U.S. patent application number 11/088342 was filed with the patent office on 2006-09-28 for safe incineration of explosive air mixtures.
Invention is credited to James L. Morrissey.
Application Number | 20060216663 11/088342 |
Document ID | / |
Family ID | 37035641 |
Filed Date | 2006-09-28 |
United States Patent
Application |
20060216663 |
Kind Code |
A1 |
Morrissey; James L. |
September 28, 2006 |
Safe incineration of explosive air mixtures
Abstract
An explosive gas incinerator system comprises a fume source
inlet for receiving a process exhaust with a volatile material that
is explosive when its concentration exceeds a lower explosive limit
(LEL). A source flowmeter measures the volume of process exhaust
being input. A temperature sensor measures the temperature of the
process exhaust. A motorized damper is used for diluting the
process exhaust with an ambient air input to produce a damper
outflow with an explosive concentration less than the LEL. A damper
flowmeter measures the total mixture volume of the process exhaust
and ambient air input in the damper outflow. Another temperature
sensor measures the temperature of the damper outflow. A gas
monitor measures the concentration of volatile material in the
damper outflow. An oxidizer burns the damper outflow in a flame to
produce a cleaner exhaust, and a fan forces gas flows through the
oxidizer. A system controller is connected to receive measurement
data from the flowmeters, gas monitor, and temperature sensors, and
is connected to control the motorized damper to maintain the damper
outflow into the oxidizer below the corresponding LEL of the
volatile material. Operator errors and attempts at sabotage can be
detected and controlled.
Inventors: |
Morrissey; James L.;
(Hayward, CA) |
Correspondence
Address: |
PATENTS PENDING
9832 LOIS STILTNER CT
ELK GROVE
CA
95624
US
|
Family ID: |
37035641 |
Appl. No.: |
11/088342 |
Filed: |
March 25, 2005 |
Current U.S.
Class: |
431/5 ;
431/10 |
Current CPC
Class: |
F23K 2900/05001
20130101; F23G 7/065 20130101; F23G 5/50 20130101; F23N 5/242
20130101; F23G 2209/141 20130101; F23G 2207/30 20130101; F23K
2400/201 20200501; F23G 7/06 20130101; F23G 2207/101 20130101 |
Class at
Publication: |
431/005 ;
431/010 |
International
Class: |
F23G 7/08 20060101
F23G007/08; F23J 15/00 20060101 F23J015/00 |
Claims
1. An explosive gas incinerator system, comprising: a fume source
inlet for receiving a process exhaust with a volatile material that
is explosive when its concentration exceeds a lower explosive limit
(LEL); a source flowmeter for measuring the volume of said process
exhaust being input; a temperature sensor for measuring the
temperature of said process exhaust; a motorized damper for
diluting said process exhaust with an ambient air input to produce
a damper outflow with an explosive concentration less than said
LEL; a damper flowmeter for measuring the total mixture volume of
said process exhaust and ambient air input in said damper outflow;
a temperature sensor for measuring the temperature of said damper
outflow; a first gas monitor for measuring a concentration of said
volatile material in said damper outflow; an oxidizer for burning
said damper outflow in a flame to produce a cleaner exhaust; and a
fan for forcing gas flows through the oxidizer; and a controller
connected to receive measurement data from the flowmeters, gas
monitor, and temperature sensors, and connected to control the
motorized damper to maintain said damper outflow into the oxidizer
below the corresponding LEL of the volatile material.
2. The system of claim 1, wherein the controller uses said
temperature, gas monitor, and flow measurement data to dilute said
damper outflow to the oxidizer to be just under the LEL
concentration.
3. The system of claim 1, further comprising: a second gas monitor
for measuring said concentration of said volatile material in said
damper outflow.
4. The system of claim 2, wherein the controller compares data
measurements from both the first and second gas monitors to improve
the accuracy of measurement of volatile material concentration.
5. The system of claim 2, wherein the controller uses data
measurements from both the first and second gas monitors to provide
a redundant measurement of the volatile material concentration.
6. The system of claim 1, further comprising: an alarm connected to
the first gas monitor that will terminate system operation if the
concentration of volatile material in said damper outflow exceeds a
predetermined limit.
7. The system of claim 1, further comprising: an operator-error and
sabotage detection function included in the controller.
8. The system of claim 7, wherein: the operator-error and sabotage
detection function includes start-up charts that are self-generated
during controlled start-ups and that are subsequently compared to
in-field operating conditions operating conditions where the
excursions of particular variables from their norms are interpreted
as operator errors or attempts at sabotage and used to signal an
alarm or shut down the process.
9. A method for operator-error and sabotage detection in an exhaust
incineration system, comprising: building start-up charts of key
operating variables in an incineration system during controlled
conditions; and subsequently comparing in-field operating condition
variables such that excursions of particular variables from their
norms are interpreted as operator errors or attempts at sabotage
and used to signal an alarm or shut down the system.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to exhaust gas incinerators,
and more particularly to the safe incineration of explosive air
mixtures.
DESCRIPTION OF THE PRIOR ART
[0002] Industrial processes exhaust various gases and particles
into the air. Modern regulations require that the emissions be
controlled and limited, and one way to scrub the exhaust is to
incinerate it before it leaves the smoke stack. But some exhausts
can be explosive if they have mixtures with the right percentages
of air-to-fuel. The lower explosive limit (LEL) is a value that
represents the minimum percent of an explosive-fume mixed in air
which will explode. The source of ignition can either be an open
flame or the auto-ignition temperature.
[0003] Some exhausts must be licensed, certified, and monitored.
The exhausts can either be permitted to be flow variable, or the
more restrictive constant flow type. Thus in the manufacturing of
fluorescent paint which generates formaldehyde, the flow of air
through the system is provided simply to remove the fumes. The
United States Food and Drug Administration (FDA) is sometimes
involved in exhaust operating permits, and these can specify a
particular flow rate.
[0004] The types of conventional systems for processing
explosive-fumes are numerous. One such system uses an LEL-monitor
set to a minimum lower explosive limit. If the LEL monitor senses
an excursion, it activates an alarm to safely shut down the
industrial process. Such type of emergency shut-down monitoring can
result in lost production time and costly cleanups and
restarts.
[0005] Another strategy used in conventional systems processes is
to inject so much excess air into the exhaust that the percentage
of explosive-fumes can never be more than a small fraction of the
LEL. Although this practice will work to prevent explosions, the
fuel costs to incinerate the much higher flowrates resulting can be
enormous. In general, the higher an LEL a system can be safely
operated at, the higher will be the efficiency of the incineration
fuels consumed. In other words, substantial operational costs
savings can be realized.
[0006] Green Oasis Environmental, Inc. (Charleston, S.C.), has a
website (www.greenoasis.com) that describes its thermal oxidizer as
comprising a burn chamber, blowers, burner, heat exchanger and
associated controls. Light end hydrocarbons in gaseous form can be
injected into the burn chamber and mixed with outside air supplied
by blowers. The mixture is burned at 1700-2000.degree. F. The
exhaust is directed to the shell of a heat exchanger to heat used
oil to the desired temperature. Dampers control the amount of heat
supplied to the heat exchanger. In the oxidation process,
hydrocarbons are converted to water and carbon dioxide with a
destruction/removal efficiency that will meet or exceed all local
or federal regulations. The thermal oxidation process is so
effective in eliminating emissions that it is principally sold as
an emissions control device. Such thermal oxidizer used in the
EnviroEconomics.TM. process has anti-flashback protection, and
operates below the lower explosive limit (LEL), qualifying the
system for operation in hazardous areas. The thermal oxidizer's
microprocessor-based control function is incorporated into the
overall system's controls, permitting fully automatic
operation.
SUMMARY OF THE INVENTION
[0007] Briefly, a fume incinerator system embodiment of the present
invention operates at any explosive level of the input flows below
or above their LEL. Monitors and controllers are used to inject
just enough air into the input mixture to prevent the flow from
exceeding the LEL as it enters the incinerator flames. The
incinerator system comprises a fume source inlet for receiving a
process exhaust with a volatile material that is explosive when its
concentration exceeds a lower explosive limit (LEL). A source
flowmeter measures the volume of process exhaust being input. A
temperature sensor measures the temperature of the process exhaust.
A motorized damper is used for diluting the process exhaust with an
ambient air input to produce a damper outflow with an explosive
concentration less than the LEL. A damper flowmeter measures the
total mixture volume of the process exhaust and ambient air input
in the damper outflow. Another temperature sensor measures the
temperature of the damper outflow. A gas monitor measures the
concentration of volatile material in the damper outflow. An
oxidizer burns the damper outflow in a flame to produce a cleaner
exhaust, and a fan forces gas flows through the oxidizer. A system
controller is connected to receive measurement data from the
flowmeters, gas monitor, and temperature sensors, and is connected
to control the motorized damper to maintain the damper outflow into
the oxidizer below the corresponding LEL of the volatile
material.
[0008] An advantage of the present invention is that a system and
method is provided to guard against accidental operator error and
deliberate sabotage.
[0009] Another advantage of the present invention is that a system
and method are provided for significant fuel savings over prior art
systems.
[0010] A further advantage of the present invention is that a
system and method are provided to control explosive-fume/air
mixtures with a built-in safety factor through the use of the
variable speed fan plus the warning message to the operator. Such
gives the operator time either to correct the out of tolerance
condition or to execute an orderly shut-down. Such also provides
further cost savings.
[0011] These and other objects and advantages of the present
invention will no doubt become obvious to those of ordinary skill
in the art after having read the following detailed description of
the preferred embodiments which are illustrated in the various
drawing figures.
IN THE DRAWINGS
[0012] FIG. 1 is a functional block diagram of a first fume
incineration system embodiment of the present invention that
operates with a standard flow from the source;
[0013] FIG. 2 is a functional block diagram of a second fume
incineration system embodiment of the present invention that
operates with a variable flow from the source; and
[0014] FIG. 3 is a perspective diagram of a gas train used in
embodiments of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0015] FIG. 1 illustrates a constant-source-flow fume incineration
system embodiment of the present invention, and referred to by the
general reference numeral 100. The system 100 is connected to
receive a fume source 102 of gases for incineration before
discharging them into the atmosphere. A standard flow is received
from fume source 102 according to the nature of the source or
government operating permits dictating such. A source flowmeter 104
monitors the volume of gases entering and reports its measurements
to a system controller 106. A first temperature sensor 108 allows
the controller 106 to convert to Standard Conditions for its
calculations. A motorized damper 110 is adjusted by the controller
106 to admit more or less ambient air 112 in order to dilute the
incoming gases enough to keep their mixture below the lower
explosive limit (LEL). Such LEL is the lowest percentage of
volatile gases in the air mixture that will explode if exposed to
open flames or sparks, or that will auto-ignite at a particular
temperature.
[0016] A principle object of the present invention is to economize
on the incineration fuels used by keeping the volume of fumes mixed
with air to be incinerated to a minimum. But such mixture needs to
be below the particular LEL, so the optimum mixture will be just
short of the LEL as it passes a damper flowmeter 114.
[0017] A second temperature sensor 116 and redundant gas monitors
118 and 120 provide measurements for the controller 106 to help it
keep the motorized damper 110 set for the optimum exhaust mixture.
Too little input air will be dangerous, and too much will waste
operating fuel. The gas monitors can be LEL-Monitors that use gas
chromatograph technology.
[0018] A detonation flame arrestor 122 prevents flames from backing
down a flume 123. A third temperature sensor 124 is located at an
oxidizer 126. For straight thermal incineration, the temperature
sensor 124 is placed at the outlet of oxidizer 126 to control the
oxidizer incineration temperature. In catalytic systems, the
temperature sensor 124 is inside and is used to control the
temperature of the catalyst bed. Such oxidizer 126 has a natural
gas or propane fuel input 127 and is operated as a fume incinerator
that may include catalysts. An extraction fan 128 pushes a clean
exhaust 130 into the atmosphere. Such fume incinerators are
commercially marketed by Conversion Products (Hayward, Calif.),
HiTemp Technology Corporation (Flemington, N.J.), Green Oasis
Environmental, Inc. (Charleston, S.C.), etc. A watchdog timer 132
periodically makes sure the controller 106 is awake and
functioning. If not, the process is shut down and an alarm is
sounded.
[0019] Many explosive-fumes are toxic. Therefore, these systems are
always run negative so that if there is a leak in the ducting,
ambient air is drawn into the duct rather than having
toxic/explosive-fumes escape into the room or the atmosphere.
[0020] Embodiments of the present invention permit the incineration
of any fume stream regardless of the initial LEL percentage. Such
includes conditions where the explosive-fume leaving the source 102
is at or above the LEL. Such systems control the explosive-fume/air
mixture so that just enough air is added to prevent the mixture
from exceeding the specified LEL percentage. Therefore, safety and
fuel economy are both improved compared to conventional
systems.
[0021] These systems can also guard against accidents, operator
error, and deliberate sabotage. Since many explosive-fumes are
toxic, these systems are run with negative internal pressures so
that if there is a leak in the ducting, ambient air will be drawn
in. The toxic/explosive-fumes are thus prevented from freely
escaping into the room or the atmosphere.
[0022] The source 102 is shown in FIG. 1 as a single item, in
reality, there may be multiple sources. Source 102 includes
processes that generate explosive substances in their exhaust
fumes.
[0023] Source flowmeter 104 measures the actual flow of the
effluent stream from the source 102. First temperature sensor 108
measures the temperature of the effluent stream as it arrives, so
all calculations can be made at standard conditions. The controller
106 calculates the speed needed by extraction fan 128 that will
keep the effluent stream flowing at its specified rate.
[0024] The measurement data from gas monitors 118 and 120 are used
by the controller 106 to calculate the amount of air necessary to
keep the fume/air mixture at a specified setpoint value. The amount
of ambient air added is controlled by a combination of the position
of the motorized damper 110 and the fan speed. The ambient air
admitted into the system is used to keep the flow entering the
oxidizer 126 at an acceptable LEL percentage.
[0025] Damper flowmeter 114 measures the effluent and ambient air
mixture flow. The amount of ambient air introduced via the
motorized damper 110 is the difference between the total flow
measured at damper flowmeter 114 minus the effluent air measured by
source flowmeter 104. Such measurement data is sent to the
controller 106 to convert the values to standard conditions.
[0026] The first gas monitor 118 determines the percent of the
explosive-fume in the stream. Such value is sent to the controller
106 which uses the measurement data to calculate the amount of
ambient air that will be needed to maintain the explosive-fume at a
safe level.
[0027] The second gas monitor 120 samples the fume stream and
calculates the percent of the explosive-fume in the effluent
stream. Any time the mixture exceeds a predetermined safety
percentage, an alarm circuit is activated. Such function is a
redundant safety device. It will only be activated if the system
fails to maintain the prescribed level of the explosive mixture in
air. It can also be used to verify the operating accuracy of the
first gas monitor 118.
[0028] The alarm circuit can be used to turn off the burner in the
oxidizer 126, or if local codes require it, the alarm circuit can
be used to shut down both source 102 and the oxidizer 126.
[0029] The detonation flame arrestor 122 is optional. It should be
used where there is a possibility that the velocity of the effluent
passing through the oxidizer could fall below normal flame
propagation speed. The detonation flame arrestor 122 can prevent a
flame from traveling back to the source 102 which may be operating
above the LEL.
[0030] The oxidizer 126 may be a straight thermal unit or a
catalytic unit. The oxidizer 126 is designed so that the
incineration temperature and residence time at incineration
temperature will reduce the percent of explosive-fumes in the
effluent to the specified level.
[0031] For straight thermal incineration, the third temperature
sensor 124 measures the temperature of the effluent stream as it
leaves the main body of the oxidizer. For a catalyst based
oxidizer, the third temperature sensor 124 measures the temperature
of the effluent stream entering the catalyst bed. The temperature
measurement data is used by the controller 106 and a burner fuel
flow valve to maintain a specified incineration temperature.
[0032] The extraction fan 128 is a variable speed type that allows
the amount of air pulled through the system to be varied over a
wide range. Such fan is used to maintain a negative system pressure
upstream to prevent explosive and/or toxic fumes from leaking out.
Fan 128 can also be located upstream of oxidizer 128.
[0033] The controller 106 is a computer capable of mathematical and
statistical quality control calculations according to data inputs,
and input/output control. The source 102 may include more than one
product, so the controller 106 preferably permits an operator to
select which product is being processed. A display screen can be
used to post system progress and graphically display the operating
parameters. The target average and the upper and lower tolerance
limits of each of the control parameters would be useful in the
display.
[0034] It necessary to include a number of safety devices in system
100. For example, a zero-speed switch and a backup air-pressure
switch connected to the extraction fan 128 to detect blade stalling
and flow malfunctions. If at any time the fan slows down below a
set minimum extraction speed, and the zero speed switch has not
activated a shut-down procedure, the air pressure switch can
override it and shut the system down. Air pressure switch on the
combustion blower is used to detect if the combustion blower motor
fails. If a failure is detected, the oxidizer flame is
extinguished. A gas pressure switch in the fuel supply 127 feeding
the oxidizer burner is used as a shut-off if the actual gas
pressure exceeds or falls below safe values.
[0035] A detonation flame arrestor 122 is placed between inlet to
the oxidizer 126 and the source 102 of the fumes. The effluent
velocity through the oxidizer is set well above the flame
propagation speed. However, if the extraction fan 128 fails, the
velocity through the oxidizer 126 may fall below the flame
propagation speed. Since the conditions in the system interior
could be above the auto-ignition point, the flame front could move
upstream all the way to the source 102 if the detonation flame
arrestor 122 were not included.
[0036] An alarm circuit is included in the first gas monitor 118
that is activated anytime the LEL percentage exceeds a design
maximum.
[0037] The watchdog timer 132 is constantly reset electrically
during normal operation. If the resets fail to come, at a minimum,
it will shut down the oxidizer 126. The maximum amount of time that
can elapse without danger is pre-determined. A time period less
than this calculated value is placed in the control program and the
watch dog timer is set for that time.
[0038] Each time, the control program elapsed time equals the
allowable time, the watch dog timer is re-set. If anything happens
to the program and it fails to execute the re-set, the watch dog
timer times out and all the outputs are turned off so that the
oxidizer flame is extinguished and the flow of explosive-fumes is
stopped.
[0039] An alarm circuit is included in the first gas monitor 118
that will be activated any time the stream mixture exceeds a safe
value. An alarm circuit in the second gas monitor 120 is included
as a back-up safety in case the alarm associated with the first gas
monitor 118 has malfunctioned.
[0040] As a further safety check, both gas meters are used to
sample the effluent stream. Both readings are sent to the
controller 106 where they are processed. Although the accuracy of
the gas meters is excellent, statistical equations can be used to
determine the true reading of each gas meter output and the natural
tolerances of the system.
[0041] True average values can be calculated to a predetermined
level of accuracy, e.g., 0.5%. One procedure takes the sum of "n"
readings and divide by "n" for the first average. Save each
individual value in a matrix for use in determining the natural
tolerance of each device. Take five additional readings and add to
the conventional sum. Then divide the new sum by the total number
of readings taken. Save each individual value in a matrix for use
in determining the natural tolerance of each device. Take the
difference between the two averages and divide by the first
average. If the error is 0.5% or less this is the average value (x)
that will be used. If the error is greater than 0.5%, the steps are
repeated.
[0042] After the true average percentage values have been
determined, the next step is to determine the natural tolerance of
each of the devices. All processes that can be measured have
variations above and below their average value which are termed the
natural control limits of the process.
[0043] To calculate the natural upper control limit (UCL) and
natural lower control limit (LCL) the following formulas can be
used: UCL = X + 3 * i = 1 i = N .times. ( X - Yi ) 2 N ##EQU1## LCL
= X - 3 * i = 1 i = N .times. ( X - Yi ) 2 N ##EQU1.2##
[0044] Where:
[0045] UCL=natural upper control limit;
[0046] LCL=natural lower control limit;
[0047] X=process average;
[0048] Yi=individual reading of the process value;
[0049] N=total number of samples.
[0050] A large percentage of the program code of controller 106
will be devoted to safety functions. So in case of an accidental or
malicious change in the process functions, the program code can
detect a change from standard conditions and shut the system down
before there can be an explosion.
[0051] The readings taken by source flowmeter 104, first
temperature sensor 108, damper flowmeter 114, first gas monitor 118
and second gas meter 116 are sent to the controller 106. From
measurement data stored internally, the controller 106 determines
if the LEL percentage downstream from the motorized damper 110 is
at a specified level.
[0052] If the fume percentage is above the upper control limit, the
controller 106 opens the motorized damper 110 and the extraction
fan 128 increases the amount of air to bring the percentage back to
within limits for safety. If the fume percentage is below the
specified level, the controller 106 decreases the motorized damper
110 and the extraction fan 128 settings to reduce the air flow.
Such brings the fume percentage back to within limits to save
fuel.
[0053] The upper natural tolerance limit and the lower natural
tolerance limit are set to include 99.73% of all readings. The
control program is capable of determining the difference between an
out-of-tolerance condition versus a random incident which can occur
and still have the process in control.
[0054] During factory calibration, controller 106 builds a series
of tables including, (1) motorized damper 110 settings and the
corresponding ambient flow, (2) fan speed (percent of maximum
setting) and the corresponding total flow, (3) user's actual LEL
percentage from the process, and (4) target percentage of the LEL.
The measurement data is saved in the controller's 106 permanent
memory and is used for a starting point when the system is
re-calibrated at the user's site. At the user's site, the
measurement data is updated if necessary to match local
conditions.
[0055] The amount of ambient air needed to keep the explosive
mixture percentage at a specified level is calculated. APLEL is the
actual LEL percentage of the effluent stream leaving the source
102. TPLEL is the target percentage of the LEL when entering the
oxidizer 126. AVC is the actual explosive-fume content of the
effluent stream in cubic feet per minute.
AVC=ESCFM.times.APLEL/100.times.LEL/100. TSCFM is the total air
flow in cfm needed to achieve TPLEL, the target LEL percentage
TSCFM=AVC/(LEL/100.times.TPLEL/100). ACFM is amount of added
ambient air in cfm needed to reach TSCFM. ACFM=TSCFM-ESCFM.
[0056] As an example, consider the following:
[0057] ESCFM=1,000 SCFM: effluent stream
[0058] LEL=2%: published LEL value
[0059] APLEL=90%: actual LEL percentage of the effluent stream as
it leaves the source 102
[0060] TPLEL=50%: target LEL percentage which will enter the
oxidizer 126. AVC=ESCFM.times.APLEL/100.times.LEL/100
AVC=1000.times.90/100.times.2/100=18 SCFM
TSCFM=AVC/(LEL/100.times.TPLEL/100)
TSCFM=18/(2/100.times.50/100)=1,800 SCFM ACFM=TSCFM-ESCFM
ACFM=1,800-1000=800 SCFM
[0061] Therefore, if the process were generating 1,000 SCFM with an
explosive-fume content of 90% of the published LEL. It would be
necessary to add 800 SCFM of ambient air to bring the mixture down
to 50% of the published LEL.
[0062] The extraction fan 128 has a variable speed motor. It is
sized so that it can remove air from the system based on the
projected worst percentage of the explosive-fume and the highest
temperature expected from the oxidizer. If the exhaust air from the
oxidizer is 1,800 SCEM at 600.degree. F., the extraction fan 128
must be capable of moving 3,600 cfm plus a safety factor. The
extraction fan 128 speed is calibrated using damper flowmeter 114
and second temperature sensor 116 with the motorized damper 110
completely open.
[0063] Starting at the fan's minimum setting, the flowmeter reports
the amount of air flowing at a series of defined steps. A typical
table would be, TABLE-US-00001 Percent of cubic maximum speed feet
delivered 10 1,120 15 2,240 20 3,360 25 4,480 30 5,600 35 6,720 40
7,840 45 8,960 50 10,080 55 11,200 60 12,320 65 13,440 70 14,560 75
15,680 80 16,800 85 17,920 90 19,040 95 20,160 100 21,280
[0064] Since the system is essentially linear, it is possible to
determine any percent setting by the following,
Cp=(((TSCFM-clva)/vstep).times.fstep)+clfa Where:
[0065] Cp=calculated percent;
[0066] TSCFM=calculated flow;
[0067] Clva=closest lower value of flow from the table;
[0068] Clfa=equivalent percent for clva;
[0069] Vstep=difference between successive volumes of air flow
steps;
[0070] Fstep=difference between successive percentages.
[0071] As an example, to move 7,000 cfm, from the above definition
and table,
[0072] TSCFM=7000
[0073] Clva=6720
[0074] Clfa=35
[0075] Vstep=1120
[0076] fstep=5 Cp=(((7000-6720)/1120).times.5)+35=36.25%.
[0077] The damper 110 is calibrated at the factory by using the
difference between damper flowmeter 114 and source flowmeter 104 to
calculate the amount air entering the duct over the range of damper
settings while maintaining the standard flow from the source 102.
For example, using settings of 5-90 degrees, a typical ambient
damper setting air flow in degrees open in SCFM, TABLE-US-00002
damper setting degrees open air flow CFM 5 500 10 1,500 15 2,500 20
4,000 25 5,000 30 5,900 35 6,500 40 7,000 45 7,500 50 8,000 55
9,200 60 10,200 65 10,900 70 11,600 75 12,300 80 12,900 85 13,400
90 13,600
[0078] There is an equivalent fan speed which moves such calculated
ambient air and the effluent flow from the source 102 at its
specified rate. Therefore, for each entry in the motorized damper
110 table, there is a corresponding fan speed, TABLE-US-00003
damper setting ambient air fan speed in degrees open flow SCFM
percentage 5 500 10 10 1,500 15 15 2,500 20 20 4,000 25 25 5,000 30
30 5,900 35 35 6,200 40 40 7,000 45 45 7,500 50 50 8,000 55 55
9,200 60 60 10,200 65 65 10,900 70 70 11,600 75 75 12,300 80 80
12,900 85 85 13,400 90 90 13,600 100
[0079] Although the change in air flow between damper settings is
not linear, the error in treating the change as linear is minor. So
the flow rates between table values can be calculated,
Cdo=((tf-clvf)/vstep).times.fstep+ed Where:
[0080] Cdo=calculated degree open;
[0081] Tf=the calculated total flow needed;
[0082] Clf=the closest lower value of flow from the table;
[0083] Ed=equivalent degree open for clf;
[0084] Vstep=difference between successive volume air flow
steps;
[0085] Fstep=difference between successive damper positions.
[0086] As an example, suppose, the damper should be open far enough
to bring in 7,200 cfm of ambient air. From the above definition and
table,
[0087] Tf=7200;
[0088] Clf=7000;
[0089] Ed=40;
[0090] Vstep=500;
[0091] Fstep=5; Cdo=(((7200-7000)/500).times.5)+40=42.0 degrees
open.
[0092] The same extrapolation equation is used to match the fan
speed to the motorized damper 110 setting.
[0093] Because there can be small variations in the actual values,
a further correction is used to achieve the most accurate flow
rates possible. For a calculated 12-degrees open, an actual flow
rate is 5,100 SCFM. The conditions required for 5,350 SCFM,
Degrees=(current degrees.times.required flow)/current flow
Degrees=(12.times.5,350)/5,100=12.6 degrees.
[0094] For safe operation, the calibration of the system is done in
two parts. A first calibration is done at the factory, while the
second calibration is done at start-up under actual running
conditions each time the system is started up. The system is
initially calibrated so that settings for the motorized damper 110,
extraction fan 128 and the oxidizer burner control have been
determined under controlled test conditions.
[0095] The settings for the motorized damper 110 are determined by
using a signal generator and the two flowmeters. The signal
generator is used to send to the calibration program the percent of
explosive-fume that the user has specified will be released at the
source 102. The oxidizer 126 is raised to incineration temperature
so that true fan speed under operating conditions can be recorded
in memory. Source flowmeter 104 measures the amount of effluent air
coming from the source 102. Damper flowmeter 114 measures the
amount of effluent air plus ambient air in the system downstream
from the motorized damper 110.
[0096] The controller 106, using the simulated fume percent and the
effluent flow rate, calculates the amount of ambient air needed to
bring the air/effluent mixture to the specified percentage of the
published LEL. The controller 106 opens the motorized damper 110 to
a position where the second flowmeter measures the total amount of
effluent plus ambient air to achieve the correct explosive
percentage while maintaining the correct effluent flow from the
source 102.
[0097] The test setting of the motorized damper 110 and extraction
fan 128 speed are put into permanent memory when the system
parameters are all correct.
[0098] To comply with air quality management district requirements,
on each start-up, the oxidizer should be brought up to temperature
before the process is started. E.g., with motorized damper 110 and
extraction fan 128 at their operating settings. The controller 106
then reads the two flowmeters and compare the two flow rates with
the flow rates in memory. If these flow rates are within the
established natural tolerances, the start-up can proceed.
[0099] The system 100 should be brought up to its operating
conditions with the thermal or catalytic oxidizer below the
auto-ignition point. However, due to air quality management
district requirements, the thermal abatement device should be at
incineration temperature before the process is started.
[0100] Under these conditions, if there were an operator error or
an act of sabotage, operating conditions must be verified as being
correct before the fume/air mixture reaches the LEL.
[0101] Operator error is more easily identified than specific acts
of sabotage where the objective is to cause an explosion. One way
to cause an explosion is to change the product formulation in such
a manner that the explosive-fume that is generated by the process
would be above the LEL. Such form of sabotage is deterred. Both gas
meters have an alarm circuit set for shut-down if the
explosive-fume meets the specified shut down level.
[0102] If the gas meters were re-calibrated so that the LEL
emergency shut-down percentage value was above the true LEL of the
fume, then the system could explode before the LEL shut-down
percentage was reached. Protecting against this event is more
complex. One procedure begins with a series of test start-ups when
it is known that all the system variables are at their specified
values. The start-up procedure is defined in such a manner that the
conditions at the point of product introduction are constant with
respect to time at every start-up.
[0103] Air quality district permits allow some deviation from
standards during commissioning. So it is possible to perform the
following procedure without being in violation of permit
conditions. When each test start-up begins, a dedicated timer is
set to zero. Then at specified time intervals, as the start-up
process proceeds, conditions for the following devices are
recorded: motorized damper 110, extraction fan 128, first gas
monitor 118, second gas monitor 120, source flowmeter 104, first
temperature sensor 108, damper flowmeter 114, second temperature
sensor 116, and oxidizer 126 burner firing rate.
[0104] From the incoming data, variable are calculated and recorded
in memory for each time interval for the motorized damper 110
setting, the extraction fan 128 speed, source flowmeter 104, damper
flowmeter 114, first gas monitor 118, second gas monitor 120, and
oxidizer 126 burner firing rate. E.g., average value, upper natural
tolerance limit, lower natural tolerance limit, and rate of change
of each device value with respect to time.
[0105] During regular start-up if one or more of the process
variables have been changed, then the values of the above eight
devices will be out of the range of either the upper or lower
natural tolerance limits. In addition, since the rate of change is
also recorded, the current rate of change is compared to the
established values in the table.
[0106] If any deviations occur, the controller 106 could shut the
system down before the process reaches the point where it is
generating explosive-fumes at a rate which exceeds the LEL.
[0107] If the extraction fan 128 were sabotaged so that the amount
of air flow was decreased, there are several ways to detect this
fault. The obvious detection would be the decreased flow reported
by the flowmeters. Also, the concentration of the explosive-fume
would be increased. Such would be caught by comparing start-up
values with the table established. It would be immediately detected
that the air flow was below the established quantity and the rate
of increase of fume concentration would also be out of tolerance.
Finally, the rate of temperature increase in the oxidizer 126 would
be above standard conditions because of the lowered air flow.
[0108] FIG. 2 illustrates a variable-source-flow system embodiment
of the present invention, and referred to by the general reference
numeral 200. An ambient air input 202 is drawn into a fume source
204. A flow of gases for incineration 206 is input to flowmeter 208
which monitors the volume of gases entering. It reports its
measurements to a system controller 210. A second temperature
sensor 112 allows the controller 210 to convert to Standard
Conditions for its calculations. A pair of redundant gas monitors
214 and 216 provide measurements for the controller 210. A
detonation flame arrestor 218 prevents flames from backing down a
flume 219.
[0109] A third temperature sensor 220 is used to control the
oxidizer temperatures. A natural gas or propane fuel input 223
supports the incineration flames inside. An extraction fan 224
forces out a clean exhaust 226 into the atmosphere.
[0110] System 200 could function without source flowmeter 208.
However, the source flowmeter will be needed to check for operator
errors and sabotage. The readings taken by the flowmeter 208, first
temperature sensor 214, first gas meter 214 and second gas meter
216 are sent to controller 210.
[0111] If the fume percentage is above an upper control limit, the
speed of the extraction fan 128 is increased to bring the
percentage back to within safety limits. If the fume percentage is
below the specified level, the speed of extraction fan 128 is
reduced. Such brings the fume percentage to within limits to save
fuel.
[0112] The upper natural tolerance limit and the lower natural
tolerance limit are set to include 99.73% of all readings. The
control program determines the difference between an
out-of-tolerance condition versus a random incident which can occur
and still have the process in control.
[0113] During factory calibration, the controller 210 builds a
series of tables, e.g., (1) fan speed (percent of maximum setting)
and the corresponding total flow, (2) user's actual LEL percentage
from the process, and (3) target percentage of the LEL. The
measurement data is saved in the controller's 210 permanent memory
and is used for a starting point when the system is re-calibrated
at the user's site and in the safety routines. At the user's site,
the measurement data is updated if necessary to match local
conditions.
[0114] The extraction fan 224 is driven by a variable speed motor.
Such type of a drive gives the fan a wide range of capacities. The
extraction fan 224 is sized so that it will remove air from the
system 200 based on the projected worst percentage of the
explosive-fume and the highest temperature expected from the
oxidizer.
[0115] For example if the exiting air from the oxidizer is 1,800
SCFM at 600.degree. F., the extraction fan 224 must be capable of
3,600 cfm plus a safety factor. The extraction fan 224 speed is
calibrated using the flowmeter 208 and first temperature sensor
214. Starting at the fan's minimum setting, the flowmeter 208
reports the amount of air flowing at a series of defined steps.
[0116] FIG. 3 illustrates a typical gas train 300 useful in systems
100 and 200 of FIGS. 1 and 2. Gas train 300 includes a gas inlet
302, a manual gas cock 304, a pilot gas cock 306, a pilot gas
regulator 308, a pilot solenoid valve 310, a gas pressure regulator
312, a low gas pressure switch 314, a pressure test connection 316,
a shut off valve 318, a normally open vent valve 320, a vent to
atmosphere 322, a second valve 324, a high gas pressure switch 328,
a second manual gas cock 330, a gas pressure test connection 332, a
pilot gas inlet 334, and a burner 336.
[0117] Two important benefits of embodiments of the present
invention are the protection against operator error and the for
detection of sabotage. They do this by building start-up charts.
The oxidizer, the fume source and motorized damper, if used, would
be at operating conditions. In a material coating example, prior to
when the first bit of the product enters the coating tunnel, the
LEL percent is zero. As the product enters and eventually fills the
coating tunnel, the percent of LEL would increase until the tunnel
is full of product.
[0118] During a series of controlled start-ups, LEL readings are
recorded, e.g., at five second intervals. For example, the oxidizer
temperature, oxidizer firing rate, motorized damper setting, and
the LEL values from the gas monitor readings. These samples can be
used to generate a curve similar to a hyperbola asymptotic to the
upper limit of the LEL percentage. Since the fumes are combustible,
they will also release heat. This will result in a gradual
reduction in the burner firing rate. These values for each reading
point are also stored. With enough points recorded during such
controlled start-ups, an upper and lower natural tolerance limit
for each point can be calculated.
[0119] On subsequent production start-ups with the same starting
zero point, the controller compares actual values for each point in
the process looking at upper and lower control limits. The
controller calculates the rate of change of the observed LEL
percentage and the rate of change in the firing rate reduction.
These values are compared to a standard rate of change. With such
information, any unexpected deviations in the control variables can
be used to signal an alarm and/or shut down the process.
[0120] Although the present invention has been described in terms
of the presently preferred embodiments, it is to be understood that
the disclosure is not to be interpreted as limiting. Various
alterations and modifications will no doubt become apparent to
those skilled in the art after having read the above disclosure.
Accordingly, it is intended that the appended claims be interpreted
as covering all alterations and modifications as fall within the
"true" spirit and scope of the invention.
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