CA1053137A - Temperature pressure activated purge gas flow system for flares - Google Patents

Abstract

ABSTRACT

A system for supplying purge gas to a flare system depending upon the temperature and the pressure in the flare, to control the flow of purge gas through the flare during the period when flaring of gas is discontinued. The rate of flow of purge gas is determined by tempera-ture measuring means. However, the control of flow depends upon the pre-ssure in the flare as well as the temperature so that when gas is being flared and the temperature is high and the pressure is above normal the purge gas is cut off, whereas when the temperature is high and the pre-ssure is normal the purge gas is flowed.

Description

1~5~ 7 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. ~ ;

L3i7 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 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

- 5 , ~,' 3'7 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 ',., ,", - ' ' .

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 `~

3'7 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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. ~ , a37 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 .: - ~ . , 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 - 12 ~
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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 .
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 .:~:. .. . .
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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~35~ 7 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 3~

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, ~ -- 15 - ~

:~S~ 37 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 - 16 ~

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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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Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. The temperature-pressure 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 ignition means and means to introduce purge gas into said system;
(b) means to measure the temperature and the pressure in said flare gas system and to determine the temperature difference between said temperature and a selected ambient temperature; and (c) means responsive to said temperature difference and to said pressure to control the flow of purge gas into said flare gas system.

2. The system as in claim 1 including control valve means responsive to said means to control the flow of purge gas into said system.

3. The system as in claim 2 in which, when said temperature difference is greater than a selected value, said control valve tends to open.

4. The system as in claim 3 in which when said pressure is higher than a selected valve, said control valve is prevented from opening.

5. The system as in claim 3 in which, when said pressure is lower than a selected valve, said control valve is permitted to open.

6. The system as in claim 2 in which said control valve is controlled proportionately by said temperature difference.

7. The system as in claim 1 including water trap means between said conduit means to introduce flare gas into said stack, and said flare stack.

8. The system as in claim 7 including in addition means to maintain a selected minimum gas pressure in said conduit means.

9. The system as in claim 7 including means for maintenance of said water trap.