Note: Descriptions are shown in the official language in which they were submitted.
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This invention is related to U.S. Patent No.
3,741,713 issued June 26, 1973r entitled: PUR~E GAS
ADMISSION CONTROL FOR FLARE SYSTEM.
In ~hemical and petroleum refining systems it is
necessary to maintain a flare stack through which waste
combustible gases can be released and burned in such a
way as safely to be disposed of, with a minimum of
pollution. Often very large quantities of flare gas ~ r~
become available in emergency situations, which gases ~;
are, at times, at a temperature i:n excess of 250 to 300 F.
A constantly burning pilot flame is provided so that any
gas introduced into the flare stack will be ignited
without fail at exit.
When the flaring of gas is stopped at the conclusion
of the emergency, the stack, and the gases in the stack,
which have been at an elevated temperature, begin to cool
to ambient temperature. As a result, the pressure of
the gas in the stack reduces proportionately to the
absolute temperature of the gas. This reduction in
pressure permits atmospheric air to be drawn into the
top of the flare stack since flow has stopped. This
creates a dangerous situation. When the next flaring
of gas is required, because of the combustible nature
of the flared gas, the pressure of the air can provide
an explosive mixture which is easily ignited because
of the constantly burning pilot.
In order to keep the system air free at all times,
it is common to admit to the system a continuous flow
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of purge, or sweep gases, to maintain CQnstant slow
movement of gases through the system and out to the
exit point. Two factors govern ~he puxge or sweep
gas movement. One factor i5 due to the passage of
wind blowing across the open discharge end of the
stack. To counter this effect it is common to use
what is called a molecular seal in which there are
two flow reversals. Such seals are described in -
U.S. Patents 3,289,729 and 3,055,417. This allows
entry of outside air only to the structure downstream
of the molecular seal which is placed generally
immediately below the point at which ignition occurs.
For large flare stacks this is an expensive addltion to
the stack and does not provide a complete solution to
the problem. ~ ;
The second factor is temperature change within the
system, due to either meteorological conditions, or due
to the flaring of waste gases at significant temperature
level. For example, in a flare stack 30 inches in
diameter and 500 feet tall the system volume is
approximately 2400 cubic feet. Assuminy such a volume
is filled with flare gases at 250 F. during a flaring
period and when the discharge of hot gases is stopped,
the system will cool to ambient temperatuxe in about
15 minutes.
The volume of gases within the flare stack at
atmospheric pressure would decrease proportionately to `~
the absolute temperature to approx~mately 1700 cubic feet. ~ ;
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Thus, air will be drawn i~nto th~ st~ck ~n the amount
of 2400 minus 1700 or approximately 700 cubic feet.
This would cause an air penetration down the stack of
appxoximately 150 feet~ Since this column of air
would travel for approximately 15 minutes it corresponds
to a velocity of 0.15 feet per second. Thus, a 3Q inch
stack would require about 700 cubic feet of purge gas,
or to provide a margin o safety, approximately 800 cubic
feet each 15 minutes. The hourly volume would be about
32~0 SCFH.
When the system is at ambient temperature there is no
longer need for this large flow of purge gas at 0.15 feet
per second. A nominal flow velocity o~ about 0.03 feet
per second is adequate to insure that air is kept out
of the stack at all times. ~-
Th prior art, as represented by the patent U.S. No.
3,741,713 is inadequate for several reasons:
1. Wastefulness of purge gases, which generally are
~ hydrocarbon gases, or fuel gases. This waste is
contrary to present day fuel-heat energy conservation
requirements.
2. Tha prior art systems do not respond to small ~ -
temperature changes in flare contained gases, whlch can
be shown to be dangerous.
3. The prior art systems do not prevent the entry or
use of purge gases when there is flow within the flare
system and purge gases are not required for flare safety.
4. In the case of flare systems which operate with
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liquid seals at the flare base, constant purge or
sweep gas flow is not required to further benefit
fuel-enexgy conservation.
The terms purge or sweep yas are used interchangeably.
Such gas can be any substance which is in gaseous phase
at any temperature to which it may be subjected in this
~ystem. Con~only used purge gases are methane, ethane,
propane, and nitrogen, but other gases such as argon can
be used. j
The purpose of the use of purge gases in flare systems
is to avoid allowing a static, or reversed Elow, in the ~ -;
entire flare system and to always maintain some small
flow toward the discharge point of the flare system.
The flare system does not become dangerous until such ~ -
time as air or oxygen is present within the system~ If
the system flow is allowed to reverse, air is drawn into
the system and there is immediate hazard of explosion
or detonation, whenever normal forward flow is re-established ?
and the air-gas mixture meets the pilot flame.
Practice in the process industries where flares are
used, has been to admit purge gas to the flare system to
cause movement within the system toward the flare at
commonly accepted flow velocity of 0.05 feet per second,
as a minimum. This flow may seem small, but for such
movement in a 24 inch flare system the purge gas required ;
is 540 SCF~I. For a 36" system the gas required i5 1,237
SCFH and for a 48" system the gas required is 2,215 SCFH. ~-
All of these are common system sizes. In some cases this
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volume of purge gas can be sa~ed, and in the case of
methane the saving is a startling $2,365 per year for
a 24" system or $9,700 per year ~or a 48 inch system.
Some pxocess plants may have as many as five flares.
The purge ratP of 0.05 feet per second is considered
satisfactory for a drop in system temperature of ~09F
in 15 minutes (such as from 80F. to 60F.) But, if
the temperature drop should be 40F. in 15 minutes it
is necessary to increase purge gas volume to maintain
the same condition of flare saf~ty because the ~as
volume is prorportional to absolute temperature at the
same pressure condition. ~mbient conditions of sunshine,
or chilling rain or wind action, plus a drop in atmospheric ~ -
temperature, combined with above-ambient stack gas ` ;~
temperature, can produce 15 minute temper~ture drops as
great as 90F. under summer conditions. At 90F. drop -
in 15 minutes the purge gas movement must be 0.083 feet ;~
per second for safety maintenance.
The data just shown are justified by calculation of ;~
the decrease in flare gas volume when there is a drop in
temperature of the gas contained in the flare system when `
there is no flow. For example, in a drop of temperature
from 80F. to 60F. the volume V within the flare system
drops to 0.963V. With a drop from 150F. to 60F. the
volume V is reduced to 0.852 ~ at the same pressure. `~
In the case of drop in temperature from 80F. to 60F. i~
this requires a volume of purge gas of 0.037 V or a flow
of purge gas at the rate of 0.050 feet per second. For
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the case of drop from l50~F. to 60F. a volume of purge gas of
0.148 V is re~uired or a ~low rate of purge gas o 0.083 feet per
second. ~ ~ ~
It is thus to be seen that in normal meteorological~ `
changes, sa~ety requires that purge gas volume be modulated to
suit the temperature condition of the gas content of the flare
system when relief venting of waste gases is not present. This is
according to normal weather conditions, but in relief of hot gases ~ ~-
from process operations the gas temperature can be as high as 500
F. or more and purge gas demand is greatly increased to 0.619 feet
per second to fur~her add to the need for gas modulation according
to ~he system temperature.
The prior art such as illustrated by Patent 3,741,713
is directed to the higher temperature condit;on but it makes no
provision for normal weather changes when, as shown, there is need ~ ?
for added purge rate. The prior art also makes no pr~vision for ;~
purge rate modulation according to the flare system gas temperature -
for purge gas conserva~ion. ~
The present invention, therefore, is a temperature-pres- ~ ;
sure activated purge gas flow system for waste gas flares comprising:
(a) a flare gas system including flare stack conduit means
to introduce flare gas into said stack, pilot ignltion means and
means to introduce purge gas into said system;
~b) means ~o measure the temperature and the pressure in
said ~lare gas system and to determine the temperature difference
between said temperature and a selected ambien~ temperature; and
(c) means responsive to said temperature difference
and to said pressure to control the flow of purge gas lnto jaid ~
flare gas system. When ~he temperature is above normal and the ~-
pressure is normal purge gas is supplied in accordance with the
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temperature. When the temperature is high an~ the pressure i5 also
high the flow of purge gas is not allowed to occur.
The invention will now be ~escribed, by way of example
only, with the use of drawings in which.
Figure 1 represents a situation in which a water seal is
provided in the flow system of the flare.
Figures 2 and 3 illustrate the situation where there is
no water seal in the flare system, but the flare gas is controlled
speci~ically by the temperature in the flare system and the pres- -
sure in the flare system.
Before explaining the present invention in detail, it
is to be understood that the invention is not limited in its appli-
cation to the details of construction and arrangement of parts
illustrated in the accompanying drawings, since the invention is
capable of other embodiments and of being practiced and carried out
in various ways.
Also it is ~o be understood that the phraseology, or ter-
minology, employed herein is for the purpose of descrip~ion and
not of limitation.
Referring now to the drawings and in particular to Pigure
1 there is shown a flare system indicated generally by the numeral
10, a purge gas system indicated generally by the numeral 12, and
a pressurè control system indicated generally by the numsral 14.
The system of Figure 1 includes as part of the flow system, a water
seal in the base of the flare stack wherein a column of water
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28 has immersed in it, the flare gas conduit 24 which
has a downwardly depending portion 26 immersed in the
water to a depth H. The depth ~I is sufficient so that
there is substantially no danger that the water level
will be drawn dawn to the point where ga~ in the stack
can leak back into the conduit 24.
The base of the stack 16 rests on the grade surface
46 and comprises a plurality of sections 18 terminating
at the top opening 22. There is a pilot ~ixture ;~
providing a constant flama for igniting the waste
combustible gases that will flow in accordance with the ;;
arrows 13 through the conduit 24-26 and bubble up
through the water 28 and flow upwardly through the
flare stack 18 to the top opening 22 where it will be
ignited and will burn.
When the flow of gas stops, the water seal at 28
prevents any of the gases in the stack 18 from re-entering -
the pipe 24. However, to further prevent this flow
backward there is a pressure connection 32 from the conduit ;~
24 to a pressure sensitive switch 34 which is supplied
with electrical power over leads 36. When the pressure
in the conduit 24 is low, power is applied through two
leads 38 to the electrically controlled valve 40 which
permits purge gas from input pipe 42 to pass through the
valve 40 and through line 44 into the conduit 24 to
maintain the pressure in the conduit at a preselected
pressure level. With this pressure at the se-l~c~d level
there is no tendency for gas to be withdrawn from the `~
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stack back into the conduit 26.
In accordance with the teachings of this invention,
measurements are made of the gas temperature in the flare .
stack and the pressure of the gas in the flare stack to ~.-
so control the flow of purge gas as to prevent any entry
of air intv the stack through the opening 22, and to
provide ~his control with a minimum total flow of purge
gas, for ~he purpose of fuel and cost saving.
A temperature sensor 48 which can be of a thermistor, :
thermocouple, capillary, or other type in a¢cordance
with the teachings of U.S. pa~ent 3,741,743 is inserted
into the flare stack 16-18. With a suitable control
means 50, as well known in the art, a signal is sent to
a thermal controller T indicated by numeral 52. Power
supplied by leads 54 to the thermal controller is `.
connected by the controller through leads 53 and 55 to
an electrically controlled valve 58. Purge gas in the .
conduit 60 is controlled by valve 58 to flow through
lead 62 into the base o the flare stack 16 and above
the level o~ the water 28. In other words when the
temperature in the stack is above normal, purge gas could
be permitted to flow through the valve 58 into the stack. ~ .
Although the prior art indicates that this thermal . :
control is satisfactory, it is in fact not fully `~
satisfactory and, therefore, it becomes desirable to
control the gas further by means of a pressure switch P
indicated by numeral 55. This is connected by conduit
64 to the flare stack 18 and the pressure switch 56 is :
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interposed between the leads 53 and 55. When the -
temperature at 48 is above normal the temperature
control device T 52 provides a signal to the valve 58
to cause purge gas to flow through lead 62. However,
when the flare gas is flowing, there is no need for
purge gas because there is flow upwardly in 18 and
the pressure in 18 is greater than noLmal. Thus when ` ;
the temperature indicated by thermal sensor 48, and
the pressure on conduit 64 are both above normal, the - ;~
pressure switch 56 prevents the passage of signal from
the temperature controller 52 to the valve, so the
valve remains closed. However, when the flow of flare ;~
gas stops, the pressure will reduce to normal, while the `
temperature still remains high. Therefore, under these
conditions`it is necessary to flow purge gas, and the ;
pressure switch 56 then closes permitting the signal ,~
from the thermal control 52 to control the valve 58 and
cause purge gas to flow into the flare stack. As the
purge gas flows and as the gas in the flare stack
cools the temperature indicated by sensor 48 eventually
reaches normal value and the thermal control 52 causes
the valve 58 to close and cut the flow of purge gas to
the stack~
In FIGURE 1 the control system indicated by numeral
12 employing thermal and pressure switches provides a
more controlIed flow of purge gas in accordance with the
temperature and pressure in the stack than does the
prior art. FIGURE 1 shows also an additional feature -~
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which has been used in prior art systems namely of
the water seal at 28. However, in FIGURE 1 there is
a furthex control indicated by numeral 14 which, as
needed, delivers a flow of purge gas in ~o the conduit
24 to maintain pressure. This involves the pressure
switch 34 and control valve 40. -;~
When flared gas flow in conduit 24 ceases~ the
immediate resting pressure in 24 is H inches water-column
and if the gas inside 24 is at ambient temperature and
if there is no leakage from 24, the resting pressure does
not change. But if the gases inside 24 are at elevated
temperature, there is reduction in both volume and -
pressure due to cooling or pressure in 24 is reduced by
leakage, correction is immediately needed.
Pressure in 24 should always be above atmospheric
pressure to avoid leakage-entry of air to 24; also to
avoid withdrawing water 28 from the base of 16 which ~
can occur if initial gas temperature is high enough ~ -
as gas flow ceases. Normal pr~ssure in 24 is grQater
than atmospheric. -
The system indicated by 14 incorporates means to
admit purge gas from substantial pressure to 24 when
the pressure internal of 24 falls to just above
atmospheric pressure. Pressure switch P senses the -
internal pressure of 24 through 32. As the pressure
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internal of 24 falis for any reason to near-atmospheric
pressure, the switch P (34~ closes. Power from 36 then
is applied through 38 to a gas valve 40 which ~hen opens
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to allow passage of purge gas from 42 to 44 and
thence to 24 for immediate restoration of pressure
within 24.
The control valve 58 is preferably one in which
the rate oE flow of purge gas through the valve ~rom
pipe 60 to pipe 62 is variably controlled in accordance
with the temperature difference ~etween the sensor
temperature and the preselected ambient temperatures,
so that as the temperature difference increases the rate
of flow of purge gas increases proportionately. The
type of control instrumentation shown is well known,
and instruments are available on the market, so that i
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further detail of the instrumentation is not re~uired. ~;~
In conduit 24, when there i5 flow of flare gas the `~
pressure is always above the normal and, therefore,
there is no flow of purge gas from line 42 through
valve 40 to line 44. However~ when flow ceases in 24
and pressure inside 24 falls to just above atmospheric
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for some reason, controls 14 immediately admit purge
gas from 42 through 40 ancl 44 to 24 for immediate
restoration of pr~ssure in 24. Control of the rate of
flow of purge gas through valve 40 is not precisely
controlled as is the flow through valve 58.
In FIGURES 2 and 3 there is shown a modification
of the system o FIGU~E 1 in which the water trap
comprising the water column 28 and inverted conduit 26,
is not present and the entire controi of purge gas is by ~;
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a system similar to that indicated generally by
numeral 12 in FIGURE 1. There is in FIGURE 2 for
example, the same sensor 48, sensor control 50, lead
51 to a temperature controller 52 supplied with power ~:
on leads 54 and connected to leads 53, and 55 to a flow
valve 76 connnected between the line 78 carrying -the ;
purge gas and line 80 connected to the conduit 72, through
which the waste flare gas passes into the flare stack 70. ~ :;
In addition, there is a pressure lead 74 connected from ~ ~:
the conduit 72 to a pressure switch 56 interposed between ~ ;~
the temperature controller 52 and the valve 76.
As in FIGURE 1 the temperature sensor 48 controls the
valve 76 as a function of the temperature measured-. : :~
However, the control signal from the thermal controller ~.
52 is controlled further by the pressure switch 56
responsive to the pressure in the conduit 72 which is
indicated on lead 74.
Here again, when the temperature of the flare gases
moving in-to the stack 70 is high, and the pressure is
low, the pressure switch 56 is closed and transmits the
signal from the thenmal controller S~ to the valve 76
permitting a controlled flow of purge gas throu~.h line 80
into the conduit 72 and to the stack. On the other hand, . ;: .
if the pressure in the conduit 72 is high and the . ~ ~.
temperature at 48 is also high, indicating that there is
a flow or flare gas at that time, there is .no need for ..
purge gas so that the pressure switch 56 will open and
prevent the signal from the ~hermal controller reaching
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the valve 76.
As in FIGURE 1 there are two main differences
between this system and the conventional priox art ;
system, namely that the control of purge gas is made ~ ;dependent upon both the gas temperature in the flare
stack and the pressure in the flare stack, whereas in ;-~
the prior art the control of purge gas was made solely
in the basis of temperature. Furthe~nore, there is
the improvement of having a variable flow value to
control the purge gas, the rate of flow being a selected
function of the temperature difference between the
existing stack gas temperature and normal temperature
depending upon the season and the weather and other .
local factors. `~
In FIGURE 3 is shown a system similar to that sf
FIGURE 2 except that it uses a type of thermal sensor
which is a liquid-filled capillary type of sensing
device well known in the art. The type of thermal `~
control 88 would be different from thé control 52 inasmuch
as the thermal sensor 86 is diferent from that of 48.
However, the commercial devices which are available on ;~
the market provide the proper control between temperature
and switch opening and closing, so that with the proper
control device any one o the conventional types of -~thermal sensors can be used, as is well known in the ;
art.
While the control system is shown as a series of ~-
separate elements, such as sensor 48, sensor control 50, ~ -
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lead 57, temperature controller 52, pressure switch 56
and valve 76, these elements could be combined in any
desired way ~o provide the equivalent, overall system. ~;
Irrespective of the individual elements and their
names, etc., the novelty o this system resides in the
dual control of purge gas based on the temperature and
the pressure in the flare system. It also includes
proportional control of the purge gas based on temperature
difference.
In FIGURE 1 where there is a water-seal formed by
immersion of 26 in 28 to depth H, immersion is critical
to avoid reversed gas flow rom the interior of 16 above
28. To provide evidence of the existence of 28 a level
lndicating device 30 is provided. Visual check o~ level
at 30 is a gesture toward safety but in the abse~ce of
visual check, an alarm can be sounded when the level of
28 falls because of leakage, drainage or other cause.
Any typical alarm device can be used. The element 30
~0 can also be adapted, by means commarcially available,
for addition of water to 28 if it is needed for any
reason and as it is needed normally.
While the invention has been described wlth a certain
degree of particularity, it is manifest that many changes
may be made in the details of construction and the
arrangement of components. It is understood that the
in~ention is not to be limited ~o the speclfic embodiments ~ ;
set forth herein by way of exemplifying the invention, but
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the invention is to be limited only by the scope of
the attached claim or claims, including the full range
of equivalency to which each elemen~ or step thereof
is entitled. ~.: :: -
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