Flare stack

Abstract

A Coanda flare is described that provides improved operation of a flare stack. A Coanda flare comprises a gas flue having a flare end, a Coanda body disposed adjacent the flare end to form an annular gap between the Coanda body and the flare end of the gas flue, such that gas emitted from the flare end passes through the annular gap and around the Coanda body, and a gas flow adjustor for adjusting pressure on gas flowing through the annular gap. The gas flow adjustor may be provided by a dual acting piston, which may for example be sensitive on one side to waste gas pressure, and on the other side to a control pressure.

Description

BACKGROUND

U.S. Pat. No. 6,960,075 describes a flare stack operating on the Coanda principle with an adjustable gap for flare gas to pass through. This was done to allow the gap to be closed off by the Coanda body at low pressures to avoid having to purge the gas flue. While this product has served its purpose, the device was designed to provide constant back pressure on gas in the flue. This meant that with different gas compositions, the resulting flame had different, and sometimes deleterious, characteristics. For example, with heavy ends and constant pressure, the flame may shorten and become more turbulent. Turbulent flow close to the flare tip can cause premature erosion of the flare tip. The spring also caused a risk of chatter at the gap, and hence increased noise.

SUMMARY

A gas flare is described that provides improved operation of a flare stack. A gas flare comprises a gas flue having a flare end, a body disposed adjacent the flare end to form a gap between the body and the flare end of the gas flue, such that gas emitted from the flare end passes through the gap and around the body, and a gas flow adjustor for adjusting the gap, hence the pressure on gas flowing through the gap. In one embodiment, the body is adjustable and connected to the gas flow adjustor. Pressure or flow adjustment allows adjustment for gas composition and therefore tailored flame characteristics. The gas flow adjustor may be provided by a dual acting piston, which may for example be sensitive on one side to waste gas pressure, and on the other side to a control pressure. The body may be a Coanda body, and gap may be annular.

There is also provided a method for operating a gas flare in which gas pressure of gas flowing through the gap of a gas flare is adjusted to provide desired flame characteristics. Thus, for example, in the case of a heavier mix of gas, the gap size may be decreased to increase flow rate and extend the flame beyond the metal of the flare tip

These and other aspects are set out in the claims, which are incorporated here by reference.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments of a gas flare stack will now be described with reference to the figures, in which like reference characters denote like elements, by way of example, and in which:

FIG. 1 is a section through a Coanda flare; and

FIG. 2 is a schematic of a gas flow adjustor for a Coanda flare, in which a solid line with arrows indicates a primary flow line, a line with repeated hash marks indicates instrument tubing and a wavy line indicates flex hose.

DETAILED DESCRIPTION

In the claims, the word “comprising” is used in its inclusive sense and does not exclude other elements being present. The indefinite article “a” before a claim feature does not exclude more than one of the feature being present.

Referring to FIG. 1, there is shown an embodiment of a flare stack 100 of the Coanda type, which has a Coanda body 10 disposed adjacent the flare end 11 of a gas flue 12. Gas flue 12 receives gas from a waste gas source 17 (FIG. 2), for example, an oil-gas separator, through pipe connection 13. Pipe connection 13 and the gas flue 12 are secured to one side of a frame 15 that is secured on its other side to a gas flow adjustor, which in the embodiment shown includes a dual acting cylinder 20. The double acting cylinder 20 incorporates a dual acting piston 19 having a lower piston face 22 that responds to pressure in a lower chamber 23 of the dual acting cylinder 20 and an upper piston face 24 that responds to pressure in an upper chamber 25 of the dual acting cylinder 25. The dual acting piston 19 is connected to the Coanda body 10 through shaft 21. The Coanda flare stack 100 is mounted in conventional fashion on a support structure.

The Coanda body 10 responds to gas pressure in the gas flue 12 by lifting off the flare end 11 to form an adjustable annular gap 18 between the Coanda body 10 and the flare end 11 of the gas flue 12. Gas emitted from the flare end 11 passes through the adjustable annular gap 18 and around the Coanda body 10. The annular gap 18 is controlled by the gas flow adjustor, which in the embodiment of FIG. 2 is provided by controlled pressure on either side of the dual acting piston 19. The weight of the Coanda body tends to close the gap, but the device can be set with a minimum gap such as 0.100 inches by providing an uneven shape to the mating surfaces of the Coanda body and flare tip. The uneven shape may be provided by any suitable means such as small bumps on the end of the gas flue.

FIG. 2 shows a gas flow adjustor for two Coanda flare stacks 100. The description of the gas flow control system will be described for just one of the two Coanda flare stacks 100. In any flare facility, there may be one, two or more Coanda flare stacks 100, such as six or eight. As shown in FIG. 2, waste gas to be flared is provided through line 26 and connection 13 to the gas flue 12. The waste gas is also provided to the dual acting cylinder 20 so that the lower face 22 of the dual acting piston 19 is acted upon by the gas pressure of the gas to be flared. The flare gas line 26 is connected to the cylinder 20 by gas line 28. Gas line 28 ends with flex hose connected into lower chamber 23 of the dual acting cylinder 20. The upper face 24 of the dual acting piston 19 of the flare stack 100 is acted upon by gas from a separate pressure source 30 connected to piston 20 by gas lines 31 and 32. Gas line 32 terminates in flex hose 33 that connects to the upper chamber 25 of the dual acting cylinder 20. The pressure in the upper chamber 25 of each dual acting cylinder 20, hence on the upper faces 23, is adjustable by a pinch valve 34, although other means of adjusting the pressure could be used. In this embodiment, the pressure source 30 used is LPG (liquid petroleum gas), which may also used to supply pilot burners 35 with fuel through line 37. Pressure in the line 37 may be controlled by a pinch valve 39. The pilot burner 35 is any suitable burner known in the art. It will be understood that other pressure sources 30 could be substituted, depending on the desired configuration.

Increasing pressure from the waste gas source 17 acting against the lower face 22 of the dual acting piston 19 acts to increase the gap 18, while increasing pressure against the upper face 24 of the dual acting piston 19 acts to reduce the gap 18. In one embodiment, there is a deadband where pressure in the gas flue from the waste gas does not increase the gap. The device thus has a deadband from 0 lb up to some set pressure where lift off of the Coanda body occurs. As the pressure of gas to be flared increases beyond the set point, for example 1 lb to 5 lb of pressure, the force against the lower face 22 causes the gap 18 to increase, in order to accommodate the higher rate of flow. As pressure in the gas flue increases, without control pressure, the gap tends to increase until the gap is at a maximum. A gap range of 0.51-0.550 inches may be used in some embodiments. The pressure at which the gap reaches a maximum can be set by the designer, as for example 10 lb. FIG. 2 also shows a flame arrestor 36 on the waste gas line 26, and various ball valves 38 for controlling flow of gas in the gas lines.

In the embodiment of FIG. 2, the control pressure against the upper face 24 tends to close the gap, and thus increase gas flow. The control pressure is thus chosen to maintain a desired rate of flow of the gas to be flared to achieve certain flare characteristics, such as flame speed, flame pattern, and flame length. The control pressure may be adjusted by operation of valve 34 manually or by an automatic system. With higher flow rates, the flame will lengthen. By increasing the flame length, for example, the heat released will not be concentrated on the Coanda body 10, which helps reduce the damage caused by heat exposure, and a relatively flat orifice velocity is obtained, which reduces noise levels. Thus, a mix of flare gas with larger amounts of propane, and relatively less methane, tends to burn with a shorter flame and more turbulence, which results in more damage to the flare tip, as compared with a lighter gas having more methane, at a given gap size. In the case of propane, therefore, the gap size may be decreased to increase flow velocity and extend the burn beyond the Coanda body.

The control pressure may also chosen to achieve a certain level of stability for the Coanda body 10, and the noise level emitted by the flare stack 100. The noise level can be reduced significantly by choosing a control pressure that allows gas to escape from the gap 18 at sub-sonic speeds. In addition, by having a regulated pressure acting against one side of the dual acting piston 19, ball flutter due to high frequency vibration of the Coanda body 10 is reduced. Not only does this increase the efficiency of the flare stack 100, it also reduces the noise as these vibrations would otherwise be transferred to the flame front since the flow of gas would be disrupted by a changing gap 18. Favorable results have been obtained by using a control pressure between 2 and 15 psig, and with the gap 18 adjustable between about 0.05 inches and 0.5 inches, where 0.05 is the minimum width of the gap 18. The control pressure that may be used to close the gap may be important in some applications such as emergency shutdown or blow down situations.

In an array of gas flares, some gas flares may be isolated by turning off gas flow to the gas flue while providing additional flow to the remaining gas flares.

Immaterial modifications may be made to the embodiments described above without departing from what is claimed. For example, the gas flow adjustor may be provided by a dual acting diaphragm, or an electronic device responsive to a pressure or flow sensor on the waste gas flow line, flow sensors, noise sensors, flame sensors and/or other sensors. While in some embodiments a Coanda body is used for its desirable gas flow characteristics, the gas flare control may also be used with a plate or other mechanism that provides an adjustable gap. While the Coanda body is shown as adjustable, it is possible in some embodiments to fix the Coanda body or other body and adjust the gap through other means such as movement of the upper end of the gas flue. Gas from the gas source may also be provided to both sides of the actuating cylinder 20 for added control.

Claims

1. A gas flare, comprising:

a gas flue having a flare end;

the gas flue being connected to a source of waste gas;

a body disposed adjacent the flare end to form a gap between the body and the flare end of the gas flue, such that gas emitted from the flare end passes through the gap and around the body; and

a gas flow adjustor connected to the gas flue and to the body to adjust the gap and control gas flow through the gap.

2. The gas flare of claim 1 in which the gap is annular.

3. The gas flare of claim 2 in which the gas flow adjustor is responsive to waste gas pressure and to a control pressure from a pressure source.

4. The gas flare of claim 3 in which the gas flow adjustor comprises:

a dual acting piston connected to the body for adjusting the annular gap, the dual acting piston being responsive on a first side of the dual acting piston to waste gas pressure and on a second side of the dual acting piston to the control pressure.

5. The gas flare of claim 1 in which the pressure source has an operating pressure range that includes 2 psig and 15 psig.

6. The gas flare of claim 3 in which the pressure source is a source of LPG.

7. The gas flare of claim 6 further comprising a gas pilot connected to the pressure source.

8. The gas flare of claim 2 in which the annular gap has an adjustment range that includes gaps of about 0.05 inches and about 0.5 inches.

9. The gas flare of claim 2 in which the body is a Coanda body.

10. The gas flare of claim 3 in which the body is a Coanda body.

11. The gas flare of claim 4 in which the body is a Coanda body.

12. A method of controlling a gas flare connected to a waste gas source, comprising:

providing a gas flare with an adjustable gap between a body and a flare end of a gas flue;

supplying the gas flue with waste gas from the waste gas source; and

controlling the pressure of gas flowing through the adjustable gap by controlling the gap size.

13. The method of claim 12 in which the body is an adjustable Coanda body.

14. The method of claim 13 in which controlling the pressure of the gas flow comprises providing a control pressure from a gas source to a first side of a pressure responsive control device.

15. The method of claim 14 in which the pressure responsive control device is a piston or diaphragm linked to the adjustable Coanda body.

16. The method of claim 15 in which:

the pressure responsive control device has a second side; and

waste gas pressure is provided to the second side of the pressure responsive control device.

17. The method of claim 16 in which the gap is adjusted to provide a desired flame characteristic.

18. The method of claim 17 in which the desired flare characteristic is selected from a group consisting of flame speed, flame pattern, flame length, Coanda body stability, and noise level.

19. The method of claim 12 in which the gap is adjusted to provide a desired flame characteristic.

20. The method of claim 19 in which the desired flare characteristic is selected from a group consisting of flame speed, flame pattern, flame length, adjustable body stability, and noise level.