Abstract:
An improved purging process in which purge gas is flowed through a flare system at a sufficient flow rate to minimize or at least acceptably control the rate of back flow of air migration into the exit port of the system. The purge gas is periodically terminated for a predetermined interval of time during which air begins to migrate into the flare system. Before this admittance of air can result in a hazardous condition within the system, the purge gas flow is re-established to sweep the air back out of the system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates generally to the field of flare gas combustion, and more particularly, but not by way of limitation, to an improved process of purging flare systems and the like. 
     2. Discussion of Background 
     Flares are devices used throughout the petroleum and chemical industries to burn combustible gases which exit the process and would otherwise flow to the atmosphere as unburned hydrocarbons. Sometimes very large volumes of these gases are released through safety devices and failure to burn these gases in a flare, thus allowing such gases to escape to the atmosphere, could result in a serious safety hazard, such as a vapor cloud explosion. A typical prior art flare system may comprise a series of conduits which connect gas sources to a vertical stack, but other types of flares also are to be understood as having difficulties that are described herein relative to a vertical stack. The stack has pilot fires burning continuously at the exit port and combustibles are ignited as they are exhausted to the atmosphere. The burning of large gas volumes can generate significant radiant heat and the flare stacks are therefore often made quite tall in order to minimize radiant heat damage at ground level. 
     Such flare stacks, as well as other types of flares, are continuously purged with a gaseous fluid to prevent air from entering the exit port and migrating into the stack which can present dangerous mixtures of air and unburned hydrocarbons. This purging usually consists of flowing a purge gas through the flare system at a rate sufficient to prevent backflow of air down the stack. The purge gas, commonly a fuel gas or nitrogen, serves to keep air out of the stack, thus preventing formation of certain mixtures of air and gas which, when ignited, can result in explosions within the flare stack. 
     At one time, the amount of purge gas used was of little concern as fuel gas and nitrogen were very inexpensive. However, basic costs of energy have risen dramatically over the past several years and the cost of purge gas has risen as well. As a consequence, several prior art devices have been used which substantially reduce purge gas flow rates required to effectively prevent air migration in flare systems. Such prior art devices serve to retard the flow of air down the stack. 
     SUMMARY OF INVENTION 
     The present invention provides an improved purging process in which purge gas is flowed through a flare system at a sufficient flow rate to substantially control the rate of back flow air migration into the exit port at the exit of the system. The flow of purge gas is periodically interrupted; that is, the flow of purge gas is ceased for a predetermined interval of time during which air begins to migrate into the flare system. Before this admittance of air can result in a hazardous condition within the system, the flow of purge gas is re-established to sweep the air back out of the system. 
     This carefully controlled interruption of purge gas flow results in a significant reduction in the amount of purge gas required and thus provides advantageous cost savings. Also, longer flare system life results since the flame at the exit of the system is extinguished during such periods of purge gas interruption; this substantially extends flare tip life because the continuous existence of flame at the exit port inevitably results in deleterious effects on the system. 
     An object of the present invention is to provide an improved purge gas process requiring a minimum amount of purge gas to achieve safe operation of a flare stack system. 
     Another object of the present invention, while achieving the above stated object, is to minimize the cost of safely purging a flare stack system. 
     Yet another object of the present invention, while achieving the above stated objects, is to provide a purging process which extends the operating life of a flare gas system tip. 
     Other objects, advantages and features of the present invention will become clear from the following detailed description when read in conjunction with the accompanying drawings and with the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a semi-detailed schematic representation of one embodiment of a flare gas system to perform the present inventive process. 
     FIG. 2 is a semi-detailed schematic representation of another flare gas system to perform the present inventive process. 
     FIG. 3 is a semi-detailed schematic representation of yet another flare gas system to perform the present inventive process. 
     FIG. 4 is a flare tip assembly that incorporates a reverse flow seal chamber which further reduces the amount of flare gas used in the present inventive process. 
     FIG. 5 is a graphical depiction of tests performed on two types of flare tip systems in the performance of the present inventive process. 
    
    
     DESCRIPTION 
     With reference to FIG. 1, waste gases are supplied to a flare system 10 having a flare stack 12 via a conduit 14. The waste gas flows upward through a purge reduction seal 16 to a tip 18 where it exits the flare system 10. The purge reduction seal 16 is not required to practice the invention but is preferred due to its ability to reduce the purge gas flow. The purge reduction seal and flare tip are discussed further hereinbelow. 
     The flare system 10 further comprises a continuous burning pilot 20 disposed near the flare tip 18. The purpose of the pilot 20 is to ignite any gas exiting the flare tip 18. 
     Purge gas flows through a conduit 22 and a motor valve 24 to the base of the flare stack 12. The flow of purge gas continuously sweeps air from the stack when no flow of flare gas via conduit 14 occurs. As discussed herein, the interruption of purge gas flow results in a slow migration of air into the stack via the exit tip 18. Research into this phenomena now allows prediction of the rate at which air migration into the system will occur and thus the length of time that purge flow may be interrupted without excessive amounts of air entering the system. A conventional timer control 26 closes the valve 24 for a predetermined time interval via an electric signal through a conduit 28 connected thereto and signals to reopen valve 24 at the end of the selected time interval. 
     FIG. 2 is another flare system 30 for the practice of the present invention. Except as now indicated, the flare system 30 is identical to the previously described flare system 10, and like numerals appear in FIG. 2 to identify the same components. As shown in FIG. 2, a conventional oxygen analyzer 32 is used to measure the oxygen content in the flare stack 12 and actuate the valve 24 based on the measured oxygen content. That is, the oxygen analyzer 32 is set to signal the opening and closing of the valve 24 via the conduit 28A connected thereto in order to effect the flow of purge gas only when the oxygen content exceeds a safe limit. 
     FIG. 3 shows yet another flare system 40 for the practice of the present invention. As for FIG. 2 above, like numerals are used in FIG. 3 to identify the same components described hereinabove for the flare system 10 and for the flare system 20. In FIG. 3, purge gas flow is periodically interrupted by the oxygen analyzer 32 causing valve 24 to selectively open and close via a signal through conduit 28B connected to the timer control 26 and thus to the valve 24. Thus the timer control 26 is interposed in the control system such that the valve 24 is opened and closed by either the oxygen analyzer 32 or the timer control 26. This adds a control redundancy, and consequently, creates a safer system. 
     Also shown in FIG. 3 is the temperature sensor 33 which senses a change of temperature condition at a selected point in the stack 12, or if preferred, the temperature sensor 33 can as well be located elsewhere, such as in the conduit 14. This can prove beneficial in the case where the flare gas passing through the stack 12 is effected by the release of a condensable vapor. Where condensation is occurring in the stack, there is a consequent pulling of air into the flare by the attendant pressure reduction. In such a case, the temperature sensor 33 (or alternatively, the sensor 33 can be a pressure/vacuum sensor, if desired) can be used to operate in combination with, or in lieu of, the oxygen analyzer 32 and the timer control 26 to control the purge gas interruption. 
     A purge reduction seal of the type discussed briefly above and enumerated 16 will now be described with reference to FIG. 4. Shown therein is a single stage flare tip assembly 50 which attaches to the upper end of a conventional, single conduit flare stack 12A and which is constructed in accordance with my U.S. patent application Ser. No. 485,623, Smoke Suppressant Apparatus for Flare Gas Combustion, filed Apr. 18, 1983 and incorporated by reference herein insofar as necessary for purposes of the present teaching. Flare gas discharge from the flare tip assembly 50 will be configured as a relatively thin layer of cylindrically shaped flare gas. The flare tip assembly 50 comprises a bolt-on flare conduit section 52 which extends upwardly from the flare stack 12A, the flare conduit 52 having an open upper end 54. A cylindrically or tubularly shaped flare housing 56 is connected to the flare conduit 52 via a pair of gusset supports 58 and by an annular bottom plate 60 welded to the lower end of the flare housing 56 and to the outer wall of the flare conduit 52. Disposed coaxially within the flare housing 56 is a liner cylinder 62 which is supported via a number of vertically extending divider members (not shown) that weldingly interconnect the liner cylinder 62 and the flare housing 56. Formed between the coaxially disposed liner cylinder 62 and the flare housing 56 is an annular orifice channel 64 which has an exit port at the upper end 66 of the liner cylinder 62, the annular orifice channel 64 being sealed at its lower end by the bottom plate 60. 
     The liner cylinder 62 has a seal plate 68 welded to the internal wall of the liner cylinder 62 and dividing same into a lower portion 70 and an upper portion 72. The flare conduit 52 extends upwardly into the lower portion 70 of the liner cylinder 62, having its upper end 54 disposed below the seal plate 68. Formed between the inner wall of the liner cylinder 62 and the outer wall of the flare conduit 52 is an annularly shaped reverse flow channel 74, the reverse flow channel 74 having fluid communication with the annular orifice channel 64 as shown. If desired, a fluid injector pipe 76 can extend through the walls of the flare housing 56 and the liner cylinder 62 and connected to and in fluid communication with an externally disposed fluid injector 78. 
     In the flare tip assembly 50, flare gas passes upwardly via the flare conduit 52 and flows from the upper end 54, the upward flow thereof being blocked by the plate 68 which serves to seal the upper portion 72 of the liner cylinder 62. The flare gas is caused to reverse its upward direction to flow downwardly through the annularly shaped reverse flow channel 74 as indicated by the arrows 80 and 82. The lower end of the liner cylinder 62 is disposed somewhat above the bottom plate 60, and the gas discharging from the reverse flow channel 74 is again caused to reverse its direction and to flow upwardly into the annular orifice channel 64, as indicated by the arrows 84; the flare gas discharges at the exit port of the annular flow channel 64 provided at the top of the flare tip assembly 50. The flare gas can be discharged from the annular orifice 64 into the atmosphere in the form of a perimeter zone discharge, or it can be passed to the tip 18 as shown in the previous figures. 
     The flare tip assembly 50 may also be equipped with an externally disposed fluid injector assembly 86 and with the conventional pilot 20. Also, the upper portion of the internal wall of the liner cylinder may be lined with a refractory (not shown) if required to protect the structure from the burning flare gas. 
     The flare tip assembly 50 provides a reverse flow seal chamber between the flare conduit 52 and the annular orifice channel 64. During purge operations, this reverse flow seal chamber serves to entrap a portion of the purge gas generally within the space formed in the reverse flow channel 74 below the seal place 68 and the lower portion of the annular orifice channel 64, and this occurs whether the purge gas is heavier or lighter than atmospheric air. The result of this purge gas entrapment is to minimize the amount of purge gas required to retard the backflow of atmospheric air into the flare stack. 
     EXAMPLES 
     A series of tests were performed to determine the rise in oxygen content versus time for two types of flare tips mounted on a reverse flow seal chamber. A basic pipe flare tip consisting of a straight section of pipe was used in one series of tests. This basic pipe flare tip is of a design well known to those skilled in the art and represents the most simple type of flare tip. A flare system of the type depicted in FIG. 4 hereinabove was used in a second series of tests; in contrast to the simple basic flare tip, the flare system 40 represents an advanced technology tip of the latest designs commercially available. Both tips were mounted on conventional reverse flow seal chambers and mounted on stacks. An oxygen analyzer was used to monitor the oxygen content below the reverse flow seal chamber. Natural gas was introduced at the base of the flare stack. 
     Purge gas was used initially to clear all oxygen from the system. The purge gas flow was then stopped and the oxygen content measured versus time. After collection of oxygen measurements, the system was purged again and the decay curve data of FIG. 5 generated. The data varied with type of tip and weather conditions but all fit within the band shown on FIG. 5 between curves 1A and 1B. 
     One series of tests were performed to specifically determine the time required to purge the air from the system versus the flow rate of the purge gas. In these tests, the purging was continued until the oxygen content was less than 1% and then interrupted for one hour. The oxygen content was recorded at the end of one hour and then the purge flow was reestablished and the time required to reach an oxygen content of less than 1% measured. Typical results for 18 inch outer diameter vertical flare stack with reverse flow seal as shown in FIG. 4 are provided in the following table. 
     
         ______________________________________Oxygen @   Oxygen after Minutes   fpsstart   1 Hour       Purge Time                          Purge Velocity______________________________________0.95%   1.45%        10        0.0040.85%   1.40%        13        0.0030.90%   1.50%        17        0.002______________________________________ 
    
     The oxygen concentrations shown in the above table of data are recognized under good engineering practices as being within acceptably safe operating conditions for vertical flare stacks. It is believed that significant deviations from these conditions can be tolerated without presenting a safety hazard. 
     While this data is for a vertical stack and will enable one skilled in the art (using generally accepted extrapolation techniques) to calculate the design requirements for any size vertical flare, it will be understood that one skilled in the art could use similar techniques to predict design criteria for other types of flares, such as, but not limited to, ground flares, pit flares and inclined boom flares now commonly found in the art. 
     Thus it is clear that the present invention is well adapted to carry out the objects and to attain the ends and advantages mentioned as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes may be made which will readily suggest themselves to those skilled in the art and which are embodied within the spirit of the invention disclosed and as defined in the appended claims.