively blocking off the flue such that exhaust
cannot exit directly. In the illustrated embodiment, an optional
heat exchanger 26 is provided in air supply line 18 to warm up the
air supply with the furnace flue 14 via lines 28 and 30. When valve
24 is closed, exhaust from the combustion chamber cannot exit the
flue directly and is forced to flow to the heat exchanger via line
28. Heat exchanger 26 extracts heat from the exhaust to warm up the
ambient air that is fed into the furnace. Supplying the furnace
with warm air, rather than ambient air, helps improve the furnace's
efficiency. The cooled exhaust then flows out of the exchanger via
line 30 back to flue 14 where it is allowed to exit into the
surroundings. Heat exchanger 26 is not necessary for the operation
of the furnace but may be optionally included to help improve the
efficiency of the furnace, based on ambient air temperature.
[0065] The system includes a plurality of monitoring devices,
including for example meters, transmitters, transducers, analyzers,
etc. for monitoring various operation variables. In the illustrated
embodiment, the system has a first flow meter 32, and a pressure
transmitter 34 in fuel gas line 16 to measure the flow rate and
pressure of the fuel gas, respectively. A second flow meter 36 is
installed in line 18 for measuring the air supply flow rate into
the furnace. A pressure transmitter 38 is provided for measuring
the box pressure of the combustion chamber. An O.sub.2 analyzer 40
is installed inside flue 14 for measuring the level of O.sub.2 in
the exhaust. Line 20 includes a third flow meter 42 for measuring
the flow rate of the feed input, and line 22 includes a temperature
transmitter 44 for measuring the temperature of the feed output.
Flow could be measured by orifice type differential pressure
measurement or vortex type devices. Temperature--Temperature could
be measured by resistance temperature detectors or thermocouple
type devices. Pressure--Pressure could be measured by diaphragm
capsule type devices. O2 Analyser--O2 analyser could be measured by
Zirconium Probes or Paramagnetic type devices.
[0066] The readings obtained by the monitoring devices may be sent
in the form of a signal, whether by wire or wireless connection, to
a control system for display and/or further processing in
real-time. The values of the operation variables generated by the
signals from the monitoring devices may be presented to an operator
of the furnace by the control system via a graphical user
interface.
[0067] Referring to FIG. 1, fuel gas line 16 and air supply line 18
have flow controllers 35 and 37, respectively, for varying the flow
rate in each of the lines. Flow controllers 35 and 37 are in
communication with the control system such that the control system
can control the fuel valve 17 and air valve 19.
[0068] FIG. 2 shows a sample graphical user interface 100 that may
be used with the system for displaying/selecting/controlling the
various operation variables by the operator. Referring to FIG. 2,
Box 102 is Feed Outlet Temperature controller. Box 116 indicates
initialized Air to Fuel Ratio from the actual Air to Fuel ratio at
the time the application turned ON. Box 104 is Trimmed Air to Fuel
Ratio calculated based on initialized Air to Fuel ratio trimmed by
Excess O2. Box 108 is Air Demand Calculation Block. Box 115 is O2
Trim. Box 124 is Fuel Calculation block as per A/F Ratio Box 104
and as per the Air Flow value 114. Box 110 is Box High Pressure
trim. Box 106 is Fuel Demand Calculation block. Box 114 is Air Flow
Controller. Box 112 is Fuel Gas Flow Controller. This figure also
has various selectors to ensure constraints are in place to avoid
nuisance trips and smooth control of all the process variables of
the furnace. Three values are shown in all the controller boxes
(Box 102, Box 112, Box 114 and Box 115). These values indicate
PV--Measured Process Variable; SP--Desired Set Point for the
Process Variable; and OP--Controller Output that drives the final
control element to achieve the Process Variable close to the Set
Point.
[0069] In one embodiment, a target temperature for the feed output
is set at a temperature T. When the temperature measured by
temperature transmitter 44 is below temperature T, it is an
indication that the furnace is not supplied with sufficient fuel
and air to heat the feed to the desired temperature. In this
situation, the box 102, Feed Outlet Temperature Controller of the
control system reacts by calculating an appropriate amount of
increase in the air supply required first (to ensure the excess O2
level is maintained) and feeds that value to Box 108 Air Demand. As
the air flow starts increasing, the box 124 Fuel as per NF Ratio
feeds the signal to Box 106 Fuel Demand that sends the signal to
Box 112 Fuel Gas Flow controller, taking into account other
real-time variables, such as the box pressure (box 110), Trimmed
A/F Ratio (box 104) and the High Selector 126 and Low Selector 128.
When the temperature measured by temperature transmitter 44 is
above temperature T, it is an indication that the furnace is
supplied with more than sufficient fuel and air to heat the feed to
the desired temperature. In this situation, the box 102, Feed
Outlet Temperature Controller of the control system reacts by
calculating an appropriate amount of decrease in the fuel gas
supply required first by Low Selector 128 and feeds that value to
Box 106 Fuel Demand. As the fuel gas flow starts decreasing, the
value selected by High Selector 126 is fed to the box 108 Air
Demand which in feeds the decreased air flow requirement to Box 114
Air Flow controller.
[0070] Initially the A/F Ratio 116 is set based on the actual
air/fuel ratio in the time that the application is turned on. The
A/F ratio can change as the quality of fuel fluctuates, even if the
fuel flow rate remains constant. More specifically, more air is
required if the fuel contains a higher portion of combustible
components (i.e. "higher quality" fuel), and vice versa. For
example, a fuel supply that contains 100% pure natural gas requires
more air (i.e. a higher NF ratio) to fully combust than a lower
quality fuel supply that contains 80% natural gas. The system
allows for the automatic adjustment of the A/F ratio when the fuel
quality fluctuates, which will be described in more detail herein
below. Preferably, the amount of air supplied to the furnace should
be slightly more than the amount required to completely combust the
fuel in the furnace, and there should not be too much excess air to
have a cooling effect on the furnace, both of which should be taken
into account in the A/F ratio used by the system.
[0071] In the sample embodiment illustrated in FIG. 2, the A/F
ratio is shown in box 104. Therefore, when an increase in the fuel
flow rate is detected, the control system automatically sends a
signal to controller 37 to increase the air flow rate in order to
maintain the A/F ratio. In the sample embodiment, box 108 shows the
required air flow rate calculated based on the measured fuel flow
rate and the A/F ratio, and a signal is sent to the air flow
controller to increase the air flow rate to the required rate
accordingly, In some cases, the quality of the fuel source can
fluctuate, which affects the amount of air required to react with
the fuel even if the fuel flow rate remains constant. In one
embodiment, the level of O.sub.2 in the exhaust stream in flue 14
is determined by analyzer 40 and is shown in box 115. The level of
O.sub.2 in the exhaust is an indication of the actual amount of air
that was consumed in burning the fuel supplied to the furnace. The
initial A/F ratio is based on the fuel quality or a hypothetical
presumed fuel quality and air quality at the time when cross limit
controller is turned on. The A/F ratio may subsequently be adjusted
based on the reading of the O.sub.2 analyzer in the flue. For
example, a high level of O.sub.2 in the exhaust indicates that the
fuel quality has decreased since the A/F ratio was last set or is
lower than the presumed value because the combustion of the fuel is
not using up most of the air supplied to the furnace. In this
situation, the system will react by lowering the A/F ratio through
O2 controller 115 based on the O.sub.2 level measured in the flue,
in order to reduce the amount of excess air in the furnace. If the
analyzer shows that there is almost no O.sub.2 in the exhaust,
which may indicate that the fuel quality has increased, the system
may react by increasing the A/F ratio through 02 controller 115
such that more air is fed into the furnace while the fuel flow rate
remains unchanged. The adjusted A/F ratio is shown in box 104.
[0072] Referring to both FIGS. 1 and 2, an embodiment of the system
is configured to monitor and control the fuel gas pressure.
Depending on the type of furnace and the type of fuel, there is
usually a safe range of fuel gas pressures within which a furnace
can operate without tripping. In the illustrated embodiment of the
system, the high and low limits of the safe fuel gas pressure range
can be set at boxes 118 and 120, respectively, and these limits may
be preset by process safety personnel in the control system and the
furnace operator can vary the control set point within these high
and low limits. Interface 100 may also display, in box 122, the
real-time fuel gas pressure that is measured and transmitted by
indicator 34.
[0073] The system may also be configured to monitor and control the
box pressure. The furnace should be operated at a box pressure
slightly below the atmospheric pressure to prevent flue gas leakage
to the surroundings, and above a certain minimum pressure to
minimize air leakage into the furnace from the surroundings, which
reduces furnace efficiency. If the box pressure is too high, the
furnace can trip. If the box pressure is too low, the furnace can
implode. Interface 100 may display, in box 110, the real-time box
pressure that is measured and transmitted by indicator 38.
[0074] Referring to FIGS. 3A and 3B, in one embodiment, the system
of the present invention is configured to set the real-time dynamic
constraints for high box pressure and high/low fuel pressure
protections. This design allows the furnace to safely run at/close
to its maximum capacity. The fuel gas pressure and box pressure of
the furnace are continuously monitored by the system in a process
300. At step 302, the system gets its initialization for dynamic
constraint function. At step 304, the system obtains respective PV
and OP for fuel gas flow, box pressure and O.sub.2.
[0075] In the first stage from step 306 to step 324, it sets the
constraints if the operation conditions are abnormal. At step 306,
the system checks the box pressure, and if the box pressure is too
high and is larger than a predefined value PV.sub.Bpmax, the system
then sets the high constraints for air flow at step 314, O.sub.2 at
step 316, fuel gas flow at step 318 and feed temperature at step
320, and goes back to step 304. Otherwise, it moves to the next
stage. At step 308, the system checks the excess O.sub.2, and if
the O.sub.2 is too low and is less than a predefined value
O.sub.2min, the system then sets the high constraints for fuel gas
flow at step 318 and feed temperature at step 320, and goes back to
step 304. Otherwise, it moves to next stage.
[0076] At step 310, the system checks the fuel gas pressure, and if
the fuel gas pressure is too high and is larger than a predefined
value PV.sub.GPmax, the system then set the high constraints for
fuel gas flow at step 318 and feed temperature at step 320, and
goes back to step 304. Otherwise, it moves to next stage. When the
system sets the high constraints, the air flow and fuel flow are
not permitted to increase any more but can be decreased. The box
pressure and fuel gas flow constraints help to prevent the high box
pressure and high fuel gas pressure in tripping the furnace. A
sharp increase (sometimes also referred to as a "spike") in the box
and fuel gas pressures may be a consequence of a huge increase in
the flow rate of the feed input. A spike in the box and fuel gas
pressures may cause the furnace to trip. By restricting the air
flow and fuel flow rates from increasing when the box pressure or
fuel gas pressure is already very high, the system may prevent
tripping of the furnace at high box pressure or high fuel gas
pressure. When the box pressure is too low, the control system
sends a message to the panel operator for his intervention.
Referring to FIG. 2, in one embodiment, if the box pressure
increases too high to reach a predefined value, the Box 110 will
start to decreases the fuel flow rate and air flow rate will follow
to decrease through the Box 115 O.sub.2 controller.
[0077] At step 312, the system checks the fuel gas pressure and, if
the fuel gas pressure is too low and is less than a predefined
value PV.sub.GPmin, the system then sets the low constraints for
fuel gas flow at step 322 and feed temperature control at step 324,
and goes back to step 304. Otherwise, it moves to the next stage.
The low constraint restricts the fuel flow rate to the current
level so that the fuel gas flow cannot be reduced any more but can
increase. By restricting the fuel flow rates from decreasing when
fuel gas pressure is already very low, the system may prevent
tripping of the furnace at low fuel gas pressure.
[0078] In the second stage from step 326 to step 344, the system
releases the constraints if all the conditions in above are back to
normal. If the box pressure is back to normal and is less than a
predefined PV.sub.BNH at step 326, then the high constraints on air
flow and 02 are released at step 340 if there are high constraints
for the air flow or O.sub.2 at step 334, and goes back to step 304.
Otherwise, it also goes back to step 304. If the box pressure is
back to normal, it checks to see if O.sub.2 is back to normal or
not, i.e., O.sub.2 is larger than a predefined (normal) value
O.sub.2NL. If the O.sub.2 is back to normal, the system goes to
step 330. Otherwise, it goes back to step 304. At step 330, if the
fuel gas pressure is back to normal and is less than a predefined
PV.sub.GNH, then the high constraints on fuel gas flow and feed
temperature are released at step 342 if there are high constraints
for the fuel gas flow or feed temperature at step 336, and goes
back to step 304.
[0079] At step 332, the system checks whether the fuel gas pressure
is recovered from low pressure or not. If the fuel gas pressure is
back to normal, i.e., it is larger than a predefined value
PV.sub.GNL, then the low constraints on fuel gas flow and feed
temperature are released at step 344 if there are low constraints
for the fuel gas flow or feed temperature at step 338, and goes
back to step 304.
[0080] In one embodiment, the system of the present invention is
configured to detect stiction in air valve in a process 400 as
shown in FIG. 4. At step 402, the system gets its initialization
for the valve stiction detection. At step 404, the system obtains
respective PV and OP for air flow. Step 406 calculates the OP and
PV differences dOP and dPV between the current time and previous
n.sup.th controller's sampling time. Here, PV is the controlled
process variable and OP is the controller's output.
[0081] At step 408 and step 410, the system checks whether the air
flow reading at meter matches the air flow controller output level
or not, i.e., dOP is larger than a predefined OP value OP.sub.max,
but dPV is less than a predefined PV value PV.sub.min. If the air
flow meter reading matches the controller output level, then air
flow controller is allowed to continue to be in CAS mode at step
416 (i.e. controlled by the control system) and restarts the
process at step 404. Here, CAS means cascade mode for a controller.
Otherwise, it sets air flow and fuel gas flow controllers in Manual
model. If the air flow meter reading does not match the controller
output level, which indicates that there may be valve stiction, the
system then switches the air flow controller at step 412 and the
fuel gas controller at step 414 to manual mode (i.e. controlled
manually by the furnace operator), and also sends notifications to
the operator warning that the controllers have been switched to
manual mode and/or air valve requires checkup or maintenance. In
one embodiment, the warning notification is sent by way of a pop-up
window in the user interface or another display visible to the
operator. Steps 412 and 414 may be implemented substantially
simultaneously. In manual mode, the operator controls the air flow
and fuel gas flow controllers, and can manually adjust the air flow
rate and the fuel gas flow rate. At step 418, it swaps data to save
the previous 1.sup.st to n.sup.th sampling OP and PV data.
[0082] For example, if air flow valve has 8% valve stiction, when
it requires to increase or decrease air flow, the air controller
will start increasing or decreasing its output until the output
increases or decreases to 8%, then the valve will suddenly open or
close at least 8%, which may cause the furnace trip at high box
pressure or cause the furnace running at the O2 deficiency
situation.
[0083] With reference to FIGS. 1 and 5, the system may be
configured to detect malfunctioning of the air flow meter in a
process 500. As described above, air flow meter 36 measures the
amount of air that flows into the furnace. The amount of air
flowing into the furnace is controlled by controller 37, which
controls valve 19. At step 502, the system gets its initialization
for the air flow transmitter failure/spike detection. At step 504,
the system obtains respective PV and OP for air flow. Step 506
calculates the OP and PV differences dOP and dPV between the
current time and previous n.sup.th controller's sampling time.
Here, PV is the controlled process variable and OP is the
controller's output.
[0084] When flow meter 36 detects a change in the air flow rate,
the system checks whether the air flow change was initiated by the
controller 37 at step 508 and step 510, i.e., dPV is larger than a
predefined PV value PV.sub.maxA, but dOP is less than a predefined
OP value OP.sub.minA or dPV is less than a predefined PV value
-PV.sub.maxA, but dOP is larger than a predefined OP value
-OP.sub.minA. If the change was initiated by the controller 37,
then the system allows the air flow controller and fuel gas
controller to remain in CAS mode at step 516 and the system
restarts process 500 at step 504. If the change was not initiated
by the controller 37, which may indicate that the air flow meter is
malfunctioning and giving false readings, the system then switches
the air flow controller 37 at step 512 and fuel gas flow controller
35 at step 514 to manual mode and also sends a notification to the
operator warning that the air flow controller and fuel gas flow
controller have been switched to manual mode and/or air flow meter
36 requires checkup or maintenance. The warning notification may be
sent by way of a pop-up window in the user interface or another
display visible to the operator. In manual mode, the operator
controls the air flow controller and can manually adjust the air
flow rate. In this situation, from the moment when the air flow
controller is switched to manual mode, the system ignores the air
flow meter readings so that they are not relied upon to change
other operation variables. At step 518, it swaps data to save the
previous 1.sup.st to n.sup.th sampling OP and PV data.
[0085] For example, if the system relies on a false reading that
indicates a 20% sudden increase in the air flow rate, but in
reality the air flow rate has not changed, then the system will
react by instructing air flow controller 37 to try to decrease the
air flow rate by 20%. Such an unwarranted decrease in the air flow
rate would lead to air deficiency in the furnace, which is unsafe
operation and may trip the furnace. Therefore, switching the air
flow controller to manual mode when there is a potential
malfunctioning of the air flow meter helps prevent the furnace from
tripping due to a false air flow rate reading.
[0086] With reference to FIGS. 1 and 6, the system may be
configured to detect malfunctioning of the fuel gas flow meter in a
process 600. As described above, fuel gas flow meter 32 measures
the amount of fuel gas that flows into the furnace. The amount of
fuel gas flowing into the furnace is controlled by controller 35,
which controls valve 17. At step 602, the system gets its
initialization for the fuel gas transmitter failure/spike
detection. At step 604, the system obtains respective PV and OP for
fuel gas flow. Step 606 calculates the OP and PV differences dOP
and dPV between the current time and previous n.sup.th controller's
sampling time. Here, PV is the controlled process variable and OP
is the controller's output.
[0087] When flow meter 32 detects a change in the fuel gas flow
rate, the system checks whether the fuel flow change was initiated
by the controller 35 at step 608 and step 610, i.e., dPV is larger
than a predefined PV value PV.sub.maxF, but dOP is less than a
predefined OP value OP.sub.minF or dPV is less than a predefined PV
value -PV.sub.maxF, but dOP is larger than a predefined OP value
-OP.sub.minF. If the change was initiated by the controller 35,
then the system allows the fuel gas flow controller and air
controller to remain in CAS mode at step 616 and the system
restarts process 600 at step 604. If the change was not initiated
by the controller 35, which may indicate that the fuel gas flow
meter is malfunctioning and giving false readings, the system then
switches the air flow controller 37 at step 612 and fuel gas flow
controller 35 at step 614 to manual mode and also sends a
notification to the operator warning that the fuel gas flow
controller and air flow controller have been switched to manual
mode and/or fuel gas flow meter 32 requires checkup or maintenance.
The warning notification may be sent by way of a pop-up window in
the user interface or another display visible to the operator. In
manual mode, the operator controls the fuel gas flow controller and
can manually adjust the fuel gas flow rate. In this situation, from
the moment when the fuel gas flow controller is switched to manual
mode, the system ignores the fuel gas flow meter readings so that
they are not relied upon to change other operation variables. At
step 618, it swaps data to save the previous 1.sup.st to n.sup.th
sampling OP and PV data.
[0088] For example, if the system relies on a false reading that
indicates a 20% sudden decrease in the fuel gas flow rate, but in
reality the fuel gas flow rate has not changed, then the system
will react by instructing fuel flow controller 36 try to increase
the fuel flow rate by 20%. Such an unwarranted increase in the fuel
flow rate would lead to air deficiency in the furnace, which is
unsafe operation and may trip the furnace. Therefore, switching the
fuel gas flow controller to manual mode when there is a potential
malfunctioning of the fuel gas flow meter helps prevent the furnace
from tripping due to a false fuel gas flow rate reading.
[0089] In one embodiment, the system is configured to determine the
reliability of the O.sub.2 analyzer reading in the flue stack.
Referring to FIGS. 1 and 7, O.sub.2 analyzer 40 provides the system
a measurement of the O.sub.2 level in the exhaust. Due to the
location of the analyzer in the furnace, the analyzer can sometimes
intermittently coated with debris from the exhaust and may be
cleared subsequently, which may cause the analyzer to give
inaccurate and fluctuating readings.
[0090] In a process 700, the system checks the O.sub.2 analyzer
readings. For example, if according to the air flow rate and A/F
(Air/Fuel) ratio there should be more (or less) O.sub.2 in the
exhaust than the level indicated by the analyzer, it is likely that
the analyzer is intermittently coated with debris and subsequently
cleared which provides an inaccurate and fluctuating O.sub.2 level
reading. At step 702, the system initialize air/fuel ratio by using
current filtered air/fuel. At step 704, the system obtains
respective PV and OP for the excess O.sub.2. At step 706, it checks
O.sub.2 analyzer's reading range with a predicted level determined
from the other operation variables, such as air flow rate as
measured by flow meter 36 and the fuel flow rate as measured by
flow meter 32. If O.sub.2 reading is out of the defined range and
DCS detects a poor PV, it will set the air flow control to manual
mode and operators notified at step 716. If the O.sub.2 level
indicated by the analyzer is much higher or much lower than the
predicted O.sub.2 level, since the system design restricts the
ratio trimmed A/F Ratio to a predetermined smaller range than the
normal full range, thus, it prevents the ratio from fluctuating by
large magnitudes. In the standard cross limit control, the O.sub.2
controller generally trims the air/fuel ratio from 10 to 20. Thus,
when the O.sub.2 analyzer fails or suddenly big change, it may trip
the unit. Referring to FIGS. 2 and 7, O.sub.2 controller just trims
the initial air/fuel ratio with a predefined small range such as
.+-.1.5 around the initial air/fuel ratio. So, when the O.sub.2
analyzer fails, the effect on the furnace will be grater reduced.
In this design, when O.sub.2 controller reaches its maximum or
minimum output, the value of air/fuel ratio can be reset so that
O.sub.2 controller recovers to its maximum regulating ability. At
step 710, the system calculates the trimmed air/fuel ratio. From
step 708 to step 714, the system checks whether the OP of O.sub.2
controller is too high (larger than a predefined value
OP.sub.maxop) or is too low (less than a predefined value
OP.sub.minop), if the OP is in the normal range, then it goes back
to step 704. If the OP is out of the normal range and is caused by
the fault O.sub.2 analyzer, it sets the O.sub.2 controller to
manual mode and notifies the operator. In step 718, operator
decides whether resets the O.sub.2 controller (goes back to step
702) to set the OP back to the normal range or remains the
operation condition (goes back to step 704) when the OP is out of
the normal range.
[0091] FIG. 8a illustrates the effect of a sharp spike in the fuel
gas flow rate on the level of excess O.sub.2 in a furnace operating
without the control system of the present invention. It can be seen
that at around 8:50 hour, the feed temperature 810 dropped, which
caused the fuel gas flow rate 812 spike, while the air flow rate
814 remained substantially constant. The spike in the fuel gas flow
rate resulted in an inversely proportional spike in the level of
excess O.sub.2 816 to zero for about 8 minutes. As discussed above,
a significant drop in the level of excess O.sub.2 close to zero
causes the furnace to run in the unsafe condition.
[0092] In contrast, it can be seen in FIG. 9a that changes in fuel
gas flow 912 rate have minimal effect on the excess O.sub.2 916 in
a furnace that uses the system of the present invention with the
feed out temperature controller 910 and excess O.sub.2 controller
916 in Auto Mode. More specifically, even the air controller's OP
(output) 918 was fully opened (105% opening) at later two periods,
i.e., the air flow 914 reached its maximum rate, but the fuel gas
flow 912 cannot be increased when the air flow 914 was limited,
thus, the excess O.sub.2 PV (process value) 916 is controlled very
close to the excess O.sub.2 SP (set point).
[0093] FIG. 8b illustrates the effect on the furnace box pressure
from changes in the air flow rate supplied to a furnace when
operated without the control system of the present invention.
Without the control system of the present invention, the effect of
changes in air flow rate on box pressure is not considered.
Therefore, the air flow rate does not have any restriction based on
the box pressure. An increase in the air flow rate 822, while the
fuel gas flow rate 826 remained substantially constant, caused a
corresponding increase in the level of excess O.sub.2 828 and
positive box pressure 824 in FIG. 8b which means the furnace is
running in an unsafe operation condition. It should also be noted
that the box pressure controller 820 was fully open (105%) to try
to operate the furnace box pressure at its set point (-0.20 PSIG),
however, even at these conditions, the controller was not able to
control box pressure.
[0094] In FIG. 8b, the box pressure controller's output 820 always
was 105%, i.e. there was no any more control on high box pressure
due to the equipment capacity limitation. The air flow rate 822 was
about 15MSCFD and it increased to about 20MSCFD from around October
23 to around October 25. The data shows that the box pressure 824
increased in substantially the same time period from -0.08 to 0.11.
The air flow rate 822 decreased between October 25 and October 26,
from 20 MSCFD to 16MSCFD. As a result, the box pressure 824 dropped
from 0.12 to -0.07. As can be seen from the graph, the box pressure
824 was positive for about two days since box pressure controller
820 had lost control.
[0095] In contrast, it can be seen that in FIG. 9b, when a furnace
is controlled by the system of the present invention and operating
close to maximum operation conditions, the air flow controller and
fuel gas flow controller has restriction regarding the box
pressure. If the box pressure is proceeding to reach a predefined
value, the application will dynamically set constraints on the fuel
and air controller to limit them increase any further thus
preventing the potential trip. It can be seen in FIG. 9b when the
box pressure 920 reaches the predefined value (-0.05PSIG) the fuel
flow 922 and air flow 924 are restricted so that the box pressure
920 cannot increase any further, which may have a slight overshoot,
while the box pressure's output 926 is 105% (fully open). Further,
if the box pressure is still increasing, the control application
110 will start trimming the fuel which eventually results in
reduction in air as well. It is worthwhile to note that the O.sub.2
928 was well maintained.
[0096] FIG. 8c illustrates the effects of changes in fuel gas flow
rate on other operation variables in a furnace operating without
the control system of the present invention. The temperature 830
dropped from 328 Deg. C to 279 Deg. C between around 16:12 hour to
16:26 hour. Consequently, the decrease in the temperature resulted
in that fuel flow 832 increased from about 0.3 MSCFD to 0.8 MSCFD,
O2 834 decreased from about 7% to 0% and fuel gas pressure 836 from
about 14 PSIG to 38 PSIG. Therefore, the furnace tripped on high
fuel gas pressure. It should be noted that the Excess O.sub.2 level
834 was close to zero prior to furnace trip, which was an unsafe
operation condition.
[0097] In contrast, as shown in FIG. 9c, when a furnace is
controlled by the system of the present invention, changes in the
feed flow rate or feed outlet temperature do not significantly
affect other operation variables. The increasing requirement for
the fuel gas flow rate 932 resulted in similar increase in the air
flow rate 938 and fuel pressure 936. Since the air flow rate
changes with the fuel gas flow rate in accordance with the A/F
ratio, the level of excess O.sub.2 934 in the furnace remained
substantially constant.
[0098] And the fuel gas flow was dynamically constraints whenever
the fuel gas pressure was above the predefined value (33 PSIG).
Thus, furnace has not ripped on high fuel gas pressure even with
increased demand in heat duty. Further, the feed output temperature
930 also remained substantially constant.
[0099] FIG. 8d illustrates the effect on the feed output
temperature from changes in the flow rate of the input feed in a
furnace that operated with the existing controls in the control
system in manual mode prior to implementation of the present
invention. The initial feed input flow rate 840 was about 75 KBPD
and it was gradually reduced to about 45 KBPD from around 23:00
hour to around 8:00 hour. The data shows that the feed output
temperature 842 increased from around 1:00 hour to around 9:00 hour
by about 20.degree. C. The feed input flow rate was then increased
from about 45 KBPD at about 12:00 hour to about 60 KBPD at 16:00
hour. The feed output temperature decreased from around 9:00 hour
to about 17:00 hour by approximately 10.degree. C. As can be seen
from the graph, the feed output temperature was significantly
affected by changes in the feed input flow rate.
[0100] In contrast, it can be seen in FIG. 9d that changes in feed
input flow rate have minimal effect on the feed output temperature
in a furnace that uses the system of the present invention with the
feed out temperature controller and excess O.sub.2 controller in
Auto Mode. When the feed rate 940 slowly changed from 65 KBPD to 55
KBPD, the feed outlet temperature PV 942 was well maintained. The
sudden variation of feed rate from 55 KBPD to 40 KBPD within 15
minutes had little effect in the feed outlet temperature (within 6
Deg. C) and in excess O.sub.2 PV (max 0.8% change) 944 as could be
clearly seen from this trend.
[0101] FIG. 8e illustrates the excess O.sub.2 comparison with one
year data before using the system of the present invention 850
(blue line) and one year data after using the present invention 852
(red line). Before using the present invention, the mean value of
excess O.sub.2 was 3 and the standard deviation of excess O.sub.2
was 0.6, and a lot of high excess O.sub.2 causing low combustion
efficiency and some low excess O.sub.2 which could result in unsafe
operation. After using the present invention, the mean value of
excess O.sub.2 was reduced to 2.5 and the standard deviation of
excess O.sub.2 was reduced to 0.3, and the excess O.sub.2 was
mostly in the desired range of excess O.sub.2 increasing the
combustion efficiency and completely eliminating the low excess
O.sub.2 which can result in unsafe operation
[0102] As shown by the sample data collected from the furnaces, one
of which operated with the control system of the present invention
and the other without, it can be seen that the control system helps
prevent significant fluctuations in the operation variables during
the operation of the furnace. The control system also helps prevent
the furnace running in the unsafe operation situation. The control
system may further help diagnose certain problems with the furnace
by monitoring the various operation variables and assist in
preventing tripping of the furnace.
[0103] From the foregoing description, one skilled in the art can
easily ascertain the essential characteristics of this invention,
and without departing from the spirit and scope thereof, can make
various changes and modifications of the invention to adapt it to
various usages and conditions. Thus, the present invention is not
intended to be limited to the embodiments shown herein, but is to
be accorded the full scope consistent with the claims, wherein
reference to an element in the singular, such as by use of the
article "a" or "an" is not intended to mean "one and only one"
unless specifically so stated, but rather "one or more". All
structural and functional equivalents to the elements of the
various embodiments described throughout the disclosure that are
known or later come to be known to those of ordinary skill in the
art are intended to be encompassed by the elements of the claims.
Moreover, nothing disclosed herein is intended to be dedicated to
the public regardless of whether such disclosure is explicitly
recited in the claims.
REFERENCES
[0104] (1) Morgan; John A. and Hachmuth; Henry K., "Control of the
fuel/oxygen ratio for a combustion process". U.S. Pat. No.
4,369,026, Jan. 18, 1983. [0105] (2) Stern; Louis P., "Boiler
control having a heating value computer and providing operation
with fuel having variable heating values". U.S. Pat. No. 4,064,698,
Dec. 27, 1977. [0106] (3) Sullivan; John D., Morales; Luis H. and
Nickeson; Robert W., "Combustion control with temperature
compensation". U.S. Pat. No. 6,984,122, Jan. 10, 2006. [0107] (4)
Hanson; Robert C., Hanson; Leon C., "Feed forward combustion
control system". U.S. Pat. No. 4,498,863, Feb. 12, 1985. [0108] (5)
Stewart; William S., "Control of a reforming furnace". U.S. Pat.
No. 4,473,490, Sep. 25, 1984. [0109] (6) Choudhury; Md Ali A.
Shoukat, Shah; Sirish L., Thornhill; Nina F., "Detection and
quantification of stiction". U.S. Pat. No. 8,145,328, Mar. 27,
2012. [0110] (7) Garcia; Octavio P., Tikkala; Vesa-Matti, Zakharov;
Alexey, Jamsa-Jounela; Sirkka-Liisa, "Integrated FDD system for
valve stiction in a paperboard machine". Control Engineer Practice,
Vol. 21, pages 818-828, 2013. [0111] (8) Choudhury; Md Ali A.
Shoukat, Shah; Sirish L., Thornhill; Nina F., "Automatic detection
and quantification of stiction in control valves". Control
Engineering Practice, Vol. 14, pages 1395-1412, 2006. [0112] (9)
Nallasivam; U., Babji; S., Rengaswamy; R., "Stiction identification
in nonlinear process control loop". Computer and Chemical
Engineering, Vol. 34, pages 1890-1898, 2010. [0113] (10) Singhal;
Ashish, Salsbury; Timonthy, "A simple method for detecting valve
stiction in oscillating control loops". Journal of Process Control,
Vol. 15, pages 371-382, 2005. [0114] (11) Chitralekha; Saneej B.,
Shah; Sirish L., Prakash; J., "Detection and quantification of
valve stiction by the method of unknown input estimation". Journal
Process Control, Vol. 20, pages 206-216, 2010. [0115] (12)
Zakharov; Alexey, Zattoni; Elena, Xie; Lei, Garcia; Octavio P., "An
autonomous valve stiction detection system based on data
characterization", Control Engineer Practice, Vol. 21, pages
1507-1581, 2013. [0116] (13) Cuadros; Marco A., Munaro; Celso J.,
Munareto; Saul, "Improved stiction compensation in pneumatic
control valves". Computer and Chemical Engineering, Vol. 38, pages
106-114, 2012. [0117] (14) Mohammad; M., Huang; B., "Compensation
of control valve stiction through controller tuning". Journal of
Process Control, Vol. 22, pages 1800-1819, 2012. [0118] (15) Smay;
Ihab, Postlethwaite; Ian, Gu; Da-wei, "Survey and application of
sensor fault detection and isolation schemes". Control Engineering
Practice, Vol. 19, pages 658-674, 2011. [0119] (16) Insermann; R.,
"Process fault diagnosis based on modeling and estimation
methods--a survey". Automatica, Vol. 20, pages 387-404, 1984.
[0120] (17) Frank; P. M., "Fault diagnosis in dynamic systems using
analytical and knowledge-based redundancy--a survey". Automatica,
Vol. 26, pages 459-474, 1990. [0121] (18) Gertler; J., Li; W. H.,
Huang; Y., McAvoy; T., "Isolation enhanced principle component
analysis". AlChE J. Vol. 45, papes 323-333, 1999. [0122] (19) Sun;
Xi, Marquez; Horacio J., Chen; Tongwen, Riaz; M., "An improved PCA
method with application to boiler leak detection". ISA
Transactions, Vol. 44, pages
* * * * *