The present invention relates to flare burning. More particularly, it relates to a system for flare burning of a combustible gas, e.g. a hydrocarbon gas.

It is known to manage and/or dispose of waste gases by flare burning. For instance, flare burning is routinely carried out in the oil and gas industry and the chemical industry, e.g. at production sites and refineries.

In onshore oil and gas operations, it is known to use ground flares or elevated flares for flare burning of gases.

Typically, a ground flare comprises an array of flare tips comprising a plurality of rows of flare tips. The flare tips are located at or close to, e.g. a few metres above, ground level within an at least partially enclosed area. Each flare tip comprises a section of pipe and at least one nozzle. In total, i.e. across all of the flare tips, a ground flare may comprise many nozzles, e.g. 150 to 1000 nozzles for a 3000 mmscfd (million standard cubic feet per day) flare. Gas for flare burning is supplied through pipes known as runners, each runner supplying gas to a plurality of flare tips. A large ground flare may comprise from three to seven runners. Typically, each runner may be provided with one, two or three pilot burners.

The at least partially enclosed area (which is rarely totally enclosed) may have a large footprint, e.g. a rectangle having sides of from 50m to several hundred metres in length. The at least partially enclosed area may be bounded by a wall or barrier. The wall or barrier may be higher at least in part than the flame(s) produced by the ground flare.

An area known as a sterile zone extends beyond the at least partially enclosed area. The sterile zone is an area into which personnel are not allowed to go, when flare burning is taking place, due to the heat flux from the flare and the additional safety risk of possible delayed ignition of unignited gas. The extent of the sterile zone may be determined in accordance with industry safety guidelines.

For instance, an industry safety guideline as outlined in API 521 sets a limit of 1.5 kWm ^"2 on the heat flux which personnel may experience. For short lengths of time, the limit may be higher, e.g. 4.7 kWm ^'2 for up to 60 seconds or 6.3 kWm ^"2 for up to 20 seconds. The sterile zone may extend a considerably shorter distance, e.g. around 25m, where the at least partially enclosed area is fully enclosed by a wall or barrier that is higher than the flame(s) and/or the ground flare is operating at a low rate.

In any case, it should be appreciated that the sterile zone may extend for several hundred metres, especially where the flame(s) are higher than the wall or barrier around the at least partially enclosed area and/or the flare is operating at a very high, e.g. emergency, flow rate.

Ground flares may be designed to operate at low pressures, e.g. 0.2 barg (3 psig) or 2 barg, high pressures, e.g. 8 barg, or at a combination of both low and high pressures, depending upon the design of the chemical plant, production facility or refinery, in particular, the processing design adopted for the combustible gases.

Ground flares operating at relatively low pressures may be especially preferred at sites such as refineries and/or sites where environmental considerations such as noise, flame height, radiation or toxicity are important.

At large, remote sites and/or sites where space is at a premium, high pressure ground flares may be preferred.

It is known that a significant portion, e.g. 25%, of the flames from a ground flare may rise above the height of the wall or barrier around the at ieast partially enclosed area.

It is known that elevated flares, in which the flare tip may be 20m or more, e.g. 50m, above the ground, may be more cost effective than ground flares. The sterile zone around an elevated flare may be larger than around a ground flare having an adequately high barrier or wall bounding the flare tips. In cases where the barrier or wall is not adequately high, the sterile zone around a ground flare may be larger than around an elevated flare of comparable combustion capacity.

The flame from an elevated flare can extend to a height of several hundred metres, e.g. 250m or more. This may be undesirable and or problematic, e.g. local communities may object to the sight and/or noise. Also, it may affect helicopter operations, which may be an important consideration at a large, remotely situated facility.

Known ground flares and elevated flares suffer from a number of other problems.

First, the size of the footprint for ground flares in particular is typically very large. Consequently, it may not be feasible to install them anywhere other than at large sites. For instance, it is known that in order to compensate for periods of flows at high rates that the size of the footprint of a high pressure ground flare is typically of the order of 2000 to 6000 m per mmscfd of gas flared. In one known case, a 3000 mmscfd ground flare has a footprint of 9000 m ² and the sterile zone extends further to approximately 150m radius, in order to reach the 1.5 kWm ^'2 heat radiation exposure limit.

It is known that delayed ignition can be a problem with ground flares and elevated flares.

By delayed ignition is meant that a quantity of gas exits the flare tip before the stream is ignited. It may be difficult to access pilot burners for maintenance in ground flares and even more so in elevated flares. This may mean that delayed ignition is more likely.

A further problem with known flares is cold venting. Cold venting may be thought of as an extension of delayed ignition and is when a quantity (large or small) of unignited gas flows out of the flare tip(s). Cold venting can lead to clouds of vapour forming, which may be unsightly and/or polluting and/or noxious and/or toxic and/or an explosive hazard, and should, therefore, be avoided or at least minimised.

A further problem associated with flares, in particular ground flares, is internal burning. This is when gas ignites within the runners. It can cause significant damage to pipework and valving.

It would, therefore be desirable to provide an improved apparatus for and method of flare burning.

A first aspect of the invention provides a system for flare burning of a combustible gas, e.g. a hydrocarbon gas, comprising a plurality of sonic flare stacks arranged at a site, each sonic flare stack comprising a sonic flare tip, wherein the sonic flare tips are angled inwardly and upwardly such that, during flare burning, flames from the sonic flare tips intersect.

The gas that is flared in the flare system is typically excess gas that has been separated from oil at an oil production facility. When an abnormal situation occurs in the production facility, for example, too high a gas pressure is detected in a separator, compressor or flow line, or leakage of gas is detected, pressure measuring instruments will register such malfunctions and open valves located in the pipework of the production facility to direct the excess gas via a main delivery conduit to the flare system.

Typically, a flare tip will comprise one or more nozzles, through which gas exits, one or more associated pilot burners and ignition assemblies for igniting gas exiting the nozzle(s). By flare stack is meant the structure upon which a flare tip is mounted. Typically, a flare stack includes an upwardly inclined, e.g. substantially vertical, pipe for transporting to-be-flared gas from ground level or below to the flare tip. Typically, the upwardly inclined pipe may have a diameter of from 100 mm to 560 mm and may be made from carbon steel, stainless steel, or molybdenum containing nickel iron chromium alloys such as Incoloy® (ex Corrotherm International). Generally, each flare stack will also include pipework and electric cabling for pilot burner(s) and ignition assemblies.

By sonic flare stack is meant a flare stack from which, during flare burning of such excess combustible gas, the flared gas exits the flare tip at a speed of about 330 ms ^"1 or more under optimal flowrate conditions, for example, at its maximum design flowrate. A sonic flare stack may have a sonic flow design. A sonic flare stack may operate at below sonic conditions, e.g. during times of reduced flare burning or during stand-by operation (also referred to as purging).

During stand-by operation of the flare system, combustible gas may be either vented into the atmosphere and/or may be burned. If combustible gas is being burned during stand-by operation, sufficient combustible gas is supplied for maintaining a flame e.g. in a pilot burner. Also, there should be sufficient stand-by purging of the flare tips with combustible gas to combat the effects of internal burning. Typically, in order to combat the effects of internal burning, purge gas exits the flare tip at a rate of at least 0.1 fps.

Sonic flare stacks are available in a range of sizes and can burn gas at a range of gas feed pressures, e.g. from 2 barg to 19 barg and above. Typically, a some flare stack may have a flare tip that is 20 inches in diameter, but larger, e.g. up to 36 inch diameter, and smaller tips are available.

In theory, the benefits of using a sonic flare tip are best realised for natural gas, when the gas is released to the atmosphere at a gas pressure of around 22 psig (1.5 barg) or higher.

Higher gas feed pressures, e.g. 8 barg or above, may be preferred as this may allow for smaller diameter piping to be used since the gas will be denser at higher pressures, thereby saving expense on construction materials and/or space.

Advantageously, a sonic flare stack may draw in air and dilute the combustible gas in cold vent situations.

An additional advantage of sonic flare stacks over conventional (non-sonic) flare stacks is that the flame typically may not be as long, due to the combustible gas burning more efficiently. The height of the flame may be from 10m to 180m, typically from 50m to 150m.

The person skilled in the art will be aware that sonic flare stacks are commercially available from a number of manufacturers.

Typically, the system may comprise one or more daily production sonic flare tips and one or more emergency sonic flare tips. A daily production sonic flare tip is operational most of the time, including during periods of low flow, i.e. when only relatively small volumes of gas need to be disposed of by flare burning. An emergency sonic flare tip is operational less frequently, generally during times when the flow of gas to the flare system is more than the daily production sonic flare tip(s) can accommodate and/or in particular wind and or climatic conditions.

Typically, a small volume of purge gas may be fed to each flare tip, during stand-by operation, such that the pressure in the flare tip is maintained at above atmospheric pressure. Purging serves to prevent air being drawn into the flare stack due to a vacuum- type effect or air turbulence at or around the tip. Accordingly, purging reduces the likelihood of internal burning occurring.

The purge gas may be an inert gas or a hydrocarbon gas. A hydrocarbon gas may be preferred and sufficient hydrocarbon purge gas should be supplied to a given flare stack to keep the nozzle(s) of the flare tip and/or the or each pilot burner ignited.

The intersection of the flames, during flare burning, is advantageous, because it may minimise or at least reduce the likelihood of undesirable cold venting events occurring. In particular, the arrangement of the system may permit cross-ignition, i.e. any unignited gas leaving a first flare tip may be ignited by a flame from a second flare tip, which may help to limit any build up of a vapour cloud.

Irrespective of any cross-ignition that may occur, if any given flare tip suffers from cold venting, then dispersion of the resultant vapour cloud may be more effective than from a ground flare, because of the upward inclination of the flare tip. Dispersion from the system of the present invention during cold venting may be comparable to that from system comprising a conventional single elevated sonic flare, because the system of the present invention comprises a plurality of sonic flare stacks.

Advantageously, since the flare tips are angled inwardly and upwardly, the sterile zone required around the system of the invention may be reduced as compared with elevated flares or ground flares (depending on the height of the wall or barrier around the flare tip(s) of a ground flare).

Further, the use of sonic flare tips and the upward inclination thereof means that if cold venting were to occur, in use, and no or insufficient cross-ignition took place, then any vapour cloud would be quickly dispersed. In particular, vapour cloud dispersion may be considerably quicker than for a ground flare.

The sonic flare stacks may be arranged in any suitable configuration, i.e. such that they bound an area having any suitable shape. The flare stacks may bound an area having a circular, elliptical or polygonal shape, such as a quadrilateral (e.g. square or rectangular shape), pentagonal or hexagonal shape. For instance, at least three, four, five or six sonic flare stacks may be arranged around the boundary of a trigonal, quadrilateral, pentagonal or hexagonal shape respectively.

The spacing between sonic flare stacks may or may not be uniform.

Preferably, the system may comprise up to 12 sonic flare tips. For example, the system may comprise two, three, four, five, six, seven, eight, nine, ten, eleven or twelve sonic flare tips.

The distances between the flare tips are selected such that, in use, the flame from a given flare tip does not cause heat damage to its neighbouring flare stacks and/or flare tips.

Preferably, the distance between a given flare tip and its nearest neighbour may be from 10m to 500m, more preferably from 10m to 130m, e.g. from 30m to 100m.

Preferably, the sonic flare tips may be angled inwardly at an angle of from 5° to 80°, more preferably from 30° to 60°, from the vertical. For instance, the sonic flare tips may be angled inwardly at an angle of 30° to 50°, e.g. around 45°, from the vertical.

Preferably, the sonic flare tips may be elevated, e.g. the sonic flare tips may be located at least 5m above ground level. For instance, the sonic flare tips may be located at a height of from 5m to 75m, preferably from 10m to 50m, more preferably from 10m to 30m, above ground level. The sonic flare tips may all be at substantially the same height above the ground. Alternatively, the sonic flare tips may be at different heights above the ground. For instance, one or more of the sonic flare tips may be at a first height above the ground, while the remaining sonic flare tips are at a second height. However, if the sonic flare tips are arranged at different heights above the ground, the heights should be selected such that the flame from a given flame tip does not cause heat damage to its neighbouring flare stacks and/or flare tips.

Preferably, each sonic flare tip may be associated with at least one pilot burner, more preferably three or more pilot burners. In comparison with known ground flares and elevated flares, there may be greater redundancy of pilot burners built in to the system, thereby reducing the likelihood of delayed ignition and/or cold venting occurring.

Preferably, the system may further comprise at least one discrete main pilot burner. The main pilot burner may be located between the flare tips, preferably, within the area bounded by the flare tips, e.g. the flare stacks may be arranged around the main pilot burner. Preferably, the main pilot burner is substantially equidistant from each of the flare tips.

The gas may be transported to the flare stacks along any suitable pipework. Preferably, the pipework may comprise a separate feed line (known as a header) for each sonic flare tip. The feed lines may branch off a main flow line. The main flow line may comprise overground and/or buried pipework. The pipework may be heat-shielded and/or insulated. For example, the pipework may be insulated using wrap-around insulation and/or the gas flow lines may be encased in a stainless steel shield or pipe having good heat expansion properties. There will be a gap between the stainless steel shield or pipe and the gas flow line, e.g. a pipe-in-pipe system having an annular gap between the inner pipe and the outer heat shielding pipe. This annular gap could be at least partially filled with a heat insulating material. Insulating, heat-shielding and/or burying the pipework may mean that the gas flow lines can be maintained at a temperature low enough to avoid a loss of mechanical integrity of the pipework leading to or in close proximity to the sonic flare tips, e.g. the header, main flow line, feed lines, flare stacks.

Preferably, one or more of the sonic flare tips may be independently controllable, e.g. by functionality built in to the flare tip assembly or remotely by way of one or more control valves and associated instrumentation. For instance, the particular flare tips in operation at a given time may be controlled in response to the amount of gas that is required to be flared and/or the pressure of gas in the flare header and/or the rate of change of the pressure of gas in the flare header and/or the wind speed and/or direction.

Signal control valves may be provided in the pipework leading to each individual sonic flare tip. The signal control valves may be adapted to open at different pre-set pressures depending upon the amount of gas being passed through the main flow line to the flare system. For example, a first signal control valve could open at a first set pressure, a second signal control valve could open at a second set pressure and a third signal control valve could open at a third set pressure, wherein the second set pressure is higher than the first set pressure and the third set pressure is higher than the second set pressure. For example, the first signal control valve could open at 8 bar pressure, a second signal control valve at 8.2 bar and a third signal control valve at 8.4 bar. The opening of these valves may be controlled in response to signals from a pressure measuring instrument, for example, a pressure sensor. Thus, the pressure sensor may register that the pressure in the main flow line is at the first set value such that the first signal control valve opens. The second signal control valve may open when the pressure sensor registers that the pressure in the main flow line is at the second set value and the third signal control valve may open when the pressure sensor registers that the pressure in the main flow line is at the third set value. Typically, each of the signal control valves is provided with a by-pass line, in which is positioned a safety valve, in particular, a bursting disc or a rupture pin valve of known type that opens automatically at a pressure that is slightly above the set pressure value for each signal control valve. Accordingly, if a signal control valve fails to operate, the safety valve automatically opens. Typically, the safety valve may open at a pressure of 1 bar or 3 bar above the set pressure for the signal control valve. A bursting disc or rupture pin valve may comprise a plate-shaped body, preferably of metal, e.g. steel, and is constructed so that it is blown at a desired set pressure value.

Each signal control valve may be configured such that it will close if the pressure of gas passing therethrough falls below a certain value. For instance, the first signal control valve may close if the pressure is less than 2 barg, the second signal control valve may close if the pressure is less than 2.5 barg and the third signal control valve may close if the pressure is below 3 barg.

The gas may have liquid droplets entrained therein. For example, droplets of water and/or oil may be entrained in the gas. Additionally or alternatively, the gas may contain chemical contaminants, e.g. toxic chemical substances such as ¾S . The amount and/or nature of the liquid entrained in the gas and/or the amount or nature of the chemical substances contained in the gas may vary over time at a particular site, e.g. there may be wide variation over the production life of an oil field. Preferably, the system may be arranged at a given site in such a way as to take account of the prevailing wind direction and/or variations in wind speed and/or direction. The flare system may be employed at sites where wind conditions are changeable. The arrangement of the flare stacks may be chosen such that the benefits of the system are maximised when the wind is blowing from any one of a plurality of directions. Further, the flow of gas to the flare stacks may be controlled and/or regulated in response to wind conditions.

This may be a particularly beneficial feature of the invention, because wind direction and wind speed may have a major effect on the efficiency and/or reliability of flare burning. Changes in wind direction and/or wind speed may increase the likelihood of internal burning or cold venting events.

Preferably, one or more of the sonic flare tips, particularly, flare tips that operate at higher pressure, may be arranged such that it emits gas in the prevailing wind direction. This may help to minimise instances of internal burning and may assist with dispersion should cold venting occur.

Preferably, the sonic flare tips are arranged in a non-uniform configuration around the boundary area of the flare system. Preferably, there are at least as many flare tips arranged to emit gas in the prevailing wind direction as against the prevailing wind direction. Preferably, there are more flare tips arranged to emit gas in the prevailing wind direction as against the prevailing wind direction e.g. three in or close to the prevailing wind direction and one substantially against the prevailing wind direction.

Additionally or alternatively, the flare tip(s) that emit gas in the prevailing wind direction may emit more gas, in use, than the flare tip(s) that emit gas against the prevailing wind direction. For instance, the flare tip that emits gas against the prevailing wind direction may be of smaller capacity and/or may be operated intermittently, e.g. in emergencies or only under certain wind conditions.

The flare system may additionally comprise one or more flare tip(s) that are not aligned either with or against (opposing) the prevailing wind direction such that gas is released from the additional flare tip(s) in a direction that deviates from the prevailing wind direction.

However, the present invention does not exclude a flare system where none of the flare tips are aligned with or against (opposing) the prevailing wind direction i.e. a system where the gas is released from the flare tips in directions that deviate from the prevailing wind direction.

As discussed above, the sonic flare tips may be independently controllable thereby allowing gas to be directed to the appropriate flare tip(s) depending on the wind conditions. Where the sonic flare tips are independently controllable by way of one or more signal control valves, these valves may be adapted to open in response to signals from instruments that record the wind speed and/or wind direction. In this scenario, the order in which the signal control valves open will vary depending on the wind speed and/or wind direction thereby directing the combustible gas to the flare tips that will achieve optimal flare burning for the given wind conditions. Accordingly, in this scenario, the safety valves may be designed to open at substantially the same set pressure.

In a preferred embodiment, a first flare stack, a main pilot burner and a second flare stack may be aligned with the prevailing wind direction at the site, e.g. the second flare stack may oppose the first flare stack with the main pilot burner positioned between and aligned with the first and the second flare stacks.

Another aspect of the present invention provides a method of flare burning of a combustible gas, e.g. a hydrocarbon gas, comprising:

• supplying the gas to a flare system comprising a plurality of sonic flare stacks arranged at a site, each sonic flare stack having a sonic flare tip, wherein the sonic flare tips are spaced apart around an area and are angled inwardly with respect to the area and upwardly relative to a horizontal plane through the area; and

• burning the gas, whereby flames from the sonic flare tips intersect.

In one aspect there is provided a gas burning flare system comprising a plurality of sonic flare stacks, each sonic flare stack comprising sonic flare tip and an elongate section of pipe for carrying gas to be burned to the flare tip for burning wherein the sonic flare tip is adapted to provide a flame from the flare tip directed at a selected angle from the direction of the elongate section of the pipe to enable the plurality of sonic flare stacks to be assembled at a site so that flames from flare tips converge toward a point of intersection. This and other examples of the invention have the advantage of providing a flare system that can be readily assembled at a site and which reduce the likelihood of cold venting. For a particular size or height of stack the selected angle can be selected based on at least one of the length and the bore of the elongate pipe for carrying gas in the stack to the tip. This has the advantage of providing a reliable passive means of reducing the likelihood of cold venting at an installation without the need for engineers installing the stack to make detailed calculations of the necessary angles and/or pipe sizes/ flow pressures. This can help to ensure that the flames converge towards a point of intersection and preferably that they fully or partially intersect.

The system and/or the stacks themselves may comprise a flow controller operable to control the flow of gas into at least one of the flare stacks based on at least one parameter selected fiom the list comprising: the pressure of gas in a flare header; the rate of change of the pressure of gas in the flare header; the wind speed; and the wind direction. The controller may include a valve and a controller coupled to a wind speed/direction sensor wherein the controller is operable to control the valve based on at least one of the sensed wind speed and/or direction. In some cases the controller is coupled to control the flow of gas in a plurality of the stacks based on their relative positions and the wind direction and/or speed. For example, the flow controller may be operable to control the flow of gas into one or more of the sonic flare tips independently from the flow of gas into at least one of the other sonic flare tips. In this way flow and pressure in the stacks can be used to compensate for wind speed and direction to reduce the likelihood of cold venting.

In order that the invention may be more readily understood, some embodiments will now be described, by way of example with reference to the accompanying drawings, in which:

• Figure 1 shows a schematic plan view of a system according to the invention;

• Figure 2 shows a vertical cross-section through the system of Figure 1 ;

• Figures 3 to 7 show computer simulations of the distribution of the heat of a flame from a sonic flare tip;

• Figure 8 shows a computer simulation of the gas dispersion during cold venting from a daily production sonic flare tip; and

• Figure 9 shows a computer simulation of the gas dispersion during cold venting from an emergency sonic flare tip.

• Figure 10 shows Figure 1 shows a schematic plan view of a flare system according to the invention. The system 1 comprises four sonic flare tips 2, 3, 4, 5 and is arranged at a site having a prevailing wind direction indicated by arrow A.

A discrete pilot burner 6 is located centrally within a sterile zone 7 (substantially equidistant from the flare tips), in which the flare tips 2, 3, 4, 5 are located with the flare tips being arranged around the perimeter of a circle. The pilot burner 6 may be a high flow rate continuously lit or on-demand pilot burner.

One of the flare tips (hereinafter the first flare tip) 5, the pilot burner 6 and another one of the flare tips (hereinafter the second flare tip) 3 are aligned within the sterile zone 7 in the prevailing wind direction A. The pilot burner 6 is located between, and approximately equidistant from, the first flare tip 5 and the second flare tip 3. The first flare tip 5 and the second flare tip 3 are located downwind and upwind respectively relative to the prevailing wind direction A, of the pilot burner 6.

The two other flare tips 2, 4 (hereinafter the third and fourth flare tips) are located within the sterile zone 7 either side of the second flare tip 3, upwind relative to the prevailing wind direction A of the pilot burner 6. The third flare tip 2 and the fourth flare tip 4 are approximately equidistant from the second flare tip 3.

The arrangement of the flare tips 2, 3, 4, 5 and the pilot burner is symmetrical about a line passing through the first flare tip 5, the pilot burner 6 and the second flare tip 3.

The system I further comprises a low pressure flare tip 28, which is located in the vicinity of the second flare tip 3. Gas is supplied to the low pressure flare tip 28 via a conduit 29. Low pressure flare tips are sometimes termed atmospheric flare tips. Gas can sometimes be supplied to low pressure flare tips at pressures of up to 20 barg.

Combustible gas for flare burning is supplied to the system 1 by a main pipeline 8. Where the system 1 is associated with an oil production facility, the gas will typically have been separated from oil and/or produced water, but may still have droplets of water and/or oil entrained therein. Such droplets may have a size of up to approximately 0.5mm after separation in flare knock-out drums or separators. Some sonic flare tips are capable of handling gas containing up to 4 wt% entrained liquid. Additionally or alternatively, the system may be associated with a gas production facility or a gas condensate production facility.

A first purge gas flow line 9 joins the main pipeline 8 upstream of a first branch line 20, which communicates with the first flare tip 5. The first branch line 20 is provided with a first signal control valve 12 operable to control gas flow to the first flare tip 5. A first safety valve 1 1 is provided in parallel with the first signal control valve 12. The first safety valve 11 fails to an open position. For example, the first signal control valve 12 may be controlled via a signal from a pressure recording instrument such that it opens at a first set pressure and the first safety valve 1 1 may be a bursting disc valve that opens at a pressure marginally above the first set valve in the event of the first signal control valve failing to open.

Downstream of the first branch line 20, a pilot burner branch line 25 branches off the main pipeline 8. The pilot burner branch line 25 provides fluid communication between the main pipeline 8 and the pilot burner 6.

Further downstream, the main pipeline 8 splits into a second branch line 21 and a third branch line 22.

The second branch line 21 communicates with the second flare tip 3 and is provided with a second signal control valve 15 operable to control gas flow to the second flare tip 3.

A second safety valve 1 is provided in parallel with the second control valve 15. The second safety valve 16 fails to an open position.

The third branch line 22 leads into a first sub-branch 24 that communicates with the third flare tip 2 and a second sub-branch 23 that communicates with the fourth flare tip 4.

Upstream of the point at which it splits into the first sub-branch 24 and the second sub-branch 23, the third branch line 22 is provide with a third signal control valve 14 operable to control gas flow to the third flare tip 2 and the fourth flare tip 4.

A third safety valve 13 is provided in parallel with the third control valve 14. The third safety valve 13 fails to an open position.

A second purge gas flow line 10 is provided, which splits into a first sub-line 26 and a second sub-line 27. The first sub-line 26 feeds into the second branch line 21 at a point downstream of the second control valve 15 and the second safety valve 16. The second sub-line 27 feeds into the third branch line 22 at a point downstream of the third control valve 14 and the third safety valve 13.

Figure 2 shows a vertical cross section through the system 1 along the line passing through the first flare tip 5, the pilot outlet 6 and the second flare tip 3.

The first flare tip 5 and the second flare tip 3 are angled upwardly and inwardly at about 45° from the vertical. In use, a first flame 18 from the first flare tip 5 and a second flame 17 from the second flare tip 3 intersect in a flame cross-over zone 19.

The flare tips, e.g. the first flare tip 5 and the second flare tip 3, are provided on flare stacks. Each flare stack comprises a substantially vertical section of pipe 33, 31 and an inclined section of pipe 32, 31 - which is inclined at approximately [] to the vertical - that leads to the flare tip 5, 3. Each flare tip typically has associated therewith a plurality of nozzles, one or more pilot burners, igniters, wind shielding apparatus and a maintenance platform.

It should be appreciated that the third flare tip 2 and the fourth flare tip are also angled upwardly and inwardly so that the flames emitted therefrom, in use, pass at least partially through or into the flame cross-over zone 19.

Figures 3, 4, 5, 6 and 7 are graphs showing computer models of the heat flux around a flame from a sonic flare tip located approximately 10 metres above ground level under selected gas flow and wind speed conditions. In each graph, height above the ground (measured in metres) is marked on the y-axis and distance from the flare tip (measured in metres) is marked on the x-axis.

In the situation illustrated in Figure 3, gas is flowing to the sonic flare tip at a rate of 0.4 bcfd (billion cubic feet per day wherein one billion cubic feet per day is equivalent to 10 ⁹ cubic feet per day) and the wind speed is 30 ms ^"1 in the direction indicated by arrow B.

The cross-sectional profile of a flame 30 produced under these conditions is shown. Around the flame 30 is a series of thermal radioactive isopleths 31, 32, 33, 34, 35 and 36 corresponding to specific heat fluxes. The heat flux at isopleth 31 is 30 kWm ^"2 ; at isopleth 32 it is 15.8 kWm ^"2 ; at isopleth 33 it is 12.5 kWm ^"2 ; at isopleth 34 it is 6.3 kWm ^"2 ; at isopleth 35 it is 4.7 kWm ^'2 ; and at isopleth 36 it is 1.3 kWm ^"2 .

In the situation illustrated in Figure 4, gas is flowing to the sonic flare tip at a rate of 1 bcfd and the wind speed is 30 ms ^"1 in the direction indicated by arrow C.

The cross-sectional profile of a flame 40 produced under these conditions is shown. Around the flame 40 is a series of isopleths 41, 42, 43, 44, 45 and 46. The heat flux at isopleths 41 is 30 kWm ^"2 ; at isopleth 42 it is 15.8 kWm ^'2 ; at isopleth 43 it is 12.5 kWm ^"2 ; at isopleth 44 it is 6.3 kWm ^"2 ; at isopleth 45 it is 4.7 kWm ^"2 ; and at isopleth 46 it is

1.3 kWm ^"2 .

In the situation illustrated in Figure 5, gas is flowing to the sonic flare tip at a rate of 4 bcfd and the wind speed is 1 ms ^'1 in the direction indicated by arrow D.

The cross-sectional profile of a flame 50 produced under these conditions is shown. Around the flame 50 is a series of isopleths 51, 52, 53, 54, 55 and 56. The heat flux at each of the isopleths 51, 52, 53, 54, 55 and 56 is 30 kWm ^"2 , 15.8 kWm ^"2 , 12.5 kWm ^'2 , 6.3 kWm ^'2 , 4.7 kWm ^'2 and 1.3 kWm ^"2 respectively.

In the situation is illustrated in Figure 6, gas is flowing to the sonic flare tip at a rate of 1 bcfd and the wind speed is 30 ms ^"1 in the direction indicated by arrow E.

The cross-sectional profile of a flame 60 produced under these conditions is shown. Around the flame 60 is a series of isopleths 61, 62, 63, 64, 65 and 66. The heat flux at each of the isopleths 61, 62, 63, 64, 65 and 66 is 30 kWm ^" \ 15.8 kWm ^"2 , 12.5 kWm ^"2 , 6.3 kWm ^"2 , 4.7 kWm ^"2 and 1.3 kWm ^"2 respectively.

In the situation illustrated in Figure 7, gas is flowing to the sonic flare tip at a rate of 1 bcfd and the wind speed is 5 ms ^"1 in the direction indicated by arrow F.

The cross-sectional profile of a flame 70 produced under these conditions is shown. Around the flame 70 is a series of isopleths 71, 72, 73, 74, 75 and 76. The heat flux at each of the isopleths 71, 72, 73, 74, 75 and 76 is 30 kWm ^"2 , 15.8 kWm ^'2 , 12.5 kWm ^'2 , 6.3 kWm ^'2 , 4.7 kWm ^'2 , and 1.3 kWm ^"2 respectively.

The results of the computer modelling depicted in Figures 3 to 7 show that the preferred selected flare tip spacings in the system according to the present invention may be appropriate, insofar as the heat flux from the flame from a given flare tip should not cause significant heat damage to its neighbouring flare stack and/or flare tip.

Figure 8 is a graph showing a computer model of gas dispersion from a daily production sonic flare tip located approximately 25 metres above ground level in a cold vent scenario. The wind direction is indicated by the arrow G and the downwind distance (measured in metres) is indicated on the x-axis.

In the situation shown in Figure 8, natural gas having a molecular weight of from 15 to 22 is flowing to the sonic flare tip at a rate of 1 bcfd and a pressure of 6 barg. The wind speed is 8ms ^"1 . The graph shown in Figure 8 illustrates that the gas disperses effectively under these conditions.

Figure 9 is a graph showing a computer model of gas dispersion from an emergency sonic flare tip located approximately 25 metres above ground level in a cold vent scenario. The wind direction is indicated by the arrow H and the downwind distance (measured in metres) is indicated on the x-axis.

In the situation shown in Figure 9, methane is flowing to the flare tip at a rate of 500 bcfd and a pressure of 6 barg. The wind speed is 2 ms ^'1 . It can be seen from Figure 9 that the gas disperses effectively under these conditions.

The system and method of the invention provide improved, especially safer, flare burning. The system can be designed to meet the requirements of and accommodate the prevailing wind conditions at a wide variety of sites.

For instance, the system will typically provide for better gas dispersion than a conventional ground flare. The capital expenditure for a system according to the invention is likely to be no more than (typically lower than) for a ground flare having the same operational capacity.

To assess a flare according to an example of the invention, a computational fluid dynamics model was constructed using the STAR-CCM+ software package (produced by CD-Adapco Limited of 200 Shepherds Bush Road, London W6 7NL).

The model assumed the combustion of 1,000 MMscfd (million standard cubic feet per day) or 847m ³ per hour at standard temperature and pressure, 273.15 K, 100 kPa. The simulation assumed 4 sonic flare tips, each lm in length and internal diameter 0.18m. The sonic flare tips were set up in a ring arrangement with a ring diameter of 40m and treated as having a stack height (height above the ground) of 100m. The flow rates and flare diameters were selected to give sonic velocity at the exit point for methane gas, flowing at 1,000 MMScfd. As will be understood by the skilled reader in the context of the present disclosure, these values are merely exemplary and other examples of sonic flare tips can be used.

The gas flow was distributed equally between the flare tips and the conditions at each tip were 263K and a pressure of 7 barg (e.g. 700 kPa above ambient pressure). Ambient conditions for the simulations (external to the nozzles) were 288K and 100 kPa. Using these parameters methane mole fraction iso-surfaces, temperature iso-surfaces, temperature profiles, velocity profiles, ground radiation profiles, and vertical plane radiation profiles were determined using the computational fluid dynamics package for two cases.

In a first case, Case A, as a comparative example the flare tips in the simulation were arranged to point vertically upwards. In a second case, Case B, the flare tips were pointed inwardly and upwardly at an angle of 45° to the horizontal plane of the ring of nozzles, and radially towards the centre of the ring.

It was found that the 1% methane mole fraction iso-surfaces for Case A extended to a height of 60m above the ring of flare tips. In Case B the 1% methane mole fraction iso- surface extended to a height of 130m above the flare tips.

The temperature iso-surfaces, selected for a temperature of 800K showed a height of 120m above the ring of flare tips for Case A, and to a height of 220m above flare tips for Case B. Studying the temperature profile within each combustion zone showed that in Case B higher temperatures were reached over a greater volume of the flare.

A consideration of the velocity profile indicated that in Case A the velocity of the gas in the flare was substantially lower than the velocity in Case B.

In Case A the ground radiation profile, (e.g. the power of radiant heat per unit area falling on a planar surface 100m below the flare tips) showed that the total radiation reaching the ground surface summed over a total ground area of 7.8 x 10 ⁶ m ² was 14.8 GW. For Case B total radiation over the same total area was 14.5 GW. The pattern of distribution of energy beneath the flare tips extended further in Case B but was more diffuse. As a result of this variation in the distribution of radiant heat, in Case A the total radiation on 1km ² of surface 100m below the nozzles was 2.8GW. Meanwhile, in Case B the total radiation on a 1km ² under the nozzles was 2.5GW. This is a reduction of approximately 10% in the total radiation power measured at ground level in close proximity to the flare.

For Case A the maximum distance from the centre point of the flare at ground level to a point having a radiation intensity of 4.7kw/m ² was 100m. The maximum distance from the centre point of the flare nozzles at ground level to a point at a radiation intensity of 4.7kw/m ² for Case B was 85m. This shows that, in examples of the invention the heat produced by the flare is more tightly confined.

Figure 10 shows the radiation profile contours for Case A. Figure 11 shows the radiation profile contours for Case B. In both Figures 10 and 11 the position and size of the calculated flares is shown at the centre of the drawing (X = 0, Y = 100) by the heavy dark region 100. A comparison of Figures X and Y clearly shows that in Case B the lateral extent of the regions of high radiation is far more tightly confined. Many modifications to the invention will be apparent to the person skilled in the art without departing from the scope of the invention.